15. Sensitivity of FDG CDET (2D Dual-Head Coincidence Gamma Camera) for the Detection of Occult or Doubtful Recurrences of Colorectal Cancer

Abstract

Purpose: The aim of this study was to evaluate the efficacy of FDG-CDET for the detection of recurrences of colorectal cancer, in occult disease or in doubtful cases at conventional imaging (CI). In all the evaluated cases, the result of FDG-CDET was compared with post surgical histology both on patient and on site bases.

Methods and patients: After fasting for 6h or more, 150-250 MBq of ¹⁸F-FDG were injected i.v. and 2D imaging (whole-body scan and at least a tomoscintigram) was started 45 min. later, using a PICKER CDET gamma camera. Among the 214 examinations (ex) performed for detection of recurrences of colorectal cancer between Jul 1997 and Feb 2000, we only considered the 58 cases with negative or questionable CI and a post surgical histologic proof.

Results: Patients were referred in three different contexts:

• 1.

  suspicion of recurrence (SR) due to raising CEA levels with normal CI i.e. occult disease (18 ex: 13 TP, 3 FN, 2 FP on patient basis, 17 TP, 9 FN, 4 FP on site basis)

• 2.

  SR due to equivocal image(s) at follow-up CI (25 ex: 18 TP, 5 FN, 2 TN on patient basis, 21 TP, 10 FN, 2 TN on site basis)

• 3.

  SR due to both raising CEA levels and equivocal CI (15 ex: 13 TP, 2 FN on patient basis, 16 TP, 8 FN, 3 TN, 1 FP on site basis).

• 4.

  The overall sensitivity was 44/54 = 81% on patient basis and 54/81 = 67% on site basis. Specificity could not be evaluated because of the very small number of patients re-operated in case of negative FDG-CDET.

• 5.

  Conclusion: When conventional imaging could not contribute, i.e. a sensitivity of zero by definition, FDG-CDET accurately diagnosed a recurrence in 81% of the patients and located 67% of all recurrent lesions. These results confirm, with post surgical histology as the only gold standard, that FDG-CDET is a powerful tool for diagnosis of recurrent colorectal cancer in difficult cases. It could be used as the first line examination as soon as a recurrence is suspected, in order to avoid less contributive imaging procedures.