Author: Kin Yin Cheung, Chen-Yu Huang, Chi Wah Kong, Ka Ki Lau, Pei-Xiong Li, Darren M. C. Poon, Bin Yang, Siu Ki Yu, Jing Yuan, Chi To Yung, Shang Peng Felix Yung 👨🔬
Affiliation: Comprehensive Oncology Centre, Hong Kong Sanatorium and Hospital, Medical Physics Department, Hong Kong Sanatorium and Hospital, Research Department, Hong Kong Sanatorium and Hospital 🌍
Purpose: This study investigates optimal treatment strategies utilizing MR-linac for extremity lesions, with a specific focus on a 0.6 cm deep lesion in the right palm. The magnetic field-induced dose effects were analyzed both with and without the use of gel to fill the gaps between the palm and bolus.
Methods: The patient was positioned head-first supine with the right hand elevated and palm facing upwards within a vaclok. CT scans were taken with a 2 cm thick, 30 by 30 cm super-flex bolus, first without then with gel. Treatment planning used the CT scan incorporating gel, prescribing doses of 30 Gy to the GTV and 25 Gy to the PTV, expanded isotropically by 3 mm from the GTV. Special attention was given to protecting the skin from high dose irradiation. Four anterior oblique beams were arranged at gantry angles 315°, 350°, 25°, and 55°.
Results: The clinical plan utilizing gel-filled gaps met all dosimetric criteria. Without gel, GTV and PTV coverage decreased due to dose perturbation at the gap. A 10% reduction in skin dose above the lesion was mainly attributed to the Electron Returning Effect (ERE), where exit electrons from the bolus are curved back to the bolus, leading to a dose re-buildup at the skin. Concurrently, an increased skin dose of 2.8 Gy at the thenar could result from the Electron Streaming Effect (ESE), where some secondary electrons traverse the air cavity are streamed away and hit the protruding thenar region. Additionally, out-of-field ESE was observed, resulting in up to 1% unintended dose to the patient's head.
Conclusion: Filling the palm-bolus gap is crucial for achieving dose coverage and mitigating effects like ERE and ESE. This approach underscores the importance of awareness of magnetic-field induced dose effect and meticulous planning for setup extremities MR-linac treatments.