SU‐E‐T‐307: Methodology for Spinal Cord Retreat Calculations

Purpose: Often in the clinical arena, physicians rely on medical physicists to provide dose estimates. This project focused on evaluating the BED to the spinal cord for patients undergoing retreatment after conventionally fractionated RT. The BED calculation method is based on published research on human and animal spinal cord reirradiations. Methods: BED is calculated separately for the first and upcoming treatments using the planning system to estimate the dose to the spinal cord in the overlap region from each course. The cumulative BED is calculated as the sum of the BEDs of the first treatment and the upcoming treatment. The tolerance BED is calculated from the QUANTEC (2010) value for 0.2% rate of myelopathy of 50 Gy in 2 Gy fractions. If the reirradiation follows the initial irradiation by at least six months, the cumulative BED is compared to the tolerance BED multiplied by 1.25 to account for repair of RT‐induced subclinical damage. If an RT course was delivered BID, then an additional factor (Hm = 0.35) was used to account for incomplete repair of sublethal damage during the BID course. Results: BED calculations were performed using this method to assist clinical decisions for spinal cord reirradiations. Patients were treated to doses as high as 59 Gy to the spinal cord while BED calculations showed the dose to the spinal cord was effectively below the QUANTEC tolerance for 0.2% rate of myelopathy. Conclusion: Application of this BED calculation methodology for spinal reirradiations places retreatment dose in the context of current clinical research. The department is analyzing spinal cord BED calculations factoring in recovery observed in a primate model predicting 50%, 60%, and 70% recovery of the initial RT course dose at 1 year, 2 years, and 3 years post‐RT respectively given an initial treatment of up to 44 Gy.