STEREOTACTIC RADIOTHERAPY FOR INTRACRANIAL MENINGIOMAS: ROLE OF INTRA-OBSERVER AND INTER-OBSERVER VARIABILITY IN CHOICE OF MARGINS Louise Lambert Radiation Oncology Department Montreal University ASTRO 2006

Intracranial Meningiomas 20% primary tumors 90% benign tumors Standard treatment:Surgery Radiotherapy possibly beneficial when: –Incomplete excision –Relapse –Patients who are not surgical candidates

Radiotherapy Better localization of tumor and immobilization techniques: prognostic factors Goldsmith BJ et al. J Neurosurgery 1994; 80:195 Fractionated Stereotactic Radiotherapy (FSRT): –Precision of position –Radiobiologic effect of fractions Alheit H. Radiother Oncol 1999;50:145-50

Goal Literature Review: –No consensus for margins with FSRT –Between 1 mm and 10 mm Alheit H. Radiother Oncol 1999; 50: Selch MT. IJROBP 2004; 59: Measure intra-observer and inter- observer variability in order to select an appropriate margin

Volumes GTV = CTV PTV = CTV + margin

Methods and Material 12 consecutive patients, treated for benign intracranial meningiomas 2 radiation oncologists and 1 neuroradiologist Contouring on merged images of CT-MRI with Brainscan Contouring target volumes: – 2 separate drawing sessions – 1 week interval – blinded

Methods and Material (2) Intra-observer variability: Mean differences between the two volumes Inter-observer variability: –Mean absolute difference between volumes –Absolute positional shift in 3D

Results: Intra-observer Mean 0.56cc (5.2%) Maximum 0.59cc (12.97%) OBSERVERMean difference cc Mean difference % Maximum difference cc A %1.36 B %0.24 C %0.18

Results: inter-observer (1) Mean maximal difference: 2.13cc (± 0.84cc) Maximal difference: 7.14cc On a mean volume of 20cc

Results: inter-observer (2) Vectorial center of mass variation: R= (x 2 +y 2 +z 2 ) Mean center of mass variation: 1.03mm  0.44mm Maximal variation: 2.79mm

Results: inter-observer (3) V = 4∏r 3 /3 dV = 4∏r 2 dr Maximal volume difference 7.14cc is within a 2mm variation in radius.

Conclusion (1) Important aspects of planning: – Accurate delimitation of tumor volumes – Choice of margins Intra-observer variability: not significant

Conclusion (2) Inter-observer variability: –Center of mass variation R = 2mm for 95% –Includes intra- and inter- observer volume variation

Limitations and Future Work Only 12 patients Exclusion volumes DVH

Acknowledgements Marie-Andrée Fortin, M.D. Carole Lambert, M.D. Jean-Paul Bahary, M.D. Christian Berthelot, M.D. Robert Doucet, M.Sc Martin Hinse,M.Sc