Investigating the Role of Adjuvant Proton Beam Therapy in Patients With Parotid Carcinoma

Malignant parotid tumours are uncommon. Whilst 85% of salivary gland tumours originate in the parotid gland, only 20-25% of these are malignant, representing only 3-6% of head and neck cancers. This is about 650 patients/year in the UK.

Standard treatment for local disease is with surgical resection followed by adjuvant radiotherapy for patients with high-risk features. Local control and 5-year survival rates are good, at >70% and >80% respectively, but radiotherapy is associated with considerable toxicity. More than 70% of patients receiving photon therapy experience significant dysgeusia (taste loss/alteration). This can be permanent, is associated with weight loss, diminished appetite, dry mouth, and negatively impact on Quality of Life (QoL).

The putative benefit of proton beam radiotherapy (PBT) relates to its characteristic deposition in the body, which limits the radiation dose received by surrounding healthy tissues. We hypothesise that irradiating the post-operative parotid bed with PBT rather than Intensity Modulated Radiation Therapy (IMRT), will reduce the dose delivered to the Organs at Risk (OAR), in particular the oral cavity (OC), leading to a reduction in acute and long term taste loss/alteration. The advantageous physical properties of PBT may also improve other side effects including fatigue, mucositis, nausea \& vomiting and potentially hearing problems, as well as overall QoL.

Radiotherapy planning studies:

Radiation planning studies have repeatedly shown statistically significant reductions in the dose delivered to healthy tissues including: the oral cavity, brainstem, spinal cord, contralateral parotid, ipsilateral and contralateral submandibular glands and ipsilateral temporal lobe. In particular, radiotherapy doses to the oral cavity are significantly reduced, typically to below 10 Gray (Gy). No routinely contoured OAR or region of interest was consistently found to have higher doses planning with protons, although skin dose may be higher.

Clinical Trials:

Whilst there are no randomised control trials comparing protons and photons for this cohort. However, there is some clinical evidence that the use of PBT leads to clinically meaningful improvements in the side effects experienced by patients. One study compared acute toxicities between matched groups receiving either protons or photons, demonstrating statistically significant reductions in dysgeusia, fatigue, mucositis and nausea and vomiting in patients undergoing proton treatment. In other studies, PBT is associated with very low toxicity level, such as less than 30% of patients experiencing any dysgeusia. This compares favourably to photon experience, such as in the phase 3 randomised controlled trial 'A Multicentre Randomised Study of Cochlear Sparing Intensity Modulated Radiotherapy Versus Conventional Radiotherapy in Patients with Parotid Tumours' (COSTAR) where approximately 60% of patients reported dysgeusia in the Head \& Neck 35 (HN35) questionnaire at 1 year. The PRONTO study is powered to identify a clinically meaningful reduction in taste dysfunction of at least 20%.

Whilst late toxicities are also likely under-reported in the retrospective international data, the published literature to date is very reassuring. The mean dose to the ipsilateral temporal lobe is reportedly reduced, and reflected in low levels of subsequent headache, fatigue and/or memory change. Similarly, whilst poorly captured, low levels of hearing dysfunction or otalgia have also been reported following PBT.

There is no rationale at all that cancer outcomes, either local control or overall survival, will be worse with PBT than photon treatment. In studies to date, local control (approximately 95%) and overall survival (89%-96%) has been excellent, and comparative to known photon experience.