Session Item

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 irradiating the entire bone marrow compartment. For TMI treatment the target consists of marrow containing bone and sanctuary sites, making the position of each part of the body important. Surface guided radiotherapy (SGRT) may aid accurate positioning (on the couch prior to irradiation). The purpose of this study was to investigate the performance of the surface guided HT-TMI and the potential reevaluation/recalculations of PTV margins for these patients.

Four patients treated during October to December 2021, in total 23 treatment fractions, were included in the study. The patients were treated with helical Tomotherapy (Radixact®, Accuray, Madison, WI) using SGRT (Catalyst HD +, C-RAD, Positioning AB, Uppsala). All patients were treated with a PVT margin ranging from 7-15 mm. The margins were recalculated based on systematic and random set-up deviations from daily imaging. The target was divided into sub-CTVs for different body parts and margins for each sub-CTV were recalculated. 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. 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.

The results suggest that SGRT for HT-TMI could allow for current PTV margins to be reduced. The SGRT-systems calculated position and the one measured after daily imaging, as an overall displacement for the whole body, is presented as a box plot (Figure 1). The calculated margins were non-isotropic and could be decreased for skull, neck, thorax, pelvis and legs, however, were kept the same for arms and increased in the vertical direction of the neck. The systematic (Σ) and random (σ) errors for each sub-CTV are presented in Table 1. 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.

Table 1: Calculated systematic, Σ, and random, σ, error for each sub-CTV, measured from daily kVCT imaging.

Surface Guided Helical Tomotherapy for Total Marrow Irradiation can potentially improve patient positioning such that margins can be reduced with decreased dose to OARs while maintaining target coverage.