Novel methods to verify dose distribution at the junction region in large-field helical total body Irradiation

Helical tomotherapy (HT) systems have a maximum treatment length of 135 cm, which poses a challenge in Total Body Irradiation (TBI) for adult patients exceeding this length. To overcome this, TBI delivery is split into two separate plans, resulting in a junction region where the plans overlap. Accurate dose delivery in this region is critical to avoid inhomogeneities that could lead to over or underdosage. This study evaluated the dose homogeneity in the junction area by comparing planning system calculations with actual dose delivered in HT-TBI.

Twenty large-field TBI patients treated on the Radixact machine were included. Patients total body was contoured as PTV, further dividing PTV into head, thorax, abdomen, pelvis, lower extremity, and junction for planning and verification purposes. The junction was subdivided into 7 × 2 cm strips in both plans. Clinically accepted plans were calculated on ArcCHECK and ART phantoms for verification. OSLDs were inserted at 18 locations in the overlapping region on the ART phantom to measure point doses.

For all patients, ArcCHECK-measured doses in all sub-volumes, including junction regions, showed 2Dγ passing rates > 94% and point dose differences were within ± 3.0%. Similarly, non-junction and junction point doses difference on the ART phantom were ± 5.0% and ± 10.1%.

Junction dose measurements showed excellent agreement with calculated dose, confirming accurate and homogeneous dose delivery in large-field HT-TBI. The consistency across phantoms indicates effective junction matching and demonstrates the planning and verification approach, supporting the clinical reliability of the system in managing complex TBI junctions.