Peripheral Dose Heterogeneity Due to the Thread Effect in Total Marrow Irradiation With Helical Tomotherapy

Abstract

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.

Dose Escalation of Total Marrow Irradiation in High-Risk Patients Undergoing Allogeneic Hematopoietic Stem Cell Transplantation

“…All patients received fludarabine (25 mg/m 2 × 3 consecutive days) and cyclophosphamide (60mg/kg/day IV × 2 days) followed by dose escalated TMI. Details of the TMI technique have been previously described 15–17 . Briefly, the patient was immobilized using the Body Pro-Lok™ system (CIVCO, Orange City, IA) to assure a consistent positioning during treatment.…”

“…The resulting images and contours were then transferred to the Tomotherapy HiArt Planning Station (Tomotherapy, Inc., Madison, WI). An optimal treatment plan was created to deliver the prescribed dose (300 cGy/fraction in 5 or 6 fractions) to the PTV and the reduced radiation dose to vital organs, as described previously 17 . The rationale for the selection of 3 Gy/fraction was derived from our previously study using TMI simulation, where we considerd the lungs as single most vital organ for toxicity 16 .…”

Dose evaluation indices for total body irradiation using TomoDirect with different numbers of ports: A comparison with the TomoHelical method

“…We found statistically significant improvements of D2, D50, and HI in the TomoDirect plans compared with TomoHelical plan. These differences may be caused by the thread effect in TomoHelical mode, which is known to be a dose‐variation pattern that manifests as a ripple, which is the result of helical beam junctioning . Takahashi et al .…”

Evaluation of Functional Marrow Irradiation Based on Skeletal Marrow Composition Obtained Using Dual-Energy Computed Tomography

“…For standard TMI, the entire skeleton (includes both red and yellow marrow) was used as a clinical target volume (CTV) and the planning target volume (PTV) was generated with 5 mm margin to CTV. The goal was to deliver 18 Gy to the PTV and minimize dose exposure to all critical organs, namely lungs, eyes, kidneys, liver, gut following recent reports (18,19). For differential targeted radiation, the PTV was separated into two groups: i) RMI: higher dose (18 Gy) irradiation to RM and lower dose (12 Gy) to rest of the skeletal system, and ii) YMI: 18 Gy irradiation to YM and 12 Gy to remaining skeleton.…”

“…To compare PTV volume coverage of RM or YM relative to total marrow, we defined “PTVrm/PTVst”, “PTVym/PTVst” where PTVrm, PTVym, PTVst refers to PTV of RMI, YMI, and standard TMI, respectively. The prescription of 18 Gy in 6 fractions was used for planning simulation to cover primary target with the 85% isodose line (18,19). The dose constraints for normal tissues were kept the same for each patient in order to keep median dose of all OARs < 10 Gy.…”