Lab 09 Brachytherapy Source Calibration: Farmer and Well-chamber Objectives: Become familiarize with the use of farmer chamber in a full-scatter phantom and the well chamber methods for Brachytherapy source calibration. Apparatus: • Ion chambers – Farmer and Well type, • Electrometer • Thermometer, Barometer • HDR unit Introduction: According to AAPM TG-56, the well-type chamber is required for the source strength measurement of radioactive afterloading sources. The recommended calibration factor is the air kerma strength (cGy m²/h). The PTW calibration certificate also includes factors for apparent activity (GBq or Ci) and exposure strength (R m² /h). Suitable applicator adapters and calibrations are available for the commercial standard afterloading systems. For measurements, the chamber is connected to a sensitive PTW electrometer (UNIDOS, UNIDOS E, MULTIDOS, TANDEM). The well-type chamber is suitable for the calibration of high dose rate (HDR) and pulsed dose rate (PDR) sources such as Ir-192. Experiment: Exercise I – Source activity verification with Well chamber: • Place the chamber on a horizontal surface near the brachytherapy unit. • Connect the 3.2mm Tube Catheter to channel 1 of the indexer and place in insert in chamber. • Switch the dosimeter on, a stabilisation period of at least fifteen minutes must be allowed for. • Set Bias to +400V. • Switch the dosimeter into electrical units and set it to the pA range • Current (A) = Charge (C) per second. • Stepsize = 1cm; Channel No# = 1; Dwell Position 1; Origin = 48mm • Nominal Time (s) = 10/(factor/100); to give 10 seconds of real time • Run the plan three times to get readings Exercise II – Source activity verification with Farmer in full scatter phantom: The Afterloading Calibration Phantom is an acrylic cylinder with a diameter of 20 cm and a height of 12 cm. It is a practical tool for afterloading source strength measurement in a solid state phantom. In addition, it is used for calibration of semiconductor probes for afterloading dosimetry. For both calibration purposes, the radioactive afterloading source is positioned into the afterloading applicator in the center hole of the phantom by remote control after the reference chamber has been placed into one of the peripheral holes. On a circle with a radius of 8 cm, there are four holes situated 2 cm from the rim of the phantom for detector positioning at 0°, 90°, 180° and 270° by using appropriate adapters. • Place the chamber in perspex insert and at position 1 • Connect the SGT and 3mm Steel Applicator to channel 1 of the indexer and place in insert at position 0 (ie the centre of the phantom) • Switch the dosimeter on make measurements in electrical units • Perform the measurement in nC and set the intergration interval on the dosimeter to 60 seconds • Stepsize = 1cm; Channel No# = 1; Dwell Position 1; Origin = 0mm • Nominal Dwell Time large enough so that 3 x 60 seconds actual time measurements can be performed • Run the plan 4 times with the chamber in the 4 different positions to get all readings