Total body irradiation: Significant dose sparing of lung tissue achievable by helical tomotherapy.

Purpose: Total body irradiation (TBI) is an important procedure in the conditioning for bone marrow and hematopoietic stem cell transplantation. Doses up to 12Gy are delivered in hyperfractionated regimes. TBI performed with helical Tomotherapy® (Accuray, Madison, Wisconsin, USA) is an alternative to conventional techniques to deliver dose in extended target volumes with the possibility of simultaneous dose sparing to organs at risk. In this study we focused on maximum dose reduction to the lungs in TBI using helical Tomotherapy®.

Material And Methods: Forty treatment plans of patients who received TBI were calculated with TomoH® (Accuray, Madison, Wisconsin, USA, Version 2.0.4) with a dose of 12Gy delivered in six equal fractions (2×2Gy/day). Planning iterations necessary to accomplish ICRU 83 report should be less than 250. Treatment time should be practicable in daily routine (<60min.). Besides the usual contouring of organs at risk special contouring was required for optimization processes which focused on maximum dose sparing in the central lung tissue. Dose constraints (D2, D98, D99) were predefined for target volumes (i.e. PTV TBI D99: 90% of prescribed dose). Homogeneity index <0.15 was defined for acceptability of the treatment plan.

Results: For all patients acceptable treatment plan fulfilling the predefined constraints were achievable. An average time of 46min is required for treatment. Thirty-four of forty patients fulfilled D2 in the PTV TBI. Four patients failed D2 due to a high BMI >28 (maximum dose 13.76Gy=114.7%). The D98 in the PTV TBI was not reached by 2/40 patients due to BMI>31 (minimum dose 11.31Gy=dose coverage of 94.2%). Also these two patients failed the homogeneity index <0.15. The mean lung dose over all patients of the right lung was 7.18Gy (range 6.4-9.5Gy). The left lung showed a median (D50) dose of 7.9Gy (range 6.7-9.3Gy). Central lung dose showed a mean dose (D50) of 5.16Gy (range 4.02-7.29Gy). The D80 of the central lung showed an average dose of 3.87Gy.

Conclusions: Total body irradiation using helical Tomotherapy® can be delivered with maximum lung tissue sparing (<6Gy) but without compromise in adjacent PTV TBI structures (i.e. ribs, heart). High conformity and homogeneity in extended radiation volumes can be reached with this technique in an acceptable planning and treatment time. Limitations may occurred in patients with high body mass index.