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Session: Space-time Modulation of Treatment [Return to Session]

Multi-Field 3D MLC-Based Spatially Fractionated Radiotherapy for Large and Bulky Masses: Risk of Intrafraction Motion Errors

C Lemos*, M Bernard, W St Clair, M Kudrimoti, D Pokhrel, University of Kentucky, Lexington, KY


TU-G-BRA-6 (Tuesday, 7/12/2022) 1:45 PM - 2:45 PM [Eastern Time (GMT-4)]

Ballroom A

Purpose: To demonstrate the feasibility, safety and efficacy of image-guided multi-field 3D MLC-based spatially fractionated radiotherapy (SFRT) as same day treatment for large and bulky tumors (≥8cm) and to examine the potential risk of target coverage loss and normal-tissues toxicity due to intrafraction motion errors.

Methods: For large and bulky non-resectable tumors, we have developed a CBCT-guided 3D-conformal MLC-based SFRT method, utilizing an in-house MLC fitting algorithm to GRID-GTV producing 1 cm diameter holes at 2 cm center-to-center distance at isocenter, generating brachytherapy sieve-like dose distribution without post processing the physician-drawn GTV contour within an hour-enabling for same day SFRT treatment as CT simulation. For 15 Gy prescription, more than 50 extracranial patients were treated using this algorithm comprising of multiple treatment sites. Risk of intrafraction motion errors was simulated for head&neck, breast, liver and pelvis patients with isocenter shifts from ±1.0-±3.0mm along all 3-directions (total, 18 plans/patient). Dosimetric parameters evaluated include: peak-to-valley dose ratio (PVDR)=GTVD10%÷GTVD90%, GTV7.5Gy, mean GTV dose and maximum dose to organs-at-risk (OAR).

Results: Target dose parameters such as PVDR, V7.5Gy, and mean target dose were minimally affected by <0.1, <1.0% and <2.0% respectively with intrafraction motion error of up to 3mm in any direction. Although, for site-specific cases, such as head&neck, maximum dose to spinal cord was increased by >1.0Gy in the shifted plans.

Conclusion: Due to the tumor size, location and nature of 3D MLC-based SFRT, isocenter shifts of up to ±3mm in any direction had a small effect on target coverage. Although, effect on OAR doses were more pronounced depending on treatment site. For liver, abdominal and pelvis tumors, additional use of immobilization devices may not deem necessary, however, for head&neck treatment it is important use larger-thermoplastic masks to minimize this effect. Dosimetric impact of intrafraction motion error between treatment fields will be investigated.


Setup Errors, Image-guided Therapy


TH- External Beam- Photons: Motion management - intrafraction

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