Feasibility of spine stereotactic body radiotherapy for patients with large tumors in multiple vertebrae undergoing re-irradiation: Dosimetric challenge using 3 different beam delivery techniques

Tomohisa Furuya, Jun H Phua, Kei Ito, Katsuyuki Karasawa
Medical Dosimetry: Official Journal of the American Association of Medical Dosimetrists 2019 March 28
The objective of the present study was to report whether the adequate spine stereotactic body radiotherapy (SBRT) plans were generated with feasible treatment duration for patients with large vertebral metastases undergoing re-irradiation. For 5 cases, the re-irradiation plans using static-field intensity-modulated radiation therapy (SIMRT), volumetric-modulated arc therapy (VMAT), and CyberKnife with a total prescribed dose (PD) of 24 Gy applied in 2 fractions were generated. A minimum dose to 95% of the evaluated planning target volume (PTVevl ) that was >70% of PD (D95 > 70% PD) was defined as minimum criterion. For the dose tolerance of the spinal cord or thecal sac, which could affect the risk of radiation myelopathy, a volume-dose constraint of 12.2 Gy was set for the planning organ-at-risk volume of the spinal cord (PRVcord ) or thecal sac and limited to 0.035 cc (D0.035 cc < 12.2 Gy) on the re-irradiation plans. For assessing the impact of the stricter dose constraint of PRVcord on the plan quality, we generated plans with a PRVcord dose constraint of D0.035 cc < 17.0 Gy, which was employed for patients with no previous history of radiation therapy (RT). Dose-volume histogram (DVH) analysis was performed for the PTVevl and spinal cord. Median PTVevl of all cases was 242.3 cc (range; 159.2 to 722.4 cc). Two out of 5 cases had a PTVevl >500 cc. The constraint of the PRVcord D0.035cc was met in all re-irradiation plans; however, a comparison between the re-irradiation plan for patients with large vertebral metastases and the plan for cases with no RT history showed that the decrease of the target dose coverage was correlated with the stricter dose constraint of the PRVcord . For SIMRT and VMAT, the re-irradiation plans met the goal of the PTVevl D95 . On the other hand, CyberKnife plans could not achieve the constraints of the PTVevl D95 . This discrepancy is due to the constraint of treatment duration, which is defined as the comfortable duration for patients with large spinal metastases. Regardless of the delivery method used, treatment plan quality is impacted to a greater extent by the dose tolerance of the spinal cord than by the size of the tumor.

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