Author: Darwin Abraham Garcia, Michael P. Grams, Scott C Lester, Maryam Mashayekhi, Douglas John Moseley, Robert W Mutter, Sean S Park, Chrystian Quintero Mateus 👨🔬
Affiliation: Mayo Clinic - Rochester, Mayo Clinic 🌍
Purpose: MBRT delivers sub-millimeter wide regions of high “peak” (~10s Gy) and low “valley” (~1 Gy) doses to treat cancer. MBRT commissioning is performed in flat, uniform plastic water. Thus, different scatter conditions associated with nonuniform, heterogeneous patient anatomy may lead to differences in delivered doses. To develop a methodology for patient-specific dosimetry, we evaluated the water-equivalence of 3D printing plastic polylactic acid (PLA) and the accuracy of gafchromic films (EBT4 and EBT-XD) between 0-70Gy.
Methods: Films were placed in 30x30x20cm3 kV-rated plastic water and exposed to known doses using an Xstrahl orthovoltage unit (180kVp). Calibration curves were generated and tested for each film model using doses between 0-70Gy. Film calibration and analysis was performed using an in-house software for single-channel dosimetry. Films were irradiated in an 8x8x12cm3 PLA volume and compared against measurements in kV-rated plastic water. A PLA phantom modeling actual patient anatomy was manufactured to replicate clinical MBRT delivery and doses measured using film.
Results: Film measurements were compared against expected doses based on the calibrated orthovoltage output. For test doses <10Gy, the percent difference was within ±5.3% (EBT4) and ±2.8% (EBT-XD), and for test doses ≥10Gy, within ±5.3% (EBT4) and ±1.6% (EBT-XD). The doses measured in PLA and plastic water were within ±1.6%. Delivery of 2475 monitor units in flat plastic water corresponds to peak/valley doses of 19.4/2.4 Gy, while exposures on the patient-specific phantom resulted in 18.1/1.8 Gy (EBT4) and 18.5/2.0 Gy (EBT-XD) vs. 17.5/1.7 Gy (EBT4) measured in-vivo.
Conclusion: Both film models may be suitable for MBRT dosimetry with peak doses up to 70Gy and valley doses as low as 1.5Gy, with a trend to higher accuracy for EBT-XD. The patient-specific PLA phantom presents a methodology for modeling MBRT heterogeneous scattering and in-vivo dosimetry, which will be critical for commissioning Monte Carlo calculations.