3D Secondary Verifications for VMAT Prostate Treatments Based in DVH-Metrics and 3D Gamma Analysis

Abstract

Independent monitor unit (MU) verification of intensity-modulated radiation therapy (IMRT) treatments is traditionally performed with Monte Carlo calculations or comparing point-dose calculations in homogeneous media. Recent verifications systems permit to perform secondary checks of treatment plans calculating dose on patient CT images. Single-point MU check is replaced with 3D treatment plan verification, incorporating DVH-based metrics with clinical relevance to the QA process. This work shows the 3D dosimetric verification of several volumetric modulated arc therapy (VMAT) prostate treatment plans by means of dose-volume parameters comparison with a new 3D treatment plan QA software. VMAT prostate plans are generated and delivered with a single VMAT arc technique. DICOM RT information (CT, RT Struct, RT Plan and RT Dose) is transferred from TPS to verification system. This software uses a new independently developed collapsed cone algorithm to calculate dose distributions in CT. A stock reference beam model is used for calculations. Beam data can be fitted with key parameters (PDDs, OFs, OARs). A dosimetric leaf gap of -2 mm is needed to scale the results. 25 prostate plans have been verified using this solution, comparing DVH parameters (Dmed, D90 and eventually Dmax) and performing 3D gamma tests for PTVs (prostate, seminal vesicles, pelvic lymph nodes) and OARs (rectum, bladder, femoral heads, penile bulb). Mean relative deviations (Dmed/D90) are (-0.22±0.97/-0.62±1.20)% for prostate PTV, (0.29±0.90/0.56±0.90)% for seminal vesicles PTV and (1.14±0.53/0.90±0.58)% for pelvic lymph nodes PTV. 3D gamma passing rate values for PTVs are (97.99±3.12)%, (99.23±1.90)% and (97.54±2.90)%, respectively. Mean deviation and gamma passing rate value (Dmed/% of points) is (-0.00±0.81/99.50±0.71)% for rectum, (0.80±0.90/99.60±1.21)% for bladder and (2.84±2.28/100.00±0.00)% for penile bulb. Mean deviations and gamma passing rates for right and left femoral heads (Dmed/Dmax/% of points) are (0.03±0.43/-1.27±2.35/100.00±0.00)% and (0.05±0.38/-1.49±2.24/100.00±0.00)%, respectively. Deviations are below 1.5% for all parameters and volumes, excepting penile bulb (due to the limited number of contoured slices). 3D gamma passing rates are above 97.5% for all structures, reaching 100.0% in some cases. The tested system is a quick, effective and reliable tool to perform independent TPS-quality dose calculations of complex treatments, such as those used in VMAT techniques.