Variation in accumulated dose of volumetric‐modulated arc therapy for pancreatic cancer due to different beam starting phases

PURPOSE: To assess the effects of different beam starting phases on dosimetric variations in the clinical target volume (CTV) and organs at risk (OARs), and to identify the relationship between plan complexity and the dosimetric impact of interplay effects in volumetric‐modulated arc therapy (VMAT) plans for pancreatic cancer. METHODS: Single and double full‐arc VMAT plans were generated for 11 patients. A dose of 50.4 Gy in 28 fractions was prescribed to cover 50% of the planning target volume. Patient‐specific Digital Imaging and Communications in Medicine–Radiation Therapy plan files were divided into 10 files based on the respiratory phases in four‐dimensional computed tomography (4DCT) simulations. The phase‐divided VMAT plans were calculated in consideration of the beam starting phase for each arc and were then combined in the mid‐ventilation phase of 4DCT (4D plans). The dose‐volumetric parameters were compared with the calculated dose distributions without consideration of the interplay effects (3D plans). Additionally, relationships among plan parameters such as modulation complexity scores, monitor units (MUs), and dose‐volumetric parameters were evaluated. RESULTS: Dosimetric differences in the median values associated with different beam starting phases were within ± 1.0% and ± 0.2% for the CTV and ± 0.5% and ± 0.9% for the OARs during single and double full‐arc VMAT, respectively. Significant differences caused by variations in the beam starting phases were observed only for the dose‐volumetric parameters of the CTV during single full‐arc VMAT (P < 0.05), associated with moderate or strong correlations between the MUs and the dosimetric differences between the 4D and 3D plans. CONCLUSIONS: The beam starting phase affected CTV dosimetric variations of single full‐arc VMAT. The use of double full‐arc VMAT mitigated this problem. However, variation in the dose delivered to OARs was not dependent on the beam starting phase, even for single full‐arc VMAT.