Purpose
To report potential dose heterogeneity leading to underdosing at different skeletal sites in total marrow irradiation (TMI) with helical tomotherapy due to the thread effect, and provide possible solutions to reduce this effect.
Methods and Materials
Nine cases were divided into two groups based on patientsize, defined as maximum left-to-right arm distance (mLRD): small mLRD (≤47 cm) and large mLRD (> 47 cm). TMI treatment planning was conducted by varying the pitch and modulation factor while a jaw size (5 cm) was kept fixed. Ripple amplitude, defined as the peak-to-trough dose relative to the average dose due to the thread effect, and the DVH parameters for 9 cases with various mLRD was analyzed in different skeletal regions at off-axis (e.g. bones of the arm, or femur), at the central axis (e.g. vertebrae), and PTV, defined as the entire skeleton plus 1 cm margin.
Results
Average ripple amplitude for a pitch of 0.430, known as one of the magic pitches that reduce thread effect, was 9.2% at 20 cm off-axis. No significant differences in DVH parameters of PTV, vertebrae, or femur were observed between small and large mLRD groups for a pitch of ≤0.287. Conversely, in the bones of the arm, average differences in the volume receiving 95% and 107% dose (V95, and V107, respectively) between large and small mLRD groups were 4.2% (p=0.016), and 16% (p=0.016), respectively. Strong correlations were found between mLRD and ripple amplitude (rs=0.965), mLRD and V95 (rs=−0.742), and mLRD and V107 (rs=0.870) of bones of the arm.
Conclusions
Thread effect significantly influences DVH parameters in the bones of the arm for large mLRD patients. By implementing a favorable pitch value and adjusting arm position, peripheral dose heterogeneity could be reduced.