A recent trial of 149 non-small cell lung cancer patients involved comparing the effects of radiation therapy and proton therapy as part of their treatment[i]. There were no differences between the treatments in either lung toxicity or treatment efficacy. Non-small cell lung cancer has many similarities with mesothelioma, and interestingly, a study of three mesothelioma patients treated with proton therapy last year found that it was a safer alternative to radiation therapy[ii]. Treatments for non-small cell lung cancer are monitored with interest by the mesothelioma community, such as proton therapy in the present study[iii], and the immunotherapy drug pembrolizumab (Keytruda).
Although the standard of treatment for mesothelioma is chemotherapy, many mesothelioma patients also undergo radiotherapy. Radiotherapy uses high energy X-ray radiation to destroy cancer cells. The objective is to reduce the size of the tumour prior to surgery, or to reduce the symptoms of mesothelioma. However, the radiation can also damage healthy cells. Radiotherapy for pleural mesothelioma and lung cancer is challenging, because the cancer sites are close to critical organs like the lung and heart.
Proton therapy is another form of radiotherapy that uses a type of high energy particles, protons, instead of X-rays. Proton beam therapy is able to target cancer more precisely than conventional radiotherapy. X-rays travel through tissue in a straight line, so tissue in front of and behind the tumour is also exposed. Protons, however, can deposit all their energy at a particular depth within the tissue, and the depth can be controlled so that only the tumour tissue receives radiation[iv]. This means that proton therapy can target the tumour, but adjacent, healthy tissues are spared. The tumour can receive a higher dose of radiation than could be given in X-ray therapy, because the dose of X-rays would need to be limited, to protect adjacent tissue.
The UK currently does not have a high energy proton beam facility. However, two new centres based at University College Hospital in London and the Christie Hospital in Manchester are currently under development. These will treat 1,500 patients a year, primarily those with the most challenging tumours of the head and neck and central nervous system[v]. In some cases, generally complex childhood cancers, the NHS can fund travel abroad for proton treatment.
The goal of the new trial in non-small cell lung cancer patients was to determine whether proton therapy was less toxic to the lungs than conventional radiotherapy. Proton therapy did not improve the amount of radiation received by healthy lung tissue, but improved the amount of radiation received by the heart. Serious side effects were reported in a greater proportion of proton therapy patients (10.5 %) compared with conventional radiotherapy patients (6.5 %). Both groups of patients benefited from their treatment, but there was no increased benefit in those treated with protons.
This is in contrast to a study of three mesothelioma patients who were treated with proton therapy. In all three cases, patients tolerated the treatment well and received lower doses of radiation in the surrounding tissues than would be expected from conventional radiotherapy. None of the three patients developed the serious side effect seen in the lung cancer trial.
In order for proton therapy to be considered as a treatment option for mesothelioma, more studies that demonstrate advantages over radiotherapy need to be reported. At this stage, any patients wishing to undergo proton therapy need to travel abroad. Currently, the NHS would not fund travel overseas for proton beam treatment for mesothelioma, because: (1) such funded cases are usually brain or neurological cancers in children, and; (2) there is insufficient evidence that proton beam is more beneficial for mesothelioma patients than conventional radiotherapy.
[i] Liao, Z. et al. Bayesian Adaptive Randomization Trial of Passive Scattering Proton Therapy and Intensity-Modulated Photon Radiotherapy for Locally Advanced Non–Small-Cell Lung Cancer. JCO JCO.2017.74.0720 (2018). doi:10.1200/JCO.2017.74.0720 http://ascopubs.org/doi/abs/10.1200/JCO.2017.74.0720?journalCode=jco (Accessed 31 January 2018)
[ii] Lee, H., Zeng, J., Bowen, S. R. & Rengan, R. Proton Therapy for Malignant Pleural Mesothelioma: A Three Case Series Describing the Clinical and Dosimetric Advantages of Proton-Based Therapy. Cureus 9, e1705 (2017). https://www.ncbi.nlm.nih.gov/pubmed/29159011 (Accessed 31 January 2018)
[iii] Proton Therapy for Mesothelioma: A Safer Radiation Treatment? Surviving Mesothelioma. 27 January 2018. https://survivingmesothelioma.com/proton-therapy-mesothelioma-safer-radiation-treatment/ (Accessed 31 January 2018)
[iv] Proton beam therapy.. Macmillan Cancer Support. https://www.macmillan.org.uk/aboutus/healthandsocialcareprofessionals/newsandupdates/macvoice/spring2015/protonbeamtherapy.aspx (Accessed 31 January 2018)