Third‐party brachytherapy source calibrations and physicist responsibilities: Report of the AAPM Low Energy Brachytherapy Source Calibration Working Group

Abstract

The AAPM Low Energy Brachytherapy Source Calibration Working Group was formed to investigate and recommend quality control and quality assurance procedures for brachytherapy sources prior to clinical use. Compiling and clarifying recommendations established by previous AAPM Task Groups 40, 56, and 64 were among the working group's charges, which also included the role of third-party handlers to perform loading and assay of sources. This document presents the findings of the working group on the responsibilities of the institutional medical physicist and a clarification of the existing AAPM recommendations in the assay of brachytherapy sources. Responsibility for the performance and attestation of source assays rests with the institutional medical physicist, who must use calibration equipment appropriate for each source type used at the institution. Such equipment and calibration procedures shall ensure secondary traceability to a national standard. For each multi-source implant, 10% of the sources or ten sources, whichever is greater, are to be assayed. Procedures for presterilized source packaging are outlined. The mean source strength of the assayed sources must agree with the manufacturer's stated strength to within 3%, or action must be taken to resolve the difference. Third party assays do not absolve the institutional physicist from the responsibility to perform the institutional measurement and attest to the strength of the implanted sources. The AAPM leaves it to the discretion of the institutional medical physicist whether the manufacturer's or institutional physicist's measured value should be used in performing dosimetry calculations.

Source strength assay of iodine‐125 seeds sealed within sterile packaging

“…If a difference of more than 5% is found, consulting the manufacturer to resolve the difference is recommended. ( 14 ) None of the cases investigated in this study exceeded the 3% deviation from the reference value that is generally tolerated, using our batch assay on an intact SCBP (Fig. 7).…”

A method for confirming a third‐party assay of I‐125 seeds used for prostate implants

“…That type of error should be detectable in our batch assay. The question of how to proceed if the measurement indicated a deviation beyond 5% has been discussed 5. A decision by the authorized physician user, informed by the uncertainty in the measurement, to continue or not with that particular batch of seeds would need to be made.…”

“…This report says “Third party assays do not absolve the institutional physicist from the responsibility to perform the institutional measurement and attest to the strength of the implanted sources”,5 The American College of Radiology (ACR) Technical Standard for the Performance of Low‐Dose‐Rate Brachytherapy Physics6 also states that each facility should have the instrumentation and methods for verifying source strength in brachytherapy. This does not mean that the third‐party assay should not be used, but rather that the medical physicist is also responsible for verifying the activity of the seeds as defined in the previously cited Task Group reports.…”

“…Two general solutions have been proposed. One is to order extra seeds from the same lot as those to be used in the implant and then assay them 5. Another way is to assay the seeds in sterile packaging using a calibrated well chamber7 or an imaging plate 8.…”

NCS Report 20: Dosimetry and quality control of brachytherapy with low-energy photon sources (I-125)

“…AAPM recommends a maximum deviation of 5% from the manufacturer's stated value for individual seeds. The responsibility of the medical physicist to perform an independent verification of the source strength of procured seeds assigned by the manufacturer is additionally emphasised by the AAPM LEBSC working group [48]. These verification measurements have to be performed with dosimetric equipment with traceable calibration to national standards (e.g.…”