ESTRO 38 Abstract book

S31 ESTRO 38

the irradiation and in the prolonged gap between the end of hypofractionated 5x5 Gy radiotherapy and surgery may allow to obtain the R0 resection rate, high pathological response rate and/or a higher rate of limb- sparing/conservative surgery as well as to increase patients' survival. Material and Methods A single-arm prospective clinical trial was conducted (NCT03651375). Treatment consisted of one cycle of doxorubicin and ifosfamide (AI), then 5x5 Gy RT, and two cycles of AI in seven-eight weeks gap between the end of RT and surgery. STS response was assessed in DWI-MR imaging (Fig. 1) and pathologically by EORTC STBSG criteria. The primary endpoint is rate of limb-sparing surgeries and R0 resections.

unnecessary doses to organs at risk. Despite use of larger PTV margins, verification of tumor position is essential. However, visualization of intra-abdominal tumor position is suboptimal using cone-beam CT scans. Daily MR- guidance can enable improved tumor verification, and adaptive radiotherapy may also allow for optimal target coverage. We report on our initial experience of MR- guided adaptive radiotherapy in intra-abdominal lymphoma patients. Material and Methods Five patients underwent both a planning CT and MRI scan (MRIdian; ViewRay Inc) during shallow inspiration breath hold after at least 3 hours of fasting. The gross tumor volume (GTV) was contoured on the planning MR, and a planning target volume (PTV) generated using a 10 mm margin. The liver, kidneys, spinal cord, and spleen were contoured as avoidance structures for initial treatment planning, but were not contoured daily due to low doses used. Daily breath-hold MR scans were performed before each fraction, and the GTV was manually adjusted when necessary. Re-optimized plans were routinely created and compared to the original plan that was recalculated on the anatomy of the day. Clinicians selected the most optimal of both plans. Treatment was delivered during video- assisted breath-hold based on tracking of the GTV, using the PTV as gating boundary. Inter-fractional changes in GTV were assessed, and the frequency of using re- optimized plans and the rationale for this were analyzed. Results Five patients, of whom three had a gastric lymphoma and two a mesenterial lymphoma, have been treated with MR- guided adaptive radiotherapy. A total of 67 MR-guided fractions were delivered, with total doses ranging from 24- 36 Gy in 12-18 fractions. Daily adjustment of the GTV was performed in 4 patients (49 fractions), with mean GTVs ranging from 244cc to 296cc. Limited inter-fractional changes in GTV were seen in 3 patients, with relative GTV differences between the mean and 10 th -90 th percentiles of <11%. The relative difference was 38% in the fourth patient, because fasting instructions were not complied with. Re-optimized plans were superior to recalculated original plans in each fraction studied, with the reason being reduced PTV coverage in 4 patients (mean V95% of 85%, 10 th -90 th percentile: 74%-95%). In another patient, the recalculated original plan would have led to a V107% >5% in 12 fractions, whereas inadequate PTV coverage was the reason for using the re-optimized plans in the remaining 8 treatment fractions. Conclusion MRI guided treatment of intra-abdominal lymphoma allows for improved visualization of the GTV and permits daily plan re-optimization, which led to superior PTV coverage and reduction in plan inhomogeneity. OC-0069 5x5 Gy with chemotherapy in borderline resectable soft tissue sarcomas: early results of a trial M. Spalek 1 , H. Koseła-Paterczyk 1 , A. Borkowska 1 , M. Wągrodzki 2 , A. Szumera-Ciećkiewicz 2 , A. Cieszanowski 3 , P. Castaneda-Wysocka 3 , T. Świtaj 1 , M. Dudzisz-Śledź 1 , A. Czarnecka 1 , E. Dąbrowska-Szewczyk 4 , P. Rutkowski 1 1 Maria Sklodowska‐Curie Institute ‐ Oncology Center, Department of Soft Tissue/Bone Sarcoma and Melanoma, Warsaw, Poland; 2 Maria Sklodowska‐Curie Institute ‐ Oncology Center, Department of Pathology and Laboratory Diagnostics, Warsaw, Poland; 3 Maria Sklodowska‐Curie Institute ‐ Oncology Center, Department of Radiology I, Warsaw, Poland; 4 Maria Sklodowska‐Curie Institute ‐ Oncology Center, Department of Medical Physics, Warsaw, Poland Purpose or Objective There is a lack of standard treatment of unresectable and marginally resectable sarcomas (STS). Results of commonly used approaches are unsatisfactory. The addition of neoadjuvant/induction chemotherapy before

Results 30 patients(pts) met eligibility criteria, 23 received the whole planned protocol treatment, four are currently receiving the treatment, in three pts the treatment was prematurely stopped. 23 pts underwent limb-sparing or conservative surgery, 3 pts underwent extremity amputation, two after 1st AI cycle due to poor tolerance, one due to extensive tumour invasion without a possibility of vessels reconstruction. Among patients who underwent conservative treatment, in 15 of them resection margin was R0, in 7 pts R1. One toxic death occurred outside our centre related to severe bone marrow suppression with septic shock after the second AI cycle. Early tolerance of chemotherapy was acceptable. Grade 3+ CTCAE4.03 toxicity occurred in 11 pts. Early RT tolerance was good. EORTC grade 1 radiation dermatitis occurred in 14 and grade 2 in three pts. Postoperative wound complications occurred in 7 pts, in two were severe. Very good pathological response (<1% of stainable tumour cells; grade A/B) was found in 5 pts (Fig. 2). Good pathological response (<50% tumour cells; grade C/D) was found in 13 pts (Fig. 2).

Conclusion Preoperative AI combined with hypofractionated radiotherapy is a feasible method of the management of