The DBCG Proton Trial. Adjuvant Breast Proton Radiation Therapy for Early Breast Cancer Patients: The Skagen Trial 2, a Clinically Controlled Randomised Phase III Trial

Adjuvant breast cancer radiation therapy (RT) is standard for all patients operated with breast conservation and for patients diagnosed with large tumours and/or node-positive disease. Around 65% of all breast cancer patients treated with RT have whole breast RT without nodal RT, whilst the remaining 35% are treated with loco-regional RT (target is breast / chest wall and regional nodal volumes). RT leads to fewer local and regional recurrences, a decrease in breast cancer death and improves overall survival. Since 2014, when the DBCG IMN study showed overall survival gain from internal mammary node (IMN) RT, IMN RT has been standard for all high-risk patients in Denmark. IMN RT causes a significant increase in dose to the heart and lung, thus heart and lung sparing RT techniques based on deep inspiration breath hold (DIBH), volumetric arc therapy and tomotherapy are increasingly used to lower dose to heart and lung whilst maintaining dose to breast and nodal targets. These advanced techniques are used in all DBCG departments routinely. Despite using advanced RT techniques, some patients still receive high RT dose to heart and lung.

Proton therapy (PT) has not been widely used nor investigated for adjuvant breast cancer RT, because there are only few proton centres. However, due to the properties of PT it is possible to achieve optimal dose coverage of relevant targets and at the same time ensure low dose to organs at risk compared with photon RT. In an energy-dependent manner, PT will deposit the majority of its dose in tissue depths defined by the Bragg peak. In practice, this translates into i) the ability to deliver the peak energy to target volumes of irregular 3-dimensional shape using pencil-beam scanning technology, ii) a sharp dose fall-off following deposition of energy in the target and iii) reduction of the integral dose to the patient. Within millimeters, the exit dose drops off from 90% to 10%, resulting in the virtual absence of an exit dose. The effectiveness, safety and feasibility of PT has been reported in few small cohort studies with limited follow up, and there is a lack of clinically controlled randomised trials documenting benefit from PT, evaluated either as higher tumour control and/or as fewer morbidities.

This trial tests standard photon RT versus experimental proton RT for selected early breast cancer patients.