ESTRO 2020 Abstract book

S400 ESTRO 2020

PD-0658 Suboptimal dosimetric coverage of PET/CT hotspots is associated with recurrence for cervical cancer F. Lucia 1 , D. Visvikis 2 , V. Bourbonne 1 , J. Bert 2 , R. Abgral 3 , G. Dissaux 1 , O. Miranda 1 , O. Pradier 1 , M. Hatt 2 , U. Schick 1 1 Radiation Oncology department- University Hospital- Brest- France, Radiation Oncology department- University Hospital- Brest- France, Brest, France ; 2 LaTIM- INSERM- UMR 1101- Univ Brest- Brest- France, LaTIM- INSERM- UMR 1101- Univ Brest- Brest- France, Brest, France ; 3 Nuclear Medicine department- University Hospital- Brest- France, Nuclear Medicine department- University Hospital- Brest- France, Brest, France Purpose or Objective Patients with locally advanced cervical cancer (LACC) treated with chemoradiotherapy (CRT) have local recurrence rate of ~15%. Areas of high uptake on Pre- treatment 18 F-fluorodeoxyglucose ( 18 F-FDG) positron emission tomography/computed tomography (PET/CT), denoted as “hotspots”, have been identified as preferential sites of local relapse. The purpose of this matched-pair study was to analyze the dosimetric coverage of these hotspots with high dose-rate brachytherapy (BT). Material and Methods Forty-two patients with recurrence after CRT for CC (20 with pelvic recurrence and 22 with distant recurrence) were matched to 42 patients without recurrence randomly selected from a larger population. Mean (±standard deviation) follow-up was 26±11 months. For each patient, a rigid registration of the CT from the pre-treatment PET/CT with the radiotherapy planning CT was performed using the 3D Slicer TM Expert Automated Registration module optimized with the Mattes mutual information metric. The fuzzy locally adaptive Bayesian (FLAB) algorithm was applied using 3 classes (one for the background and the other two for tumour) in PET to simultaneously define an overall tumour volume and the high-uptake sub-volume V1. The inclusion of V1 in the high-risk clinical target volume (CTV HR) and its dosimetric coverage were evaluated using the “Dose Volume Histogram” module in 3D Slicer TM . The average of the 3-4 BT sessions was reported. Results In the recurrence group, V1 was not included in the CTV HR and not covered by the 85Gy isodose in 17/42 patients (41%) (1 with pelvic recurrence and 16 with distant recurrence) and not by the 80Gy isodose in 7/42 patients (17%) (all with distant recurrence). The mean coverage of V1 by isodoses 85, 80 and 78Gy were 92.1±12.1%, 98.4±4.0% and 100%, respectively. In the non-recurrence group, V1 was not included in CTV HR and not covered by the 85 Gy isodose in 3 patients only (7%). The mean coverage of V1 by isodose lines 85, 80 and 78 Gy were 99.4±2.2%, 100% and 100%, respectively. The hotspots coverage by the 85 Gy isodose was significantly better in patients who did not recur, but only when compared to patients with distant relapse (p<0.0001). Conclusion Suboptimal dosimetric coverage of high FDG uptakes on pretherapeutic PET seems to be associated with an increased risk of recurrence. The identification of these hotspots on PET could contribute in optimizing the BT procedure in patients with LACC.

PD-0659 Looking Beyond D90: Correlating EUBED, gBEUD with Outcome in Cervical Cancer Brachytherapy D. Todor 1 , E. Fields 1 , A. Allen 1 , D. Batchelar 2 , E. Brouillard 2 , J. Ding 2 , M. Kudla 2 , F. Bachand 2 , G. Hajdok 3 , D. D'Souza 3 1 Virginia Commonwealth University, Radiation Oncology, Richmond- VA, USA ; 2 British Columbia Cancer Center, Radiation Oncology, Kelowna, Canada ; 3 London Health Sciences Center, Radiation Oncology, London, Canada Purpose or Objective The current formalism for reporting and adding the effects of BT and external beam RT doses is based on a single metric, D90 to CTV_HR and its EQD2. Our previously published work however, has demonstrated that BED(D90) is unable to distinguish between plans that are optimized differently or created with different applicator systems. In comparison, equivalent uniform BED (EUBED) and generalized biologically equivalent uniform dose (gBEUD), which integrates dose inhomogeneity, are better quantities for discerning these important differences. We sought to measure local control in women being treated with radiotherapy for locally advanced cervix cancer and assess its correlation with EUBED and gBEUD integrated over the whole volume of CTV_HR compared to the traditional, single point metrics D90. Material and Methods 163 women with FIGO 2018 IB3-IIIC2 cervical cancer from 3 institutions were included. All women received EBRT ± chemotherapy and 3-5 fractions of intracavitary brachytherapy. The median follow up for all women was 59.5 months (range 0-87 months). At this time 6.1% have had local recurrences, 12.2% distant recurrences, 1.2% both local and distant. Data was extracted from TPS and dosimetric and radiobiological quantities computed. EUBED and gBEUD were calculated for each fraction after a voxel-based conversion from dose to BED (α/β=10Gy). EUBED uses an extra parameter α (range 0.05-0.5Gy -1 ) with smaller values typically associated with radioresistant tumors. Similarly, gBEUD uses a parameter a which allows a variable emphasis on hot/cold spots (range –5,5). Distributions were compared using the 2-sample Kolmogorov-Smirnoff test at 5% significance. Results Distributions of all dosimetric quantities were compared among institutions, starting with CTV size and D90: