Dose Distribution of HDR Brachytherapy using Different Sources, Treatment Planning Systems and Applicators

Abstract

Three dimensional radiotherapy techniques propose the opportunity of delivering the dose distribution which is well conformed to target volume while minimizing the exposure by radiations to nearby organs. However, second cancer incidences from the radiations are rare and much late effects after radiotherapy techniques. The purpose of dose distribution is vital as imprecision in dose parameters can produce further complications. The critical organs of the patients are usually received low radiations due to the complex radiation fields which are close or relatively far from target and may be a cause to produce secondary cancer risk. Therefore, numerous parameters regarding to the radiotherapy treatment planning must be discussed. This study aims to calculate the dose distribution, dose volume histograms (DVHs), life time secondary cancer risk, solid cancer risk or excess absolute risk (EAR) using the different modalities of radiotherapy (APBI – accelerated partial breast irradiation, EBRT – external beam radiotherapy). Breast cancer is global dilemma and considerable improvements have been made in the outcomes with early stage breast cancer patients. In our analysis, due to the steep dose gradient and the prescription to 10 mm tissue depth, Accelerated partial breast irradiation (APBI) with 50 kV x-ray miniature source and MammoSite brachytherapy with Ir-192 Source delivered the highest maximal dose to the ipsilateral breast. MammoSite brachytherapy delivered higher maximal dose to heart than after three dimensional conformal radiotherapy (3D-CRT) and significantly lower using the tangential IMRT. The risk of solid cancer is generally related to radiotherapy. In general, the incidences of secondary cancers are related to the actual dose received by the irradiated organ. In our study, secondary cancer risk is estimated from the breast radiotherapy techniques. IORT associated with less second cancer risk (0.02%) for ipsilateral lung in comparison to APBI and EBRT. Second cancer risk observed by IORT for contralateral breast is less in comparison to APBI and EBRT. The risk calculated from EBRT for the ipsilateral lung (2.9%) and contralateral lung (0.2%). Tangential IMRT and 3D-CRT irradiated the higher volume of contralateral breast and ipsilateral lung for the dose (<1 Gy). EAR decreased for contralateral breast, ipsilateral lung and contralateral lung for 3D-CRT and tangential IMRT viiiin comparison to multibeam IMRT and VMAT by using linear, linear-exponential and plateau models. In order to get better accuracy in dose distribution, it is point of interest to compare the dose distribution by treatment planning system, applicators and sources. High dose-rate (HDR) brachytherapy techniques are commonly used to treat the tumours such as the breast, cervix and prostate. In these brachytherapy techniques, applicators are inserted close to tumours while the radioactive sources are adjusted at suitable position for the desired delivery of dose. Intracavitary brachytherapy (ICBT) constitutes an essential component in the radiation therapy of cervical cancer. With high-dose-rate (HDR) afterloading units, ICBT procedures require multiple applications spread over time. In our study, The GammaMedplus HDR remote brachytherapy afterloading system with single high active (usually about 10 Ci) cylindrical 192 Ir source having active dimensions of 0.6 mm diameter and 3.6 mm length was used. The planning was done using treatment planning software (ABACUS 3.1). It is noted that total average dose and standard deviation of dose for nine patients by Ring applicator at bladder, rectum, RPWRP and LPWRP was of 4.26 ± 0.35 Gy, 2.72 ± 0.29 Gy, 0.98 ± 0.11 Gy and 0.83 ± 0.16 Gy respectively whereas total average dose and standard deviation of dose for five patients by Fletcher applicator at bladder, rectum, RPWRP and LPWRP was of 4.05 ±0.73 Gy, 3.02 ± 0.76 Gy, 1.22 ± 0.25 Gy and 1.23 ±0.38 Gy respectively.