A Practical Radiotherapy Treatment Planning Technique for Second-Incidence Cancers That Incorporates Complete Organ-At-Risk Dose History.

Introduction: Patients requiring treatment for second cancer incidences present unique radiotherapy plan development challenges. Historical dose delivered to organs at risk must be accounted for to properly estimate lifetime toxicity risks, but historical dose delivered to the region now occupied by tumours does not contribute to the prescription dose. Treatment planning systems permit inclusion of a base plan but do not provide the ability to manipulate it. We present a technique, dose cropping, which incorporates organ-at-risk dose history into the base plan while selectively excising dose history to diseased tissues now occupied by tumours. A retrospective plan comparison is performed to assess the effectiveness of dose cropping.

Methods And Materials: Nine patients who received a second course of radiotherapy for cancers of the head-and-neck were replanned using the proposed technique. Clinical second courses and replans were compared on the basis of conformity index, heterogeneity index, maximum point dose, tissue control probability (TCP), normal tissue complication probability (NTCP), and whether the planning guidelines could be met by the treatment planning system. Replan constraints and guidelines followed the clinical treatment. In addition, a tissue recovery model was incorporated, applied to both clinical and replan courses, and compared to estimate the relevance of the dose cropping technique in such regimes.

Results And Discussion: Replans had reduced organ-at-risk maximum point doses (5 Gy for spinal cord, 4 Gy for brainstem), NTCP (2.9% median reduction), and were able to more consistently achieve the V > 98% coverage target regardless of the tissue recovery model. At the same time, replans using the dose cropping technique were statistically indistinguishable from clinical second courses on the basis of plan conformity, heterogeneity, or TCP (P > .31 in all cases).

Conclusions: Dosimetric history cropping is a valuable and widely applicable technique for second cancer radiotherapy planning. It also provides a natural means to incorporate tissue recovery models, biologically effective dose conversion, and NTCP and TCP model evaluation.