Radiation and Cancer Physics

PD 19 - Physics 12 - Poster Discussion - Treatment Planning

1169 - A Pilot Study of MR-Guided Mid-Treatment Adaptive Proton Therapy for Pediatric Low-Grade Gliomas

Wednesday, October 24
3:57 PM - 4:03 PM
Location: Room 217 A/B

A Pilot Study of MR-Guided Mid-Treatment Adaptive Proton Therapy for Pediatric Low-Grade Gliomas
S. Acharya, K. Coca, E. E. Bowers, M. Gargone, and T. E. Merchant; St. Jude Children's Research Hospital, Memphis, TN

Purpose/Objective(s): Intensity-modulated proton therapy (IMPT) is associated with reduced integral brain dose compared to photon therapy in the treatment of pediatric low-grade gliomas (pLGG). Mid-treatment imaging and response evaluation may allow for further reductions in dose to organs at risk (OAR) and improvement of the therapeutic ratio. This study investigates potential dosimetry advantages to MRI-guided mid-treatment adaptive planning for patients treated using IMPT.

Materials/Methods: From 2006 to 2017, patients less than 21 years of age with intact pLGGs were treated with definitive radiotherapy and underwent mid-treatment MRI. Clinically delivered regimens were 54Gy/30 fractions (n=26) or 52.2GyRBE/29 fractions (n=2). Simulated initial IMPT plans were created with prescribed dose of 52.2GyRBE/29 fractions. Target volumes and OARs were retrospectively contoured on mid-treatment MRI. Tumors with mid-treatment response (>20% reduction in GTV) were replanned using the response-based contours. Coverage, dose constraints, and robust optimization parameters were identical for the initial and adapted plans. The initial and adapted plans were compared for OAR sparing.

Results: Mid-treatment imaging response was observed in 6/28 (21%) patients: five were located in the diencephalon/optic pathway region and one in the brainstem. Mean (± SD) GTV reduction was 32%±8%. Mean reduction in the volume of the brainstem receiving 40 GyRBE (V40GyRBE), V45GyRBE and V50GyRBE was 9.3%±4.1%, 8.2%±3.8%, and 5.5±5.1% respectively. Four of six adapted plans showed reductions in right and left cochlea V25GyRBE (right cochlea mean reduction: 29.6%±15.6%, left cochlea mean reduction: 20.9%±21.8%). Two of six adapted plans showed reductions in optic chiasm V50GyRBE (mean reduction: 9.8%±5.4%).

Conclusion: Based on mid-treatment response to IMPT, MRI-guided adaptive planning may allow for reductions in brainstem V40GyRBE-V50GyRBE. Select patients may benefit from reductions in optic chiasm V50GyRBE and cochlea V25GyRBE. Prospective evaluation of adaptive therapy in pLGG is warranted to avoid late brainstem and optic nerve toxicities.

Author Disclosure: S. Acharya: None. K. Coca: None. E.E. Bowers: None. T.E. Merchant: None.

Sahaja Acharya, MD

Disclosure:
Employment
St Jude Children's Research Hospital: Assistant Member: Employee

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