Dosimetric effect of photon beam energy on volumetric modulated arc therapy treatment plan quality due to body habitus in advanced prostate cancer

Purpose: The purpose of this study was to dosimetrically compare 6- and 10-MV photon beam energies in high-risk prostate cancer patients of various body habitus using a volumetric modulated arc therapy (VMAT) radiation delivery technique. The objectives of the study were to evaluate whether dosimetric differences exist and to investigate whether differences are dependent on patient body habitus.

Methods and materials: Forty patients with various body habitus who had previously received treatment to the prostate and pelvic lymph nodes with VMAT techniques were chosen. Patients were planned in the Pinnacle(3) treatment planning system with double or triple SmartArc plans with 6- and 10-MV photon energies. All patients were optimized with the same planning objectives and normalized such that 95% of the planning target volume (PTV) received the prescription dose. Patients were evaluated for PTV and organ at risk (OAR) parameters for the bladder, rectum, small bowel, penile bulb, and sigmoid colon. Metrics used for comparison were D2%, D98%, homogeneity, conformity, and dose falloff for the PTV and D(2%), D(mean), V(80%), V(60%), and V(40%) for OARs. Statistical differences were evaluated with a paired-sample Wilcoxon signed rank test with a significance level of .05.

Results: For the PTV, there were no statistically significant differences in D(mean), D(2cc), conformation number, and homogeneity index values, but the dose falloff parameters, R50 and R25, showed a median improvement of 6.7% (P<.01) and 6.2% (P<.01), respectively, with 10 MV. A correlation between patient anterior-posterior distance (d(AP)) and percentage reduction in R50 of 0.436% per centimeter (P<.01) was determined. For OARs, statistically significant reductions in dose metrics were found in the small bowel and bladder, but increases in the D(2cc) of 3.5% in the penile bulb (P<.01) and 0.2% in the rectum (P=.02) were shown with 10 MV. The use of 10 MV also demonstrated a statistically significant reduction in the total number of monitor units of 15.9% (P<.01) compared with 6 MV.

Conclusions: The study showed that 10 MV provides a faster dose falloff than 6 MV for patients whose prostate and pelvic lymph nodes are treated using a VMAT technique irrespective of body habitus; however, the improvement in dose falloff is dependent on body habitus and increases as the patient body habitus increases.