ESTRO 2020 Abstract Book

S461 ESTRO 2020

Results The highest mean Dice coefficient and lowest coefficient of variation was observed for the brainstem. Poor agreement was observed for normal all other OARs. Poorest agreement was observed for the target- approximate optic chiasm. While preoperative GTV demonstrated a high degree of agreement between observers, there was poor agreement in postoperative planning GTV and CTV (Table 1). There was no difference in the mean degree of variability observed between GTV and CTV (p=0.09).

were deemed to be radiation induced according to standard criteria previously established by Cahan et al. Lesions previously treated with surgery had histologic grade available; those that were unresected were presumed to be grade 1. Results A total of 37 patients were identified, with 72 lesions treated with radiosurgery. Median follow up after radiosurgery was 44 months (1.4-150.7 months). Median age at initial radiotherapy was 5 years (4 months – 48 years), and at radiosurgery was 38 years. Median interval between prior radiotherapy and radiosurgery was 31 years (7-65 years). The most common prior conditions treated were acute lymphoblastic leukemia (12 patients), medulloblastoma (7) and astrocytoma (6). Of the 72 lesions, 62 were WHO grade 1 (4 based on histology and 58 based on radiology), six were grade 2 and four were grade 3. The median lesion volume was 2.13cc (0.04-13.8cc), with a median radiosurgery prescription dose of 13 Gy (12-16 Gy) at the 50% isodose line. Local control, on a per lesion basis, was 88.6% at 5 years (95% confidence interval [CI], 72.3-95.6). For grade 1 or radiologically-diagnosed lesions, local control was 96.6% at 5 years (95% CI, 77.9-99.5), whereas those with grade 2 or 3 lesions had a local control of 40% at 5 years (95% CI 5.2- 75.3, p = 0.005). Estimated 5 year overall survival was 100%. Radiologic edema developed in 17 lesions (23.6%) and was symptomatic in 12 (16.7%). Doses above 12 Gy were not associated with improved local control rates (log- rank p=0.292), nor toxicity (radiologic or symptomatic edema). Conclusion Radiosurgery is an effective treatment option for grade 1 or radiologically-diagnosed radiation-induced meningiomas, with a prescription dose of 12 Gy appearing to be sufficient in this cohort. PO-0856 Analysis of inter-observer variation from meningioma eContouring session S. Weiss 1 , C. Hansen 2 1 Fox Chase Cancer Ctr, Radiation Oncology, Philadelphia- PA, USA ; 2 Fox Chase Cancer Center, Radiation Oncology, Philadelphia, USA Purpose or Objective Inter-observer GTV and CTV contour variability may be a surrogate for consensus of the region at risk from the disease process. We sought to quantify the inter-observer variation in delineating of the target GTV and CTV, and organs at risk for a case of atypical meningioma. Material and Methods Permission was obtained from the American Society of Radiation Oncology (ASTRO) to analyze inter-observer variation in physician attendee contours of an ASTRO 2017 eContouring session of atypical meningioma. A waiver was granted from institutional IRB. 29 radiation oncologists voluntarily participated. Each was provided a case history of a patient with partially resected atypical meningioma abutting the left optic nerve. Pre- and post- operative MRI imaging with diagnostic reports were available. Each participant was asked to contour the gross tumor volume (GTV), clinical target volume (CTV), brainstem, optic chiasm, and optic nerves. The data sets were anonymized. Dice statistics to assess the degree of overlap between an individual’s contour against a referenced expert, descriptive statistics, and coefficients of variation (SD/mean) were calculated for the GTV, CTV, and organs at risk (OAR). Zero overlap would yield a Dice coefficient of 0.00, while perfect overlap a coefficient of 1.00. Based on similar contouring trials, where Dice coefficients of 0.85-0.90 indicate consistency between observers, we set a Dice coefficient of <0.80 to indicate meaningful variation.

Pre- Sx Tumo r

Brainste m

Chias m

CTV GVT LON RON

0.68 8 0.11 6

0.74 6 0.12 7 0.17 0

0.59 0 0.16 3 0.27 5

0.60 8 0.12 3 0.20 2

Mean

0.889

0.483

0.910

SD

0.072

0.220

0.014

Coefficie nt of Variation

0.081

0.414 0.16 9

0.016

Conclusion There there is considerable inter-observer variability in the definition of GTV and CTV in the targeting of atypical meningioma. This may represent either inconsistent physician training in meningioma treatment planning or a deeper knowledge deficit regarding delineation of gross and risk for subclinical disease. Inaccurate contouring may result in both under-treatment and OAR overexposure. This has particularly serious implications for meningioma radiosurgery, which is increasingly offered in spite of a paucity of long-term follow-up. Evidence-driven guidelines to define optimal GTV and CTV target delineation of meningiomas are warranted. PO-0857 Leptomeningeal disease following stereotactic radiotherapy for resected brain metastases S. Rogers 1 , N. Lomax 1 , S. Alonso 1 , T. Hancock 1 , S. Khan 1 , J. Schürkens 1 , E. Kessler 1 , B. Eberle 1 , S. Gomez Ordonez 1 , O. Riesterer 1 , J. Fandino 2 , S. Bodis 1 1 Kantonsspital Aarau, Radiation Oncology, Aarau, Switzerland ; 2 Kantonsspital Aarau, Neurosurgery, Aarau, Switzerland Purpose or Objective Nodular leptomeningeal disease can occur after resection of brain metastases due to dissemination of tumour cells into the cerebrospinal fluid. Postoperative whole brain radiotherapy reduces the incidence of leptomeningeal relapse but the associated neurocognitive impairment and lack of survival benefit underlie the preference for postoperative stereotatic radiosurgery (SRS) or hypofractionated stereotactic radiotherapy (hfSRT). We evaluated clinical outcomes, particularly leptomeningeal disease, following hfSRT to the surgical cavity. Material and Methods 45 consecutive patients (48 surgical cavities) were evaluated for rates of leptomeningeal disease (LMD), local control (LC), overall survival (OS) and radionecrosis (RN). Prior to 2015, the dose prescription was to the isocentre (100%) with a 5 mm margin (10 patients) and is currently to the 80% isodose with a 2mm margin and 5mm extension along the dura accoprding to the consensus guidelines (Soliman et al, IJROBP 2018). The median dose was 5 x 6 Gy = 30 Gy (range 20Gy-30Gy), according to dose to volume brain-CTV . Follow-up MRI was performed every 3

months. Results

24/44 patients are alive, of whom 7 were treated prior to 2015 [med. OS 52 mths (29-83 mths)]. 1 patient was lost to follow-up. The12 month LC rate is 86% (30/34 cavities).