National reference levels of CT procedures dedicated for treatment planning in radiation oncology

To present results of the first national survey on reference levels of CT imaging performed for the treatment planning purposes in radiation oncology in Croatia.

Data for CT protocols of five anatomical regions including head, head and neck, pelvis, breast, and thorax were collected at eight radiation oncology departments in Croatia. Data included volume CT dose index (CTDI_vol), dose-length product (DLP), scan length and set of acquisition and reconstruction parameters. Data on a total of 600 patients were collected. Median values of scan length, DLP and CTDI_vol were calculated for each acquisition protocol. Third quartiles of the median CTDI_vol and DLP values were proposed as the national radiotherapy planning reference levels (RPRL).

The largest CoV were assessed for RT Breast (63.8% for CTDI_vol), RT Thorax (79.7% for DLP) and RT H&N (21.2% for scan length). RT Head had the lowest CoV for CTDI_vol (1,9%) and DLP (17,2%), while RT Breast had the lowest coefficient of variation for scan length (12.8%). Proposed national RPRLs are: for RT Head CTDI_vol16cm = 62 mGy and DLP_16cm = 1738 mGy.cm; for RT H&N CTDI_vol16cm = 35 mGy and DLP_16cm = 1444 mGy.cm; for RT Breast CTDI_vol32cm = 16 mGy and DLP_32cm = 731 mGy.cm; for RT Thorax CTDI_vol32cm = 17 mGy and DLP_32cm = 865 mGy.cm; for RT Pelvis CTDI_vol32cm = 20 mGy and DLP_32cm = 1133 mGy.cm.

Results of this study show variations in CT imaging for treatment planning practice at the national level which call for optimization of procedures.