Initial Results of 68Ga-FAPI-46 PET/MRI to Assess Response to Neoadjuvant Chemotherapy in Breast Cancer

Estrogen Receptor-targeted PET Imaging for Breast Cancer

Two complementary patient cases are presented to highlight the importance of estrogen receptor (ER)-targeting imaging in treatment planning and selection for endocrine therapy in breast cancer patients. This article will discuss the radiopharmaceuticals and biology, imaging interpretation, and current clinical applications of ER-targeting imaging using fluorine 18 fluoroestradiol PET.

The Current and Future Roles of Precision Oncology in Advanced Breast Cancer

Breast cancer is a common but heterogeneous disease characterized by several biologic features, including tumor grade, hormone receptor status, human epidermal growth factor receptor 2 status, and gene expression assays. These biologic and genomic features drive treatment decisions. In the advanced disease setting, inter- and intrapatient tumor heterogeneity is increasingly recognized as a challenge for optimizing treatment. Recent evidence and the recent approval of novel radiopharmaceuticals have increased recognition and acceptance of the potential of molecular imaging as a biomarker to impact and guide management decisions for advanced breast cancer.

PET/MRI and Novel Targets for Breast Cancer

Breast cancer, with its global prevalence and impact on women's health, necessitates effective early detection and accurate staging for optimal patient outcomes. Traditional imaging modalities such as mammography, ultrasound, and dynamic contrast-enhanced magnetic resonance imaging (MRI) play crucial roles in local-regional assessment, while bone scintigraphy and 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) aid in evaluating distant metastasis. Despite the proven utility of 18F-FDG PET/CT in various cancers, its limitations in breast cancer, such as high false-negative rates for small and low-grade tumors, have driven exploration into novel targets for PET radiotracers, including estrogen receptor, human epidermal growth factor receptor-2, fibroblast activation protein, and hypoxia. The advent of PET/MRI, which combines metabolic PET information with high anatomical detail from MRI, has emerged as a promising tool for breast cancer diagnosis, staging, treatment response assessment, and restaging. Technical advancements including the integration of PET and MRI, considerations in patient preparation, and optimized imaging protocols contribute to the success of dedicated breast and whole-body PET/MRI. This comprehensive review offers the current technical aspects and clinical applications of PET/MRI for breast cancer. Additionally, novel targets in breast cancer for PET radiotracers beyond glucose metabolism are explored.

Innovations in Positron Emission Tomography and State of the Art in the Evaluation of Breast Cancer Treatment Response

The advent of hybrid Positron Emission Tomography/Computed Tomography (PET/CT) and PET/Magnetic Resonance Imaging (MRI) scanners resulted in an increased clinical relevance of nuclear medicine in oncology. The use of [18F]-Fluorodeoxyglucose ([18F]FDG) has also made it possible to study tumors (including breast cancer) from not only a dimensional perspective but also from a metabolic point of view. In particular, the use of [18F]FDG PET allowed early confirmation of the efficacy or failure of therapy. The purpose of this review was to assess the literature concerning the response to various therapies for different subtypes of breast cancer through PET. We start by summarizing studies that investigate the validation of PET/CT for the assessment of the response to therapy in breast cancer; then, we present studies that compare PET imaging (including PET devices dedicated to the breast) with CT and MRI, focusing on the identification of the most useful parameters obtainable from PET/CT. We also focus on novel non-FDG radiotracers, as they allow for the acquisition of information on specific aspects of the new therapies.

68Ga-Labeled Fibroblast Activation Protein Inhibitor PET/CT for the Early and Late Prediction of Pathologic Response to Neoadjuvant Chemotherapy in Breast Cancer Patients: A Prospective Study

68Ga-labeled fibroblast activation protein inhibitor (68Ga-FAPI) PET/CT has demonstrated promising clinical results, with a higher SUVmax and tumor-to-background ratio (TBR) in breast cancer (BC) patients than 18F-FDG PET/CT. Here, we aimed to evaluate the suitability of 68Ga-FAPI PET/CT for the early and late prediction of the pathologic response to neoadjuvant chemotherapy (NAC) in BC. Methods: Twenty-two consecutive patients with newly diagnosed BC and an indication for NAC were prospectively included. All patients underwent standard chemotherapy and 68Ga-FAPI PET/CT at baseline, after 2 cycles of NAC (PET2), and 1 wk before surgery (PET3). SUVmax was measured in the primary tumor region and positive regional lymph nodes. The expression of fibroblast activation protein in the primary lesion was analyzed by immunohistochemistry. Results: Seven patients (31.8%) achieved a pathologic complete response (pCR), and 15 (68.2%) had residual tumors. Thirteen patients (59.1%) showed concentric withdrawal of the primary tumor, and 9 (40.9%) showed diffuse withdrawal. Between PET2 and PET3, the ΔSUVmax of the primary tumor (R 2 = 0.822; P = 0.001) and metastatic lymph nodes (R 2 = 0.645; P = 0.002) were significantly correlated. The absolute values of SUVmax and TBR at PET2 and PET3 were lower in patients with pCR than in those without pCR (P < 0.05). Moreover, a larger ΔSUVmax at any time point was strongly associated with pCR (P < 0.05). Similar downward trends in SUVmax, TBR, and ΔSUVmax were observed in the pattern of primary tumor reduction. For predicting pCR, the optimal cutoff values for ΔSUVmax after 2 chemotherapy cycles, ΔSUVmax before surgery, TBR after 2 chemotherapy cycles, and TBR before surgery of the primary tumor were 3.4 (area under the curve [AUC], 0.890), 1.1 (AUC, 0.978), -63.8% (AUC, 0.879), -90.8% (AUC, 0.978), 7.6 (AUC, 0.848), and 1.4 (AUC, 0.971), respectively. Immunohistochemistry showed that the SUVmax and TBR of 68Ga-FAPI PET/CT were positively correlated with fibroblast activation protein expression (P < 0.001 for both). Conclusion: Assessment of early changes in 68Ga-FAPI uptake during NAC by 68Ga-FAPI PET/CT can predict pCR and primary tumor concentric withdrawal in BC patients. 68Ga-FAPI PET/CT has great potential for the early and late prediction of the pathologic response to NAC in BC.

Metabolic Imaging as a Tool to Characterize Chemoresistance and Guide Therapy in Triple-Negative Breast Cancer (TNBC)

After an initial response to chemotherapy, tumor relapse is frequent. This event is reflective of both the spatiotemporal heterogeneities of the tumor microenvironment as well as the evolutionary propensity of cancer cell populations to adapt to variable conditions. Because the cause of this adaptation could be genetic or epigenetic, studying phenotypic properties such as tumor metabolism is useful as it reflects molecular, cellular, and tissue-level dynamics. In triple-negative breast cancer (TNBC), the characteristic metabolic phenotype is a highly fermentative state. However, during treatment, the spatial and temporal dynamics of the metabolic landscape are highly unstable, with surviving populations taking on a variety of metabolic states. Thus, longitudinally imaging tumor metabolism provides a promising approach to inform therapeutic strategies, and to monitor treatment responses to understand and mitigate recurrence. Here we summarize some examples of the metabolic plasticity reported in TNBC following chemotherapy and review the current metabolic imaging techniques available in monitoring chemotherapy responses clinically and preclinically. The ensemble of imaging technologies we describe has distinct attributes that make them uniquely suited for a particular length scale, biological model, and/or features that can be captured. We focus on TNBC to highlight the potential of each of these technological advances in understanding evolution-based therapeutic resistance.

Comparison of MRI vs. [18F]FDG PET/CT for Treatment Response Evaluation of Primary Breast Cancer after Neoadjuvant Chemotherapy: Literature Review and Future Perspectives

The purpose of this systematic review was to investigate the diagnostic accuracy of [18F]FDG PET/CT and breast MRI for primary breast cancer (BC) response assessment after neoadjuvant chemotherapy (NAC) and to evaluate future perspectives in this setting. We performed a critical review using three bibliographic databases (i.e., PubMed, Scopus, and Web of Science) for articles published up to the 6 June 2023, starting from 2012. The Quality Assessment of Diagnosis Accuracy Study (QUADAS-2) tool was adopted to evaluate the risk of bias. A total of 76 studies were identified and screened, while 14 articles were included in our systematic review after a full-text assessment. The total number of patients included was 842. Eight out of fourteen studies (57.1%) were prospective, while all except one study were conducted in a single center. In the majority of the included studies (71.4%), 3.0 Tesla (T) MRI scans were adopted. Three out of fourteen studies (21.4%) used both 1.5 and 3.0 T MRI and only two used 1.5 T. [18F]FDG was the radiotracer used in every study included. All patients accepted surgical treatment after NAC and each study used pathological complete response (pCR) as the reference standard. Some of the studies have demonstrated the superiority of [18F]FDG PET/CT, while others proved that MRI was superior to PET/CT. Recent studies indicate that PET/CT has a better specificity, while MRI has a superior sensitivity for assessing pCR in BC patients after NAC. The complementary value of the combined use of these modalities represents probably the most important tool to improve diagnostic performance in this setting. Overall, larger prospective studies, possibly randomized, are needed, hopefully evaluating PET/MR and allowing for new tools, such as radiomic parameters, to find a proper place in the setting of BC patients undergoing NAC.

What radiolabeled FAPI pet can add in breast cancer? A systematic review from literature

To provide an overview of the current available data about FAPI PET in breast cancer patients, with a perspective point of view. A literature search for studies about FAPI PET in the last 5 years (from 2017 to January 2023) was carried out on MEDLINE databases, such as PubMed, EMBASE, Web of Science and Google Scholar using the following keywords: "PET" AND "FAPI" AND "Breast Cancer" AND "Fibroblast imaging". The Critical Appraisal Skills Program (CASP) checklist for diagnostic test studies was used for testing the quality of selected papers. 13 articles were selected, including 172 patients affected by breast cancer who underwent FAPI-based PET images. CASP checklist was used in 5/13 papers, demonstrating a general low quality. Different types of FAPI-based tracers were used. No difference in terms of FAPI uptake was reported based on the histopathological characteristics, such as immunohistochemistry and grading of breast cancer. FAPI demonstrated more lesions and yielded much higher tumor-to-background ratios than 2-[18F]FDG. Preliminary experiences with FAPI PET in breast cancer showed some advantages than the current available 2-[18F]FDG, although prospective trials are needed to further evaluate its diagnostic utility in clinical practice.