A semiempirical method for the description of off-center ratios at depth from linear accelerators

Comparison of measured and calculated skin doses in CT-guided interventional procedures

OBJECTIVE

The objective of our study was to compare theoretic estimations of the dose to the patient's skin during CT-guided interventional procedures with measurements performed using radiation therapy verification films.

MATERIALS AND METHODS

In each of the 12 interventions studied, a Kodak EDR2 film was positioned under the patient's anatomic area of concern. After processing, each film was scanned with a medical-grade scanner to produce a digital image from which the gray-scale profiles were obtained using the appropriate software. From these data and respective data from a series of calibration films, the entrance skin dose (ESD) profiles were derived. These ESD profiles were compared with the ESD profiles produced using a theoretic model and its revised version, which utilizes the DICOM data of each slice (i.e., tube potential, tube loading, slice thickness, slice location, pitch, and table height) and air-kerma output measurements from the CT tube.

RESULTS

In general, the ESD profiles calculated using the revised theoretic method were in better agreement with the profiles derived from the verification films than the ESD profiles derived from the original theoretic method. The deviations from the peak skin doses (PSDs) derived from the digital film images were within -3% and 27% of the PSDs derived from the verification films. The respective deviations of the ESD profiles calculated with the original theoretic method were quite larger.

CONCLUSION

The theoretic model provides a useful tool for estimating skin doses during CT-guided interventions with a reasonable level of accuracy. With further refinement and a little automation this method could be implemented for daily use.