Review on early-stage laryngeal cancer soft tissue imaging protocol

Abstract

Aim

Historically, early-stage laryngeal cancer has been treated with lateral opposing fields, centred at thyroid cartilage, with a field size of approximately 6 × 6cm. Since 2017, our centre has adopted the volumetric modulated arc therapy (VMAT) technique, followed by daily cone beam computed tomography (CBCT) for soft tissue matching while adhering to departmental protocol of 0.3cm spinal cord tolerance. Precision of target volume irradiation is crucial to avoid compromising tumour control and overdosing adjacent healthy organs. Due to the concerns about geographical misses from laryngeal motion from swallowing and breathing, we aim to review the critical steps in soft tissue matching for early-stage laryngeal cancer radiotherapy and update our current imaging protocol.

Methods

Fifty-three patients treated with VMAT and daily CBCT imaging were audited. CBCT images of patient re-setups were reviewed and matched manually on MOSIAQ. Bone-based registration covered C3-6 vertebrae while soft tissue-based registration focused on planning target volume (PTV) and thyroid cartilage. Magnitude of shifts (x-, y-, z-directions) were recorded and the variances were calculated. Further observations on the onset and number of re-setups were made.

Results

Thirty-seven out of the 53 patients required re-setups, with the highest being 18 re-setups for a particular patient. A total of 205 repeated CBCTs were analyzed and it was observed that 82% of the re-setups was due to the z-direction [0.64mm ± 0.32 (0.00 - 2.00)], followed by y-direction [0.16mm ± 0.14 (0.00 - 1.00)] and x-direction [0.09mm ± 0.09 (0.00 - 0.40)]. The larger variance in the longitudinal direction is due to the swallowing motion. Majority of re-setups occurred during week 1 and from week 3 onwards. This could be due to patients’ uneasiness or difficulty following instructions at the start of radiotherapy course. Gradual laryngeal shifts contributing to setup errors over the course of radiotherapy are common, often from laryngeal oedema or weight loss.

Conclusion

CBCT has enabled precise soft tissue anatomical-based matching, especially for early-stage laryngeal cancer, because inferior dosimetry has been shown to compromise survival in head and neck cancer patients. Initially, a conservative approach limiting spinal cord variance from target to 0.3cm was to ensure minimal dosimetric impact on spinal cord dose. However, considering the frequency of re-setups and a significant portion of discrepancies exceeding 0.3cm, increasing the allowable tolerance up to 0.5cm is justifiable. This would accommodate anatomical changes, reduce re-setups and maintain clinically acceptable dosimetric outcomes