SU‐E‐T‐489: Radiobiological Evaluation of the Inter‐Fractional Organ Deformations in Cervical Cancer Treatments with HDR Brachytherapy

Purpose: The proper combination of the doses by separate brachytherapy fractions affects the estimation of the effectiveness of the treatment since the patient anatomy is not the same between the different brachytherapy fractions. The purpose of this study is to estimate the variation in the effectiveness of the interfractional HDR brachytherapy procedures using dosimetric and radiobiological measures. Methods: The treatment plans of 10 cervical cancer patients (stage III), who were treated with external beam radiotherapy and HDR brachytherapy (using tandem and ovoids), were evaluated. Dose‐volume indices for the cervix and organs‐at‐risk (OARs), along with radiobiological measures, were considered in determining the clinical effectiveness of each fractional dose distribution. The biological effective doses (BED), biologically effective uniform doses (BEUD) and generalized equivalent uniform doses (gEUD) were additionally obtained. Results: For a sample patient, the average physical and BED doses by the external radiotherapy were 46.9 Gy and 45.6 Gy for the target, 35.2 Gy and 30.7 Gy for the rectum, 46.0 Gy and 42.7 Gy for the bladder, respectively. Similarly, for the HDR brachytherapy, the average physical and BED doses were 42.3Gy (SD:1.8) and 65.4Gy (SD:4.6) for the target, 12.8Gy (SD:1.3) and 14.0Gy (SD:2.2) for the rectum, 11.7Gy (SD:1.3) and 12.4Gy (SD:1.8) for the bladder, respectively. The respective gEUD and BEUD values were 36.5Gy (SD:0.5) and 49.2Gy (SD:1.0) for the target, 6.9Gy (SD:1.7) and 35.6Gy (SD:5.8) for the rectum, 7.1Gy (SD:1.2) and 39.9Gy (SD:2.2) for the bladder, respectively. Conclusion: It is shown that the variation in the effectiveness of the different HDR brachytherapy fractions can vary considerably. The calculation of the total dose distribution should be performed by summing up the deformed fractional dose distributions.