CyberKnife Boost in Pediatric Ependymoma of the Fourth Ventricle

Background: Ependymoma of the fourth ventricle is a challenging disease. Brain stem tolerance dose usually hinders the delivery of adequate radiation dose to target volumes. Aim: To present the treatment outcome of pediatric ependymoma of the fourth ventricle using combined CyberKnife and volumetric modulated arc therapy (VMAT) and compare it to VMAT plans. Methods: The medical charts of 15 patients were retrospectively reviewed. All patients underwent surgery and received radiotherapy in 2 phases using combined plans of VMAT and CyberKnife boost aiming for a total prescription dose of 59.4 Gy in 33 fractions. These plans were compared to a cone-down two-phase VMAT plans. At least 95% of the target volume was required to be encompassed by the 95% isodose level of prescription dose. Results: It was not feasible to achieve adequate target coverage using VMAT without exceeding the brain stem tolerance doses in all but one case. Brain stem maximum point dose, V59 Gy, V55.8 Gy and mean doses were 60.46 ± 0.3 Gy, 1.62 ± 0.42 cc, 37.49 ± 5.78% and 54.79 ± 0.64 Gy in VMAT plans compared to 59.67 ± 0.21 Gy, 0.55 ± 0.22 cc, 25.49 ± 3.84% and 52.86 ± 0.88 Gy in combined technique plans (p = 0.002, 0.001, 0.001 and 0.001; respectively). The estimated 5-year progression-free and overall survival rates were 53.3% and 63.6%. Performance status and extent of surgery significantly influenced overall survival. None of the patients had serious toxicities. Conclusion: On the contrary to VMAT cone-down plans, it was possible to achieve adequate target coverage without violating brain stem constraints using the combination of VMAT and CyberKnife techniques in pediatric ependymoma of the fourth ventricle. The treatment protocol was well tolerated with no reported serious radiation toxicities.