Simple and effective immobilization for radiation treatment of choroidal melanoma

Monique van Prooijen; Lily Chau; Grace Tsui; Valerie Kelly; Michael Holwell; Tony Tadic; Derek S Tsang; Hatem Krema; Normand Laperriere

doi:10.1016/j.meddos.2023.03.002

Simple and effective immobilization for radiation treatment of choroidal melanoma

Med Dosim. 2023;48(3):154-160. doi: 10.1016/j.meddos.2023.03.002. Epub 2023 Apr 27.

Authors

Monique van Prooijen¹, Lily Chau², Grace Tsui², Valerie Kelly², Michael Holwell², Tony Tadic³, Derek S Tsang⁴, Hatem Krema⁵, Normand Laperriere⁴

Affiliations

¹ Department of Medical Physics, Princess Margaret Cancer Centre, Toronto, ON, M5G 1Z5, Canada. Electronic address: Monique.vanProoijen@rmp.uhn.ca.
² Department of Radiation Therapy, Princess Margaret Cancer Centre, Toronto, ON, M5G 2M9, Canada.
³ Department of Medical Physics, University of Toronto, Princess Margaret Cancer Centre, Toronto, ON, M5G 1Z5, Canada; Department of Radiation Oncology, University of Toronto, Princess Margaret Cancer Centre, Toronto, ON, M5G 1Z5, Canada.
⁴ Department of Radiation Oncology, Princess Margaret Cancer Centre, Toronto, ON, M5G 1Z5, Canada.
⁵ Department of Ocular Oncology, Princess Margaret Cancer Centre/ UHN, University of Toronto, Toronto, ON, M5G 2M9, Canada.

PMID: 37120386
DOI: 10.1016/j.meddos.2023.03.002

Abstract

At our institution, patients diagnosed with choroidal melanoma requiring external beam radiation therapy are treated with two 6 MV volumetric-modulated arcs delivering 50 Gy over 5 daily fractions. The patient is immobilized using an Orfit head and neck mask and is directed to look at a light emitting diode (LED) during CT simulation and treatment to minimize eye movement. Patient positioning is checked with cone beam computed tomography (CBCT) daily. Translational and rotational displacements greater than 1 mm or 1° off the planned isocenter position are corrected using a Hexapod couch. The aim of this study is to verify that the mask system provides adequate immobilization and to verify our 2-mm planning target volume (PTV) margins are sufficient. Residual displacements provided by pretreatment verification and post-treatment CBCT data sets were used to assess the impact of patient mobility during treatment on the reconstructed delivered dose to the target and organs at risk. The PTV margin calculated using van Herk's method¹ was used to assess patient motion plus other factors that affect treatment position, such as kV-MV isocenter coincidence. Patient position variations were small and were shown to not cause significant dose variations between the planned and reconstructed dose to the target and organs at risk. The PTV margin analysis showed patient translational motion alone required a PTV margin of 1 mm. Given other factors that affect treatment delivery accuracy, a 2-mm PTV margin was shown to be sufficient for treatment of 95% of our patients with 100% of dose delivered to the GTV. The mask immobilization with LED focus is robust and we showed a 2-mm PTV margin is adequate with this technique.

Keywords: Dose reconstruction; PTV margin assessment; Patient motion.