Spinal cord tolerance for stereotactic body radiotherapy

Area of Science:

• Radiation Oncology
• Neurosurgery
• Medical Physics

Background:

• Stereotactic body radiotherapy (SBRT) is increasingly used for spinal tumors.
• Radiation-induced myelopathy is a serious complication of spinal radiotherapy.
• Understanding dose-response relationships is crucial for safe SBRT delivery.

Purpose of the Study:

• To analyze dosimetric data in patients who developed radiation myelopathy after SBRT for spinal tumors.
• To compare dose-volume parameters between patients with and without radiation myelopathy.
• To evaluate the utility of the biologically effective dose (BED) model in predicting SBRT-related myelopathy.

Main Methods:

• Retrospective analysis of five patients with radiation myelopathy and 19 controls post-SBRT.
• Contouring of the thecal sac to represent the spinal cord.
• Analysis of doses to the maximum point and various volume subsets (0.1- to 5-cc).
• Calculation and comparison of normalized 2-Gy-equivalent BEDs (nBEDs) using an alpha/beta ratio of 2.

Main Results:

• Radiation myelopathy occurred at maximum point doses ranging from 10.6 Gy (1 fraction) to 30.9 Gy (3 fractions).
• A significant interaction was found between patient subsets (myelopathy vs. no myelopathy) and dose-volume parameters (p = 0.0003).
• The mean maximum point nBED showed a significant difference between the groups (p = 0.01).

Conclusions:

• Maximum point dose is a critical factor in spine SBRT to avoid myelopathy.
• A single fraction of 10 Gy to the maximum point is considered safe.
• An nBED of 30-35 Gy 2/2 to the thecal sac for up to five fractions is associated with a low risk of radiation myelopathy.