Elimination of the Set-Up Margin Using Daily Adaptive Radiotherapy for Head and Neck Radiotherapy: A Feasibility Study
M Lin*, J Park, X Zhong, L Chen, H Morgan, Z Trivedi, A Godley, A Pompos, S Jiang, D Sher, UT Southwestern Medical Center, Dallas, TX
Presentations
TU-IePD-TRACK 3-4 (Tuesday, 7/27/2021) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]
Purpose: Involved Nodal Radiotherapy using AI-based Radiomics (INRT-AIR) for head-and-neck cancer eliminates the elective neck treatment altogether and focus treatment on the nodes themselves. This pilot dosimetric study leverages adaptive therapy to further reduce PTV margins and assess resulting dosimetric gain.
Methods: A total of 10 patients treated with INRT-AIR were recruited and replanned with Ethos treatment-planning-system. Ethos plans were created with planning-target-volume (PTV) margin 5mm and 1mm (for intrafractional-motion), respectively. The plans were virtually delivered in the Ethos emulator using the CBCT acquired in the patient’s prior treatments. The 5mm plans were delivered with image-guidance-radiation-therapy (IGRT) to mimic the current delivery while the 1mm plans were delivered with daily adaptive therapy. The initial plan quality was compared to quantify the dosimetric improvements. The adaptive plan quality was compared with the initial plan to evaluate the robustness of the 1mm INRT-AIR plan against patient’s daily anatomy change. Independent fractions of delivered dose of the two cohorts were compared to evaluate the efficacy of 1mm adaptive therapy over the conventional 5mm IGRT. The gross-tumor-target (GTV) and clinical-target-volume (CTV) coverages along with the organs-at-risk (OAR) dose were the denominators. A dose difference of 300cGy was considered clinically significant.
Results: Among the high impact OARs, the 1mmINRT-AIR plans exhibited clinically significant improvements in contralateral-submandibular-gland (Avg.1775cGy), constrictor muscle (Avg.457cGy), contralateral-parotid-gland (Avg.363cGy), and mandible(Avg.335cGy). Despite the adaptive workflow is fully automatic and user has very minimum control of adaptive plan optimization, over 83% frequency that 1mm adaptive plan recovers the initial plan quality. While 5mmIGRT delivery degraded both the target coverage and OAR sparing, 1mm adaptive delivery holds the promise of dosimetric benefit from margin reduction.
Conclusion: We demonstrated the significant dosimetric benefit of dramatic PTV margin reduction made viable using daily-adaptive-radiotherapy, even in the context of INRT-AIR planning that has improved OAR dosimetry.
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