ESTRO 2021 Abstract Book

S873

ESTRO 2021

metastases (<3 sites) with good performance status (PS 0-1) and controlled primary were accrued. Frameless stereotactic based immobilization was ensured with a thermoplastic mask, a contrast CT simulation was done with 0.625 mm slices and fused with T1 contrast/T2 Flair MRI images for contouring. PTV margin of 2-3 mm and a dose of 20-30Gy in 1-5 fractions was decided based on the treatment volume. 12Gy normal brain volume and marginal dose was considered for dose prescription. Response to treatment, new brain lesion free survival, overall survival and toxicity profile after CK was evaluated. Normogram for Indian patient population for ‘new lesion free’ survival was considered. Results In 100 accrued patients with 167 brain lesions [mean age 57 yrs(29-82yrs), female 54%, primary cancer- lung 42 %,breast 32%, RCC 9 % ,other primary site 17%]. 15% received previous WBRT. Solitary metastasis was in 59%, upto 3 in 32% and >3 in only 9% patients. One, 3 & 5 fraction SRS was done in 61%, 15% and 24%. SRS dose was 12 Gy in 4%,13-18Gy in 19%,20-24Gy in 42% and 25-30Gy in 35%. Mean PTV was 22 cc. Mean 12Gy normal brain volume was 5.1cc,Brainstem max dose 643cGy, Optic chiasm max dose 229cGy,Mean HI 1.16,CI 1.23.Mean monitor units 18749, treatment time 52 minutes and number of beams was 203. At mean follow up 58.8 wks (4-164 wk), 86 had follow up evaluation (39 patients were alive, 47 expired). Among 32 patients who had recurrent brain metastases, 20 underwent treatment (12 received re-SRS, 5 WBRT, 3 had surgery) while 12 had progressive systemic and intracranial recurrence and were treated with only palliative/ supportive care. Only ‘Out of field’ and only ‘in-field’ recurrence noted in 13% and 5% respectively. Both in/out of field recurrences was in 14%.Long term steroid usage was noted in 19%. Radiological radiation necrosis was diagnosed in 11 % and 3 patients required surgery. Conclusion In selected patient cohort, SRS in Indian patients with brain metastasis is similar to published literature. Recurrence rate at 6 months is 20% and 80% of recurrent cases are re-treated with SRS. Radiation necrosis rate is 11% and only 3% require surgical intervention. PO-1048 FluorEthyl-l-Tyrosine PET in glioma radiotherapy planning: an isotoxic dose prescription approach C. Delli Paoli 1 , M. Esposito 2 , B. Grilli Leonulli 1 , I. Laghai 3 , G. Muscas 4 , M. Betti 5 , M. Perna 6 , V. Baldazzi 6 , A. Konze 7 , A. Della Puppa 4 , S. Sestini 3 , S. Russo 2 , S. Scoccianti 1 1 Ospedale Santa Maria Annunziata; Azienda USL Toscana Centro, Radiation Oncology Unit; Department of Oncology, Bagno a Ripoli, Firenze, Italy; 2 Azienda USL Toscana Centro, Medical Physics Unit , Firenze/Empoli, Italy; 3 Nuovo Ospedale Santo Stefano; Azienda USL Toscana Centro, Nuclear Medicine Unit, Prato, Italy; 4 Azienda Ospedaliero Universitaria Careggi, Neurosurgery Unit, Firenze, Italy; 5 Azienda USL Toscana Centro, Medical Physics Unit , Prato/Pistoia, Italy; 6 Ospedale Santa Maria Annunziata; Azienda USL Toscana Centro, Medical Oncology Unit; Department of Oncology, Bagno a Ripoli, Firenze, Italy; 7 Azienda USL Toscana Centro, Neuroradiology Unit; Department of Radiology, Firenze, Italy Purpose or Objective To use a personalized isotoxic dose prescription (IDP) approach for dose escalation on the FET-PET-defined biological tumor volume (BTV FET-PET ) in patients with newly diagnosed high grade gliomas (HGG). Materials and Methods 7 patients were included in this preliminary study. Simulation CT was coregistered with a CE-3D T1 weighted MRI scan and a FET-PET CT scan, using the deformable registration algorithms of MIM (7.0.6). CTV MR included the area suspected for residual disease in the CE-3D T1 weighted MRI scan (GTV MR ) and the operative bed +1.5 cm safety margin, individually adapted to anatomical barriers. PTV MR was generated by adding a 3 mm margin to CTV MR . The BTV FET-PET was defined as the volume within a tumor-to-background ratio cut-off value ≥2.5. Prescribed dose to PTV MR was 60 Gy in 30 fractions. Two planning strategies with VMAT using two arcs in non coplanar geometry were generated for each patient: 1) a plan with homogeneous dose prescription (HDP) on the PTV, and 2) a plan with an inhomogeneous dose (nHDP) prescription where a SIB on the BTV FET-PET was performed. Dose constraints based on QUANTEC review were considered as hard constraints. Following an isotoxic approach, the SIB dose prescription was determined by constraints on OARs, without any limit for the maximum dose into the BTV FET-PET . Plans were automatically generated using the template approach of Monaco 5.12 with radiobiological and multi-criterial cost functions. Results There was large variation between BTV FET-PET and GTV MR of the seven patients (median volume: 8.18 ± 17.2 cc vs 24.6 ± 13.0 cc, respectively). Dosimetric difference between HDP and nHDP strategies are summarised in table 1. The OARs dose constraints were met in all plans. Dose to healthy tissues was not significantly different in the two classes of plans.