Fast Neutron Therapy for Breast Cancer Treatment: An Effective Technique Sinking into Oblivion.

Adjuvant radiation therapy significantly decreases breast cancer mortality [1, 2], but firstgeneration techniques, which relied on large irradiation fields, were associated with an increased cardiotoxicity risk. Fortunately, breast radiotherapy has evolved, and state-ofthe-art radiation therapy techniques are currently able to efficiently limit heart exposure without altering tumor control, even in complex anatomic situations. In this context, particle radiation therapy is of particular interest: depth-dose curves of proton and carbon ion beams sharply increase when those particles come to rest. This physical feature, known as the Bragg peak, can be efficiently used to limit radiation doses delivered to organs at risk. However, among the particle radiotherapy techniques evaluated to date for breast cancer treatment, fast neutron therapy (FNT) currently seems to be sinking into oblivion, despite promising clinical data. Although . 35 000 patients have been treated with FNT for half a century, only a few FNT facilities are still operating worldwide (in the United States, Germany, and Russia). However, neutrons have specific radiobiologic advantages that deserve consideration. Their linear energy transfer is about 200 times greater than that of photon beams [3, 4], ranging somewhere between 20 and 100 keV/lm, and the relative biologic effectiveness (RBE) of fast neutron beams is estimated to be between 3.0 and 8.0. Lethal DNA breaks are consequently rapidly caused for a short distance, which is of particular interest when treating superficial tumors, such as chestwall recurrences. Finally, the oxygen enhancement ratio of FNT is evaluated to be around 1.3 [3], and this minimal sensitivity to hypoxic conditions [5] may be valuable when irradiating unresectable breast tumors, which are often characterized by significant areas of hypoxic tissues [6].