(P133) Spinal Cord Dose Comparison With and Without Contrast Density Correction on CT Myelogram Simulations for Patients Treated With Stereotactic Body or Conventional Radiotherapy

Sukhjeet S. Batth, MD, MS, Raymond Chiu, CMD, Nicholas Trakul, MD, PhD; USC Norris Cancer Center

PURPOSE/OBJECTIVES: Patients treated with stereotactic body radiotherapy (SBRT, also known as stereotactic ablative radiotherapy) or conventional RT to spinal targets often undergo a computed tomography (CT) simulation with a myelogram to accurately identify the spinal cord. Treatment planning algorithms use Hounsfield units and a CT density table to calculate radiation dose. Accuracy of these calculations may be improved by correcting for the contrast’s actual density, which is not present at the time of treatment delivery. We hypothesize that the spinal cord receives a higher radiation dose than calculated after correction of the contrast’s physical density.

MATERIALS AND METHODS: CT simulations with a myelogram for treatment of 10 spinal targets from nine patients were retrospectively reviewed. Seven of these targets were treated with SBRT, one was treated with intensity-modulated RT, and two were treated with three-dimensional conformal RT. The contrast from the myelogram was contoured in the treatment planning software, and the physical density was adjusted to 1 g/cm³. The uncorrected spinal cord maximum and mean doses were then compared with the doses after correction of the density. A two-tailed, paired t-test was used to compare the calculated doses.

RESULTS: The corrected spinal cord doses were all significantly higher than the uncorrected doses based on paired t-test for the maximum dose (P = .0073) and the mean dose (P = .00087). The mean maximum cord dose was 2,065 cGy and 2,077 cGy after correction. The mean of the average cord dose was 1,302 cGy and 1,315 cGy after correction.

CONCLUSIONS: These data suggest that the use of a CT myelogram with simulation significantly underestimates the spinal cord maximum and mean doses. The clinical significance of this finding is uncertain, and this study is limited by its small sample size. Density correction should be further investigated in a larger study, given the high doses used in SBRT and potential morbidity of spinal cord injury.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org