Answer to Question #11015 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:


We are operating an 18 MV linear accelerator (LINAC). The maximum instantaneous neutron dose at the door is about 150 µSv h-1. The maximum instantaneous photon dose on the door is about 50 µSv h-1. The patient load is 50 patients per day (on average). We only have TLD (thermoluminescent dosimeter) badges for our workers which are good for photon dosimetry. Could you suggest how we can assess whether the door shielding is sufficient? Are there any publications on this matter?


There are three independent steps that need to be followed to ensure that the door shielding is adequate:

  1. Perform shielding calculations for the total (photon and neutron) weekly and hourly doses at the door for the site specific workload and usage factors. Compare these doses to the applicable regulatory limits.
  2. Perform careful photon and neutron radiation surveys outside the door. Calculate total weekly and hourly doses using site specific workload and usage factors.
  3. Install passive radiation monitoring devices (e.g., TLDs for photons, track-etch dosimeters for neutrons, etc.) outside the door. Process these at some preestablished frequency.

Please refer to National Council on Radiation Protection and Measurements Report No. 151, Structural Shielding Design and Evaluation for Megavoltage X and Gamma-ray Radiotherapy Facilities, which provides recommendations on shielding calculations, radiation surveys, and neutron monitoring equipment.

Nisy Ipe, PhD, CHP

Answer posted on 19 June 2014. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.