Is Monte Carlo Calculation Necessary In Patients With Thoracic Spinal Tumors Treated With Robotic Radiosurgery-Based Stereotactic Body Radiation Therapy (Sbrt)?

Ray Tracing (RaTr) is known to be a suboptimal planning algorithm for tumors located in the chest and is not approved for use in all Radiation Therapy Oncology Group (RTOG) lung SBRT trials, for which Monte Carlo (MC) calculation is required. This study attempts to determine whether RaTr is acceptable for use for SBRT in patients with thoracic spinal tumors. We selected 27 patients with thoracic spinal tumors treated with robotic radiosurgery SBRT in our database. All patients were planned using Ray Tracing. Most patients had at least a small portion of the PTV touching lung parenchyma. A Monte Carlo plan was run using the monitor units obtained from the Ray Tracing plan. Dose volume histograms of the spinal cord, esophagus and skin and dosimetric parameters of the plans based on RaTr and MC were then compared. The percentages of PTV covered by the prescribed dose with RaTr and MC were 90.4% (range 67.7-99.4%) and 83.5% (52.1-94.7%; p = .008). There was a loss of 8.5% (0.8-28%) coverage in 88.9% of patients. Maximum dose to PTV via MC with respect to RaTr ranged from 14% lower to 7% higher doses. Median maximum doses to the spinal cord based on RaTr and MC were 8.75 Gy (4.7-21.4 Gy) and 8.21 Gy (4.9-21.0 Gy). Median maximum doses to the esophagus based on RaTr and MC were 3.75 Gy (0.24-17.8 Gy) and 3.45 Gy (0.2-16.9 Gy), while the median maximum doses to the skin based on RaTr and MC were 11.3 Gy (5.1-27.5 Gy) and 10.9 Gy (5.3-26.1 Gy). The % differences in maximum doses to the spinal cord, esophagus and skin were 4.2% (−37% to 14%), 1.7% (−16 to 14%) and 1.9% (range, −4.7 to 7.1%). For patients with thoracic spinal tumors treated with robotic radiosurgery-based SBRT, the PTV coverage can be diminished by as much as 28% and the spinal cord dose can be underestimated by up to 14%. This is particularly crucial in reirradiation scenario where there is very safety margin for cord tolerance. It is prudent to consider using MC algorithm when treating patients with thoracic spinal tumors with robotic radiosurgery SBRT.