[ RadSafe ] Use Factor Limit

[Robert Barish]

Rafi:

Use factors for medical accelerators generally are chosen as a result of considerable experience with therapy techniques. In the latest NCRP Report 151, on the design of shielding for therapy facilities, the long-standing values of close to ¼ for beams pointing laterally and upward and are maintained (See Table 3.1 in the report). In my own practice, I use a value of 1/8 for beams at any other angles. Report 151 bins the angles into 45-degree intervals and presents values between 1/7 and 1/20. I have seen others use a value of 1/10, but the difference in calculated shielding is minimal with those two choices. Of course if there are specific procedures to be done in the vault that would call for individualized use factors, then they must be included in the shielding calculations. As an example, total-body irradiation performed at a distance of four to five meters from the linac results in a workload sixteen to twenty-five times greater for that single patient in the lateral direction used for the treatment. If a large fraction of treatments were tangential breast fields, the fraction at those oblique angles would be increased.

Because the weekly limit of 20 microsievert for unrestricted areas (those that are limited to an annual dose of 1 millisievert) is the same as the hourly dose limit, the “0.02mSv in any hour” restriction only comes into play for “ordinary” treatments if the occupancy factor chosen for a particular location is less than 1/40.  This does not, however, mean that special procedures such as stereotactic radiosurgery or other high-single dose techniques won’t necessitate an evaluation of the hourly dose restriction. It just means that for standard, evenly-distributed through the week, standard-dose treatments, the hourly limit would only come into play if one were to assume an occupancy factor less than 1/40.

In your post you say: “the user can declare a use factor and occupancy factor.” That is indeed true, but given the large number of facilities where “typical values” of these factors (U and T in most shielding texts) have been employed, and the current recommendations of NCRP and the British IPEM in their “Guidance Notes”, it would require considerable justification for any site to use numbers smaller than are typically encountered in those publications. Again, it may be possible, but justification would have to be provided.  As RSO you have the right as well as the obligation to make the shielding designer justify their choice of those factors if they are atypically low. And the final judgment on whether the choices were appropriate would obviously occur once the linear accelerator was put into clinical use and the actual use was established.

This response ignores certain considerations of “instantaneous” dose rate restrictions that exist outside of the USA in some countries. If Israel is one of those place than IDR calculations would also be required. But that’s not the point your question addresses.

Hag Sameach

Robert Barish, Ph.D., CHP
Shielding Consultant


Date: Tue, 11 Dec 2007 13:24:35 +0200
From: "srebro" <srebro at bgumail.bgu.ac.il>
Subject: [ RadSafe ] use factor limit
To: <radsafe at radlab.nl>
Message-ID: <007001c83be8$68b1e6c0$3a15b440$@bgu.ac.il>
Content-Type: text/plain; charset="us-ascii"

Hi

When checking shielding calculations for medical accelerators can the use
factor be any number according the user statement or there is a minimum
number.

The problem that I have is how to check that the public dose limit of 0.02
mSv in any 1 hour is kept. The user can declare on a use factor and
occupancy  factor that are smaller than the actual work.



Rafi Srebro

RSO Ben-Gurion University