Surface Guided Helical Tomotherapy for Total Marrow Irradiation

Sammanfattning: Background: Total marrow irradiation (TMI) with helical tomotherapy treatment (HT-TMI) can be used as conditioning regimen for leukemia patients prior to stem cell transplantation by radiating the skeletal tissue. To ensure adequate dose coverage a margin is applied to the clinical target volume (CTV), i.e. planning target volume (PTV) margin. By applying as small margin as possible the dose to organs at risk (OAR) is minimized. The PTV margin depends on the positioning deviation of the CTV. To aid accurate positioning, (on the couch prior to irradiation) an optical surface scanning system (OSS) can be used. By projecting visual light onto the patient surface and detecting the reflective light, the position of the patient can be calculated. The live surface is compared to a reference surface, i.e. the planned position, and any deviations can manually be corrected for. This application is called surface guided radiotherapy (SGRT). Purpose: The purpose of this study was to investigate if SGRT for HT-TMI can improve patient positioning and allow for smaller PTV margins that may reduce the dose to OARs. Method and Material: A method was developed to position TMI patients using SGRT (Catalyst HD +, C-RAD, Positioning AB, Uppsala) on a Tomotherapy unit (Radixact, Accuray Inc., Madison, WI, USA). Three patients treated during October to December 2021 were included in the study. The patients were positioned using the developed surface guided method and the treatment margins were evaluated. The positioning deviations for the patients positioned using SGRT were retrospectively compared with the deviation observed for 16 TMI-patients previously treated and positioned using 3-point localization at another Tomotherapy unit (TomoTherapy, Accuray Inc., Madison, WI, USA). All patients were immobilized in the same way and treated with the same PVT margins. For both groups the margins were recalculated based on systematic and random set-up deviations measured from daily imaging. The target was divided into sub-CTVs for different body parts and treated separately when calculating the margin. The margin calculations were based on a population-based margin recipe derived by M. van Herk et al. 2000 to ensure a minimum dose to the CTV of 95% for 90% of the patients. The recipe was adjusted to be valid for treatments with few fractions. For the three patients positioned using SGRT, the calculated margins were evaluated by re-planning the treatment and comparing the dose to OARs. The reliability of the new margins was evaluated by recalculating the treatment plan onto the daily images and evaluating the 99.5 % CTV dose coverage of the new plan in comparison to the original plan. Result: The results support that the positioning deviation decrease when using SGRT compared to conventional 3-point localization. The calculated margins were non-isotropic and could be decreased for skull, neck, thorax, pelvis and legs, however, kept the same for arms and increased in the vertical direction of the neck. With the new margins, in comparison to the original plan, the mean dose to OARs was reduced for the following organs: bladder, bowel bag, genitalia, heart, liver, kidneys, lungs, rectum. The median target coverage for D99.5% CTV per fraction was 1.92 (range 1.81 – 1.94) Gy and 1.92 (range 1.71 – 1.94) Gy for the original plans and the new plans, respectively. In the D99.5% CTV coverage no significant difference (p<0.05) was observed. Conclusion: The developed method for Surface Guided Helical Tomotherapy for Total Marrow Irradiation can potentially improve patient positioning such that margins can be reduced which consecutively can reduce dose to OARs while maintaining target coverage.