thyroid incidentalomas are newly detected thyroid nodules , discovered during a computed tomography ( ct ) , magnetic resonance imaging ( mri ) , ultrasound ( us ) , or positron emission tomography ( pet ) exam for nonthyroid diseases . focal type is defined as an area of uptake of 2-fluorodeoxyglucose ( fdg ) in less than one lobe , whereas diffuse type is fdg uptake in the entire thyroid gland . there are guidelines for managing palpable thyroid nodules but there are no such guidelines for nonpalpable nodules . fdg - pet has a reported sensitivity of 7590% and a specificity of 90% for detecting thyroid malignancies . whole body fdg - pet / ct combines the technology of whole body ct scan with fdg uptake localization to assess glucose utilization rates . the vast majority of solid tumors have an enhanced glycolytic rate , and they are therefore amenable of being imaged with fdg - pet . fdg - pet has become the standard of practice for staging , restaging , and assessment of therapy response in a variety of malignant solid tumors . normal thyroid gland shows very low fdg uptake , and some data suggest that a moderate diffuse uptake can represent a normal variant . however , fdg has high accumulation in tumors because of increased glucose transport and glycolysis . benign diseases of the thyroid such as graves ' disease and thyroiditis can have high standard uptake values ( suv ) because of increased blood flow , increased glucose metabolism , autoimmune response by lymphocytes , and inflammation . therefore , areas with high fdg uptake can be nonspecific and can range from benign processes to malignant neoplasms . thyroid nodules have different morphologic appearances , and they can be seen as diffuse or focused lesions on the pet / ct . us is usually the first step in the management of thyroid abnormalities found in physical and laboratory exams . the overall incidence of thyroid incidentalomas is on the rise with 2.4 times increase in the last decade . based on the pet / ct alone , definite diagnosis can not be made since there are no established criteria to diagnose benign or malignant incidentalomas . current practice is to biopsy all nodules > 1  cm and all nodules < 1  cm with risk factors for malignancy . this study also aims to assess the value of pet / ct as a screening tool to screen for thyroid incidentalomas . there is no report on the prevalence of pet / ct thyroid incidentalomas in the general population . yet , thyroid incidentalomas are quite common , and their prevalence depends on the population studied , region or country where the study was conducted , and the diagnostic methods used . we performed a retrospective analysis of all whole body fdg - pet / ct done at our institution between 1/1/2000 and 8/20/2008 ( figure 1 ) . an inclusion criteria was that the lesions were not visible or palpable during routine clinical exam . we excluded from our study all cases with prior histories of thyroid neoplasms and thyroidectomies or any type of thyroid surgery . a total of 156 patients who had abnormal fdg uptake were further selected for secondary analysis . from this group , univariate one - way analysis of variance ( anova ) was performed to characterize differences between segments classified according to the results of diagnostic methods . a p value of < 0.05 was considered statistically significant . in this study , only 40 out of 156 incidentalomas ( 25.6% ) had biopsies . there are lesions with low , hyperdense , and a mixture of low and high attenuations . our results show no correlation between the size of the lesion and the suv value . there are papers that show the correlation between size and suv value while some show no correlation . in the group of cancerous incidentalomas , there are 6 males ( mean age 53.3 , age range 3378 ) and 9 females ( mean age 56.2 , age range 3873 ) . for the group with benign incidentalomas , there are 4 males ( mean age 76.5 , age range 6983 ) and 21 females ( mean age 59.1 , age range 3986 ) . 14 out of 15 ( 93% ) malignant lesions have focal uptake , and the remaining 1 has diffuse uptake . this finding agrees with prior studies in that focal uptake lesions have > 30% increase in malignancy rates compared to diffuse uptake lesions . among cancerous nodules , the highest suv value is 9.9 and the lowest is 2.5 ( figure 2 ) , and for the benign lesions , the highest is 46 and the lowest is 0.8 . from the 40 biopsy reports , there are 15 cancerous nodules , corresponding to about 38% . there are 11 papillary carcinomas ( 6 right lobe , 3 left lobe , 2 both lobes ) , 1 follicular carcinoma ( left lobe ) , 1 anaplastic carcinoma ( right lobe ) , 1 adenocarcinoma from the breast primary ( left lobe ) , and 1 metastasis of unknown origin ( right lobe ) . among benign incidentalomas , there are 2 nonspecific benign lesions ( 1 left lobe and 1 right lobe ) , 18 colloid lesions ( 10 left lobe and 8 right lobe ) , 1 chronic lymphocytic thyroiditis ( right lobe ) , 3 follicular adenomas ( 2 left lobe and 1 right lobe ) , and 1 hurthle cell lesion ( right lobe ) . generally , incidentalomas found in males at young age carry a risk of being malignant . we also find that incidentalomas are far more common in females than males , and this is in agreement with other studies . framingham population - based study reported the thyroid nodule prevalence of 6.4% for females versus 1.6% for males . the average size of nodules in the malignant group ( n = 12 ) is 2.0  cm ( range 0.66  cm ) , and that in the benign group ( n = 20 ) is 1.4  cm ( range 0.22.4  cm ) . the graphical representation of sizes in figure 3 shows that the majority of sizes of benign and malignant incidentalomas are in the same range ( i.e. , below 3  cm ) with only 2 of the lesions in the malignant group ( 5  cm and 6  cm , resp . ) outside this range . we can not safely conclude that small nodules are benign and large nodules are cancerous , and there is no agreement yet on the cutoff value of the size of the lesion to warrant further workup . adequate samples are difficult to obtain from lesions < 8  mm . on the other hand , studies have shown that cancer prevalence for lesions < 1 and > 1  cm is the same , and so we must take the other risk factors into consideration besides the size . moreover , papini et al . who examined us features of thyroid nodules also found no correlation between malignancy and the dimensions of the lesion or the multinodularity . attenuation is a feature of ct , and low attenuation means that a particular area is less intense than the surrounding . all of the malignant nodules confirmed by biopsy have low attenuation , with the exception of two which have a mixture of high and low attenuation . for this study , some studies associate low attenuation with malignancy while some suggest high attenuation for malignancy [ 4 , 6 ] . diffuse fdg uptake is often due to hypothyroidism or thyroiditis , especially hashimoto 's or autoimmune thyroiditis . one study reported higher likelihood of cancer when the average suv is greater than 5.69   but most other studies can not find a correlation between suv and malignancy [ 9 , 19 ] . in our study , the average suv value of the malignant group ( n = 15 ) is 5.5 , and that of the benign group ( n = 25 ) is 7.2 . as a result , considering these information , we can not set a threshold suv value for malignant lesions . our prevalence of 38% falls within the range of 1850% reported in the literature for prevalence of malignancy among incidentalomas . in our study , most of the malignant incidentalomas are primary thyroid malignancies , and this is similar to other studies where most cancerous incidentalomas are thyroid primary . papillary carcinomas are most common , and the majority of them occur as right lobe lesions . prevalence of incidentalomas found by pet / ct at our institution is 1.84% , which is within the range of 1.24.3% reported by liu . the results of our study are compared to studies done on incidentalomas at other institutions ( table 3).the investigations at various hospitals mentioned in table 3 reflect different study designs , patient populations , and institutional practices . the prevalence of incidentalomas range from 1.1% to 7% and the prevalence of malignancy among incidentalomas ranges from 14% to 66% . a new thyroid nodule appears at the rate of 0.08% per year in the general population , and the incidence of thyroid malignancy is 0.0040.1% per year . not surprisingly , the incidence and prevalence of thyroid cancer in the patient population we see at the nuclear medicine department at ucsf is much higher than the general population . if we perform pet / ct scans to screen for incidentalomas in the general population , it will increase the health care costs with little benefit to the patients . the cost of pet / ct far outweighs the reduced mortality associated with early diagnosis of thyroid cancers . a paper by ohba et al . , who prospectively followed patients for 3 years , mentioned that repeated fdg - pet to follow up patients with thyroid nodules is ineffective . diffuse - type incidentalomas with the absence of risk factors can be managed by physician visits , lab tests , and monitoring with us . yet , we can not rule out malignancy based on the diffuse pattern alone because the diffuse uptake can mask the focal lesions . one of the limitations of our study is that out of 156 abnormal pet / ct thyroid exams , only 40 had biopsy reports . the prevalence of malignancy among incidentalomas may be different from the current one if all underwent biopsy . we must be aware that pet / ct can not detect small lesions such as micropapillary thyroid carcinomas which can have normal or low suv uptake . our data suggest that focal fdg uptake representing incidentalomas should be followed by pathologic diagnosis , especially in those with chronic conditions or known diagnosis of a solid tumor . this study did not support the idea for using pet / ct to screen the general population for incidentalomas .
objectives . to determine the prevalence of incidentalomas in a patient population with no known thyroid malignancy who underwent whole body fdg - pet / ct for staging or restaging of neoplasia . 
 the additional aim of the study was to evaluate the feasibility of using petct as a screening tool for malignant thyroid incidentalomas . 
 methods . 
 retrospective review of medical records of all the thyroid exams done at our institution between january 1 , 2000 and august 20 , 2008 . 
 we made a criterion of pet / ct as the primary method of detection of incidentalomas . 
 results . from a total of 8464 thyroid exams , 
 156 incidentalomas were found and 40 incidentalomas underwent anatomopathology analysis , which was used as gold standard . 
 chi - square analysis was used to analyze the data . 
 there is no significant association between suv value and the prevalence of incidentalomas . discussion . from january 1 , 2000 to august 20 , 2008 
 , incidentalomas have a prevalence of 1.84% at our institution . 
 38% of the incidentalomas that were biopsied were characterized as representing malignant tumors . conclusion . 
 focal , abnormal fdg uptake representing incidentalomas must be followed up with biopsies . 
 it is impractical to use pet / ct as a screening tool to detect incidentalomas for the general population but it must be done in patients with history of any type of cancer .