Purpose: This study attempts to evaluate Ray Tracing (RT) and Monte Carlo (MC) algorithm for CyberKnife treatments of spine lesions and determine whether MC algorithm is necessary for all spine treatment and analyze which sites of spine lesion for which RT algorithm is comparable to MC algorithm.
Methods: The CyberKnife is used for stereotactic body radiotherapy for lesions in the cervical spine (30), thoracic spine (50), lumbar spine (30) and sacral spine (15). The PTV volume ranged from 51.7 cc to 319.8cc. Patients were treated in 3 fractions with a dose of 24-30Gy. Treatment plans were generated using 1-4 IRIS aperture collimators and an average 148 (range 104-227) non-coplanar non-isocentric beams. Dose was calculated using RT and MC algorithms for patients planned with the same beam angles and monitor units. DVH of the target and selected critical structures are evaluated, including V100 of PTV, D0.1cc, D1cc and D5cc of the esophagus, spinal cord and bowels, and Dmean and V16 of kidneys.
Results: V100 of PTV with MC was 94.80%, 88.47%, 92.52% and 93.41% respectively in cervical, thoracic, lumbar and sacral spine. For thoracic spine, RT algorithm significantly overestimates the percentage volume of target covered by the prescribed dose, and overestimates doses to organs at risk in most cases, including lung, spinal cord and esophagus. For cervical, lumbar and sacral spine, the differences of the target coverage of prescription dose were generally less than 3% between the RT and MC. The differences of doses to organs at risk varied with lesion sites and surrounding organs.
Conclusion: In the thoracic spine lesions with beams through air cavities, RT algorithm should be limited and verified with MC algorithm, but the RT algorithm is adequate for treatment of cervical, lumbar and sacral spine lesions without or small amount of beams passing through the lungs.