Dosimetric study of volumetric modulated arc therapy fields for total marrow irradiation

Normal organ toxicity is the leading factor that limits dose escalation in total body irradiation for bone marrow transplant (BMT) patients. In recent years, total marrow irradiation (TMI) using the helical tomotherapy (HT) system is being used as a more targeted form of TBI to treat BMT patients. In this study, we evaluated the feasibility of using volumetric modulated arc therapy (VMAT) to deliver TMI treatment.

CT data sets from two female and two male patients who had received prior total marrow and lymphatic irradiation with HT were selected for this study. The target volumes included skeletal bones from the skull to the mid-thigh level, major lymph nodes, and spleen. Twelve Gray in 8 fractions was prescribed to the target volumes. Each VMAT plan was generated with eight arc fields to cover the entire target volumes. Both the VMAT and the HT plans were normalized so that 85% of the skeletal bone volume was covered by the prescription dose. For each patient, more than 20 normal organs were included in plan optimization.

Both the VMAT and HT plans showed comparable dose coverage to the target volumes and significant sparing of normal organ dose. The median dose to the skeletal bone volume was 13.4 and 12.6 Gy in the VMAT and HT plans, respectively. The VMAT plans showed >10% and >20% reduction of average median dose compared to the HT plans in 16 and 11 organs, respectively. The average beam-on time was 628±32 s and 1122±106 s in the VMAT and HT plans, respectively.

The VMAT plans provided adequate target dose coverage and normal organ sparing compared to helical tomotherapy plans for efficient delivery of total marrow and lymphatic irradiation. This study demonstrates that the VMAT technique could be a feasible alternative to helical tomotherapy for TMI treatment.