Interfractional Dosimetric Variations of the Target and OARs for HDR GYN Brachytherapy
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PO-GePV-T-34 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To investigate the interfractional dose variations of the target and organs at risk (OAR) with respect to the prescription and its potential effect.
Methods: We retrospectively analyzed 29 cervical cancer patients treated with intracavitary high dose rate (HDR) brachytherapy using tandem and ovoids from January 2020 to November 2020. Prescriptions ranged from 5 Gy to 7 Gy per fraction for 2 to 5 fractions. Treatment planning were conducted in the Varian Eclipse brachytherapy treatment planning system (TPS). The HRCTV dosimetric parameter of interest was the dose that covers 90 % of the HRCTV (D90). The dosimetric parameter for OAR evaluation was the absorbed dose by 2 cc of the volume of interest (D2cc). Each fraction’s dosimetric parameter was normalized to the prescription and first fraction value to determine the proportional, and thus, percentage difference. For prescription normalization, OARs were normalized to the recommended OAR tolerance values from the International Commission on Radiation Units report 89 (ICRU 89) of 80% of the prescription. Statistical analysis was performed in Python 3.7 with the utilization of the Statistics package.
Results: The mean variations for the HRCTV were 23.9 ± 17.3 % for prescription, respectively. Mean dose variations from the dose constrains were 19.1 ± 19.9, 16.6 ± 17.3, and 26.4± 17.5% for rectal, bladder, and sigmoid, respectively. The dose deviation from the first fraction followed a similar pattern. A shift to high dose regions indicated the dose delivered to the target and critical structures was higher than the prescription.
Conclusion: A pattern of dose variation including a shift to higher dose in both the HRCTV and OARs was observed. Such deviations may have potential effect on dose prescription and plan evaluation, which will be further investigated.
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