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FDG PET/CT in carcinoma of unknown primary.

Kwee TC, Basu S, Cheng G, Alavi A - Eur. J. Nucl. Med. Mol. Imaging (2009)

Bottom Line: Carcinoma of unknown primary (CUP) is a heterogeneous group of metastatic malignancies in which a primary tumor could not be detected despite thorough diagnostic evaluation.Because of its high sensitivity for the detection of lesions, combined (18)F-fluoro-2-deoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) may be an excellent alternative to CT alone and conventional magnetic resonance imaging in detecting the unknown primary tumor.This article will review the use, diagnostic performance, and utility of FDG PET/CT in CUP and will discuss challenges and future considerations in the diagnostic management of CUP.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands. thomaskwee@gmail.com

ABSTRACT
Carcinoma of unknown primary (CUP) is a heterogeneous group of metastatic malignancies in which a primary tumor could not be detected despite thorough diagnostic evaluation. Because of its high sensitivity for the detection of lesions, combined (18)F-fluoro-2-deoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) may be an excellent alternative to CT alone and conventional magnetic resonance imaging in detecting the unknown primary tumor. This article will review the use, diagnostic performance, and utility of FDG PET/CT in CUP and will discuss challenges and future considerations in the diagnostic management of CUP.

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Bar graph showing percentages of primary tumors detected by FDG PET/CT per location [12]
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Fig3: Bar graph showing percentages of primary tumors detected by FDG PET/CT per location [12]

Mentions: Several meta-analyses have assessed the diagnostic performance of stand-alone FDG PET in patients with CUP [30–32], reporting primary tumor detection rates of 41% [30], 24.5% [31], and 43% [32], with a generally higher sensitivity than specificity using histopathological analysis of tissue obtained by biopsy or surgery and/or other imaging procedures or clinical follow-up as the standard of reference [30–32]. However, as mentioned previously, combined PET/CT systems have many advantages over the stand-alone PET scanner and are rapidly replacing the latter. One recent meta-analysis investigated the diagnostic performance of FDG PET/CT and reported a primary tumor detection rate of 37% [12]. Diagnostic performance of FDG PET/CT was not significantly different between patients presenting with cervical metastases and patients presenting with extracervical metastases, while lung, oropharyngeal, and pancreatic cancer were reported to represent the most frequently detected primary tumors (Fig. 3) [12]. Lung and pancreatic cancer are indeed also the most common primary tumor locations in autopsy studies [16–19]. The rather unexpected high rate of primary oropharyngeal cancers in the aforementioned meta-analysis [12] can be explained by the fact that it included a large proportion of patients with cervical metastases, whose primary tumors are most frequently located in the oropharynx [33]. On the other hand, breast cancer appears to be the most common cause of a false-negative FDG PET/CT result (Fig. 4) [12]. This is due to the well-documented inability of FDG PET(/CT) to consistently demonstrate small (<1.0 cm) and low-grade lesions with low or no FDG uptake (e.g., tubular carcinoma and noninvasive cancers such as ductal or lobular carcinoma in situ) [34]. This limitation of FDG PET/CT should be kept in mind and in case of a high suspicion of a primary breast cancer (e.g., in case of axillary lymph node metastases) [23], a dedicated MRI scan may be considered [35, 36]. Similar to that in the breast, false-negative FDG PET/CT results in other locations are most likely attributable to small lesion size and low or no FDG uptake. In contrast, the oropharynx and the lung are the two most common locations of false-positive FDG PET/CT results [12] (Fig. 5). Literature on the exact causes of false-positive FDG PET/CT results is scarce, although benign inflammatory lesions [7, 9, 37] and pulmonary infarction [9] have been reported etiologies. Furthermore, of special note is the fact that incidental pulmonary emboli have been reported to occur in 4% of patients with cancer [38]. It can be speculated that this may, in part, explain the rate of false-positive FDG PET/CT results in the lung. Figures 1 and 2 show two examples in which FDG PET/CT was able to detect the unknown primary tumor.Fig. 1


FDG PET/CT in carcinoma of unknown primary.

Kwee TC, Basu S, Cheng G, Alavi A - Eur. J. Nucl. Med. Mol. Imaging (2009)

Bar graph showing percentages of primary tumors detected by FDG PET/CT per location [12]
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2822231&req=5

Fig3: Bar graph showing percentages of primary tumors detected by FDG PET/CT per location [12]
Mentions: Several meta-analyses have assessed the diagnostic performance of stand-alone FDG PET in patients with CUP [30–32], reporting primary tumor detection rates of 41% [30], 24.5% [31], and 43% [32], with a generally higher sensitivity than specificity using histopathological analysis of tissue obtained by biopsy or surgery and/or other imaging procedures or clinical follow-up as the standard of reference [30–32]. However, as mentioned previously, combined PET/CT systems have many advantages over the stand-alone PET scanner and are rapidly replacing the latter. One recent meta-analysis investigated the diagnostic performance of FDG PET/CT and reported a primary tumor detection rate of 37% [12]. Diagnostic performance of FDG PET/CT was not significantly different between patients presenting with cervical metastases and patients presenting with extracervical metastases, while lung, oropharyngeal, and pancreatic cancer were reported to represent the most frequently detected primary tumors (Fig. 3) [12]. Lung and pancreatic cancer are indeed also the most common primary tumor locations in autopsy studies [16–19]. The rather unexpected high rate of primary oropharyngeal cancers in the aforementioned meta-analysis [12] can be explained by the fact that it included a large proportion of patients with cervical metastases, whose primary tumors are most frequently located in the oropharynx [33]. On the other hand, breast cancer appears to be the most common cause of a false-negative FDG PET/CT result (Fig. 4) [12]. This is due to the well-documented inability of FDG PET(/CT) to consistently demonstrate small (<1.0 cm) and low-grade lesions with low or no FDG uptake (e.g., tubular carcinoma and noninvasive cancers such as ductal or lobular carcinoma in situ) [34]. This limitation of FDG PET/CT should be kept in mind and in case of a high suspicion of a primary breast cancer (e.g., in case of axillary lymph node metastases) [23], a dedicated MRI scan may be considered [35, 36]. Similar to that in the breast, false-negative FDG PET/CT results in other locations are most likely attributable to small lesion size and low or no FDG uptake. In contrast, the oropharynx and the lung are the two most common locations of false-positive FDG PET/CT results [12] (Fig. 5). Literature on the exact causes of false-positive FDG PET/CT results is scarce, although benign inflammatory lesions [7, 9, 37] and pulmonary infarction [9] have been reported etiologies. Furthermore, of special note is the fact that incidental pulmonary emboli have been reported to occur in 4% of patients with cancer [38]. It can be speculated that this may, in part, explain the rate of false-positive FDG PET/CT results in the lung. Figures 1 and 2 show two examples in which FDG PET/CT was able to detect the unknown primary tumor.Fig. 1

Bottom Line: Carcinoma of unknown primary (CUP) is a heterogeneous group of metastatic malignancies in which a primary tumor could not be detected despite thorough diagnostic evaluation.Because of its high sensitivity for the detection of lesions, combined (18)F-fluoro-2-deoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) may be an excellent alternative to CT alone and conventional magnetic resonance imaging in detecting the unknown primary tumor.This article will review the use, diagnostic performance, and utility of FDG PET/CT in CUP and will discuss challenges and future considerations in the diagnostic management of CUP.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands. thomaskwee@gmail.com

ABSTRACT
Carcinoma of unknown primary (CUP) is a heterogeneous group of metastatic malignancies in which a primary tumor could not be detected despite thorough diagnostic evaluation. Because of its high sensitivity for the detection of lesions, combined (18)F-fluoro-2-deoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) may be an excellent alternative to CT alone and conventional magnetic resonance imaging in detecting the unknown primary tumor. This article will review the use, diagnostic performance, and utility of FDG PET/CT in CUP and will discuss challenges and future considerations in the diagnostic management of CUP.

Show MeSH
Related in: MedlinePlus