The roles of PET and CT/PET as preoperative studies

Introduction

Positron emission tomography (PET) with ¹⁸F-fluorodeoxyglucose (FDG) is a functional imaging technique that has demonstrated advantages over anatomically based imaging modalities in oncology in the detection of malignant lesions. The recent introduction of combined computed tomography (CT)/PET systems allows the coregistration of functional PET and anatomical CT images, which will very likely improve accuracy. The role of PET and CT/PET in breast cancer is reviewed.

Methods

In breast cancer, FDG-PET has been used for detection, staging, and response monitoring; one of its main clinical applications is defining the extent of recurrent or metastatic disease [1]. We have reviewed the evidence regarding the roles of PET and CT/PET as preoperative studies in breast cancer.

Results

The review of the evidence shows that PET and CT/PET can contribute to patient diagnosis and management as preoperative studies in breast cancer in the following situations:

• 1. Detection of primary breast cancer. When evaluating suspicious breast abnormalities, FDG-PET has a sensitivity of 80–100% and specificity of 75–100%. However, its role in primary tumor detection is not clear when compared with conventional imaging methods and remains to be determined [1].

• 2. Prognostic value of FDG uptake in primary tumor. Most studies suggest that the level of FDG uptake in primary breast tumors carries clinical and biological information, and that a higher FDG uptake is correlated with more clinically aggressive tumors [1]. This information may help to stratify patients according to prognosis and risk for recurrence, and may help to tailor treatments to the individual patient.

• 3. Axillary node staging. In this situation, FDG-PET had a sensitivity of 57–100% and specificity of 66–100%. FDG-PET underestimates the number of tumor-involved nodes compared with pathologic evaluation from conventional dissection. Therefore, FDG-PET should not replace axillary node sampling for routine staging of the axilla because even microscopic nodal involvement may be important for prognosis and treatment planning [1]. However, FDG-PET may be complementary to sentinel lymph node mapping and other standard axillary procedures in patients with more advanced tumors or equivocally palpable axillary nodes.

• 4. Detection of locoregional and distant metastases. Functional imaging with FDG-PET is more accurate than CT for the detection of nodal involvement in the mediastinum; the sensitivity of FDG-PET was significantly higher (85%) than CT (50%), with nearly the same specificity (90% for FDG-PET versus 83% for CT). Regarding the detection of distant metastases, FDG-PET can accurately detect sites of distant disease with a sensitivity of 80–97% and specificity of 75–94% [1].

• 5. Evaluation of therapy response. In locally advanced breast cancer (LABC), the assessment of response to neoadjuvant chemotherapy with conventional imaging methods is often inaccurate or slow because it depends on morphological criteria. Initial studies have shown the utility of FDG-PET in the evaluation of treatment response, specifically in its ability to discriminate responders from nonresponders more accurately and earlier than conventional imaging methods [1]. Changes in FDG uptake after a single course of chemotherapy can predict pathological response in primary LABC tumors [2, 3]. Histopathological response could be predicted with an accuracy of 88–91% after the first and second course of chemotherapy [3]. Other PET tracers may be used in the evaluation of the primary tumor; preliminary results suggest that applying PET in this way may help to identify physiologic manifestations of drug resistance, which would help to tailor systemic therapy [1]. Preliminary data also suggest that FDG-PET may be useful in the assessment of sites of disease other than the primary tumor for monitoring response to chemotherapy in advanced breast cancer. Initial studies suggest other possible applications of FDG-PET, such as evaluation of the response of skeletal metastases to therapy, and prediction of the response to antiestrogen therapy in patients with advanced estrogen receptor positive breast cancer [1]. Regarding CT/PET, to date only few studies have been reported, but the advantages of CT/PET compared with PET alone may be taken to indicate that CT/PET may improve the accuracy in the evaluation of treatment response by directly defining metabolic and morphological changes [4].

• 6. Future applications. FDG is the most important radiotracer for PET in breast cancer and therefore it is analyzed in most studies. However, in the near future more specific PET radiopharmaceuticals may help to guide treatment, individualizing therapies to a particular patient depending on the tumor's biologic characteristics [1]. PET may help in management decisions by quantifying the therapeutic target, identifying resistance factors, and measuring early response to therapy.

Conclusion

The clinical application of PET and CT/PET in breast cancer will help to predict clinical behavior, and allow one to choose the appropriate treatment and to tailor local treatment options to the individual patient. PET and CT/PET are also likely to play key roles in monitoring systemic therapy and evaluating the response to therapy at an earlier stage than conventional methods. In the future, PET may be applied with other tracers in addition to FDG, to improve characterization of tumor biology and more effectively measure response to therapy.