Abstract
BackgroundFor radiotherapy of the head and neck, 5-point mask immobilization is used to stabilize the shoulders. Still, the daily position of the shoulders during treatment may be different from the position in the treatment plan despite correct isocenter setup. The purpose of this study was to determine the interfractional displacement of the shoulders relative to isocenter over the course of treatment and the associated dosimetric effect of this displacement.MethodsThe extent of shoulder displacements relative to isocenter was assessed for 10 patients in 5-point thermoplastic masks using image registration and daily CT-on-rails scans. Dosimetric effects on IMRT and VMAT plans were evaluated in Pinnacle based on simulation CTs modified to represent shoulder shifts between 3 and 15 mm in the superior-inferior, anterior-posterior, and right-left directions. The impact of clinically observed shoulder shifts on the low-neck dose distributions was examined.ResultsShoulder motion was 2-5 mm in each direction on average but reached 20 mm. Superior shifts resulted in coverage loss, whereas inferior shifts increased the dose to the brachial plexus. These findings were generally consistent for both IMRT and VMAT plans. Over a course of observed shifts, the dose to 99% of the CTV decreased by up to 101 cGy, and the brachial plexus dose increased by up to 72 cGy.Conclusionshe position of the shoulder affects target coverage and critical structure dose, and may therefore be a concern during the setup of head and neck patients, particularly those with low neck primary disease.
Highlights
For radiotherapy of the head and neck, 5-point mask immobilization is used to stabilize the shoulders
In general, shoulder positional variation is approximately 2-6 mm; large shifts up to 2 cm may be possible even when 5-point masks that extend over the shoulders are used
No trend was seen for large shifts with time, so patients are at risk for having large shoulder displacements early and late in treatment
Summary
For radiotherapy of the head and neck, 5-point mask immobilization is used to stabilize the shoulders. The purpose of this study was to determine the interfractional displacement of the shoulders relative to isocenter over the course of treatment and the associated dosimetric effect of this displacement. Extensive effort is spent in positioning and immobilizing the patient, the focus is on target alignment; the position of the body away from isocenter is often ignored. Such distant body positions may affect the delivered dose distribution. Previous studies have found shoulder displacement (in conjunction with isocenter setup) in excess of 1 cm [3,4]. The dosimetric impact of these shoulder shifts was speculated upon, it was not quantified
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