A study on dosimetry accuracy of Strut-Adjust Volume Implant (SAVI) brachytherapy

Introduction: Accelerated Partial Breast Irradiation (APBI) is an effective treatment for breast carcinomas because of its limited number of fractions. Strut-adjusted volume implant (SAVI) is a HDR brachytherapy applicator to deliver the prescription dose with good PTV coverage and acceptable dose to skin, chest wall and organ at risks. Acceptable clinical outcomes depend on accurate dosimetry methods. Currently water is used as the reference dosimetry phantom material in treatment planning systems. The purpose of this study is to consider the corrections on phantom material and geometry in dose distribution of SAVI brachytherapy simulation using MCNP5 Monte Carlo code. Materials and Methods: SAVI with 8 peripheral source channels were considered expanded inside the lumpectomy cavity in breast. Three realistic phantoms were employed to perform the step by step corrections and dose distributions were compared with that of water. Considerations were included using the ICRU-44 defined breast material as the real breast composition and moreover the air-tissue interferences at the skin, lung and the air cavity inside the SAVI were studied. Results: According to the results, the dose differences due to the breast material corrections were less than 1% in the most points of breast and only 2-3% overestimation were observed close to the skin. Breast phantoms containing air cavity have shown up to 2% decreases in regions around it. Finally a phantom with both noted corrections and a curved surface for breast skin, corrected the calculations with about 3-5% underestimations at skin. Conclusion: According to the results of this study, dose distributions in breast phantoms containing different materials does not differ significantly with the dose distribution in water phantom. Conservative approximations can be provided with water phantom and it doesn’t seem necessary that the current treatment planning systems be replaced with the model-based patient specific dosimetry methods.