Validation of an in vivo transit dosimetry algorithm using Monte Carlo simulations and ionization chamber measurements.

PURPOSE: Transit dosimetry is a safety tool based on the transit images acquired during treatment. Forward-projection transit dosimetry software, as PerFRACTION, compares the transit images acquired with an expected image calculated from the DICOM plan, the CT, and the structure set. This work aims to validate PerFRACTION expected transit dose using PRIMO Monte Carlo simulations and ionization chamber measurements, and propose a methodology based on MPPG5a report. METHODS: The validation process was divided into three groups of tests according to MPPG5a: basic dose validation, IMRT dose validation, and heterogeneity correction validation. For the basic dose validation, the fields used were the nine fields needed to calibrate PerFRACTION and three jaws-defined. For the IMRT dose validation, seven sweeping gaps fields, the MLC transmission and 29 IMRT fields from 10 breast treatment plans were measured. For the heterogeneity validation, the transit dose of these fields was studied using three phantoms: 10 , 30 , and a 3 cm cork slab placed between 10 cm of solid water. The PerFRACTION expected doses were compared with PRIMO Monte Carlo simulation results and ionization chamber measurements. RESULTS: Using the 10 cm solid water phantom, for the basic validation fields, the root mean square (RMS) of the difference between PerFRACTION and PRIMO simulations was 0.6%. In the IMRT fields, the RMS of the difference was 1.2%. When comparing respect ionization chamber measurements, the RMS of the difference was 1.0% both for the basic and the IMRT validation. The average passing rate with a γ(2%/2 mm, TH = 20%) criterion between PRIMO dose distribution and PerFRACTION expected dose was 96.0% ± 5.8%. CONCLUSION: We validated PerFRACTION calculated transit dose with PRIMO Monte Carlo and ionization chamber measurements adapting the methodology of the MMPG5a report. The methodology presented can be applied to validate other forward-projection transit dosimetry software.

Gamma passing rates of daily EPID transit images correlate to PTV coverage for breast cancer IMRT treatment plans.

PURPOSE: The use of the transit image obtained with the electronic portal-imaging device (EPID) is becoming an extended method to perform in-vivo dosimetry. The transit images acquired during each fraction can be compared with a predicted image, if available, or with a baseline image, usually the obtained in the first fraction. This work aims to study the dosimetric impact of the failing fractions and to evaluate the appropriateness of using a baseline image in breast plans. MATERIAL AND METHODS: Twenty breast patients treated in a Halcyon were retrospectively selected. For each patient and fraction, the treatment plan was calculated over the daily CBCT image. For each fraction, the differences respect to the treatment plan values of OARs and PTV dosimetric parameters were analyzed: ΔDmean , ΔD95%, ΔD98%, ΔD2%, ΔV36Gy, ΔV38.5Gy, and ΔV43.5Gy. Daily fractions were ranked according to the differences found in the dosimetric parameters between the treatment plan and the daily CBCT to establish the best fraction. The daily transit images acquired in every fraction were compared to the first fraction using the global gamma index with the Portal Dosimetry tool. The comparison was repeated using the best fraction image as a baseline. We assessed the correlation of the dosimetric differences obtained from the CBCT images-based treatment plans with the gamma index passing rates obtained using first fraction and best fraction as baseline. RESULTS: Average values of -11.6% [-21.4%, -3.3%] and -3.2% [-1.0%, -10.3%] for the ∆PTVD98% and ∆PTVD95% per every 10% decrease in the passing rate were found, respectively. When using the best fraction as baseline patients were detected with failing fractions that were not detected with the first fraction as baseline. CONCLUSION: The gamma passing rates of daily transit images correlate with the coverage loss parameters in breast IMRT plans. Using first fraction image as baseline can lead to the non-detectability of failing fractions.