ESTRO 2023 - Abstract Book

S313

Sunday 14 May 2023

ESTRO 2023

PD-0411 Intra-applicator target volume and OAR dose variation in MRI guided adaptive BT for cervix cancer V. Sim 1 , R. Gonzalez Vaz 2 , E. Jones 2 , B. Taylor 3 , I. White 3 1 Guy's and St Thomas' NHS Foundation Trust, Clinical Oncology, London, United Kingdom; 2 Guy's and St Thomas' Foundation Trust, Medical Physics, London, United Kingdom; 3 Guy's and St Thomas' Foundation Trust, Clinical Oncology, London, United Kingdom Purpose or Objective Evaluate intra-applicator target volume and organ at risk (OAR) dose variation in MRI guided adaptive brachytherapy (MRIGABT) for cervix cancer. Calculate cost implications of replan at each treatment fraction. Materials and Methods Radiotherapy treatment planning MRI (RTPMRI) were acquired before each brachytherapy fraction (1-4), in 14 patients treated with intrauterine brachytherapy (IUB) using the VeneziaTM applicator. Each patient had 2 IUB applicator insertions and received 28 Gy/4# following EBRT 25 Gy/25# (EMBRACE II protocol)1 A replan based on anatomy on MRI acquired on day 2 (D2) of each IUB applicator insertion was planned and delivered (D2-replan). Dose to the high risk clinical target volume (HRCTV) and OARs were compared to conventional treatment where the treatment plan on day 1 of IUB applicator insertion is also delivered on day 2 (no replan). A paired students t-test was used to assess for statistical differences. Total biologically equivalent doses for EBRT and brachytherapy were calculated in 2 Gy equivalents using the EQD2 equation and reported as total dose (Gy). Cost of a replan at each treatment fraction in terms of additional radiology and physics resources required was calculated. 1) EMBRACE - https://www.embracestudy.dk/UserUpload/PublicDocuments/EMBRACE%20II%20Protocol.pdf Results Forty-six RTPMRI and IUB HDR treatment plans were analysed in 14 patients. The mean (±SD) total dose (EBRT + HRCTV 90) received for D2-replan and D2-no replan was 90.5 (±4.3) and 88.7 (±4.8). There was a statistically significant difference in target dose between D2-replan and no replan P<0.05). OAR dose constraints exceeded EMBRACE dose constraints in 50% of patients if no replan was performed (rectum 18%, sigmoid 11%, bowel 17% and bladder 7%). No OAR total dose constraints were exceeded with D2-replan. Conclusion Daily MRIGABT replan at each treatment fraction results in improved HRCTV dosimetry and reduced risk of exceeding OAR dose constraints. A replan ensures HRCTV D90 of >90Gy and better OAR sparing. In our institution, the cost of a replan is € 603 (additional physics and radiology resources). Based on current activity, this amounts to € 18,090 per year (n=30 patients treated/year). It could be argued that 50% of patients did not need a replan because OARs were within tolerance and HRCTV dose achieved. Additional resources required for a replan mean that selection criteria for patients who will benefit most needs to be defined. PD-0412 Impact of vaginal dilators in G2-late vaginal complications in brachytherapy for endometrial cancer. A. Rovirosa 1 , Y. Zhang 2 , F. Noorian 1 , R. Abellana 3 , R. José 4 , A. Herreros 4 , G. Antelo 4 , A. Torne 5 1 Hospital Clinic-Universitat de Barcelona, Radiation Oncology, Barcelona, Spain; 2 Henan Provincial People’s Hospital. China, Cancer Center,, Zhengzhou, China; 3 Hospital Clínic-Universitat de Barcelona, Fonaments Clínics, Barcelona, Spain; 4 Hospital Clínic-Universitat de Barcelona, Radiation Oncology, Barcelona, Spain; 5 Hospital Clínic-Universitat de Barcelona, Gynecological cancer Unit, Barcelona, Spain Purpose or Objective To evaluate possible prognostic factors for G2-late vaginal complications after vaginal brachytherapy (VBT) with or without external beam radiotherapy in postoperative endometrial cancer (PEC). Materials and Methods Between 2013 and 2018, 126 PEC patients treated with VBT±EBRT were retrospectively analysed considering possible prognostic factors for vaginal complications including age, body mass index, applicator diameter, clinical target volume (CTV), use of dilators, chemotherapy and EQD2( α / β =3) at the most exposed 2cm3 of the vagina. Late vaginal complications were evaluated using objective criteria of LENT-SOMA. Statistical analysis: The Chi-square or Fisher exact test and Student’s test were used to study the association between G2 late vaginal complications and prognostic factors. The variables associated at a univariate level were considered in the multiple logistic regression model. Results The median age was 66 years (40-86), and the median follow-up was 66 months (8-104). Among 126 patients, no one showed vaginal-cup relapse in the present series but one in the outer third. 19/126 (15%) showed G2 late vaginal complications,