Evaluating variation of Hot Spot Location in order to radiobiological calculation of Organ at Risk in High-dose- rate Brachytherapy of Cervical Cancer

Introduction: Gynecological malignancies present challenges in determining an appropriate volumetric dose due to the highly variable physiologic activity of the surrounding tissue. Because of the high doses used in brachytherapy, surrounding structures have the potential to move around in the dose region and receive unknown amounts of radiation. Calculating hotspot region from reference point such as PIBS (Posterior Inferior Border of Samphys pubis) location could overcome challenges in determining the true delivered dose through a dose accumulation process. Materials and Methods: Data for patients treated with a vaginal cylinder or tandem and ovoids were imported into HDRPLUS software. The bladder, rectum and applicators were contoured and reconstructed on each computed tomography (CT) scan. The distance between high dose area and samphyspubis were reported. The PIBS is a fixed point and we can measure the distance from it for comparison of changing location in several fraction of treatments. . Point doses, D۲cm۳ , were used as a comparison value for each method Results: The cylinder set had a DSP (Distance from Samphys Pubis) to hotspot ranging from ۱.۲ to ۵ cm for bladder and ۱ to ۶ cm for rectum. The tandem and ovoids set of patients had a DSP ranging from ۰.۶ to ۳ for rectum and ۱.۵ to ۷cm for Bladder. Conclusion: From this study we can use radiobiological EQD۲ model for computing tolerance dose to OAR, so this method is a strict but sure approach for calculation.