Purpose: To determine the inter-fractional dosimetric variation of accelerated partial breast irradiation (APBI) using the SAVI applicator due to daily patient positioning variations, through the first and second day of treatment
Methods: A CT is taken for treatment planning purpose, the PTV_eval is created according to the ASTRO recommendations and a treatment plan is made with a prescription of 22.5Gy in 3 fraction to the PTV_eval. Short CT scans were taken before the first treatment on day 1 and second fraction treatment on day2 to ensure no change in the integrity of the applicator and its position with respect to the anatomy. For the purpose of this study. Plans were created on the CTs of the first and second fractions, the dwell times and the source activity of the treatment plan were used in the comparison plans to keep the activity-hour intact.
Results: The deviation of the PTV_Eval coverage -D90% varied between 0.08% and 6%. The variation in dose to skin ranged from 0.4Gy - 2.1Gy for 2cc and from 1.1 - 2.8Gy for 0.03cc. For the CW/ribs, the deviation of dose for the 2cc ranged from 0.3 to 4.6Gy and for the 0.03cc was from 0.7Gy to 6.5Gy.
Conclusion: The positioning of the ipsilateral arm plays an important role in the variation of the proximity of the critical structures to the applicator. This variation determines changes in the new PTV_EVAL. Greater variation in dose was noted for 0.03cc as compared to 2cc for both CW/ribs and Skin. Variation in the coverage of PTV-Eval can be as high as 6%. As result of our finding, the highest grade of reproducibility in the patient setup is recommended to minimize the dosimetric variations.