Incidental finding of synchronous malignancy on PET/CT

Issue: January 10, 2009

ByMunir Ghesani, MD, FACNM

ByRobert Chen, MD

A 62-year-old man with a 40-year history of smoking presented with shortness of breath and weight loss. Initial chest X-rays were unremarkable and the patient subsequently underwent full body CT scan in search of a primary malignancy. CT scan demonstrated a large central mass in the left lower lobe inseparable from the hilum. There was complete encasement of the left pulmonary artery and left mainstem bronchus, with postobstructive consolidation in the left lower lobe. Incidentally noted were multiple hypodense masses scattered throughout the liver, which were interpreted as metastases from a primary lung carcinoma. A biopsy of the lung revealed small cell lung cancer; a PET scan/CT study was requested to stage the full extent of disease.

As expected, PET/CT scan demonstrated abnormal uptake in the lung and liver, with maximum SUVs of 7.0 and 7.2, respectively. However, there was also an incidental note of abnormal nodular area of increased metabolic activity in the sigmoid colon, demonstrating a maximum SUV of 8.1. Corresponding anatomic CT images demonstrated the possibility of a sigmoid mass. Colonoscopy and eventual histopathology of this region revealed adenocarcinoma of the sigmoid colon. The hepatic masses were revealed to be metastases from the sigmoid adenocarcinoma, and the small cell lung cancer was another synchronous primary cancer.

Figure 1: Initial PET/CT examination demonstrates hypermetabolic activity (yellow circle) associated with primary lung carcinoma. Upper left image is axial CT scan, upper right image is corresponding PET image, lower left image is fusion image containing PET images displayed on a color scale and CT images displayed on a gray scale. Lower right image is maximum intensity project (MIP) image of whole body PET study.

Figure 2: Initial PET/CT examination demonstrates hypermetabolic activity (yellow circle) within several hepatic masses. Display convention is the same as Figure 1.

Figure 3: Initial PET/CT examination demonstrates abnormal focal hypermetabolic activity within the sigmoid colon with a maximum SUV of 8.1. Corresponding anatomic images demonstrate an eccentric soft tissue mass arising from the colonic wall. Display convention is the same as Figures 1 and 2.

Source: M. Ghesani

DISCUSSION

PET scan has been documented to incidentally detect unsuspected precancerous and cancerous lesions. Agress et al studied a group of 1,750 patients who underwent PET scanning for a variety of known or suspected malignancies. Forty-two patients had unexpected findings of increased metabolic activity that could not be explained by physiologic uptake and were in an unusual location for spread of metastases given the type of primary tumors. Of these 42 patients, 30 (71%) had malignancies that were different than the known malignancy. These incidental cancers included colonic adenomas/adenocarcinomas, breast carcinoma, laryngeal squamous cell cancer, endometrial adenocarcinoma, ovarian carcinoma, papillary thyroid cancer and fallopian tube adenocarcinoma.

Naydich and Ghesani et al performed a similar study evaluating incidental abnormal metabolic uptake on PET scans that could not be explained by physiologic uptake and were unlikely to be related to the primary tumor. In 21 patients in whom histopathologic correlation was available, 14 (66%) patients were found to have malignant/premalignant conditions, including 10 colonic adenomas/adenocarcinomas, two papillary thyroid cancers, one renal cell carcinoma, and one parotid.

PET scan has been a useful adjunct for staging cancer and monitoring therapy in oncology patients. However, several studies are demonstrating the incidental detection synchronous malignancies by PET. This is not surprising, given that the molecular marker used for PET, F-18 FDG, is an analogue of glucose that reflects increased GLUT-1 transporters on cell surface and increased glycolytic activity intracellularly, a process shared by several malignancies. The studies cited demonstrate that a significant majority of cases with abnormal nonphysiologic uptake that is unrelated to the primary tumor lead to the eventual discovery of occult cancerous and precancerous lesions. In the remainder of the patients, this abnormal localization represents increased glucose metabolism in the inflammatory/infectious process. In our case, it led to the discovery of unsuspected colonic adenocarcinoma. This stresses the vital importance of working up suspicious and nonphysiologic metabolic activity because incidentally discovered malignant and/or premalignant conditions could have a potential impact on a patient’s clinical outcome and long-term survival.

Munir Ghesani, MD, is Associate Clinical Professor of Radiology at Columbia University College of Physicians and Surgeons and Attending Radiologist at St.Luke’s-Roosevelt Medical Center.

Robert Chen, MD, is a Radiology Resident at Beth Israel Medical Center.

For more information:

Agress H, Cooper B. Detection of clinically unexpected malignant and premalignant tumors with whole-body FDG PET: histopathologic comparison. Radiology. 2004. 230;417-422.

Naydich M, Ghesani M, et al. Incidental findings on whole body F-18 FDG PET scans are frequently due to malignant of premalignant conditions. ACNP abstract.