Estimation of uncertainty budget in cross calibration of small volume ionization chambers in ⁶⁰Co beam - an approach towards quality control in dosimetry

The advance radiotherapy (RT) involves high precision treatments like SRT, SRS, IMRT and VMAT which includes small fields. The dosimetry with the benchmark small volume detectors is extremely desired. Ionization Chambers (IC), supposed to be the benchmark detectors for such dosimetry need to be monitored for its valid calibration coefficient provided by Primary Standard Dosimetry Laboratory (PSDL) or Secondary Standard Dosimetry Laboratory (SSDL) traceable to PSDL. Due to ageing or post-major repairs and possible damage, response of the IC and in turn the calibration coefficient may change. Due to the limitations and logistics, PSDL may have challenges to calibrate small volume chambers for the entire country. At busy institutes, it may be challenging to keep the track of the calibration validity of the detectors and this may raise a question or there may be an ambiguity to use these detectors for absolute dosimetry for high precision RT to maintain high quality control in dosimetry. This can be overcome by cross calibrating chambers against a reference chamber traceable to PSDL. The cross calibration alone is not sufficient, hence one need to estimate the uncertainty budget as well. At our institute we have multiple ICs (with different volumes), out of which more than 10 chambers were found with calibration date expired