SU-E-T-210: Surviving a Visit by the Radiological Physics Center.

Area of Science:

• Medical Physics
• Radiological Sciences
• Radiation Therapy Quality Assurance

Background:

• Discrepancies in dosimetry measurements can arise from various sources, including instrument calibration and procedural errors.
• The Radiological Physics Center (RPC) provides external quality assessment for radiation therapy centers, reporting potential errors exceeding 10%.

Purpose of the Study:

• To establish an objective method for evaluating the validity and clinical significance of negative reports from the RPC.
• To investigate the sources of discrepancies in Cs-137 source activity measurements.

Main Methods:

• Compared measurements from an institution's well counter against RPC data for Cs-137 sources (3M and Amersham).
• Utilized well counters calibrated by different Accredited Dosimetry Calibration Laboratories (ADCLs) and a Cs-137 check source.
• Performed measurements during two separate visits, 51 days apart, to assess consistency.

Main Results:

• An initial visit revealed discrepancies in 9 out of 25 sources (8-11%), attributed to factors like incorrect straw positioning and ADCL calibration variations (2%).
• An unexplained 2.3% error in RPC data was identified post-visit.
• Using a 2004 check source calibration factor reconciled all sources to within 0.5% of their 2002 decayed values.

Conclusions:

• Simple constancy check systems are vital components of a robust quality assurance program in radiation physics.
• Acknowledging measurement uncertainties and inter-laboratory calibration differences is essential.
• Constancy checks provide a means to resolve measurement discrepancies and validate dosimetry program integrity.