ESTRO 2022 - Abstract Book

S124

Abstract book

ESTRO 2022

Materials and Methods We retrospectively analyzed the outcome of 21 patients with MPNST treated between July 2013 and December 2020. Nineteen pts (90.5 %) were treated with post-operative RT. Residual disease was macroscopic in 11 pts (52 %), 8 pts (40.9 %) were treated for local recurrence after surgery. Two unresectable pts underwent definitive radiotherapy after biopsy. The most frequent tumor site was brachial plexus. The patients were irradiated with a median total dose of 63.9 Gy(RBE) (range, 54-76.8). Primary endpoints were overall survival (OS), local control (LC) and progression-free survival (PFS), calculated with Kaplan Meyer method. Secondary endpoint was toxicity, assessed according to Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Results Median follow-up was 25 months (range, 2 – 88). Three patients were lost to follow-up after median time of 27 moths (range, 16-41). Twelve local progressions (57%) were observed with a median time to local recurrence of 11 months (range, 1 – 66 months). Two distant metastases (9,5%) were recorded. OS at 2 and 3 years was 72% and 43%, respectively. LC at 2 and 3 years was 48% and 34%, respectively. PFS at 2 and 3 years was 48% and 34%, respectively. Acute toxicities of grade 2 (erythema and paresthesia) and grade 3 (erythema) in 4 patients were observed. Severe late radiation related toxicity of grade 3 (peripheral motor neuropathy and brachial plexopathy) was recorded in 2 patients. Conclusion High dose CIRT shows favorable results with acceptable toxicities in patients with gross residual and local recurrence after surgery, or unresectable malignant peripheral nerve sheath tumors. Patients accrual by multidisciplinary approach is still ongoing to confirm these findings and investigate on this challenging disease. M.R. Fiore 1 , A. Ghirelli 2 , S. Molinelli 3 , G. Magro 3 , A. Chalaszczyk 4 , A. Mairani 5 , A. Donatelli 6 , S. Imparato 7 , M. Ciocca 8 , E. Orlandi 4 1 National Center Oncological Hadrontherapy (CNAO) , Radiotherapy, Pavia, Italy; 2 Istituto Oncologico del Veneto, Radiotherapy, Padova, Italy; 3 National Center for Oncological Hadrontherapy (CNAO), Physics, Pavia, Italy; 4 National Center for Oncological Hadrontherapy (CNAO), Radiotherapy, Pavia, Italy; 5 Heidelberg Ion Beam Center, Radiation Oncology, Heidelberg, Germany; 6 I.R.C.C.S. San Matteo Hospital, Radiology, Pavia, Italy; 7 National Center for Oncological Hadrontherapy (CNAO), Radiology, Pavia, Italy; 8 National Center for Oncological Hadrontherapy (CNAO), Physics, Pavia, Italy Purpose or Objective There is little data on sacral insufficiency fracture (SIF) incidence following pelvic carbon-ion RT (CIRT). We aim at analyzing: i) the occurrence and the pattern of SIFs in patients underwent curative CIRT for sacral chordomas; ii) the correlation between SIFs and RBE-weighted dose (D RBE ) and dose-averaged LET distributions (LET d ). Materials and Methods We retrospectively analyzed 76 patients, consecutively treated between 2013 and 2021, with radical CIRT. The prescribed dose ranged from 70.4 to 73.6 Gy(RBE), in 16 fractions. No patients reported pre-existing fracture at the treatment site. The For patients reporting a SIF during their follow-up, the volume of the fracture (VF) was contoured on the corresponding appropriate MRIsequence and graded according to the common terminology criteria for adverse events (CTCAE) v 5.0. MRI studies of VF were registered with the original MRI/CT study used for treatment planning. We analyzed, DVH D RBE and LET d distributions of the sacral bone, looking for a correlation between VF and the volume of sacrum (V D ) receiving a dose D (ranging from 20 to 75 Gy(RBE), in steps of 5 Gy(RBE)). For the VF, the minimum, median and maximum D RBE and LET d and the volume percentage receiving a LET d > 50 keV/ m (V 50keV/ m ) were compared to the healthy bone average values. Results The median follow-up was 34 months (range 2 – 86 months). SIF was diagnosed in 27 patients (36%) after a median follow- up of 26 months (range 2 – 67 months). The majority of the patients experienced multiple (69%) or single (31%) fractures located in the sacral wings (Figure 1). Twenty-three (85%) patients were scored G1, without associated clinical symptoms, the remaining four (15%) were classified G2, reporting walking pain as the main symptom. The V D of sacrum receiving doses D ≥ 55 Gy(RBE) were significantly higher in the fractured patient cohort. When comparing LET d distributions in the fracture volume, against the healthy bone region, the average minimum LET d was significantly higher; while no difference was found in V 50keV/mm . Conclusion The SIF occurrence recorder in our patient population is in agreement with previous study using particle therapy. The volume of sacral bone receiving high doses significantly affects the risk of developing SIF. Other clinical factors (I.E. age, sex, prior surgery, concomitant medication, body mass index) are currently under evaluation in order to develop a patient- specific risk stratification to minimize severe clinical complications and set-up (pre)-rehabilitative strategies. MO-0150 Sacral insufficiency fractures after CIRT for sacral chordoma: dosimetric and LET analysis.