ESTRO 2023 - Abstract Book

S363

Sunday 14 May 2023

ESTRO 2023

Oncology, Genolier, Vaud, Switzerland; 9 Swiss Medical Network, Clinique Générale -Beaulieu, Radiation Oncology, Geneva, Switzerland; 10 Swiss Medical Network, Clinique de Genolier, Radiation Oncology, Genolier, Vaud, Switzerland Purpose or Objective The paradigm of early stage rectal cancer treatment is rapidly changing with attempts to avoid unnecessary surgery. Radiotherapy (RT) with chemotherapy (RCT) play a major role in terms of local control (LC). RT dose escalation would improve the rate of complete responses (CR), but due to the potential toxicity of the surrounding tissues, this strategy remains limited even with modern external beam RT techniques. We report our institutional short-term outcome of patients treated with endocavitary low energy contact RT, better known as Papillon. Materials and Methods Between 2015 and 2022, we used Papillon to treat 50 rectal cancer patients; however, for the purpose of this report, we excluded patients treated with Papillon after neo-adjuvant RCT, in a salvage setting after local relapse, and as an adjuvant treatment of a malignant polyp, We treated 23 patients in our institution with Papillon after external beam RT or RCT with the intent to achieve a pre- planned organ preservation strategy. We report the outcome of 20 patients with > 3 months follow-up (FU). Toxicities were systematically reported. Papillon was commonly delivered in 3 fractions (median; range, 1-4), and the most common prescribed dose was 90 Gy (range, 40 – 90). Results Median FU was 26 months (range, 4-48). Median age was 70 years (range, 49-91). Nine patients had Stage I (T1-T2N0), 6 patients stage II (T3N0), 4 patients stage III (T1-3N0-N2), and 1 patient stage IV (T3N0M1). Response to treatment was evaluated at 6 weeks (digital examination, DRE) and at 3 months (MRI, rectoscopy, DRE). FU with MRI, DRE, rectoscopy, and endosonography (when judged necessary), was performed every 3 months for the first 2 years and every 6 months thereafter. A thoracic and abdominal CT scan was performed every 6 months. All of our patients had a complete clinical response at their 3 months assessment. Biopsies were taken in 3 patients with dubious lesions, confirming no residual disease. So far, 7 patients achieved long term (> 3 years) organ preservation. Acute toxicity was very mild with most patients experiencing erratic bowel movements within the first 3 months. G1 rectitis was the most common late effect (n= 4), except for one patient who needed argon cauterization (G2). There were no treatment related deaths. Conclusion Our experience with Papillon x-ray contact radiotherapy shows in our selected patients a complete local response achieving long-term organ preservation. Our results are in line with the recently proffered 3-year results of the OPERA randomized trial. This unique treatment modality may help future patients with early stage rectal cancer benefit from low toxicity RT dose escalation, to achieve complete local response, and avoid surgery. OC-0459 Brachytherapy VS External Radiotherapy: A Dosimetric Comparison for rhabdomyosarcoma treatments A. Traore-Diallo 1 , K. Boualaoui 1 , F. Goudjil 2 , S. Bolle 1 , S. Espenel 1 , C. Robert 1,3 , C. Charghari 1,3 1 Gustave Roussy, Radiotherapy, Villejuif, France; 2 Curie, Protontherapy, Orsay, France; 3 Gustave Roussy, INSERM, Radiothérapie Moléculaire et Innovation Thérapeutique, Villejuif, France Purpose or Objective The choice of the optimal therapeutic strategy in pediatric cancers remains an important issue, especially in radiotherapy. Indeed, due to the age of the patients, it is important to drastically reduce the doses received by the healthy organs. In external beam radiation therapy (EBRT), a prospective comparison between protons, electrons and photons is possible. However, the dosimetric result of brachytherapy (BT) is highly dependent on the quality of the implantation. In this study, a retrospective dose comparison was carried on pediatric patients with rhabdomyosarcoma treated with brachytherapy. Materials and Methods For all patients, 7 irradiation techniques were compared, including brachytherapy (Oncentra) and 6 EBRT techniques: 3D conformational EBRT with electrons and photons (Raystation), intensity-modulated radiation therapy (IMRT, Raystation), volumetric modulated arc therapy (VMAT, Raystation), tomotherapy (Precision), and proton therapy (Eclipse). The tumors of the 4 considered patients were located in the trunk (right humerus, supraclavicular region), in the head and neck region or in the vesico-prostatic region. The prescribed dose was equal to 60 Gy, except for the supraclavicular tumor (prescription equal to 55 Gy in brachytherapy and 54 Gy in EBRT). The brachytherapy treatment was delivered using a pulsed dose rate technique (PDR-BT) with 0.42 Gy pulse per hour. Data were compared considering equivalent doses of 2 Gy fractions (EQD2). Results All dosimetries were produced except for patient suffering from the tongue tumor for whom the proton therapy plan was not feasible due to the presence of teeth. Figure 1 compares dose maps obtained by VMAT and tomotherapy, protons and brachytherapy for the patient treated for a vesico-prostatic tumor. As expected, VMAT resulted in a large bath of low doses. The 40 Gy isodose decreased from 149.3 cc to 129.9 cc and again to 59.6 cc when switching from VMAT to protons and then to brachytherapy. For the patient treated for the tongue tumor, maximum dose to the mandible was decreased from 60.9 for 3D CRT photons to 47.4 Gy for brachytherapy treatment. The same was observed for cartilage volumes for rhabdomyosarcoma located inside the humerus with maximum dose values less than 38.3 Gy for cartilage in the surrounding bones for BT. The corresponding maximum values were equal to 60.6 and 60.7 Gy for 3D CRT and proton therapy photons, respectively.