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Matching Protocol and Practice: The Challenge of Meeting Lung and Kidney Total Body Irradiation Constraints for
Bing-Hao Chiang1, Kent Wallner1, Ralph Ermoian1
1Department of Radiation Oncology, University of Washington, Seattle, Washington.
Purpose:
Total body irradiation (TBI), a form of immunomodulation, improves treatment outcomes for rapidly progressive scleroderma. The landmark Scleroderma: Cyclophosphamide or Transplantation (SCOT) trial used strict 200-cGy lung and kidney dose restrictions to limit the likelihood of normal tissue toxicity. The protocol as written did not specify how or where the 200-cGy limit was to be measured, opening the door to variable techniques and outcomes.
Methods And Materials:
Following the SCOT protocol, a validated 18-MV TBI beam model was used to evaluate lung and kidney doses with varying Cerrobend half-value layers (HVLs). Block margins were constructed per the SCOT protocol.
Results:
Using the 2 HVL SCOT block guidelines, the average central point dose under the lung block center was 353 (±27) cGy, almost double the mandated 200 cGy. The mean lung dose was 629 (±30) cGy, triple the mandated 200 cGy. No block thickness could achieve the mandated 2 Gy due to contribution from unblocked peripheral lung tissue. With 2 HVLs, the average kidney dose was 267 (±7) cGy. Three HVLs were needed to reduce it <200 cGy, meeting the mandated SCOT limit.
Conclusions:
There is considerable ambiguity (and inaccuracy) in lung and kidney dose modulation for TBI. It is not possible to achieve the mandated lung doses using the protocol-specified block parameters. Future investigators are encouraged to take these findings into account to develop more explicit, achievable, reproducible, and accurate TBI methodology.
