Lung dose depending on exact patient positioning during total body irradiation (TBI) - Isoeffective considerations to assess the risk of interstitial pneumonitis after TBI

Purpose: In this case report, we studied the effect of patient's movements on total lung dose during total body irradiation (TBI). The dose-effect relationship regarding the development of interstitial pneumonitis and the problem of defining a threshold value are discussed. Based on considerations about the isoeffects we calculated the pneumonitis risk in dependence of increasing lung dose. Patient and Method: We calculated dose-volume histograms of the lung for defined lateral. deviations (0-3 cm) from the isocenter. Total dose was 12 Gy, given in six fractions over 3 days. Lung shields were used after a total dose of 9 Gy. Lung shields were transferred into the Helax-TMS(R) planning system to quantify the influence of lateral deviation to lung dose. Results: The child's lateral deviation amounted up to 3 cm. Median dose of the whole lung amounted up to 11.64 Gy depending on lateral deviation. Discussion: In TBI, the lung limits the total dose. To estimate the risk of radiation pneumonitis, we calculated the isoeffective lung dose of our TBI regime for a fractionation scheme of 2 Gy daily using a formalism of van Dyk. The increase of median lung dose from 9.76 to 11.64 Gy would isoeffectively correspond to the increase from 19 Gy (no deviation) to 20.9 Gy (3 cm lateral deviation) with conventional fractionation. According to Burman, a pneumonitis risk of approximately 20% could be expected. Conclusion: With an estimated pneumonitis risk of approximately 20%, an indication for irradiation in general anesthesia seems to be reasonable. This is practicable in cooperation with radiation oncologists, anesthesists and pediatricians and should be included into therapeutic concepts.