This study aimed to quantify the difference in setup margin in cone beam computed tomography (CBCT) setup imaging, utilising the van Herk formula for two different image registration methods. Two alternative techniques of registration, bony landmark (BL) matching and soft tissue matching (ST) for head and neck cancer patients, were investigated.

This study included 30 head and neck cancer patients who received a simultaneous integrated boost of 54–60–66 Gy in 30 fractions, using volumetric modulated arc treatment. A total of 867 CBCT images were acquired during patient setup and further analysed for setup margin calculation. A region of interest was described using a clip box between the reference and CBCT image to calculate the patient’s positional inaccuracy in three translational directions, X, Y and Z, where X was mediolateral, Y was the cranial-caudal, and Z was the anterior-posterior direction in the patient-based coordinate system, respectively. The shifts were captured by altering the BL and ST matching, and the setup margin was calculated using the van Herk formula (=2·5Σ + 0·7σ where Σ was the systematic and σ was the random error).

The difference between bony and ST matching in most cases was observed to be 1·4 mm in all translational directions at a 95% confidence interval and <1° in all rotational directions. The rotational error was found to be below the action level (±3°); hence, no corrections related to rotational error were made. The translational setup margin for bone and ST-based registration was X (BL) = 4·6 mm, X (ST) = 4·4 mm, Y (BL) = 6·3 mm, Y (ST) = 4·7 mm, Z (BL) = 3·0 mm, Z (ST) = 3·6mm.

Two distinct registration approaches for head-neck patient setup did not yield any significant difference in the setup margin calculation. A suitable approach for CBCT and reference CT registration technique was required for the setup margin calculation. Confusion in selecting the correct image registration procedure can result in incorrect treatment execution. The compatibility of the two registration approaches was established in this study. Image fusion was neutralised before the second match (ST) to avoid hysteresis. For setup verification using CBCT for the head and neck region, both bone and ST registration were compatible for setup verification.

The impact of modern high-precision conformal techniques on rare but highly morbid late complications of head and neck radiotherapy, such as necrosis of the bone, cartilage or soft-tissues, is not well described.

Medical records of head and neck cancer patients treated in prospective clinical trials of definitive high-precision radiotherapy were reviewed retrospectively to identify patients with necrosis.

Twelve of 290 patients (4.1 per cent) developed radiotherapy necrosis at a median interval of 4.5 months. There was no significant difference in baseline demographic (age, gender), disease (primary site, stage) and treatment characteristics (radiotherapy technique, total dose, fractionation) of patients developing radiotherapy necrosis versus those without necrosis. Initial management included antibiotics or anti-inflammatory agents, tissue debridement and tracheostomy as appropriate followed by hyperbaric oxygen therapy and resective surgery for persistent symptoms in selected patients.

Multidisciplinary management is essential for the prevention, early diagnosis and successful treatment of radiotherapy necrosis of bone, cartilage or cervical soft tissues.