ESTRO 2021 Abstract Book

S1061

ESTRO 2021

identified in the superior and inferior slices, and therefore Σ deform, σ deform and σ treat could not be calculated in these directions (Fig. 2). The mean volumes for sCTV 205.06cm 3 and sPTV 301.16cm 3 are larger than rCTV 161.89cm 3 and rPTV 244.86cm 3 .

Conclusion Target deformation and intrafraction motion errors are smaller if only the anterosuperior 1.5cm of the lower uterus is included in LRCTV for cervical cancer irradiation. Using an MRgRT pathway to define reduced volumes and treat could result in reduced internal margins and irradiated volumes. PO-1288 Sequential external beam RT boost with two simultaneous dose levels in advanced cervical cancer C. Mariucci 1 , L. Vicenzi 1 , S. Costantini 1 , C. Di Carlo 1 , F. Cucciarelli 1 , M. Valenti 2 , G. Mantello 1 1 Azienda Ospedaliero Universitaria Ospedali Riuniti, Radiotherapy, Ancona, Italy; 2 Azienda Ospedaliero Universitaria Ospedali Riuniti, Medical Physics, Ancona, Italy Purpose or Objective External beam radiotherapy with concomitant chemotherapy (CH), followed by interventional radiotherapy (brachytherapy), is the gold standard in locally-advanced cervical cancer (LACC). The aim of our study is to assess the external beam boost feasibility in patients (pts) unfit for interventional radiotherapy. Materials and Methods We retrospectively analyzed data of 15 pts with median age of 51 years (range, 32-83), affected by LACC treated between July 2017 and July 2020. Ten out of 15 pts (66%) had squamous cell carcinoma, 4 pts (27%) adenocarcinoma and 1 patient (7%) neuroendocrine carcinoma. The stage at diagnosis, according to FIGO 2018 classification, ranged between IB3-IIIC2. Pts underwent pelvic +/- para-aortic lymph nodes external beam radiotherapy with Volumetric Modulated Arc Therapy (VMAT) and concomitant weekly platinum-based CH. MRI was performed before starting radiotherapy and at the fourth week of treatment to evaluate tumor response. Afterwards, owing to interventional radiotherapy limitations (cervical canal not detectable or unsolvable stenosis), a sequential VMAT boost to residual primary tumor was delivered. Two different concentric boost Clinical Target Volumes (CTV) were identified as the whole cervix (CTV1) and MRI residual tumor (CTV2). During treatment a daily CBCT was performed. A routine follow up was planned with MRI every three months as well as periodic TC and PET-CT scans. Results The median total dose to the pelvis +/- para-aortic lymph nodes was 45 Gy (range, 45–50.4 Gy) with daily 1.8 Gy fractionation. In 7/15 pts, a median 54 Gy (range 50-56.25 Gy) simultaneous integrated boost was administered to positive PET-CT lymph nodes. Concerning the cervical boost, the median prescribed dose to the CTV1 was 19.8 Gy (range 17-27.5 Gy) in 10- 11 fractions, while a median total dose of 30 Gy (range 20-33 Gy) in 10-11 fractions was delivered to CTV2. We estimated to join a total EQD2 dose range of 76.8-82.1 Gy (a/b 10) to the CTV2. After a median follow-up of 12 months (range, 6-29), we observed a local complete response in 7 pts; a partial response in 6 patients: one patient had hepatic progression while a second one pelvic lymph node progression. Additionally, one patient with partial local response underwent surgical hysterectomy with complete residual tumor excision. Local and systemic progression occurred in only 2 pts. No gastrointestinal and genitourinary acute or late toxicity > G2 was detected, according to Common Terminology Criteria for Adverse Events v.4 scoring criteria .