Dysprosium detector for neutron dosimetry in external beam radiotherapy

Radiotherapy treatments with high-energy (> 8 MeV) photon beams are a standard procedure in clinical practice, given the skin and near-target volumes sparing effect, the accurate penetration and the uniform spatial dose distribution. On the other hand, despite these advantages, neutrons may be produced via the photo-nuclear (gamma,n) reactions of the high-energy photons with the high-Z materials in the accelerator head, in the treatment room and in the patient, resulting in an unwanted dose contribution which is of concern, given its potential to induce secondary cancers, and which has to be monitored. This work presents the design and the test of a portable Dysprosium dosimeter to be used during clinical treatments to estimate the "in vivo" dose to the patient. The dosimeter has been characterized and validated with tissue-equivalent phantom studies with a Varian Clinical iX 18 MV photon beam, before using it with a group of patients treated at the S. Anna Hospital in Como. The working principle of the dosimeter together with the readout chain and the results in terms of delivered dose are presented.