Fractionated stereotactic radiotherapy of benign skull-base tumors: a dosimetric comparison of volumetric modulated arc therapy with Rapidarc® versus non-coplanar dynamic arcs.

BACKGROUND: Benign tumors of the skull base are a challenge when delivering radiotherapy. An appropriate choice of radiation technique may significantly improve the patient's outcomes. Our study aimed to compare the dosimetric results of fractionated stereotactic radiotherapy between non-coplanar dynamic arcs and coplanar volumetric modulated arctherapy (Rapidarc®).

METHODS: Thirteen patients treated with Novalis TX® were analysed: six vestibular schwannomas, four pituitary adenomas and three meningioma. Two treatment plans were created for each case: dynamic arcs (4-5 non coplanar arcs) and Rapidarc® (2 coplanar arcs). All tumors were >3 cm and accessible to both techniques. Patients had a stereotactic facemask (Brainlab) and were daily repositioned by Exactrac®. GTV and CTV were contoured according to tumor type. A 1-mm margin was added to the CTV to obtain PTV. Radiation doses were 52.2-54 Gy, using 1.8 Gy per fraction. Treatment time was faster with Rapidarc®.

RESULTS: The mean PTV V95 % was 98.8 for Rapidarc® and 95.9 % for DA (p = 0.09). Homogeneity index was better with Rapidarc®: 0.06 vs. 0.09 (p = 0.01). Higher conformity index values were obtained with Rapidarc®: 75.2 vs. 67.9 % (p = 0.04). The volume of healthy brain that received a high dose (V90 %) was 0.7 % using Rapidarc® vs. 1.4 % with dynamic arcs (p = 0.05). Rapidarc® and dynamic arcs gave, respectively, a mean D40 % of 10.5 vs. 18.1 Gy (p = 0.005) for the hippocampus and a Dmean of 25.4 vs. 35.3 Gy (p = 0.008) for the ipsilateral cochlea. Low-dose delivery with Rapidarc® and dynamic arcs were, respectively, 184 vs. 166 cm(3) for V20 Gy (p = 0.14) and 1265 vs. 1056 cm(3) for V5 Gy (p = 0.67).

CONCLUSIONS: Fractionated stereotactic radiotherapy using Rapidarc® for large benign tumors of the skull base provided target volume coverage that was at least equal to that of dynamics arcs, with better conformity and homogeneity and faster treatment time. Rapidarc® also offered better sparing of the ipsilateral cochlea and hippocampus. Low-dose delivery were similar between both techniques.