Room: Exhibit Hall | Forum 3
Purpose: To calculate and compare the dosimetric quantities, Dose-to-water-in-medium (Dw,m) and dose-to-medium-in-medium (Dm,m), from commercially available Model Based Dose Calculation Algorithms and Monte Carlo simulations for five APBI brachytherapy cases.
Methods: Clinical APBI brachytherapy cases were selected since APBI brachytherapy patients typically have more heterogeneous geometry. Treatment plans from Acurosâ„¢ BV ver. 13 (Varian Medical Systems, Inc., Palo Alto, CA) were simulated with MCNP6.1.1. Acuros and MCNP can calculate Dm,m and via the large cavity theory approach, report Dw,m. This method utilizes the ratio of mass-energy absorption coefficients (medium/water). The patient geometry was modified to consist of 4 materials (water, lung, bone and air). Dose ratios were computed and compared amongst the algorithms. Target coverage, highest dose to ribs, skin, and lung were compared.
Results: Dw,m and Dm,m values differ around 1% for lung and up to 8% for bone for with the same algorithm. Differences are constant for soft tissues varying with the distance from the source for bone. The dose discrepancy between MCNP and Acuros was 1 Â± 2% (one standard deviation) for the calculated volume and better than 0.5% for a majority of the voxels within the PTV_EVAL. However, differences of up to 10% for Dm,m were observed near material interfaces. Clinical dose metrics demonstrate agreement within 1.5% between MCNP and ACUROS (Dw,m and Dm,m) whilst D0.1cc differences are up to 1.7% (Dm,m) for the ribs. Differences between clinical dose metrics determined by Dw,m and Dm,m were up to 2.3% (PTV-V150) and 3.4% (Ribâ€“D0.1cc) respectively. The mean difference for the PTV_EVAL is <1% and becomes negligible if air cavities are properly excluded.
Conclusion: Differences between (Dw,m and Dm,m) can be relevant and should be considered during planning. Dm,m is more sensitive to tissue segmentation with large differences close to interfaces with bone and air.