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ObjectivesTo explore the feasibility of multi‐isocentric 4π volumetric‐modulated arc therapy (MI4π‐VMAT) for the complex targets of head and neck cancers.MethodsTwenty‐five previously treated patients of HNC underwent re‐planning to improve the dose distributions with either coplanar VMAT technique (CP‐VMAT) or noncoplanar MI4π‐VMAT plans. The latter, involving 3–6 noncoplanar arcs and 2–3 isocenters were re‐optimized using the same priorities and objectives. Dosimetric comparison on standard metrics from dose‐volume histograms was performed to appraise relative merits of the two techniques. Pretreatment quality assurance was performed with IMRT phantoms to assess deliverability and accuracy of the MI4π‐VMAT plans. The gamma agreement index (GAI) analysis with criteria of 3 mm distance to agreement (DTA) and 3% dose difference (DD) was applied.Results CP‐VMAT and MI4π‐VMAT plans achieved the same degree of coverage for all target volumes related to near‐to‐minimum and near‐to‐maximum doses. MI4π‐VΜΑΤ plans resulted in an improved sparing of organs at risk. The average mean dose reduction to the parotids, larynx, oral cavity, and pharyngeal muscles were 3 Gy, 4 Gy, 5 Gy, and 4.3 Gy, respectively. The average maximum dose reduction to the brain stem, spinal cord, and oral cavity was 6.0 Gy, 3.8 Gy, and 2.4 Gy. Pretreatment QA results showed that plans can be reliably delivered with mean gamma agreement index of 97.0 ± 1.1%.Conclusions MI4π‐VMAT plans allowed to decrease the dose‐volume‐metrics for relevant OAR and results are reliable from a dosimetric standpoint. Early clinical experience has begun and future studies will report treatment outcome.

Background: Aim of this study is to evaluate the accuracy of the gated volumetric modulated arc therapy (VMAT/RapidArc) using 2D planar dosimetry, DynaLog files and COMPASS 3D dosimetry system. Materials and Methods: Pre-treatment quality assurance of 10 gated VMAT plans was verified using 2D array and COMPASS 3D dosimetry system. Advantage of COMPASS over 2D planar is that it provides the clinical consequence of error in treatment delivery. Measurements were performed for non-gated and different phase ga.ng window level (80%, 50%, 30% & 20%) to know the impact of ga.ng in VMAT dose delivery. Results: In 2D planar dosimetry, gamma agreement index (GAI) for all measurements were more than 95%. DynaLog file analysis shows the average devia.ons between actual and expected posi.ons of monitor units, gantry and mul.-leaf collimator. The STDVs MU and gantry posi.on were less than 0.10 MU and 0.33° respec.vely. Root mean square (RMS) of the devia.ons of all leaves were less than 0.58 mm. The results from COMPASS show that 3D dose volume parameters for ten pa.ents measured for different phase ga.ng window level were within the tolerance level of ±5%. Average 3D gamma of PTV and OAR's for different window level was less than 0.6. Conclusion: The results from this study show that gated VMAT delivery provided dose distribu.ons equivalent to non-gated delivery to within clinically acceptable limits and COMPASS along with Matrix Evolu.on can be effec.vely used for pretreatment verifica.on of gated VMAT plans.

VMAT plans by TPSP and TPSB offered clinically acceptable dose distributions. TPSB-based optimization showed enhanced sparing of serial organs whereas TPSP-based optimization showed superior sparing of parallel organs.