IRS provides annual patient dose audits as a routine service for most of its customers. In 2013, we provided breast dose audits to 28 sites with mammography DR. Of these sites, we noticed that no systems had doses anywhere near the National Diagnostic Reference Level (N-DRL) of 3.5mGy (applicable to oblique images of compressed breast thicknesses (CBT) between 50-60mm). In fact, most of our mammography DR systems were achieving doses at less than half the N-DRL, and only one system had an average dose of 2mGy or more. Therefore, all 28 dose audit reports went out to our customers explaining their doses were below the national limits and so acceptable, and Local DRLs were advised. We decided we should do a more in depth analysis at the data. We had many questions, and wanted to know the answers…
- What are the typical doses for the different manufacturer or model types?
- Is the N-DRL useful for modern day systems? If not could it be revised down?
- Should we be setting up L-DRLs based on sites, or could we propose them by manufacturer and/or model?
- How accurate are the system-displayed doses compared to those we calculate?
- In physics testing we have remedial tolerances for a ‘new standard breast’ – a 53mm breast equivalent phantom consisting of Perspex and spacers. The remedial tolerance is 2.5mGy. However, the N-DRL is 3.5mGy for CBTs of 50±5mm, which is only slightly larger than the breast equivalent breast. So why the large discrepancy between N-DRLs and the 53mm breast phantom tolerance?
Our study set out to try answer all these questions. The first publication of this work is being presented at as a Poster at Mammographic Symposium in Bournemouth form Monday 30th June. Hopefully you can make it there!
The poster has been sent to the printers today. I am going to Glastonbury tomorrow, and then straight to the conference next Monday, bright-eyed and bushy-tailed, ready for an early morning start. If you want to catch up and discuss the Poster or any other matter whilst am there, please get in touch.