Answer to Question #3418 Submitted to "Ask the Experts"

What is the method for doing the absorbed dose calculation for a patient treatment by a ⁶⁰Co machine?

There are several ways to calculate the dose in patient treatment using a ⁶⁰Co source and the one I will outline for you is the oldest approach, which is still used in many circumstances today. First I should clarify that while your question restricts the radiation source to ⁶⁰Co, the method I will outline works just as well for accelerator therapy sources.

[Editor’s note: The following is a basic description of the techniques used for depth dose determination and patient treatment. In no way does this imply that such therapy should be performed, as a patient-specific treatment plan must be developed and approved by a physician. Such therapy plans need to be developed by an experienced medical physicist.]

First, we have to set the geometry used to measure the radiation levels and characteristics of the source. A large tank of water, typically a 50 cm cube (2.54 cm = 1 inch) is set on the floor of the treatment room and centered about the radiation beam as the beam is pointing downward. The distance from the ⁶⁰Co source to the surface of the water is adjusted to be 100 cm, which is the source to surface distance (SSD). Then a waterproof ion chamber is positioned at the center of the radiation beam in water and the field size of the radiation beam incident on the tank is adjusted to be a 10 x 10 cm square. The position of the ion chamber below the surface of the water is adjusted up and down the central axis until the system displays a maximum signal. The depth at which this occurs for ⁶⁰Co sources is 5 mm. This is the D_max depth and is the depth at which the dose rate for the radiation source is measured.

The theory for calculating the dose rate is based on the Bragg-Gray cavity theory and it essentially converts the measured ionization caused by the radiation field in the gas cavity of the ion chamber to dose to tissue at that location with the ion chamber absent. The unit of dose is the gray (Gy). Thus, after this measurement, you have an accurate tissue dose rate at D_max for a 10 x 10 cm square field.

Then, without moving the ion chamber, the field size is adjusted to different square sizes ranging from 40 x 40 cm to 3 x 3 cm. At each field size the D_max dose rate is measured. The value changes with field size due to radiation scatter in the phantom and increases slowly with increasing field size and decreases with smaller field sizes. As tumors vary in size, you now have the dose rate at D_max for a series of square field sizes.

As tumors are not typically at this 5 mm depth in the body, we next have to measure the variation of the radiation intensity as the ion chamber is moved to greater depths in the water phantom. This is referred to as a depth/dose measurement and one has to perform it for the full sequence of field sizes referenced above. Each series of measurements for a specific field size in the table of data is normalized to 100% at D_max. Lower readings will be observed with increasing depth due to the attenuation of the radiation by the water.

With the above information, you are now ready to treat your first patient. Set the patient at an SSD of 100 cm as you had set the water phantom and adjust the collimators to create a field size that encompasses the tumor volume. A typical daily dose is 1.8 Gy. Divide the desired daily dose by the D_max dose rate (Gy/min) for the field size you set and then divide by the percent depth dose for the field size chosen (in fractional form). This will result in a time that the ⁶⁰Co unit should be turned on and the patient exposed.

As ⁶⁰Co is a radioactive source that decays in intensity with time, the measured Dmax dose rate (Gy/min) has to be corrected for this decay (decrease in intensity) and the correction is about 1% per month since the measurement was made.

I should caution you that this is a very basic description that outlines the rudiments of a calibration. Corrections for nonsquare irregular fields, the fact that the human body is not water, oblique incidence of the radiation beam, surface irregularities on the patient (for example, the nose) and many other corrections have to be made as well.

One source of information that covers the fine points of calibration of a ⁶⁰Co system as well as accelerators is the Task Group report #51 published by the American Association of Physicists in Medicine, One Physics Ellipse, College Park, Maryland.

James B. Smathers, PhD, CHP

Editors Note: The report referenced in the last paragraph is AAPM Report 67, “Protocol for Clinical Dosimetry of High-Energy Photon and Electron Beams.”