ESTRO 2021 Abstract Book

S1092

ESTRO 2021

Purpose or Objective As our population ages, the number of patients with hip prostheses receiving pelvic radiotherapy is increasing presenting challenges due to artefacts in CT images, difficulties in determining the prosthesis density and limitations of dose calculation algorithms. Minimising the proportion of the beam entering the PTV laterally through the prosthesis may reduce the uncertainties, but results in higher doses on average to the rectum and bladder. We investigated this hypothesis for 17 patients with bilateral hip prostheses who received radiotherapy for prostate cancer and compared the rates of acute and late GU and GI toxicities to a control group of 50 patients. Materials and Methods All patients received radiotherapy to their prostate only with a prescription of 60Gy in 20# or 57Gy in 19#. Patients in the protheses group were treated using a fixed field IMRT approach. Beam angles and field edges were chosen so that the beam did not enter the PTV through the prostheses. Patients in the control group were treated using VMAT. All patients were scored for acute and late GU and GI toxicity using the CTCAE (version 5) system. Median follow up times to determine late toxicity were 3.2 and 4.0 years for the prostheses group and control group respectively. The incidence rates of grade 2 or worse (G2+) toxicity were compared between the groups. Mean bladder and rectal DVHs were also calculated for each group. Results The incidence of acute toxicity and late GU toxicity was similar. G2+ late GI toxicity incidence was 31% for the prostheses group and 14% for the control group (p=0.14). There were no G3+ GU events across both groups and 1 late G3 GI event in the control group. The figures show the mean rectal and bladder DVHs with confidence intervals and p values. Statistically significant differences in rectal DVH were seen at 14-56Gy and in bladder

DVH

at

28-51Gy.

Conclusion The incidence of G2+ late GI toxicity at 31% for the prostheses group is quite high, but no excess G3+ events were observed. The mean rectal DVH is substantially higher for the prostheses group, for doses 14-56Gy. A previous study 1 reported a crude incidence of 9% for late GI G2+ RTOG adverse events in a mixed group of patients with unilateral and bilateral prostheses and using VMAT with avoidance sectors, whereas we used fixed field IMRT. Optimising the planning technique to minimise the doses to OARs for patients with bilateral hip prostheses is an important goal to prevent normal tissue toxicity and enable dose escalation. 1 Ng WL et al Rep Pract Oncol Radiother 20 :273-7; 2015 PO-1331 SBRT prostate cancer boost of 9 Gy, prospective phase II trial. F. Ferrer Gonzalez 1 , A. Pont 2 , R. De Blas 3 , A. Boladeras 4 , O. Garin 2 , N. Garcia Expósito 5 , M. Stefanovic 5 , M. Garcia Marqueta 6 , M. Juarez 7 , M. Olivas 8 , L. Armijos 9 , M. Alonso Becerra 5 , A. Alvarez 5 , A. Slocker 4 , M. Ventura 10 , M. Castells 11 , E. Merino 12 , F. Suarez 13 , C. Gutiérrez 7 , J. Delgado 5 , E. Zardoya 14 , D. Najjari 7 , C. Picon 14 , M. Ferrer 15 , F. Guedea 6 1 Catalan Institute of Oncology, Radiation Oncology, L'Hospitalet. Barcelona, Spain; 2 Institut Municipal d'Investigacions Mèdiques, Health Services Research Unit, Barcelona, Spain; 3 Catalan Institute of Oncology,