Comparison of Dynamic Trajectory Radiotherapy and Volumetric Modulated Arc Therapy for Loco-Regionally Advanced Oropharyngeal Cancer

Abstract

Dynamic trajectory radiotherapy (DTRT) extends volumetric modulated arc therapy (VMAT) with dynamic table and collimator rotation. We investigate the potential benefit of DTRT over VMAT with respect to organ-at-risk (OAR) sparing for patients with loco-regionally advanced oropharyngeal cancer (OPC). We created DTRT and VMAT plans for 46 cases with prescribed doses of 50-70 Gy (2 Gy fractions, sequential boost) using 6 MV flattened beam on c-arm Linacs. For DTRT: case-specific collision avoidance maps were created and gantry-table paths were determined based on contoured structures to minimize fractional target/OAR overlap in beam's eye view. Gantry-collimator paths minimized field width in the leaf-travel direction. DTRT paths were imported in a research version of a commercial treatment planning system for intensity modulation optimization using the same optimizer and dose calculation algorithms as for VMAT. Plans at each dose level were normalized with 100% of the prescribed dose to 95% of the planning target volume (PTV). PTV coverage and OAR sparing for DTRT and VMAT were compared using Wilcoxon matched-pair signed rank test (5% significance level). The correlation between the fractional OAR/PTV50Gy volume overlap and difference in OAR Dmean between the two techniques was evaluated using Spearman's correlation coefficient. Plans at each dose level were all acceptable and had D0.03cc<110% prescription dose (difference between DTRT and VMAT were not statistically significant). In the combined plans, PTV50Gy D95% and D98% and PTV70Gy D95% were significantly higher with DTRT compared to VMAT. Differences in PTV66Gy coverage and PTV70Gy D5% were not statistically significant. Mean Dmean to the contralateral (CL) parotid gland was 13.99 / 15.28 Gy for DTRT / VMAT respectively, it was 25.75 / 28.05 Gy for the CL submandibular gland, 44.88 / 45.82 Gy for the pharynx and 30.90 / 33.93 Gy for the oral cavity (all with p<.001). Mean Dmean to the ipsilateral (IL) parotid gland was 29.00 / 29.28 Gy for DTRT / VMAT respectively (p = .77) and 57.71 / 57.91 Gy for the IL submandibular gland (p = .99). Significantly higher doses were observed with DTRT than with VMAT for the optical and auditory OARs and for the brain, but well below the clinical goals. The difference in Dmean (VMAT-DTRT) was negatively correlated to the fractional OAR/PTV50Gy overlap for the CL parotid and submandibular gland and the IL submandibular gland (r = -0.55 to -0.51, p<.001) but not for IL parotid gland (r = -0.17, p = .26), oral cavity (r = -0.13, p = .41) or pharynx (r = -011, p = .47). For at least the same target coverage, DTRT statistically significantly improves sparing for OARs related to salivary and swallowing functions compared to state-of-the-art VMAT. The advantage of DTRT decreases with increasing fractional OAR/PTV50Gy volume overlap for some salivary glands but not for pharynx or oral cavity. DTRT is promising to reduce treatment-related toxicity for patients with OPC.