Electronic Brachytherapy Organ at Risk Dose: A Non-Inferiority Comparative Study between Iridium-192 and 50 Kilovoltage X-Ray Source Models at Steve Biko Academic Hospital

Introduction: The use of the Xoft Axxent electronic Brachytherapy (eBx) system has become more common due to advantages, like lower doses to the Organs At Risk (OARs), than the traditionally used radioactive sources like Iridium-192 (Ir-192). Xoft’s eBx uses a miniaturised X-ray tube instead of a radioactive source. At Steve Biko Academic Hospital, the EMBRACE-II protocol is followed when planning and treating gynaecological brachytherapy patients to ensure that the OARs such as bladder and rectum reach the compliance of the dose constraints. Studies have shown that Ir-192 source models adhered to these constraints. Hence, with eBx becoming more popular, it is vital to ensure that the new technology is not inferior to Ir-192 when comparing the OARs doses. Materials and Methods: The study considered ten cervical cancer patients, planned with the 50 kV eBx source model and retrospectively planned with an Ir-192 source model. The delineated Planning Target Volume (PTV) and OARs were kept identical between the models. The retrospective planning aimed to replicate the PTV D90% of the initial plan. The difference in doses (D2cc) to Organs At Risk (OAR) between the models was calculated and expressed as a percentage. Statistical significance was determined using a paired t-test with a 95% confidence level. Results: For the rectum, the mean D2cc was 2.52 ± 1.05 Gy for eBx and 2.78 ± 1.16 Gy for Ir-192. For the bladder, the mean D2cc was 4.27 ± 1.46 Gy for eBx and 4.36 ± 1.24 Gy for Ir-192. The average difference in planned doses (D2cc) for the rectum and bladder between two models was 5.31% for the rectum and 0.11% for the bladder. These results for rectum doses were rendered statistically significant, whereas bladder results were insignificant. In both cases, the Ir-192 model delivered the higher OARs doses. Conclusion: No significant differences were found in bladder dose (D2cc) when comparing the two source models. The eBx model delivered on average 5.31% less rectum dose (D2cc). eBx proves to be non-inferior to Ir-192 for gynaecological brachytherapy treatments.