Abstract
Objective:This study investigates the impact of a restricted craniocaudal (CC) field length of <20 cm on the selection of head and neck cancer (HNC) patients who can be treated on the MR-Linac using a single isocentre technique. We also assess the effects of anthropometric factors and the neck position on the CC field length.Methods:110 HNC patients who underwent radical primary or adjuvant radiotherapy were retrospectively analysed. We assessed the proportion of treatment fields with a CC length of <20 cm and the effects of gender, height, hyo-sternal neck length (distance from superior surface of hyoid to sternal notch measured on the coronal reconstruction of the planning CT) and neck position on CC length.Results:95% of HNC patients had a CC field length <20 cm. Female patients showed a significantly shorter median CC length than male patients in both extended (p = 0.0003) and neutral (p = 0.008) neck positions. Neck position influenced the median CC length with neutral neck being significantly shorter than extended neck (p = 0.0119). Patient height and hyo-sternal neck length showed positive correlation with the CC length, with neck length in neutral position having the strongest correlation (r = 0.65, p = 0.0001 and r = 0.63, p < 0.0001, respectively for extended neck; r = 0.55, p = 0.0070 and r = 0.80, p < 0.0001, respectively for neutral neck). A hyo-sternal neck length of <14.6 cm predicted a CC length of <20 cm in neutral neck position.Conclusion:The majority of patients with HNC at the Royal Marsden Hospital have anthropometric features compatible with their being treated on the MR-Linac using a single isocentre technique. The absolute CC field size may vary according to primary tumour site, patient factors and neck position. A hyo-sternal neck length cut-off of 14.6 cm in the neutral neck position can be used as a surrogate marker for suitability of treatment on MR-Linac.Advances in knowledge:This paper highlights the potential impact of a restricted CC field in HNC patient selection for the MR-Linac treatment. This is the first report to suggest the use of neck length as a surrogate marker for suitability of treatment on the MR-Linac.
Highlights
The Elekta Unity magnetic resonance-linear accelerator (MR-Linac) is a hybrid system that integrates the imaging capability of a 1.5 T MR scanner (Philips Healthcare, Best, The Netherlands) with a linac (Elekta, AB, Stockholm, Sweden)
The CC field restriction imposed by a modified MR coil means that not all head and neck cancer (HNC) patients will be suitable for treatment using a single isocentre
Inferior half of the sphenoid sinus need to be included in the low dose clinical target volume (CTV) for T1-2 nasopharyngeal cancer and the whole sphenoid sinus if T3-4.11 In this study, no paranasal cancer patients and only one of three nasopharyngeal cancer patients had a treatment field length of
Summary
The Elekta Unity magnetic resonance-linear accelerator (MR-Linac) is a hybrid system that integrates the imaging capability of a 1.5 T MR scanner (Philips Healthcare, Best, The Netherlands) with a linac (Elekta, AB, Stockholm, Sweden). As the magnetic field remains active during treatment delivery, scattered secondary electrons can bend back at the air–tissue interfaces (electron return effect) or spiral along the magnetic field (air-electron streaming effect). These electrons can deposit in the skin and lung,[2] and on surfaces perpendicular to the magnetic field such as the jaw, armpits and arms.[3] These must be accounted for at the planning and optimising stages to reduce unwanted radiation dose deposition outside of the treatment field.[3,4]
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