PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s):To evaluate the variation in the radiobiological dose delivered to the critical organs (skin and ribs) during accelerated partial breast irradiation (APBI) using SAVI applicators in high dose rate (HDR) brachytherapy.
Materials/Methods:A retrospective analysis of 10 patients’ data treated with SAVI applicators at a single center were considered for this study. The patients were chosen from the groups that required multiple re-planning due to incorrect size of the applicator. The CT scans, taken before each treatment were separately imported into the treatment planning system and registered with the initial CT scan respective to the applicator for dosimetric evaluation. Three radio channel markers of the applicator are used as reference points to conduct landmark registration on each CT scan. The plans that were generated for future fractions were also used. Radiobiological factors were calculated for the dose delivered to 0.1 cc of the skin and ribs. This was done for composite plans and for individual plans. All the measurements where charted and compared to the accepted tolerances.
Results:Differences between the planned doses to the critical structures and the actual delivered doses over the 10 fractions were found. Variations in the delivered dose to the skin and ribs were statistically significant for fractions 1 and 2 respectively were over 5%. Variations were statistically significant for 4 and more fractions for both organs. Some data indicate, that in some instances the patients received more than 145% of the prescribed dose at the skin surface. Such variations, although acceptable by the ABS, are not accepted at the cancer center where I conducted this study.
Conclusion:The differences recorded in the radiobiological dose to the critical organs indicate that the delivered doses to these organs differ substantially from the prescribed dose calculated in the initial treatment plan from fraction to fraction, especially when the applicator is not of the correct size. Similarly, the delivered dose to the PTVeval varies from the calculated dose from fraction to fraction affecting the quality of the treatment. We conclude that taking a correct evaluation of the applicator size is absolutely necessary and a better method than the printed charts must be used. The CT scan taken before each treatment is required, not only in these cases but also in every APBI case due to the mobility of the breast and the reproducibility uncertainties that arise in this type of treatment.
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