3074 - Optimizing Isocenter Spacing for Total Marrow and Lymphoid Irradiation on a Ring-Gantry Linac: A Dosimetric and Robustness Analysis

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 7

POSTER

Presenter(s)

Chunhui Han, PhD - City of Hope National Medical Center, Duarte, California

A. Magliari¹, L. Rosa¹, J. Y. C. Wong², T. M. Williams², A. Liu², and C. Han²; ¹Varian Medical Systems, Palo Alto, CA, ²Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA

Purpose/Objective(s): Volumetric-modulated arc therapy (VMAT)–based Total Body Irradiation and Total Marrow and Lymphoid Irradiation (TMLI) are increasingly implemented on both C-arm and ring-gantry linacs. However, no consensus exists regarding the optimal superior–inferior overlap between adjacent arc sets. This study systematically evaluates the trade-offs between minimal arc overlap—expected to reduce treatment time but potentially compromise dosimetry, robustness, and peripheral target coverage—and maximal overlap, which could improve robustness at the cost of longer delivery. We aimed to quantify how isocenter spacing influences dose quality, setup robustness, and delivery efficiency.

Materials/Methods: A publicly available knowledge-based planning (KBP) model was used to generate TMLI VMAT plans for the upper body portion of a 50-cm-wide male patient on a ring-gantry linac with a 28 × 28 cm² MLC aperture. A reference plan was created with full 14-cm superior–inferior field overlap to ensure each arc fully overlapped half of its adjacent arcs. Additional plans were generated with progressively reduced overlaps of 12, 10, 8, 6, and 4 cm.

Robustness was evaluated by simulating systematic superior–inferior isocenter misalignments. For each plan, alternating isocenter pairs were shifted 1 mm closer and 1 mm farther (net 2-mm add/gap). This process was repeated for misalignment magnitudes of 2, 4, 6, 8, and 10 mm (corresponding to 2–10 mm add/gap), yielding 36 total plans spanning all overlap conditions and misalignment scenarios.

Each plan underwent scorecard-based quality analysis for a composite dose quality score as well as: Dmax, Mean lung dose (MLD), 12-Gy coverage of PTV in the arms (PTVarms), estimated beam-on time, and required number of isocenters.

Results:

Increasing isocenter spacing did not reduce treatment time in proportion to the reduced overlap due to increased modulation required to maintain coverage. A meaningful decrease in beam-on time occurred only when spacing increased enough to eliminate an isocenter group (20-cm spacing in this patient), resulting in a 46-second reduction (674 s ? 628 s). However, this came with notable dosimetric degradation:

Composite score: 79.7% ? 76.4%
Dmax: 133% ? 164%
12-Gy PTVarms coverage: 85% ? 81%
MLD remained essentially unchanged (6.5 Gy ? 6.4 Gy)

Importantly, the severity of misalignment-induced dosimetric degradation was comparable across all isocenter spacing conditions, indicating that reduced overlap does not meaningfully exacerbate head-to-foot misalignment sensitivity.

Conclusion: Maximal isocenter overlap provides the highest overall dosimetric quality for TMLI on a ring-gantry linac. While increasing isocenter spacing can reduce treatment time when it eliminates an entire isocenter group, this benefit is small and accompanied by clinically meaningful reductions in target coverage and dose quality. The authors therefore recommend maintaining maximal feasible overlap to ensure optimal dosimetric results.