Background: Volumetric Modulated Arc Therapy (VMAT) is an advanced radiotherapy technique that enables highly conformal dose delivery while minimizing radiation exposure to surrounding healthy tissues. The collimator angle is a critical planning parameter that influences beam modulation, dose distribution, and treatment efficiency. However, its quantitative impact on treatment outcomes remains insufficiently defined across different anatomical sites and planning techniques. 

Purpose: This study aimed to systematically review and meta-analyze the impact of collimator angle on target coverage, dosimetric parameters, organ-at-risk (OAR) sparing, treatment efficiency, and clinical outcomes in VMAT. 

Methods: A systematic review was conducted following PRISMA guidelines, including peer-reviewed studies published. Databases searched included PubMed, Scopus, Web of Science, Embase, and Google Scholar. Studies were included if they evaluated collimator angle effects on VMAT dosimetry, including parameters such as D95, Dmean, conformity index (CI), homogeneity index (HI), monitor units (MUs), and OAR dose. A narrative and thematic meta-analysis was performed. 

Results: Across multiple studies, target coverage remained largely stable over a wide range of collimator angles (0°–90°), demonstrating the robustness of VMAT optimization algorithms. However, moderate collimator angles between 15° and 45° consistently produced optimal dosimetric outcomes, including improved CI, HI, and reduced MUs. In prostate VMAT, optimal performance was observed at 20°, 30°, and 45°, while nasopharyngeal carcinoma studies reported improved PTV coverage of approximately 92–94% within the 15°–30° range. Advanced optimization techniques such as SACAO reduced monitor units by approximately 15.7% and field size by 41.1%, significantly improving efficiency and dose conformity. Higher collimator angles (>45°) demonstrated mixed effects, including improved OAR sparing in some cases but reduced delivery accuracy, with γ-index passing rates decreasing by up to 5.6%. Furthermore, collimator misalignment exceeding ±1° resulted in substantial degradation of target coverage and homogeneity, with variations in HI reaching up to 75% in extreme cases. OAR sparing was primarily influenced by multileaf collimator (MLC) design and advanced delivery techniques rather than collimator angle alone. MLC-based shielding reduced OAR dose by up to 20%, while narrower leaf widths (2.5 mm vs 5 mm) improved dose gradients and normal tissue sparing. 

Conclusion: Collimator angle has minimal direct impact on target coverage but significantly influences plan quality, efficiency, and treatment robustness. Moderate angles (15°–45°) provide optimal dosimetric performance, while advanced optimization techniques are particularly beneficial in complex cases. Accurate delivery and strict quality assurance are essential, as mechanical errors greater than ±1° can negate dosimetric advantages. Collimator angle should therefore be integrated into a comprehensive, patient-specific VMAT planning strategy.

Volumetric Modulated Arc Therapy (VMAT) is a radiotherapy technique that delivers radiation in a continuous arc while simultaneously modulating the intensity of the radiation beam. It offers several advantages over other techniques. Firstly, VMAT allows for rapid treatment delivery, minimizing the time patients spend on the treatment table. Secondly, it provides better sparing of organs-at-risk (OARs), reducing the radiation dose to healthy tissues surrounding the tumor. Thirdly, VMAT offers improved target coverage, ensuring that the tumor receives the prescribed dose while minimizing the dose to surrounding healthy tissues. Lastly, VMAT has shown to be robust and reproducible, providing consistent treatment outcomes. These advantages have been demonstrated in various studies, including those by Zhang et al. (2023), Hunte et al. (2022), Chen et al. (2020) and Zhang et al. (2022). Volumetric Modulated Arc Therapy (VMAT) is a modern technique used in radiotherapy that offers superior conformity, efficiency, and reproducibility in the treatment of various cancers. It involves delivering radiation in an arc-shaped trajectory while simultaneously modulating the intensity of the radiation beam. VMAT has several advantages over traditional techniques such as Three-Dimensional Conformal Radiotherapy (3DCRT). It has been shown to improve dosimetric measures and treatment outcomes in prostate, cervical carcinoma, and head and neck cancer patients. VMAT has also been found to reduce the mean dose to critical organs such as the bladder and bowel bag in rectal cancer patients, resulting in lower complication probabilities. Additionally, VMAT has been associated with lower radiation doses to the remaining kidney and better coverage of the planning target volume (PTV) in whole abdominal irradiation for Wilms tumor treatment. Overall, VMAT offers improved treatment precision, reduced toxicity, and better treatment outcomes compared to traditional radiotherapy techniques The collimator angle plays a crucial role in VMAT planning. Different collimator angles can affect various dosimetric parameters such as target coverage, conformity index (CI), homogeneity index (HI), gradient index (GI), and monitor units (MUs) (Hashir, et al., 2023; Rose et al.,2023; Jiuling et al., 2022; Wuji et al., 2021). Optimal collimator angles have been found to improve dose conformity, homogeneity, and MUs (Lauren et al., 2021). In nasopharynx carcinoma, collimator angles between 15°-30° have shown better target coverage and reduced MUs . A novel algorithm of sub-arc collimator angle optimization (SACAO) has been developed to improve dosimetric quality and treatment delivery efficiency in single-isocenter coplanar VMAT stereotactic radiosurgery (SRS) for multiple metastases. For whole-brain radiotherapy with hippocampus and inner ear sparing, increasing the collimator angle towards 90° has shown significant improvements in plan quality, including better CI, HI, and reduced doses to organs-at-risk . However, collimator angle optimization may have limited benefit in reducing island blocking in single-isocenter multiple-target (SIMT) SRS plans. Understanding the impact of collimator angle on treatment outcomes is important in radiation therapy. Different collimator angles can affect the surface dose, dose conformity, homogeneity, and dose to organs at risk (OARs) (Jeong-Ho et al., 2022; Hashir et al., 2022; Jiuling et al., 2022; Yong et al., 2017). Studies have shown that the surface dose increases as the collimator angle increases, and the length of the tumor also affects the surface dose. Optimal collimator angles have been found to improve dose conformity, homogeneity, and treatment efficiency. Additionally, collimator angle optimization can lead to better target dose distribution and sparing of OARs. The choice of collimator angle can impact dosimetric parameters such as target coverage, CI, HI, GI, and MUs. Therefore, understanding the impact of collimator angle is crucial for optimizing treatment plans and achieving better treatment outcomes in radiation therapy. The optimal collimator angle in radiation therapy has been a topic of controversy. Several studies have investigated the impact of collimator angle on treatment plan quality and organ-at-risk sparing. Battinelli et al. found that optimizing the collimator angle can reduce the exposed area between lesions in non-isocentric treatments, leading to more efficient treatment and potential time savings (Cecilia et al., 2021). Sun et al. demonstrated that increasing the intersection angle between collimator angles in dual-arc VMAT plans for whole-brain radiotherapy can improve plan quality, including conformity and homogeneity indices, as well as reduce monitor units and dose to organs-at-risk (Wuji et al., 2021). Saeed et al. concluded that different collimator angles have minimal impact on target coverage and dose to organs-at-risk in prostate carcinoma patients, suggesting the need for solid decision-making regarding collimator angles (Hashir et al., 2023). Mancosu et al. highlighted the significance of collimator angle rotation in optimizing dose distribution for vertebral metastases, with leaf travel parallel to the spinal cord primary orientation yielding the best plans (Pietro et al., 2010). Andersen et al. recommended maintaining collimator angles within 15-30 degrees based on various dosimetric parameters and quality assurance results (Andersen et al., 2016). The main objective of the paper is to systematically evaluate the available literature on the impact of collimator angle on treatment outcomes in VMAT. To assess the influence of collimator angle on dose distribution to the target and critical organs and to identify the optimal collimator angle for various treatment scenarios and potential recommendations for clinical practice.

Study Design and Search Strategy

This study followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to conduct a systematic review on the impact of collimator angles on treatment outcomes in Volumetric Modulated Arc Therapy (VMAT). A comprehensive literature search was performed in major databases, including PubMed, Scopus, Web of Science, Embase, and Google Scholar, from their inception to January 2025. The search terms included: Collimator angle, Volumetric Modulated Arc Therapy" OR "VMAT, Treatment outcomes, Dosimetric impact and Radiotherapy planning. Boolean operators (AND, OR) were applied to refine search queries and retrieve relevant studies. Additional hand-searching of the reference lists of included articles was conducted to ensure comprehensive coverage.

Inclusion and exclusion Criteria

Data extraction and Synthesis 

Data extraction was performed using a standardized form that comprised of Title, Objective, Methods Used, Results, Conclusions, Contributions, Practical Implications and References. Any discrepancies in data extraction were resolved through discussion and consensus among the reviewers. A narrative synthesis approach was employed to summarize and interpret the findings from the selected studies (Samaila et al., 2024; Samaila et al., 2025).

Ethical Considerations 

This review exclusively involved the narrative analysis of published data, and therefore, ethical approval was not required.

Numerous studies have explored the impact of collimator angles on dose distribution to target volumes and critical organs. Findings indicate that the optimal collimator angle varies significantly depending on the specific treatment site and technique. In prostate cancer patients, collimator angles of 20°, 30°, and 45° have demonstrated superior dose conformity, homogeneity, and monitor unit (MU) efficiency. For nasopharyngeal carcinoma, collimator angles within the 15° to 30° range have been shown to enhance target coverage while reducing MU usage. In the context of stereotactic body radiotherapy (SBRT) for non-small cell lung cancer, MLC positional errors, including collimator angle deviations, can significantly impact dose distribution, with VMAT being more susceptible than IMRT. For single-isocenter coplanar VMAT stereotactic radiosurgery of multiple brain metastases, a novel algorithm known as Sub-Arc Collimator Angle Optimization (SACAO) has been shown to improve dosimetric quality and treatment delivery efficiency. The selection of collimator angles in VMAT should be carefully evaluated for each treatment scenario to optimize plan quality, target coverage, and treatment efficiency.

Impact of collimator angle on Target Coverage 

The studies summarized in Table 1 collectively demonstrate that collimator angle has a limited influence on baseline target coverage but plays a significant role in optimizing overall plan quality in volumetric modulated arc therapy (VMAT). In prostate cancer planning, both Saeed et al. (2023) and Isa et al. (2014) consistently reported that planning target volume (PTV) coverage remains largely unchanged across a wide range of collimator angles (0°–90°). This finding suggests that modern inverse planning algorithms are sufficiently robust to maintain clinically acceptable coverage regardless of collimator orientation. However, while coverage remains stable, key dosimetric parameters such as conformity index (CI), homogeneity index (HI), and monitor units (MUs) are sensitive to angle selection, indicating that collimator angle primarily influences plan quality refinement rather than fundamental dose delivery. A consistent trend across multiple studies and anatomical sites is that moderate non-zero collimator angles, typically within the range of 15° to 45°, provide the most favorable dosimetric outcomes. Saeed et al. (2023) identified optimal plan performance at 20°–45°, while Isa et al. (2014) reported improved conformity and homogeneity at 45°. Similarly, Thomas et al. (2023) demonstrated that angles between 15° and 30° achieved superior PTV coverage and reduced monitor units in nasopharyngeal carcinoma, whereas very low angles (5°–10°) resulted in poorer coverage. These improvements are likely attributable to enhanced multileaf collimator (MLC) modulation and reduced tongue-and-groove effects, which enable more effective dose shaping. Consequently, moderate collimator angles appear to provide an optimal balance between dosimetric quality and delivery efficiency and may serve as a practical default in routine VMAT planning. Despite these advantages, important trade-offs related to deliverability, mechanical accuracy, and case complexity must be considered. Kim et al. (2015) reported a negative correlation between collimator angle and gamma index passing rates, suggesting that higher angles may compromise delivery accuracy. Furthermore, Puchades-Puchades et al. (2015) demonstrated that collimator angle misalignment exceeding ±1° can significantly degrade target coverage and homogeneity, particularly for large PTVs, highlighting the importance of rigorous quality assurance and precise mechanical calibration. In complex clinical scenarios, such as craniospinal irradiation and stereotactic radiosurgery, collimator angle assumes a more critical role. Li et al. (2015) showed that non-zero angles eliminate dose gaps at field junctions, while Battinelli et al. (2021) demonstrated that sub-arc collimator angle optimization significantly improves plan quality and efficiency. These findings underscore the need for individualized, context-specific collimator angle selection that balances dosimetric benefits with delivery feasibility.

Table 1: Impact of collimator angle on Target Coverage.

Organ at risk (OAR) sparing

The studies presented in Table 3 demonstrate that advanced collimation techniques, particularly the use of multileaf collimators (MLCs), play a crucial role in reducing radiation dose to organs at risk (OARs) across a range of clinical applications. In breast cancer radiotherapy, significant reductions in mean dose to critical structures such as the heart, left anterior descending artery (LAD), and lungs were achieved through MLC-based shielding (Mahani et al., 2023). Similarly, MLC-shielded fields have been shown to reduce out-of-field doses compared to conventional cerrobend blocks, indicating improved protection of healthy tissues during treatment (Momeni, 2023). These findings highlight the importance of precise beam shaping in minimizing unintended radiation exposure while maintaining effective target irradiation. Beyond the presence of MLCs, their configuration and integration with advanced treatment techniques further enhance OAR sparing. Narrower MLC leaf widths have been associated with improved conformity and steeper dose gradients, resulting in superior normal tissue sparing, particularly in stereotactic irradiation for brain metastases (Ohira et al., 2021). Additionally, technique-driven innovations such as 90° angled collimated dynamic IMRT (A-IMRT) demonstrated improved OAR protection compared to conventional IMRT, although volumetric modulated arc therapy (VMAT) remained superior overall in terms of conformity and sparing (Serarslan et al., 2023). Furthermore, dynamic trajectory radiotherapy (DTRT), which incorporates coordinated motion of the gantry, couch, and collimator, achieved substantial improvements in OAR sparing in head and neck cancers compared to VMAT (Bertholet et al., 2022). These findings suggest that both hardware characteristics and delivery techniques are critical determinants of optimal organ protection. Optimization strategies involving collimator orientation and beam geometry also contribute to achieving a balance between target coverage and OAR sparing. While variations in collimator angle alone may not significantly influence OAR dose in certain cases, such as prostate VMAT (Saeed et al., 2023), more advanced approaches can yield meaningful improvements. For example, the split X-jaw technique has been shown to enhance target coverage while simultaneously reducing dose to surrounding normal tissues (Sarma et al., 2023). Collectively, the evidence indicates that effective OAR sparing is achieved through a combination of appropriate collimator design, optimized beam modulation, and advanced planning strategies. These findings underscore the need for individualized, technique-specific planning approaches to maximize the therapeutic ratio in modern radiotherapy.

Table 3: organ at risks.

Impact of collimator angle on Clinical outcomes

The reviewed literature in table 4 demonstrates that collimator angle optimization has a measurable and clinically relevant impact on treatment quality in VMAT and SRS planning, with the degree of influence varying across anatomical sites and planning techniques. In prostate VMAT planning, Saeed et al. (2023) reported that while target coverage and organ-at-risk (OAR) doses remained largely unchanged, optimal angles between 20° and 45° significantly enhanced dose conformity and homogeneity, emphasizing that collimator settings primarily refine dose distribution rather than altering overall plan safety. In cases involving multiple brain metastases, the influence of collimator angle selection becomes more pronounced. Shen et al. (2022) showed that the Sub-Arc Collimator Angle Optimization (SACAO) algorithm greatly improved conformity, homogeneity, and gradient indices while reducing monitor units and field size by more than 40%, thereby improving treatment efficiency. Supporting this, Battinelli et al. (2021) demonstrated that dynamic collimator trajectories outperform fixed angles, achieving exposed-area reductions of up to 71%, which can substantially improve lesion selectivity and reduce unnecessary normal-tissue irradiation. Other studies presented more nuanced perspectives based on planning context and disease site. Pudsey et al. (2021) found that collimator angle optimization and jaw tracking provided limited clinical benefit in single-isocenter, multiple-target SRS, suggesting that geometric complexity can overshadow the effects of angle adjustment. In whole-brain radiotherapy with hippocampal and cochlear sparing, Sun et al. (2021) reported that increasing the angular separation between dual-arc collimator configurations improved plan quality and reduced monitor units, highlighting angle selection as a modifiable parameter for complex sparing scenarios. For locally advanced nasopharyngeal carcinoma, Kim et al. (2017) found that angles of 20°–30° produced the best conformity and homogeneity indices, although higher angles were associated with increased optic-apparatus dose, demonstrating the need for balanced optimization. Foundational contributions by Zhang et al. (2010) and Kim et al. (2015) further showed that personalized and dynamically optimized collimator strategies can enhance dosimetric performance while improving quality-assurance γ-index outcomes. Collectively, these studies establish that collimator angle optimization is integral to maximizing plan conformity, improving treatment efficiency, minimizing unnecessary exposure, and strengthening overall VMAT plan quality.

Table 4: Impact on Clinical outcomes.

Potential factors influencing the impact of collimator angle 

The influence of collimator angle on radiotherapy planning is shaped by several interrelated factors, including anatomical site, treatment modality, and the range of angular rotation applied during optimization. Each of these factors plays a critical role in determining how effectively the collimator orientation contributes to improving dose conformity, sparing organs at risk, and enhancing overall treatment efficiency. Understanding these parameters is essential for ensuring that collimator adjustments are not used generically but tailored to the specific clinical scenario. This tailored approach ensures that treatment plans achieve adequate tumor coverage while minimizing unintended dose to surrounding tissues. One of the most important determinants is the treatment site, as different anatomical regions respond differently to changes in collimator orientation. Prostate cancer planning, for example, has shown limited sensitivity to collimator angle variations, with negligible differences in conformity and homogeneity across multiple angles, suggesting that the relatively simple geometry of the target reduces the influence of angular adjustments (Kim et al., 2024; Saeed et al., 2023). Conversely, brain metastases present a more complex dosimetric environment, where optimized collimator angles have been shown to significantly improve OAR sparing and inter-lesion dose separation due to the irregular and spatially separated targets involved (Shen et al., 2022). These findings emphasize that anatomical complexity and target geometry strongly dictate the potential benefits of collimator optimization. The treatment technique further modulates the importance of collimator angle selection. In conventional VMAT, specific collimator angles have been shown to enhance dose distribution by aligning more effectively with the MLC leaf motion and the geometry of the target volume. However, more advanced techniques such as hybrid VMAT (HVMAT) introduce additional flexibility that allows for a wider range of acceptable angles without significantly compromising plan quality, owing to the technique’s ability to incorporate static-field components that support dose shaping (Kim et al., 2024; Saeed et al., 2023). Additionally, the range of collimator rotation itself plays a crucial role. Research indicates that intermediate angles—typically around 20°, 30°, and 45°—produce superior dosimetric outcomes by balancing leaf travel efficiency and beam modulation, whereas very small or extreme angles can lead to suboptimal distributions, including elevated surface doses in small-target scenarios (Saeed et al., 2023; Kim et al., 2022). These combined findings show that while collimator angle adjustments hold significant potential for improving treatment efficacy, their actual impact is highly dependent on treatment site, planning technique, and selected angle range.

Implications of Collimator Angle in VMAT: Comparisons of literature findings

The collective body of evidence demonstrates that collimator angle selection in VMAT significantly influences dose distribution, although the magnitude of its impact varies by anatomical site, treatment geometry, and planning strategy. Several studies, including Yoo et al. (2014) and Jeong et al. (2014), reported that modest angles—typically around 15°—can enhance dose conformity and improve target coverage in prostate VMAT, reflected in metrics such as increased D95 and improved planning target volume (PTV) dose uniformity. For more complex scenarios, such as whole-brain radiotherapy, Niemierko et al. (2013) found that dual-arc configurations using orthogonal or near-orthogonal collimator angles (e.g., 350°/80°) produced superior dosimetric performance. These variations highlight that optimal collimator settings depend heavily on patient anatomy and the spatial characteristics of the target structures. In contrast, studies such as Xing et al. (2016) showed limited or no dosimetric improvement when adjusting collimator angles for multi-target SRS, suggesting that in highly conformal high-dose treatments, arc geometry and MLC modulation may play a more dominant role than the angle itself. The literature also reveals a critical balance between dose distribution and treatment delivery efficiency. Dynamic collimator rotation approaches (DC-VMAT) have been investigated as a means of improving efficiency by adjusting the angle throughout gantry rotation. Niemiec et al. (2016) demonstrated reductions in treatment time with DC-VMAT compared with static-angle VMAT (SC-VMAT). However, not all findings support routine adoption of dynamic approaches. Zhang et al. (2023) reported that in single-isocenter multitarget SRS, DC-VMAT provided limited clinical benefit and introduced additional mechanical complexity that could outweigh its efficiency advantages. Similarly, algorithm-based collimator angle optimization (CAO), explored by Wang et al. (2020), shows potential for automating angle selection and improving plan quality, but its effectiveness is highly dependent on case complexity and the relative weighting of planning objectives. Together, these findings indicate that while angle optimization and dynamic strategies can improve workflow and potentially enhance precision, they must be applied judiciously and tailored to the clinical scenario. Another recurring theme in the literature is the impact of collimator angle on sparing organs at risk. Yoo et al. (2014) observed that certain angle combinations in double-arc VMAT led to reduced rectal doses in prostate cancer treatments, illustrating how beam orientation influences the interplay between MLC projection and organ geometry. Thomas et al. (2023) further supported the trend that angles between 15° and 30° often strike an effective balance between target coverage and OAR protection across several anatomical sites. However, some investigations—such as Xing et al. (2016) in multitarget SRS—found negligible differences in OAR dose with varying angles, reinforcing that the benefits of angle selection are not universal and depend strongly on spatial complexity. Advancements such as dynamic collimator adjustment, as explored by Zhang et al. (2016) and supported by work from Sun et al. (2021), suggest that real-time modulation of angle could further optimize both OAR sparing and delivery efficiency. Nonetheless, widespread clinical integration requires additional validation due to challenges related to treatment verification, mechanical reliability, and integration into existing planning workflows. Overall, current evidence supports collimator angle optimization as an important but context-dependent component of VMAT planning that warrants continued investigation.

Meta-analysis of the Impact of Collimator Angle on Target Coverage

The synthesis of studies in Table 5 indicates that collimator angle has a limited direct influence on baseline target coverage in volumetric modulated arc therapy (VMAT), but plays a significant role in optimizing plan quality, efficiency, and robustness. Across prostate cancer studies, target coverage remained largely unchanged over a wide range of collimator angles (0°–90°), demonstrating the robustness of modern VMAT optimization algorithms (Isa et al., 2014; Saeed et al., 2023). Despite this stability, important differences were observed in plan quality metrics such as conformity index (CI), homogeneity index (HI), and monitor units (MUs), suggesting that collimator angle primarily functions as an optimization parameter rather than a determinant of target dose adequacy. This distinction is clinically relevant, as improved conformity and homogeneity contribute to better dose shaping and reduced irradiation of surrounding normal tissues. A consistent and well-supported finding across the included studies is the superiority of moderate non-zero collimator angles, typically within the range of 15°–45°. Multiple investigations demonstrated that this range yields improved conformity, homogeneity, and treatment efficiency. For instance, optimal plan performance was reported at 20°–45° in prostate VMAT (Saeed et al., 2023), while 45° produced the best conformity and homogeneity indices in phantom-based analyses (Isa et al., 2014). Similarly, in nasopharyngeal carcinoma, angles between 15° and 30° achieved superior target coverage and reduced monitor units, whereas very low angles (5°–10°) resulted in suboptimal coverage (Thomas et al., 2023). These findings suggest that moderate collimator rotation enhances multileaf collimator (MLC) modulation and reduces geometric inefficiencies, such as tongue-and-groove effects, thereby improving dose distribution without compromising coverage. The impact of collimator angle is further influenced by anatomical complexity and treatment technique. In relatively simple geometries, such as prostate VMAT, angle selection primarily refines plan quality. However, in more complex scenarios, including craniospinal irradiation and stereotactic radiosurgery (SRS) for multiple brain metastases, collimator angle becomes a more critical planning variable. For example, non-zero collimator angles eliminated dose gaps at field junctions in craniospinal irradiation, thereby improving treatment robustness (Li et al., 2015). In addition, sub-arc collimator angle optimization (SACAO) significantly enhanced conformity, homogeneity, gradient index, and normal tissue sparing, while reducing monitor units and field size in multi-target SRS (Battinelli et al., 2021). These findings indicate that the clinical importance of collimator angle increases with treatment complexity and modulation demands. Another important dimension highlighted by the meta-analysis is the influence of collimator angle on treatment deliverability and mechanical accuracy. Higher collimator angles have been associated with reduced gamma index passing rates, indicating potential discrepancies between planned and delivered dose distributions (Kim et al., 2015). More critically, collimator angle misalignment exceeding ±1° has been shown to significantly degrade target coverage and homogeneity, particularly for large planning target volumes, with substantial variations in dosimetric indices (Puchades-Puchades et al., 2015). These findings emphasize that the benefits of optimal angle selection are contingent upon precise mechanical calibration and rigorous quality assurance, as inaccuracies in collimator positioning can negate dosimetric advantages. Overall, the evidence supports a hierarchical interpretation in which collimator angle has a modest direct effect on target coverage but a substantial indirect effect on plan quality, robustness, and clinical reliability. Moderate angles (15°–45°) consistently provide the best balance between dosimetric performance and efficiency, while advanced or automated optimization strategies are particularly beneficial in complex cases. Importantly, the clinical gains associated with collimator angle optimization depend on accurate delivery and quality assurance. Therefore, collimator angle should be considered as part of an integrated treatment planning strategy rather than an isolated parameter, with selection tailored to anatomical complexity, treatment technique, and machine performance.

Table 5: Meta-analysis of the Impact of Collimator Angle on Target Coverage.

Meta-Analysis results Summary for Organ-at-Risk (OAR) Sparing

The meta-analysis presented in Table 6 demonstrates that organ-at-risk (OAR) sparing in modern radiotherapy is predominantly influenced by multileaf collimator (MLC) utilization and advanced treatment delivery techniques, with additional contributions from geometric and planning optimizations. Across multiple studies, MLC-based approaches consistently achieved significant reductions in dose to critical structures such as the heart, lungs, and left anterior descending artery compared to conventional shielding methods (Mahani et al., 2023; Momeni, 2023). Furthermore, improvements in hardware design, particularly the use of narrower MLC leaf widths, were associated with enhanced dose conformity, steeper gradients, and superior normal tissue sparing (Ohira et al., 2021). Advanced delivery techniques, including volumetric modulated arc therapy (VMAT) and dynamic trajectory radiotherapy (DTRT), further strengthened OAR protection, with DTRT showing notable improvements over VMAT in complex head and neck cases (Bertholet et al., 2022; Serarslan et al., 2023). While collimator angle variation alone appears to have a limited and context-dependent impact on OAR dose (Saeed et al., 2023), more sophisticated planning strategies such as split X-jaw techniques and dynamic beam modulation provide meaningful enhancements in both OAR sparing and target coverage (Sarma et al., 2023). Overall, the evidence supports a multifactorial approach in which optimal OAR protection is achieved through the combined use of advanced MLC design, innovative delivery techniques, and individualized treatment planning strategies.

Table 6: Meta-Analysis results Summary for Organ-at-Risk (OAR) Sparing.

Meta-Analysis Summary Results for the Impact of Collimator Angle on Clinical Outcomes

The meta-analysis summarized in Table 7 demonstrates that collimator angle optimization influences clinical outcomes primarily through improvements in plan quality, treatment efficiency, and normal tissue protection rather than direct enhancement of target coverage. A consistent finding across multiple studies is that moderate collimator angles, typically within the range of 15°–45°, provide optimal dosimetric performance, resulting in improved conformity index (CI), homogeneity index (HI), and, in some cases, reduced normal tissue complication probability (NTCP) (Andersen et al., 2016; Kim et al., 2017; Saeed et al., 2023). Importantly, target coverage remains largely unchanged across varying collimator angles, indicating that collimator angle serves as a refinement parameter rather than a primary determinant of dose delivery. These improvements in dosimetric quality are clinically relevant, as enhanced conformity and homogeneity are associated with better tumor control and reduced radiation-induced toxicity, thereby improving the overall therapeutic ratio. Furthermore, the analysis highlights the growing importance of advanced and automated collimator angle optimization strategies, particularly in complex treatment scenarios such as multiple brain metastases and whole-brain radiotherapy. Techniques such as sub-arc collimator angle optimization (SACAO) and dynamic trajectory optimization have demonstrated substantial improvements in plan quality, including reductions in monitor units and unnecessary irradiation of normal tissue (Battinelli et al., 2021; Shen et al., 2022). However, the benefits of collimator angle optimization are not universal, as some studies report limited impact in specific treatment configurations, such as single-isocenter multiple-target stereotactic radiosurgery (Pudsey et al., 2021). Additionally, higher collimator angles may introduce trade-offs, including increased dose to sensitive structures and reduced delivery accuracy, as reflected by lower gamma index passing rates (Kim et al., 2015; Kim et al., 2017). Overall, these findings suggest that while collimator angle optimization can enhance clinical outcomes, its effectiveness is context-dependent and should be integrated with other planning strategies to achieve optimal and clinically reliable results.

Table 7: Meta-Analysis Summary Table: Impact of Collimator Angle on Clinical Outcomes.

This systematic review and meta-analysis provide strong evidence that collimator angle plays a secondary yet clinically significant role in VMAT planning, primarily influencing plan quality, efficiency, and treatment robustness rather than directly affecting baseline target coverage. Across multiple studies, target coverage remained consistently stable over a wide angular range (0°–90°), highlighting the robustness of modern inverse planning algorithms. However, optimal dosimetric performance was consistently achieved within moderate collimator angles of 15°–45°, with specific optimal values of 20°, 30°, and 45° identified in prostate cancer planning, and 15°–30° in nasopharyngeal carcinoma, achieving PTV coverage levels of approximately 92–94%. The analysis demonstrates that collimator angle optimization significantly enhances key plan quality metrics, including conformity index (CI), homogeneity index (HI), and monitor unit (MU) efficiency. Advanced optimization strategies, such as Sub-Arc Collimator Angle Optimization (SACAO), further improved treatment quality by reducing monitor units by approximately 15.7% and field size by 41.1%, while enhancing dose conformity and normal tissue sparing. These improvements are particularly pronounced in complex treatment scenarios, such as stereotactic radiosurgery (SRS) for multiple brain metastases and craniospinal irradiation, where optimized collimator angles eliminate dose gaps and improve treatment robustness. Despite these benefits, the findings highlight important trade-offs related to treatment deliverability and mechanical accuracy. Higher collimator angles (>45°) were associated with decreased γ-index passing rates, with reductions of up to 5.6%, indicating potential discrepancies between planned and delivered dose distributions. More critically, collimator misalignment exceeding ±1° was shown to significantly degrade target coverage and homogeneity, with variations in HI reaching up to 75%, particularly for large planning target volumes. These results underscore the necessity of rigorous quality assurance protocols and precise mechanical calibration to ensure safe and effective treatment delivery. In terms of organ-at-risk (OAR) sparing, the analysis confirms that collimator angle alone is not the primary determinant. Instead, OAR dose reduction is predominantly influenced by multileaf collimator (MLC) design, leaf width, and advanced delivery techniques. MLC-based approaches achieved dose reductions of up to 20% in critical structures such as the heart and lungs, while narrower MLC leaf widths (e.g., 2.5 mm) provided superior dose gradients and normal tissue sparing compared to wider leaves (5 mm). Additionally, advanced techniques such as VMAT, dynamic trajectory radiotherapy (DTRT), and split X-jaw planning further enhanced OAR protection and treatment efficiency. Overall, this study establishes that collimator angle should be considered as part of a multifactorial, patient-specific optimization strategy in VMAT planning. Moderate angles (15°–45°) are recommended as a practical default for most clinical scenarios, while advanced and automated optimization techniques should be employed for complex cases. Future research should focus on large-scale clinical trials and the development of standardized, automated collimator angle optimization protocols to further improve treatment precision, efficiency, and long-term clinical outcomes.