2053 - Reirradiation Composite Plans and Recovery Factors for Spinal Cord

Purpose/Objective(s): We hypothesize that over time spinal cord can often recover sufficiently from radiation therapy to enable salvage radiation, but recovery factors are not known well enough to optimize the benefit risk tradeoffs. Therefore we present 3D composite plan dose from SBRT spinal reirradiation cases overlapping the spinal cord in our institution and pool them with data from the literature to create a spinal cord dose response model including time recovery.

Materials/Methods: Hotspots may occur in different locations in each course of treatment, so a simple dose volume histogram (DVH) based method was used to account for the differing dose contribution at each volume: In the composite plan the volumes corresponding to the hottest Dx values D1cc, D0.1cc and D0.03cc of spinal cord were autocontoured. Then the average values within each Dx volume were used to account for the average contribution at each of those regions from each course. All spinal cord maximum point doses (Dmax) were converted to biological effective dose (BED) with the linear quadratic model (LQ) and alpha/beta=2Gy. Recovery was modeled as a raised exponential function applied to prior BED values. Dose response was modeled as a logistic function of the sum of the recovered Dmax values plus the reirradiation Dmax values. All model parameters were fitted using maximum likelihood.

Results: Spinal cord recovery was modeled from 292 treatments. The literature search revealed 6 myelopathy cases among 282 reirradiated lesions in 13 studies with published spinal cord maximum point doses (Dmax) from each course including the time interval between courses. To this we added ten patients from our own institution, for which composite plans were available. In our own patient cohort, median time interval between courses was 12 months (range: 3 – 32 months). The initial prescription was 10-30Gy in 1-5 fractions, with the average being 22.8Gy and 4 fractions. The reirradiation used 5 fractions in all except two patients, with a median 5-fraction equivalent prescription of 30Gy. Spinal cord Dmax for the initial course was below BED2 = 120Gy per TG101, with median 72Gy (range: 56-120Gy). Initial median spinal cord Dx values were D1cc=63Gy (25-79), D0.1cc=68Gy (48-100), D0.03cc=72Gy (50-111). For reirradiation spinal cord was constrained to lower dose when the time interval was short or the prior spinal cord dose was high, with the resulting median 39Gy (range: 9-78Gy). Reirradiation median spinal cord Dx values were D1cc=27Gy (6-42), D0.1cc=34Gy (8-54), D0.03cc=35Gy (9-58). Spinal cord recovery after 1 year was estimated to be greater than 50%.

Conclusion: If radiation recovery factors were known, the implications would be the ability to offer salvage radiation to more patients safely. Full dose distributions are needed because hotspots from each course many occur in different locations, but we found only Dmax values for spinal cord reirradiation in the literature. Therefore we present the full dose distributions of our patient data for future data pooling.