Radiation protection for an intra-operative X-ray device

Abstract

Objectives: Therapeutic partial breast irradiation can be delivered intra-operatively using the Intrabeam 50 kVp compact X-ray device. Spherical applicators are added to the source to give an isotropic radiation dose. The low energy of this unit leads to rapid attenuation with distance, but dose rates are much greater than for diagnostic procedures.

Methods: To investigate the shielding requirements for this unit, attenuation measurements were carried out with manufacturer-provided tungsten-rubber sheets, lead, plasterboard and bricks. A prospective environmental dose rate survey was also conducted in the designated theatre.

Results: As a result of isotropic geometry, the scattered dose around shielding can be 1% of primary and thus often dominates measured dose rates compared with transmission. The absorbed dose rate of the unshielded source at 1 m was 11.6 mGy h(-1) but this was reduced by 95% with the shielding sheets. Measured values for the common shielding materials were similar to reference data for the attenuation of a 50 kVp diagnostic X-ray beam. Two lead screens were constructed to shield operators remaining in the theatre and an air vent into a service corridor. A lead apron would also provide suitable attenuation, although a screen allows greater flexibility for treatment operators. With these measures, staff doses were reduced to negligible quantities. Survey measurements taken during patient treatments confirmed no additional measures were required, but the theatre should be a controlled area and access restricted.

Conclusion: Results from this study and reference data can be used for planning other facilities.

Figure 1

Typical intra-operative breast treatment set-up, with shielding in place.

Figure 2

Diagram of one set-up for attenuation measurements, with both source and detector shielded by lead collimation.

Figure 3

Diagram of theatre layout. Numerals refer to measurement locations in Tables 1 and 2. Areas with solid shading are designated controlled and those with hatching are designated supervised. Solid lines within the theatre room show the approximate positions of the two screens.