Surface Dosimetry of Patients Undergoing Total Body Irradiation: A Retrospective Analysis for Quality Assurance

Total body irradiation (TBI) is used prior to bone marrow transplantation as part of the conditioning regimen in selected patients. A linear accelerator-based technique was used at our treatment centre between June, 2004 and August, 2015. Patients were treated supine with extended source-to-surface distance (SSD) lateral fields, and prescription dose was 12 Gy delivered in six fractions, two fractions per day. Dose was prescribed to midplane at the level of the umbilicus and monitor units were calculated manually based on measured beam data. Dose variation within 10% of the prescribed midplane dose is considered acceptable for TBI treatment. This was achieved in our clinic by using compensators to account for missing tissue in the head and neck and lower leg regions. Lung attenuators were routinely used to correct for internal inhomogeneity, which resulted from low density lung tissue. The purpose of this study was to determine whether dose variation was within acceptable limits for these patients as part of a quality assurance process. Following chart review, 129 patients who received six-fraction TBI from 2004 to 2015 were included in this study. Patients receiving single fraction treatment were excluded. Metal oxide semiconductor field effect transistors (MOSFET) dosimetry was used to measure surface dose at four or five locations during patients’ first fraction of TBI. Dosimetry was repeated during the second fraction for any site with variation greater than 10%. Statistical analysis was carried out on patient data, diagnosis and dosimetry measurements. Of the 129 patients who met the inclusion criteria, 50 were diagnosed with acute myelogenous leukemia, 30 with acute lymphoblastic leukemia and 11 with chronic myelogenous leukemia. The rest of the patients were diagnosed with lymphoma or myelodysplastic syndromes. The mean percent variation in dosimetry measurements taken at the specific locations ranged between 3.5% and 8.3%. The highest variation was found in measurements performed on the cheek. A high percentage of all dosimetry readings (85.5%) was within the acceptable range of +10% from the expected value. The highest number of individual readings taken at a specific location that fell outside this range were found at the cheek. We conclude that the linear accelerator delivered TBI at our centre meets the acceptable limits of dose variation over an 11-year period.