Author: Salim Balik, Eric Chang, Kristine Garcia, Andrew Islas, Lijun Ma, Zhilei Liu Shen, Elizabeth Zhang-Velten 👨🔬
Affiliation: Radiation Oncology, Keck School of Medicine of USC 🌍
Purpose: NRG protocols recommend simulating and treating head and neck (HN) cancer patients in a supine position for optimal setup reproducibility. However, certain patients cannot tolerate the supine position due to pain or excessive secretions. This study evaluates the reproducibility of a unique prone HN setup using an in vivo EPID dosimetry system.
Methods: A patient with cutaneous squamous cell carcinoma on the posterior scalp was unable to tolerate supine positioning due to pressure on the bleeding and painful tumor as well as extensive right neck nodal lymphadenopathy forcing a kyphotic posture. To stabilize the patient in a prone position, a Vacbag was placed under the body, a Moldcare head cushion supported the forehead, and a thermoplastic mask was applied over the back of the head and shoulders. Damp towels were used on the back of the head to serve as a 0.5 cm bolus and to improve patient comfort under the mask. Sun Nuclear PerFRACTION EPID dosimetry software was employed to verify the prone setup and detect any daily setup errors.
Results: EPID images were captured during treatment, and the PerFRACTION software automatically analyzed the EPID images and trajectory log files. The relative differences in composite point dose at the isocenter for five treatment fractions were 0%, -1.7%, 0.2%, -9.5%, and 0.6%, with only one fraction exceeding the 3% point dose difference criterion. A 2D gamma analysis (3%/2mm criteria and 20% threshold) showed composite passing rates of 97.18%, 94.01%, 98.46%, 93.87%, and 91.26% for the five fractions, all exceeding the 90% criterion.
Conclusion: In vivo EPID dosimetry results indicate that prone HN setups are reproducible, with excellent 2D gamma passing rates and small point dose variations observed in only one fraction. This demonstrates the feasibility of prone positioning as an alternative for HN patients who cannot tolerate supine setup.