Optimal Combination of Pitch, Modulation Factor and Dosimetric Considerations in Treatment Planning for Total Body Irradiation Using Helical Tomotherapy

OBJECTIVE: The aim of this study is to makethe standard total body irradiation (TBI) protocol for Helical tomotherapy© (HT) and to analyze the optimal pitch and modulation factor (MF) with respect to dose homogeneity index (HI), target dose coverage, target overdose, beam on time (BOT) and mean lung dose. MATERIALS AND METHODS: Ten patients who underwent high-dose TBI were taken for this study. For each patient, 35 dose plans were created by different combination of pitch and MF. The optimal pitch and MF were deduced using scatter plot and regression methodology based on target coverage, HI, target volume receiving 103%(V(103%)), 105%(V(105%)) and 107% (V(107%)) of the prescription dose and BOT. Using these optimal pitch and MF, the final dose plan was made and the planning aim and achieved dose was compared using two tailed student’s t-test. Radiochromic films and ionization chambers were used to measure the delivered dose using anthropomorphic phantom on various points for the head and pelvis regions to verify the skin flash margin and its effect on skin dose. RESULTS: The optimal pitch and MF value were 0.287 and 2.4 respectively. Based on optimal pitch and MF, the mean BOT was 1692 seconds with optimal inhomogeneity (7.4%). For target, D95 and D98 were 97.09% (range:94.7-99.6%, p=0.002) and 93.9% (range:91.5-94.4%,p=0.007) respectively, and mean D2 was within 107% with SD of ±1.22% (p=0.04). The mean of PTV receiving V(103), V105 and V107 was 24.48% (range=7.7-36.6%, p=0.03), 5.76% (range=1.4-12.1%, SD=±3.3%), 1.93% (range=0.1-4.6%, p=0.008) respectively. Our measurements show that the flash margin did not increase the skin dose. CONCLUSION: In our study, the optimal combination of pitch value of 0.287 and MF value of 2.4 provided acceptable plans for all patients planned for TBI in HT. The flash margin can provide adequate coverage during patient position uncertainty without increasing the skin dose.