Room: AAPM ePoster Library
Purpose: To evaluate the importance of dosimetric differences between TG43 formalism and Monte Carlo (MC) simulations in breast brachytherapy with a ¹?²Ir source.
Methods: Treatment plans for 34 breast cancer patients were recalculated with the MC-based treatment planning software (TPS) RapidBrachyMCTPS. The dwell times and dwell positions were imported from Oncentra TPS and microSelectronV2 (Elekta, Sweden) source was used. Patient tissue was segmented according to TG-43 formalism, where patient tissue was assigned to water with unit density and on a voxel by voxel assignment of tissue and mass density derived from a CT-to-density curve. Tissue elemental compositions were assigned according to TG-186. Dose was computed and reported as dose to water in water, Dw,w, dose to medium in medium, Dm,m, as well as dose to a water cavity embedded in tissue Dw,m. The following dose volume histogram (DVH) metrics were evaluated: Dose Homogeneity Index (DHI), CTV D100%, PTV V200%, 100% and 90% and D0.1cc for lung, chest wall and skin.
Results: The differences between Dm,m and Dw,m were less than 1% for all DVH metrics, except for the PTV V200%, where Dm,m was lower than Dw,m by 3.40%. The largest differences between Dm,m and Dw,w were seen for PTV V200% (-8.01%), lung D0.1cc (-4.88%) and skin D0.1cc (-4.01%). The differences between Dm,m and Dw,w were statistically significant (p<0.01) for all investigated DVH metrics (p<0.01). The smallest deviation between Dm,m and Dw,w amongst studied target DVH metrics was for PTV V90%, <1%.
Conclusion: With PTV V90% having the smallest deviation and being the clinical goal for coverage based on the NSABP protocol B-39, the treatment of breast cancer with HDR brachytherapy and the TG-43 approximation is efficient. However, there are significant overestimations between prescribed and delivered dose when using simplified TG-43 dosimetry compared to MC-based dosimetry.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Collaborative Health Research Projects (grant number 523394-18) and Natural Sciences and Engineering Research Council of Canada (grant number 241018).