MAXIMUM  PERMISSIBLE  DOSE  and  DOSIMETRY

MAXIMUM PERMISSIBLE DOSE (MPD)

No limited skeletal radiographer should be receiving any significant radiation if he/she consistently stands behind a well-constructed operator's booth and observes radiation safety procedures.

In other types of diagnostic imaging procedures (in hospitals and radiology labs), particularly those involving fluoroscopy or the handling of radioisotopes, the technician may be exposed to radiation. In these cases, the standard is to limit the radiation worker's maximum permissible dose (MPD) to the whole body or its most critical organs to 1.25 rem per calendar quarter.  The long-term permitted exposure is determined by allowing a total accumulative exposure of 5 rem for each year after age 18.  There has been a move over the last few years to lower this MPD even further.  No one under age 18 can be legally employed in work that exposes him/her to radiation to a greater degree than the general public would receive.

Maximum cumulative whole body dose equals 5 (N-18) [N equals age in years at last birthday].

These limits apply to exposure to the most critical organs (gonads, bone marrow, head/trunk, and lens of the eye).  For the less critical organs, higher calendar quarter exposures are allowed.

Remember, MPDs are established for radiation workers.  There are no MPDs for patients.  The reason for this is that patients are not exposed to radiation on a routine daily basis; and when they are exposed, it is for a diagnostically-important reason (benefit/risk ratio).

PERSONNEL DOSIMETRY

Dosimetry is required by state law if the x-ray operator is expected to receive 1/10 the MPD in any quarter (decreased from 1/4 in 1993).  Since the MPD is 5 rem/year, or 1250 mrem per quarter, this means that dosimetry is required if the operator is likely to receive 125 mrem per quarter.  

Because limited skeletal radiographers (who produce only plain films in a permanent site with a leaded operator’s booth) are not expected to receive any significant radiation (as explained above), personnel dosimetry is not needed in private office-based practices; however, some facilities choose to use it just to assure peace of mind of both the doctor/employer and the x-ray technicians.  Some facilities utilize a dosimetry service when the facility is first constructed, then abandon the service when the reports continue to come back showing no radiation exposure. 

Personnel dosimetry is, of course, needed in situations where the operator is not able to consistently stand behind a permanent operator’s shield, or who handles radioactive isotopes. 

Personnel dosimetry service contracts require a year of service.

The most common type of dosimeter is a film badge that makes use of the density of the developed film as a measure of its exposure.  The film badge is typically in the form of a small rectangular case which contains radiosensitive film with superimposed absorbers to permit determination of the characteristics of the radiation which strikes the film.  A laboratory periodically collects and reads such badges, replacing them with new ones, and sending to the subscriber a report of radiation accumulated during the quarter, the year, and the lifetime of the operator.  If the reading is to be a valid measure of the technician's exposure, the dosimeter must be worn at all times while the technician is working, but never when the technician is x-rayed as a patient, and not at other times.  The location of the dosimeter is also important.  The dosimeter must be worn at the front of the body, preferably at waist level, or on the collar.  Each operator must have his/her own badge.