Dosimetric Evaluation of Helical Tomotherapy Low-Dose Total Body Irradiation (TBI) for Mini Allogeneic Transplants

Abstract

Reduced intensity conditioning (RIC) regimens use less toxic dosages of chemotherapy and/or radiation to prepare the body for allogeneic transplant. When total body irradiation (TBI) is used it is frequently given with a small single dose of 1 - 2 Gy. We speculated that helical tomotherapy could offer simplified treatment planning and that the dose homogeneity and coverage should be excellent with IMRT use. In addition patient comfort and treatment simplicity may be improved because patients can be treated supine on a standard treatment couch without requiring any beam modifying devices. A pre-clinical dosimetry study was performed, utilizing pre-existing full-body CT simulation scans from three patients. Patients had been simulated in full body vacuum immobilization bags and head masks. Patients were not actually treated with these plans. GPU-based planning software was used to generate upper-body and lower-body plans. The treatment machine has a maximum field length of 160 cm, so the upper part of the body would typically be treated first from head to thighs, and then the lower body would be treated from feet to thighs. A gap is calculated between the two fields to create a uniform dose at the junction. In this study the entire body was used as the target volume with no added margins. No avoidance structures were required due to the low dose. Prescription dose was 2.0 Gy in a single fraction, aiming for 95% PTV coverage. Approximately 60 minutes of dosimetry time was required per patient. The planning parameters included a beam field width of 5 cm, a pitch of 0.3 and a modulation factor of 3.0 - 3.6. Average beam-on times were 35.3 minutes upper body and 19.3 minutes lower body, for a total of 54.6 minutes. The upper body received a mean and median dose of 2.11 Gy (SD 0.07), with a D80 of 2.07 Gy and a D20 of 2.15 Gy. The dose heterogeneity index (D20 - D80) / (Rx Dose) was 4.13%. The hottest 5% of the volume (D5) received over 2.24 Gy. The uniformity index (D5 / D95) was 1.115. The proportion of body that received at least 95% of the prescribed dose (V95) was 98.8%. Specific organs were contoured as reference structures, and the median dosages to these organs were: skin 2.11 Gy (SD 0.14), lenses 2.17 Gy, lungs 2.11 Gy (SD 0.04), and kidneys 2.13 Gy. Low dose TBI planning for helical tomotherapy was dosimetrically simple and elegant. Dose coverage and dose homogeneity were excellent. No external shields, bolus, spoilers, or compensating devices were required in the planning process due to IMRT’s ability to achieve homogenous target coverage. Beam-on times were reasonable and patient treatments should be comfortable because patients can lie supine on a standard treatment couch. Helical tomotherapy appears to be a promising technique for delivering low dose TBI in reduced intensity conditioning regimens before mini transplant.