[External exposure of hot cell operators in nuclear medicine].

PURPOSE

The coming into effect of decrees No. 626/94, 242/96 and 230/95 has once again brought out the problem of the radiation exposure of hot cell operators in nuclear medicine.

MATERIAL AND METHODS

With regard to the activity of the Division of Nuclear Medicine of the Istituto Nazionale Tumori in Milan, a map has been produced of the radiation fields in the hot cell in- and outside the working station by measuring the rate of exposure and evaluating the radiation energy using film dosimeters in multifilter containers. The individual doses were measured with film dosimeters for the sternum, the back of the hand and the wrist, and with thermoluminescent dosimeters for the fingers and forehead. The thermoluminescent ring was worn with the detector towards the palm and towards the back of the hand in order to identify the side that was exposed most; the film dosimeter on the sternum was worn both underneath and above the lead apron to reveal a possible reduction in overall exposure due to attenuation of the lower-energy components.

RESULTS

The diffuse radiation field in the hot cell during the usual working activity amounts to 1 microSv/h. Assessment of the energy of the radiation fields within the working station revealed a higher energy (90 to 140 keV) in the source storage area than in the area where the syringes are prepared, the latter being affected by diffuse radiation with components of approximately 35, 90 and 110 keV. The hand of the operator is unevenly exposed to the diffuse radiation field and the fingers are more exposed than the back of the hand and the wrist: when the thermoluminescent ring was worn with the detector towards the palm of the hand the measured values did not show a higher exposure than when it was worn with the detector towards the back of the hand. The equivalent of the overall dose measured underneath the lead apron did not show any relevant reduction of the exposure due to attenuation of the lower-energy component (approx. 35 keV). We report the dosimetric findings regarding the total and partial exposure of four different operators during their regular weekly shift, with the dosimeters for the sternum, ring finger and forehead being replaced daily. The average equivalent of the dose to the hand for the manipulation of 37 GBq of 99mTc, measured with a thermoluminescent ring on the proximal phalanx of the ring finger, ranged 3.9 mSv to 2.0 mSv. Except in one case, the sternum and forehead proved to be well protected by the shielding of the working station.

DISCUSSION

The operator who stay in the hot cell for 5 hours/week accumulates, due to the presence of diffuse radiation, 5 microSv/week to the whole body; when his/her hands are inside the working station for 2.5 hours/week in the most unfavorable conditions as regards the presence of radioactive sources, they will be exposed to 1250 microSv/week, independently of dose preparation. The exposure of hot cell personnel is highly dependent on the ergonomics of the operator (build, height, arm spread, hand size, etc.) with respect to the position of the apertures and the inspection windows of the working station; as a consequence, the three dosimetric values (exposure of the sternum, the hands and the forehead) cannot be correlated. The fingers are the most exposed part of the hand, which confirms the appropriateness of our choice of the thermoluminescent ring to measure the partial exposure of the hands. Our results have been compared with those reported in the literature and with statistical data relative to three years of regular activity (1994-1996), during which the hot cell operators were monitored according to the same parameters; female operators proved to be more exposed than males, with average yearly equivalents of the total dose of 2764 microSv and 860 microSv, respectively, and average yearly equivalents of the partial dose to the hands of 32,288 microSv and 9460 microSv, respectively. The average total partial dose equivalent rati