The importance of the heterogeneity effect in TPS dose calculation in HDR 192Ir Brachytherapy of Breast cancer

Brachytherapy plays an important role in the curative management of cancers in a variety of disease sites, especially breast cancer. In this form of radiotherapy, a high dose can locally be delivered to the small tumor volume, with rapid fall-off dose in the surrounding healthy tissues and spare OARs.The place of brachytherapy (BCT) especially in breast conserving therapy, as a part of standard treatment for a vast majority of early stage breast cancer patients, is remarkable and established; that’s proven to reduce local recurrence, toxicity, treatment time, it was used as definitive treatment or as a boost.There has been a growing interest for BCT-APBI (Accelerated partial breast irradiation ) and various approaches have been developed, which include Multicatheter interstitial , intracavitary with balloons (Mammosite™), hybrid applicators and irradiation with low energy X-ray.Although HDR -APBI has excellent clinical & cosmetic Outcomes with low rates of long-term toxicity, but inaccurate dose plan of treatment, can lead to many serious effect on local tumor control and side effects.Current HDR-brachytherapy dose calculations are based on a single source dose distribution in water that is represented in the TPS by specific parameters (TG-43) guidelines. This method is fast and practical in the clinic. however, influence of the heterogeneities of patient s anatomy, applicator shielding, interseed attenuation, and finite patient dimensions are all ignored.The TG-43 dose calculation formalism assumes an infinite scattering medium and can result in overestimation of absorbed dose at a low-density interface. While in many clinical settings, the actual scatter conditions may deviate from these reference conditions, leading to significant overestimates of dose, e.g. when the source is near the surface of the skin. Moreover, comparing the calculated dose distributions to the TG43 and MC calculations for HDR brachytherapy APBI, showed that for air cavity and scatter due to patient size and different densitis of rib and lung, there’s Significant changes in dose to the target and OARs. Detected dosimetric differences in target DVH parameters (e.g. target coverage and homogeneity indices) clinically is still controversy. However dose close to the lung which is at relatively large distance from the implant lead to a TG-43 dose overestimation for the organs at risk. This mismatch between the calculated dose and the actual dose, will certainly, affect the tumor control probabilities and normal tissue complication probabilities.The development of modern HDR- TPS that belong to the model-based dose calculation algorithms(MBDCAs) e.g. Acuros BV, has improved the accuracy of calculated dose distributions as an alternative to TG-43 in 192Ir breast brachytherapy. Unlike TG-43, the main advantages of Acuros BV are that they account for tissue inhomogeneities, scatter disequilibrium, and applicator attenuation, so that they calculate dose by using the materials’ mass density to determine the atomic cross section and rely heavily on dose optimization software which can tailor doses to specific clinical needs without knowing the composition of the tissue through which radiation transport is taking place in actual treatment conditions.

کلیدواژه ها:

HDR- breast brachytherapy ، APBI ، Inhomogeneity ، TG-43 brotocol ، model-based dose calculation algorithm(MBDCA) ، Acuros BV-TPS