Brachytherapy

ARPANSA currently does not maintain primary standards for the dosimetry of brachytherapy sources. Here we discuss the options available for clinics seeking traceable measurements for brachytherapy sources. The bulk of brachytherapy in Australia is performed with Iridium-192 high dose rate afterloaders and Iodine-125 radioactive seed implants.

The Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) has recently published guidelines which include recommendations on the calibration of the source strength [1].

Iridium-192

The source output of Iridium-192 is specified in terms of the reference air kerma rate (RAKR) [2] at 1 m (in the absence of air scatter and attenuation). This quantity is used in treatment planning to determine the dose distribution around the source, and hence the dose profile delivered to the patient.

The RAKR can be measured directly by placing a calibrated ionisation chamber at 1 m (and removing the effects of air scatter and attenuation). However, this is not usually performed in a clinic because the air kerma rate is below the rates usually measured with Farmer-type chambers. To overcome this problem, a special jig can be used to hold a Farmer-type chamber at a fixed distance close to the source. Using correction factors, the RAKR can be determined from the measurement in the jig [2]. ARPANSA provides calibrations for air kerma which can be interpolated to Iridium-192 and used with a jig to determine the air kerma rate. This method is suggested as a secondary check by the ACPSEM. In such cases, ARPANSA calibrates a Farmer-type chamber with the build-up cap on at Cobalt-60 and one of the higher energy kV X-ray beams. The air kerma calibration coefficients can be interpolated to Iridium-192. Details of this method are provided in references [2] and [3].

The ACPSEM recommends that the RAKR is measured in is a well chamber. Such chambers must be calibrated for the specific source-type (manufacturer and model). At present, this calibration can only be performed by sending the well chamber overseas. ARPANSA recommends that users check that overseas calibrations are:

1. traceable to a primary standard
2. performed for the source type for which the chamber will be used
3. performed by a laboratory accredited to ISO/IEC17025 or equivalent standard.

The calibration certificate should include a statement of the traceability of the measurement and its uncertainty.

Iodine-125

The output of Iodine-125 seeds are usually specified in terms of the radioactivity of each seed. ARPANSA provides dosimetry standards, which do not necessarily apply in this case. ANSTO has the authorisation to maintain standards for radioactivity and has at times provided calibration services for well chambers for certain types of brachytherapy seed.

Well chambers used to characterise brachytherapy seeds need to be calibrated for the seed type (manufacturer and model). The same recommendations apply for getting a well chamber calibrated overseas as for Iridium-192.

References

[1] ACPSEM brachytherapy working group recommendations for quality assurance in brachytherapy, Claire Dempsey, Ryan Smith, Thulani Nyathi, Abdurrahman Ceylan, Lisa Howard, Virendra Patel, Ram Das, Annette Haworth, Australas. Phys. Eng. Sci. Med. (2013) 36:387–396.
[2] Calibration of photon and beta ray sources used in brachytherapy IAEA Tecdoc 1274, Vienna 2002.
[3] Comparison of ¹⁹²Ir air kerma calibration coefficients derived at ARPANSA using the Interpolation method and at the National Physical Laboratory using a direct measurement, Butler D, Haworth A, Sander T, Todd S (2008) Australas. Phys. Eng. Sci. Med. 31(4):332–338.