Answer to Question #6785 Submitted to "Ask the Experts"

Q

According to National Council on Radiation Protection and Measurements Report No. 151 for shielding design of radiotherapy facilities, the workload (W) for radiotherapy equipment is defined as "the time integral of the absorbed-dose rate determined at the depth of the maximum absorbed dose, 1 m from the source."

This parameter is supposed to be proportional to the total weekly equivalent dose delivered outside of the radiation vault, using the appropriate parameters proposed in the text.

However the prescription dose in radiotherapy is generally not at the depth of the maximum but rather at the depth of the tumor being treated, which is normally greater than the maximum dose depth. This leads to bigger "beam-on-times" than would be estimated by this definition of W, therefore to an underestimation of the required shielding to meet with regulatory limit, at least in the case of leakage and scatter component evaluation.

1. Isn't this definition underconservative?
2. Wouldn't it be more appropriate to estimate the workload parameter as proportional to "the total monitor units rendered per week"?
3. What is the magnitude of the underestimation in practical terms?
4. Does this result in a real shielding deficiency or is this underestimation somehow compensated by the overestimation of some of the other parameters involved in the calculation and recommended in the document?

A

For radiation protection purposes one is interested in the maximum absorbed dose or dose equivalent to an individual outside the shielding, which is the time integral of the of the maximum absorbed dose rate outside the shielding. The maximum absorbed dose rate outside the shielding is proportional to the maximum absorbed dose rate at 1 m from the source. "The time integral of the absorbed-dose rate determined at the depth of the maximum absorbed dose, 1 m from the source" is therefore not a statement of where the patient dose is calculated.

Even when the dose is calculated at some depth, one can always recompute the dose from each beam at the depth of maximum absorbed dose, 1 meter from the source. Calculation of monitor units (MU) to deliver a certain dose at a point at depth on the central beam axis requires a number of dosimetric quantities which are measured as part of the linear accelerator (linac) commissioning. Machines are usually calibrated to deliver 1 rad (10^-2 Gy) per monitor unit at the reference depth t_o, for a reference field size 10 cm x 10 cm and a source to calibration distance of SCD. In fact, the monitor units are proportional to that dose at maximum through the calibration factor but as authors of the NCRP Report No. 151, we did not wish to use monitor units in NCRP Report No. 151 in the definition since they are unitless. So there is no "underconservatism" built into this definition. One is supposed to analyze the way one's linear accelerator is used to treat patients and then "calculate" this workload in terms of the dose that is delivered to this distance from the source.
 
James Deye, PhD, and Nisy E. Ipe, PhD, CHP