Fractionated Stereotactic Radiotherapy of Base of Skull Meningiomas: A Preliminary Comparison in the Delineation of the Gross Tumor Volume between Four Medical Specialities

Purpose: To study the interobserver variability of base of skull meningiomas delineation on computed tomography. Methods and Materials: A group of eight physicians working in the same institution (two neurosurgeons, two neuroradiologists, two radiation oncologists and two medical oncologists) were asked to delineate the gross tumor volume of two patients on sequential CT slices. The same clinical information is provided to each observer. The results of the delineation by the four specialties are compared to each other. The mean of the volumes delineated by all observers are calculated. To determine the variation in the anatomical position of the target volume, a 3D comparison between an arbitrary point determined in the tumor and the most distant point of the tumor in the three directions is realized. For the interobserver variability, the coefficient of variance is used. Results: Large differences are observed between the eight physicians both for the estimation of the tumor volume and its anatomical position. The value of the ratio of the largest to the smallest volumes is 8.8 for the first patient and 3.3 for the second. Those ratios become 1.19 and 1.66 for the medical oncologists, 1.07-1.57 for the neuroradiologists, 1.07-1.08 for the neurosurgeons and 1.14-6.5 for the radiation oncologists. The intersecting volume (on which all physicians agree) represents only 32% for the first patient and 34% for the second. Compared to radiation oncologists and medical oncologists, neurosurgeons and neuroradiologists delineate slightly smaller volumes. The results for the radiation group need to be taken cautiously because of the large delineation of one of the two observers. The lowest coefficient of variations, indicator of a smallest interobserver variation, are found for the group of neurosurgeons and neuroradiologists. Conclusions: The first step of 3D treatment planning, the delineation of base of skull meningiomas tumor is crucial. A good co-operation within a multidisciplinary team is mandatory. Different imaging modalities such as NMR and methionine PET are used. Difference between radiation oncologists and other specialties could be unconscious integration of geometrical uncertainties relevant in radiotherapy as the set-up variation, organ motion.