Part 1. Good Practice vs. Poor Practice in Screening for Tuberculosis
Tuberculosis case-finding in Detroit (Chapter 15) was an eminently sensible and useful program; it used x-ray on ostensibly healthy people who had a positive tuberculin history. Expert use of fluoroscopy was also very effective in detecting the disease (see Chapter 30), but expert use of fluoroscopy was very different from screening based on the crude "photofluorograms" described below --- a massive use of radiation with apparently meagre results.
Catherine Caufield (in her 1989 book, Multiple Exposures, pp.144-145) relates some statements made to her by Dr. Francis Curry, who was deputy director and later director of public health and hospitals in San Francisco from 1960 to 1970. He is quoted as saying: "What was so horrible about what was happening then is that so many machines had no filters, no coning, no shielding. Many people were getting total body exposures and were getting doses big enough to show clinical symptoms." (That takes doses of the order of 50 rads to the whole body.)
And further (at p.145), Caufield describes the Mass Screening program: "In 1950, San Francisco, like many other cities across the country, established a mass chest x-ray programme intended to detect tuberculosis. The survey used photofluorograms --- photograms of fluoroscope images -- rather than conventional x-rays. If a photofluorogram indicated there might be a problem, the subject was then advised to have a more detailed x-ray examination. More than 40,000 people per year visited the city's clinics or a specially equipped van that toured the city, offering free chest X-rays. `One reason for the large neighborhood programmes', explained Curry, `was that the lung associations wanted this type of continuing service. It would help them with their fund-raising. Local groups would have contests to see who had the most members screened. Lots of the black churches did this. Everybody and his brother would go. The same people went over and over again. They were getting a lot of radiation and we were getting [finding] no disease at all.' "
This monstrosity of a `programme' never should have existed, since good programs had been well demonstrated in 1932. But, as Dr. Curry stated, the neighborhood programs were probably more related to fund-raising than to health improvement or tuberculosis detection.
We can consider here one estimate of the national dose which resulted from such programs with mobile x-ray vans.
Karl Z. Morgan's group at Oak Ridge National Laboratory monitored some of the x-ray devices used in the mass screening and found they were delivering skin exposures of between 2 and 3 Roentgens while the average chest dose from a chest x-ray unit at Oak Ridge was 15 millirads. We recently checked this with "KZ" Morgan personally, and he confirmed these numbers. We presume these x-rays were always taken from back to front [rays entering the person's back and exiting the person's front][ except for infants]. Karl Morgan was not able to confirm whether all the exams were taken in this manner. Recent procedure certainly has been to take the exams postero-anterior (back to front) [except for infants]. We shudder to consider the possibility that the reverse was the case in some of the exams, since this would mean an enormous increase in dose to the breasts.
In X-Rays: Health Effects of Common Exams (Gofman and O'Connor, 1985), there is a table, Special Table C (at p.404), which permits one to know what the dose in milli-rads is to specific organs when exposure is made to a 1.0 Roentgen, free-in-air source, at a specific beam quality and kilovoltage (2.3 mm Al HVL and 30 keV). The difference between back-to-front (PA) and front-to back (AP) is enormous.Specific Organ Beam AP Beam PA Ratio Breast Pair (Female) 693 37 693/37 = 18.7
The dose to the breast-pair is 18.7 times as great for the AP beam entry.
We take the lowest value of Karl Morgan, 2 Roentgens skin dose entering in the back. Our calculation shows that the breast dose is 37 milli-rads for an entrance dose of one Roentgen. So for 2 Roentgens entrance dose, we multiply 2 R x 37 milli-rads per Roentgen, to get a breast dose of 74 milli-rads (0.074 rads). Obviously those having 3 R of entrance dose get a 50% higher dose to the breasts.
The reader may wonder why the doses were so very much higher with the photo-fluorograms than with direct recording on film. In making a photofluorogram, the x-ray beam is "on" a longer time. Direct recording on film requires only a very brief exposure to x-rays, and delivers a sharper, more informative image than a picture from a screen.Part 2. An Estimate of Doses
Which May Have Been Received in Mass Screening
Assuming that 1/2 of the San Francisco population was female, we would have 350,000 female persons.
And accepting the estimate that 1/2 of the 40,000 persons per year who took such examinations were women, we have 20,000 women receiving an average dose of 0.074 rads to the breasts. Probably this is a gross underestimate, considering Dr. Curry's statements.
Total person-rads = (20,000 persons) x (0.074 rads) = 1,480 person-rads to breasts.
Now we concern ourselves with obtaining the Population-Dose, which brings in the unirradiated females of the population of San Francisco.
Population Dose = (1,480 person-rads) / (350,000 female persons)
= 0.0042 rads to breasts.
This mass screening program may have been better or worse in other cities in the United States. We shall make the approximation that the dose of 0.0042 rads applies for all ages from childhood through 64 years. What is not clear is the total duration of the mass screening, since such screening apparently started and ended at different times in different cities. We shall assume, for a conservative underestimate of dose, that the duration was for half of the 40 years from 1920 to 1960. And, therefore, we shall cut the dose down to 0.0021 rads to breasts, for a typical year. This is the dose we transfer to Col. Eye in the Master Table.
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