[Dosimetric effect of patient arm position on Cyberknife radiosurgery for spinal tumors]

To assess the potential dosimetric effects of arms movement in patients with Cyberknife spine tumors.

In the study, 12 patients with thoracic and lumbar tumors were retrospectively selected respectively. The contour of the patient's arms was sketched and the CT density was modified to be equivalent to air in order to simulate the extreme case when the arm was completely removed from the radiation fields. The dose of simulated plan was re-calculated with the original beam parameters and compared with the original plan. The changes of V₁₀₀, D₉₅, and D₉₀, conformity index (CI) and heterogeneity index (HI) in planning target volume (PTV), as well as D_max, D_1cc and D_2cc in the spinal cord, stomach, esophagus, and intestines were analyzed by comparing with the original plans.

Compared with the original treatment plan, V₁₀₀, D₉₅, D₉₀ and CI of PTV for the simulated plan was increased by 0.86%, 2.02%, 1.97% and 0.80% respectively, the difference was statistically significant (P < 0.05). D_max, D_1cc and D_2cc of spinal cord was increased by 2.35%, 0.59% and 1.49% on average, compared with the original plan, the difference was statistically significant (P < 0.05). The difference was statistically significant only in average D_2cc of stomach, which was increased by 1.70%, compared with the original plan (P < 0.05). There was no significant difference in dose change of eso-phagus and intestine between the original and simulated plans.

This study analyzed the most extreme arm position in spinal tumor of radiation therapy based on Cyberknife. It was found that the change of arm position had little effect on dosimetry. In addition, with the change of arm position, the dose in PTV and organ at risk (OAR) increased, but the increase was relatively small. Therefore, in some special cases where the patient really can't keep the arm position consistent during treatment, reasonable adjustment can be accepted. However, in order to ensure accurate radiotherapy, patient position should be as stable and consistent as possible.