Exposure of the American People to Iodine-131 from Nevada Nuclear-Bomb Tests Review of the National Cancer Institute Report and Public Health Implications (1999) / Chapter Skim
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Appendix F. Screening for Thyroid Cancer: Background Paper
Appendix F. Screening for Thyroid Cancer: Background Paper
Pages 221-263
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From page 221... ...
The NCI findings raised concerns that individuals exposed to higher dosages of I-131 might have a high risk of developing thyroid cancer later in life and that early detection and intervention for thyroid cancer could be useful for those per sons. To justify screening, there must be evidence that screening tests can detect thyroid cancer accurately and safely in asymptomatic individuals, that early treatment decreases mortality or morbidity from thyroid cancer when compared with delaying treatment until symptoms occur, and that the benefits of screening clearly outweigh the adverse effects of the screening program itself.
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From page 222... ...
We used population-based studies as a benchmark against which the yield and benefits of clinic-based screening programs could be measured. Finally, studies of monitoring in high-risk groups describe efforts to monitor individuals with occupational exposure to radiation or with a history of head and neck irradiation for benign or malignant conditions.
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From page 223... ...
Either a physical examination or an ultrasound examination can be used to screen for thyroid cancer. Both tests identify thyroid nodules that could be cancerous.
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From page 224... ...
Many studies report the number of cases of cancer detected by screening, but few report observed health outcomes in the screened population, and none has made an unbiased comparison of the outcomes of screening with the outcomes of usual care. As a result, the reduction in morbidity and mortality associated with screening can only be inferred from indirect evidence.
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From page 225... ...
Studies of screening do provide evidence about the number of cancers found in a population (arc 3) and, to a lesser extent, the stages of these cancers at the time of detection (arc 4)
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From page 226... ...
Arcs 10 and 12 are dashed because most studies report results for usual care rather than screening programs, and so they deal with populations that might not be similar to those included in screening programs. Lack of information about current practice also makes it difficult to estimate the effectiveness of not screening; that is, of usual care.
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From page 227... ...
For example, in practice, serial ultrasound examinations, trials of levothyroxine, and scintigraphy are commonly used to evaluate or monitor patients with thyroid nodules, but we assumed that these interventions would not be part of a screening protocol and so did not attempt to assess the benefit or harm they produce. In choosing between palpation and ultrasound as the first test for screening, an important issue is whether it is worthwhile to pursue small (<1 cm or <1.5 cm)
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From page 228... ...
Thus, the foremost question in choosing between ultrasound and palpation is this: In patients with low levels of radiation exposure due to exposure to fallout from the Nevada experiments, do the potential benefits of detecting and treating occult thyroid cancers outweigh the potential harm posed by additional monitoring, surgery, and medical treatments that will result from screening? Because ambiguity about these decisions could reduce the benefits of screening, a screening recommendation about thyroid cancer should carefully specify what level of exposure makes a patient "high risk" with respect to the aggressiveness of follow-up of small nodules, what size nodules should prompt a work-up, and when aggressive surgical therapy should be applied.
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From page 229... ...
ing for congenital or familial thyroid disorders were also In all, 56 studies were included in our review: Twenty-six addressed either screening in an exposed or unexposed population or surveillance in an exposed population; 23 addressed the accuracy of palpation, ultrasound, or FNA; and 10 addressed the incidence, natural history, treated history, or predictors of mortality in patients with thyroid nodules or thyroid cancer. (The numbers do not add to 56 because of overlap among paper topics.)
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From page 230... ...
How might reducing the delay in diagnosis affect the potential complications of thyroid cancer? From each study of screening or surveillance we extracted the following information: the setting in which screening or surveillance was performed, the risk status of the target population, the screening tests used, and the prevalence of thyroid nodules and of thyroid cancer found by screening.
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From page 231... ...
SCREENING TESTS History and Physical Examination Palpation A thorough clinical evaluation for thyroid disease usually includes ascertainment of age, gender, family history of thyroid disease, history of hormonal problems or prior neck irradiation, and visual inspection and palpation of the thyroid gland by a clinician (Ashcraft and Van Herle, 1981; Brander et al., 1992~. The goal of palpation is to detect nodules and to assess the size of the thyroid gland.
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From page 232... ...
, it is usually not possible to tell when nodules are benign or malignant by palpation alone (Ashcraft and Van Herle, 1981~. Three studies of screening in asymptomatic individuals used palpation and ultrasound to detect thyroid nodules (Table Fib.
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From page 234... ...
An ultrasound test usually reveals the following details about thyroid nodules: echogenicity (reduced, normal, increased) , "geneity" (homo, hetero)
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From page 235... ...
The use of FNA has greatly reduced the need for surgical excision to diagnose the cause of thyroid nodules (Fraker et al., 1997~. Fine-needle aspiration is usually an outpatient procedure.
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From page 236... ...
The probability of nondiagnostic FNA results also is related to the size of the nodule being aspirated. One study found that 27% of the nodules smaller than 1.5 cm were nondiagnostic (Khafagi et al., 1988~.
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From page 237... ...
Longitudinal studies of patients who had a benign FNA and did not have surgery demonstrate that a benign FNA is a reliable finding. In four studies, patients with untreated, cytologically diagnosed benign thyroid nodules were followed for several years for evidence of thyroid cancer (Jones et al., 1990; Kuma et al., 1994;
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From page 240... ...
Notes: Prob (cancer) is probability of thyroid cancer; PV+, positive predictive value, is the fraction of patients with a positive test who have thyroid cancer; PV-, negative predictive value, is the fraction of patients with a normal test who do not have thyroid cancer.
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From page 241... ...
, and 3 were screening programs of healthy adults using casefinding (Bruneton et al., 1994; Miki et al., 1993; Woestyn et al., 1985~. Eight studies concerned individuals exposed to fallout from nuclear detonations.
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From page 242... ...
In unexposed populations, palpable thyroid nodules are more common in women, and their likelihood increases with age. In a population-based study in Whickham County, England (Tunbridge et al., 1977)
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244 Ct ED to .
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247 ~ oo ~ ~ 0 0 0 0 0 0 0 0 0 0 oo ~ ~ 0 oo 0 0 ~ 0 0 0 0 oo ~0 ~ cat 8 8 8 8 8 o 8 .
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From page 249... ...
. Irradiation for benign medical conditions has been shown to result in an increased probability of thyroid nodules.
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From page 250... ...
Unexposed Populations In the general population, 1-5% of nodules are found to be carcinomas; the probability that a nodule is cancerous appears to be higher for larger nodules detected by palpation than for smaller nodules found by ultrasound (Ishida et al., 1988; Miki et al., 1993)
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From page 251... ...
Size of Neoplasms Found Through Screening Exposed Populations Because screening is intended to find cancers in an earlier stage than would be found in the course of usual care, the size of cancers found in a screening program is important. Early detection of thyroid cancers will improve survival only if the cancers found have a better prognosis than would have occurred if they were detected in usual care.
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From page 252... ...
, all cancers found were papillary and follicular. Probabilitv of Undergoing Surgery Unexposed Populations Two studies of healthy, unexposed individuals provide information on surgery performed as a result of screening by palpation in unexposed populations.
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From page 253... ...
In the observational study, Ishida and co-workers (1988) compared sizes of thyroid cancers found through screening with those found in the course of usual care.
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From page 254... ...
In any case, this finding argues (as the authors conclude) for more aggressive initial management of large nodules, once they are diagnosed, but it does not necessarily mean that diagnosing these nodules earlier than was done with usual care would substantially improve outcomes.
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From page 255... ...
reports a lower recurrence rate in patients diagnosed after a screening program was begun compared with those diagnosed through usual care reflecting factors such as smaller tumor size in cancers discovered through screening. Factors associated with risk of recurrence, such as age at diagnosis (Krausz et al.,1993; Mazzaferri and Jhiang, 1994; Schindler et al., 1991; Schneider et al., 1986; Simpson, 1987; Tubiana et al., 1985)
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From page 256... ...
Tumor size and spread at initial diagnosis do not appear to be associated with mortality from recurrence. Even though recurrent thyroid cancers are usually highly curable, Mazzaferri (1987)
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From page 257... ...
Complication (recurrent laryngeal nerve injury) 0.0005 Complication (hypocalcemia)
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From page 258... ...
The improvement in survival could be estimated if we knew the distribution of tumor sizes in the usualcare population and in a screened population; the average duration of delay in diagnosis in usual care, relative to screening; the rate at which tumors in one size category progress to a larger size category; and the relationship between the size of the cancer and the likelihood that the cancer is localized. Because data about these variables are lacking studies of screening have not demonstrated to what degree the cancers found by screening have a better prognosis we could not make a reliable estimate of the benefits of screening.
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From page 259... ...
Hypothetical improvement in survival in relation to risk categories if screening increased proportion of cancers with no metasteses from 60% to 70% compared to usual care (adult females only) Usual Care ~ .
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From page 260... ...
1991. Fine needle aspiration biopsy in the management of thyroid nodules.
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From page 261... ...
1990. Comparison of fine needle aspiration cytology, radioisotopic and ultrasound scanning in the management of thyroid nodules.
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From page 262... ...
1990. Impact of fine needle aspiration cytology on the management of solitary thyroid nodules.
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From page 263... ...
1994. Thyroid nodules: clinical effect of ultrasound-guided fine-needle aspiration biopsy.
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