MLEM Neutron Spectra Unfolding in a Radiotherapy Bunker Using Bonner Sphere Spectrometer

Abstract

High-energy radiotherapy treatments of a medical Linear Accelerator (LinAc) generate secondary neutrons that can produce health damage on the human body as the induction of secondary cancers. The energy spectrum of these neutrons must be determined to estimate the extra dose received by patients inside a radiotherapy room during radiotherapy treatment. To quantify the neutron production, a Ludlum Bonner sphere spectrometer (BSS) is used for measurement at different points of a LinAc bunker at the Hospital Universitari i Politècnic La Fe de València. With the neutron measured data and a set of response detector curves obtained by Monte Carlo simulations with MCNP6.1.1, the Maximum Likelihood Expectation Maximization unfolding method is used to unfold the energy neutron spectrum. Unfolded neutron spectra at different locations were compared to those obtained by Monte Carlo simulation of the same setup, showing the same energetic behavior. The fluence rate decreases with source distance, and the shape changes from a fast neutron peak in the nearest LinAc head location to a prominent thermal neutron peak in the bunker maze region. Moreover, the neutron ambient equivalent dose was obtained from the unfolded spectra and compared to Berthold detector measurements, being consistent.

Keywords:

• Bonner sphere spectrometer
• MCNP6
• neutrons
• dose
• radiotherapy

Acknowledgments

The authors would like to acknowledge the collaboration of the radiotherapy staff of the Hospital Universitari i Politècnic La Fe de València for their constant interest, support, availability, and collaboration in the development of this work. Thanks to Varian for all the information received related to the LinAc model.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was partially supported by the program “Ayudas para la promoción de empleo joven e implantación de la Garantía Juvenil en I+D+I, Plan Estatal de Investigación Científica y Técnica e Innovación 2017-2020” from the “Iniciativa de Empleo Juvenil” (IEJ) and the “Fondo Social Europeo” (FSE), grant number: PEJ2018-001727-A-AR.