Total Body Irradiation Using Fixed-beam Tomotherapy in Patients With Width Larger Than 40cm

Abstract

To investigate the feasibility of TBI using TomoDirect (TD) for patients with width larger than 40cm and to perform a dosimetric comparison study of treatment plans using Helical Tomotherapy (HT) and TD techniques. 10 TBI patients were selected with median field width in lateral direction of 51.91cm (44.77 cm - 62.89 cm). TBI planning target volume (PTV) was defined as the external body contour minus the bilateral lung expanded by 5 mm. For optimization the lungs were divided into inner (lungs minus 1.5cm), mid lung ring (lungs minus 0.5cm) and outer lung ring (lungs minus mid lung). A virtual bolus was created by expanding the patient’s external contour by 1cm and assigning a density of 0.2 g/cm3. The TBI prescription was at least 80% of the PTV to receive 12 Gy in 6 fractions. For each patient, two treatment plans were generated using both TD, with 8 beam angles, and HT techniques. A dynamic jaw size of 5 cm, a pitch of 0.43, and a modulation factor of 2.0 were used in this study. All other optimization objectives were kept the same for both treatment techniques. The dosimetric evaluation parameters included: PTV mean dose (Dmean), 98% and 95% of PTV dose coverage (D98, D95), homogeneity index (HI), TBI beam-on time and bilateral lungs dose. Student t-test was performed to evaluate the dosimetric differences between TD and HT plans. A p-value of ≤ 0.05was considered significant. Median and ranges for PTV Dmean, D98, D95, HI and beam-on time from TD and HT plans for all ten patients are shown in Table 1.Abstract 3690; Table 1Dmean (Gy)D98 (Gy)D95 (Gy)HITime (minutes)TD12.08(12.01-12.15)10.9(10.6-11.7)11.48(11.25-11.66)0.146(0.03-0.19)22.41(18.27-30.54)HT12.10(12.08-12.23)10.8(9.7-11.5)11.46(10.89-11.65)0.17(0.11-0.28)17.79(14.96-21.49)Significant?NoYesNoYesYes Open table in a new tab Table 1 showed that there was no statistical significant difference between TD and HT techniques for all the patients (n=10) for Dmean and D95. TD plans had better dose coverage of D98 and better HI and longer beam-on times compared to HT plans. For the lung doses, there was a significant difference between HT and TD plans. HT plans spared the lungs better as compared to TD plans, with median dose and ranges of 8.37Gy (7.97Gy to 8.81Gy) for TD plans and 7.93Gy (7.45Gy to 8.26Gy) for HT plans. TD and HT plans were compared for 10 TBI patients. All plans had comparable Dmean and D95 PTV dose coverage and were acceptable for clinical use. However, TD plans had both better D98 dose coverage and dose homogeneity especially for the patients who had more than 50cm width in the lateral direction (7 out of 10 patients). HT treatment plans generally had less beam-on time and spared more lungs as compared to TD plans.