Consideration of accurate dose distribution for Contura balloon catheter in breast brachytherapy with MCNP5 simulation

Introduction:
Contura is a balloon applicator that used to deliver intra cavity radiation for Partial Breast Irradiation (PBI) treatments. Contura with 4 peripheral lumens surrounding the central, for loading HDR sources from an after loader system can provide an asymmetric dose distribution and gives a more flexible shape of dose than earlier balloon applicators such as Mammo site. Accurate dosimetry is necessary to avoid deviations in delivered dose from that prescribed for a successful treatment. In this study we tried to explore possible uncertainties in simple water phantom measurements compared to the realistic breast cancer treatment condition with Contura.
Materials and Methods:
MCNP5 simulations were performed to have investigations on dose distribution. Several trials have been introduced to study the effects of corrections for phantom geometry and content. First, dose distributions were considered for an ICRU-44 breast phantom and the obtained results were compared with water. In next trial geometry was corrected to take the effects of an air cavity as lung inside the chest into account. Investigations were followed in third trial with correction at the breast skin surface to evaluate tissue-air interferences at skin. 192-Ir HDR sources were simulated on 7 dwell positions on each lumen. Lumens were defined inside a water sphere as saline filled balloon. Results have presented as dose distribution plots in breast and dose volume histogram analysis for PTV.
Results: The ICRU-44 breast phantom demonstrated differences less than 1% in breast from water. Air cavity as the lung tissue has not shown significant effects on dose distribution in the breast, but correction at skin surface has caused up to 5% overestimations proximity near this area. However almost in other places in the corrected breast, the matched results to the water phantom have been observed.
Conclusion: The results of this investigation indicate no significant deviations from a water phantom without any corrections. Therefore, treatment planning systems can still be used without significant uncertainties in treatment of breast cancer using HDR brachytherapy with Contura device. Differences can be corrected to provide a precise dosimetry but because of underestimated values obtained in water phantom dosimetry it can be a conservative approximation.