Dosimetric Comparison of Volumetric Modulated Arc Therapy and Intensity Modulated Radiation Therapy for Soft Tissue Sarcoma of the Extremities.

PURPOSE: Radiation therapy is a standard part of limb conserving therapy for extremity soft tissue sarcoma (STS) at high risk of recurrence. Toxic effects increase with radiation dose and volume of normal tissue irradiated. This study sought to compare dosimetry of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to investigate the optimal planning technique. METHODS AND MATERIALS: Twenty patients with extremity STS who underwent preoperative radiation therapy (50 Gy in 25 fractions) between 2016 and 2020 at a specialised sarcoma center were included. The original treatment techniques were sliding window IMRT or 3-dimensional conformal. VMAT plans were retrospectively generated according to the original tumor and organ-at-risk constraints. Quality assurance was performed as per departmental protocol. Wilcoxon signed-rank test was used to compare dosimetric parameters (for planning target volume [PTV], in-field bone, and soft tissue structures), monitor units (MUs), and treatment time. RESULTS: Median patient age was 65 years and the majority were male (n = 14, 70%). The most common subtype was undifferentiated pleomorphic sarcoma (n = 14, 70%), and most tumors were located on the thigh (n = 12, 60%). Median PTV was 1110 cm3 and median volume of in-field bone 236 cm3. VMAT plans had significantly lower average MU (480 vs 862 MU, P < .001) and overall treatment time (300 vs 153 seconds, P < .001). PTV coverage favored VMAT, with marginally higher mean, minimum, and maximum doses and higher conformity index. However, differences were not statistically significant. Dose to infield bone and soft tissue structures were similar or slightly lower with VMAT. CONCLUSIONS: In extremity STS, VMAT plans demonstrated a favorable trend toward tumor coverage and dose conformity compared with IMRT along with significantly lower MUs and half the overall treatment time.