ESTRO 2023 - Abstract Book

S1937

Digital Posters

ESTRO 2023

Purpose: Toxicity from cervical brachytherapy has been demonstrated to mostly relate to the D2cc. Therefore a simplified version of knowledge based planning may be possible, only investigating the relationship of the overlap distance for 2cc and the D2cc from planning. This study aims to investigate if such a simple relationship could be used to predict the D2cc, and if this can be used to detect sub-optimal plans and improve plan quality. Materials and Methods Methods: The overlap volume histogram (OVH) method was used to determine the distance for 2cc of overlap between the OAR and CTVHR. Linear plots were used to model the OAR D2cc and OVH predicted 2cc distance. The model was used to predict D2cc, and this value entered as the maximum constraint in the inverse planning algorithm. Two datasets of 20 patients (plans from 43 insertions in each dataset) each were used to create 2 models, and the performance of each model was compared. Doses were scaled to ensure consistent CTVHR D90 doses. Results Results: Mean bladder doses decreased by 2.9% for both models, mean rectal dose decreased 14.9% for model 1 and 6.0% for model 2, mean sigmoid dose decreased 10.7% for model 1 and 6.1% for model 2, mean bowel dose decreased 4.1% for model but no statistically significant difference observed for model 2. Conclusion Conclusion: A simplified knowledge based planning method was used to predict D2cc and was able to automate optimization of brachytherapy plans for locally advanced cervical cancer. E. Yap 1 , W. Bacorro 1 , T. Sy Ortin 1 , C. Jainar 1 , M. Dumago 1 , M. Bojador 1 , S. Cabrera 1 , L. Jacomina 1 , G. Gonzalez 2 , J. Mariano 2 , L. Antonio 1 , A. Bautista 3 , J. Genson 4 1 University of Santo Tomas Hospital, Radiation Oncology, Manila, Philippines; 2 University of Santo Tomas Hospital, Gynecologic Oncology, Manila, Philippines; 3 Manila Doctors Hospital , Gynecologic Oncology, Manila, Philippines; 4 Manila Doctors Hospital , Radiation Oncology , Manila, Philippines Purpose or Objective Current literature on single application multifractionated brachytherapy(BRT) is limited. The use of MRI, which provides better soft-tissue delineation, allows more hypofractionated treatment, with a recent study showing acceptable results using three fractions. However, with CT- based treatment, the most commonly used regimen consists of 5-6 fractions with concerns of toxicity if a more hypofractionated regimen was used. The aim of the study is to evaluate the safety and clinical outcomes of single application multifractionated CT-guided interstitial high-dose-rate(HDR) BRT given in four fractions in locally advanced cervical cancer. Materials and Methods Patients with cervical cancer FIGO 2018 stage IIB-IVA that were treated definitively with external radiation (50Gy/25fractions) ± weekly cisplatin followed by single application multifractionated CT-guided interstitial HDR BRT were included. The protocol involves the use of four fractions of HDR BRT given in 3 consecutive days with 2 fractions given on the second day 6 hours apart with a total planning target of HRCTVD90 of >85Gy EQD2. Dosimetric data, clinical response, and toxicity records were reviewed. Results Twenty-three consecutive patients were included. Clinical stage distribution was as follows: IIB: 13.0% (3), IIIB: 26.1% (6), IIIC: 26.1% (6) and IVA: 34.78% (8). Median initial T score at diagnosis was 10 (range 7-15). All patients underwent Intracavitary + Interstitial BT with needles (median 11 ; range, 6-20). The mean ± standard deviation HRCTV volume was 67.88 ± 27.66 cm3 and HRCTV D90 dose was 83± 10.8Gy. The 2 cm3 to bladder, rectum, and sigmoid were 86.37 ± 7.5 Gy, 72.60 ± 6.0 Gy, and 68.18 ± 8.3 Gy, respectively. Mean overall treatment time was 65 ± 21 days. With a median follow up of 9.5 months (range, 1-39), The median survival was 16 months, and the one-year local control rate, progression free survival and overall survival were 73% (54-100, 95% CI), 60% (40-92, 95% CI), and 72% (52-100, 95% CI), respectively. For the acute toxicity, grade 3 radiation-related toxicity was observed in 4 patients: 1 hematologic, 1 diarrhea, and 2 infections. The most severe was one patient with grade 4 toxicity of sepsis. For the late toxicity, grade 3 toxicity was observed in 3 patients: 2 proctitis needing intervention and 1 genitourinary toxicity. There were 3 grade 4 genitourinary toxicity with one needing a nephrostomy tube, and two requiring dialysis. PO-2153 single application CT-based interstitial high-dose-rate brachytherapy for cervical cancer