Hemibody irradiation using helical tomotherapy planning

Purpose/Objective(s): Hemibody irradiation is a successful technique for treating widespread bone metastases that was used in the 1970s to early 1990s. The technique as used then involved treating AP-PA with 6-MV photons from an extended distance. Little to no treatment planning was done. The current series evaluated modern radiation therapy treatment planning for helical tomotherapy to determine if there is a significant decrease in dose to vital structures thereby reducing the toxicity of treatment. Materials/Methods: Computed tomographic (CT) scans were obtained from either simulation CT scans for other treatments or from diagnostic procedures. Plans for treatment were then generated using a commercially available tomotherapy treatment planning system. The upper hemibody field stopped at the bottom of L3 and the lower hemibody field started at the top of L5. The upper hemibody was given 6 Gy and the lower hemibody was given 8 Gy in 1 treatment. The target was the entire skeleton above or/and below the match point with a 3-mm expansion. All plans were done with intensity modulated radiation therapy (IMRT) using a 5-cm field width, planning pitches of 0.3 and 0.287 and a Normal Calc grid. Multiple avoidance structures were used. Dose-volume histograms were then generated for all vital structures. Treatment times were kept as short as possible while maintaining adequate dose distributions. The ability of matching upper and lower hemibody fields was studied. (Table Presented) Results: The initial planning used the match point of the original hemibody studies between fields at L1-L2. This match was at the level of the kidneys and therefore the match was moved to the bottom of L3 and top of L5. Seven patients were studied, 4 male and 2 female adults and 1 7-year-old male. Four of the adults and the child were from craniospinal irradiation and therefore only had upper hemibody irradiation but could be used for evaluating the match. Two diagnostic whole-body CT scans from PET-CT scans were used to do full lower hemibody plans. Among the many upper hemibody vital structures monitored, the mean doses included lens 1.7 Gy, brain 5.7 Gy, hippocampus 1.7 Gy, parotids 2.8 Gy, heart 4 Gy, lungs 4.8 Gy, heart 3.9 Gy, stomach 3.3 Gy, and kidneys 3.2 Gy. In the lower hemibody the bladder received a mean dose of 4.5 Gy, the rectum 3.5 Gy, and the gonads 2.5 Gy (max 2.4 Gy). The L4 dose for matched fields was a mean of 8.1 Gy and a maximum of 9.16 Gy. Conclusion: The original hemibody treatments gave at least the given dose and usually higher to all structures, including the brain and lungs. Using helical tomotherapy, these doses can be markedly decreased and may allow higher treatment doses to the skeleton to be given with less toxicity. This may allow the reintegration of hemibody irradiation into radiation therapy as a cost-effective treatment for systemic bone metastases. A trial of single hemibody IMRT treatment is being developed.