Session Item

Source strength for high-dose-rate (HDR) and low-dose-rate (LDR) brachytherapy sources should be measured by the clinical medical physicist. In general, the manufacturer provides a calibration certificate documenting the reference air-kerma rate (RAKR) or air-kerma strength (S_K). This certificate will be at a specific time point and for individual sources or for the mean of a batch of sources. The physicist is responsibility for brachytherapy dose calculations, which require input of source strengths, so physicist-measured values are often used in the clinical process. Requirements for physicist-measured values vary based on local regulatory standards, but these measurements generally have lower uncertainties than manufacturer-reported values. This is because the manufacturer transfers the calibration of a traceably-calibrated instrument to the production instrument used for calibrating customer sources while the chamber used by the medical physicist is calibrated directly by the calibration laboratory.

The equipment needed to measure brachytherapy source strength has become more uniform over time, and includes a well-type air ionization chamber with an insert to reproducibly position the source at the center of the collecting volume, a radiation source to demonstrate system constancy, an electrometer and triaxial cabling, and a barometer/thermometer/hygrometer (often all part of a single instrument). The electrometer and triaxial cabling are typically shared with the dosimetry program for external-beam radiotherapy calibrations. The radiation source to demonstrate system constancy may be a long-lived radionuclide (e.g., ¹³⁷Cs, ⁹⁰Sr, or ²⁴¹Am) having similar radiation quality and source strength as the brachytherapy source to be measured. Other constancy sources could include another brachytherapy (such as demonstrating system constancy for a new HDR source by measuring an old HDR source preceding source exchange), or even a linac with the well chamber positioned on the floor. The well chamber and electrometer are sent for calibration every two years, or shown to have constancy if intercompared with other instruments that have been calibrated within two years. When mailing equipment for calibrations, the physicist should measure before and afterwards that the instruments have not changed over the course of shipment to demonstrate constancy of the calibration. All calibrations should be directly traceable to a primary standards laboratory and documented as such.

There are several guidance documents and national laws on how to interpret physicist-measured values of brachytherapy source strength in comparison to manufacturer-reported values. For a single HDR source such as ¹⁹²Ir or ⁶⁰Co, the measurement is compared and the physicist may use their value if the two agree within 3%. For LDR seeds (¹²⁵I, ¹⁰³Pd, ¹³¹Cs), often there is more than a single source and guidance documents generally recommended that a subset be measured. When using stranded and sterile seeds to implant, additional loose and non-sterile seeds (5% or 5 seeds, whichever is fewer) from the same manufactured lot are ordered for the purpose of demonstrating agreement within 3%. When comparison of the manufacturer and physicist-measured values are not within the expected tolerance, the physicist must investigate the source of discrepancy and alert the manufacturer (Butler et al. MedPhys 2008). Sources with long half-lives (LDR ¹³⁷Cs tubes and HDR ⁹⁰Sr/⁹⁰Y ophthalmic applicators) are assayed at least annually to show predictable decay.

The formalism for deriving brachytherapy source strength is the product of the corrected reading, multiplied by the well chamber calibration coefficient for the particular source model, and decayed based on the source half-life to the reference day and time. The reading is averaged over at least three separate measurements, and corrections are made for ion recombination, atmospheric pressure and temperature, and shown to be within the acceptable range of relative humidity. Measurements are performed in a low-scatter environment to minimize increased signal due to radiation scatter from the room walls, floor, and ceiling (thus matching the chamber calibration conditions). Special calibration formalisms are available for combinations of brachytherapy sources and treatment applicators, electronic brachytherapy sources, LDR ophthalmic plaques containing ¹⁰⁶Ru/¹⁰⁶Rh, and ⁹⁰Y microspheres used for selective internal radiotherapy of hepatocellular carcinoma or liver metastases. Future brachytherapy calibration standards may employ the absorbed dose-rate to water at 1 cm as it may provide lower propagated uncertainties than the combination of RAKR (or S_K) and the dose-rate constant. However, such calibrations are not widely available for all sources, the gains realized by lower propagated uncertainties are minimal, and clinical brachytherapy treatment planning systems do not yet permit entry of this quantity.