Precision Medicine and PET/Computed Tomography in Melanoma

The diagnostic accuracy and clinical impact of FDG-PET/CT follow-up for patients on adjuvant immunotherapy for high-risk malignant melanoma

OBJECTIVE

The benefit of FDG-PET/CT in follow-up of patients treated with adjuvant immunotherapy after resection of high-risk malignant melanoma (MM) is debated. This study evaluated the diagnostic accuracy and clinical impact of FDG-PET/CT for diagnosing MM recurrence during the first year after surgery.

METHODS

We retrospectively included 124 patients with resected high-risk MM, who received adjuvant immunotherapy and follow-up FDG-PET/CT. Clinical information and AJCC-8 stage was obtained from patients' medical records. Recurrence was verified by biopsy/progression on a subsequent scan leading to change of treatment. Non-recurrence was assumed when no metastases were observed until the subsequent follow-up scan. Incidence of recurrence, sensitivity, specificity, positive and negative predictive values (PPV and NPV) were outcome measures.

RESULTS

Incidence rate of MM recurrence was 0.27 [95% CI 0.17-0.37] per person-year during the first-year. Recurrence was detected in 13 patients (10%) at 3-month FDG-PET/CT, in 10 patients (8.1%) at 6 months, 1 patient (0.8%) at 9 months, 3 patients (2.4%) at 12 months. The overall sensitivity, specificity, PPV, and NPV were 97% [86-99], 82% [78-86], 39% [29-50], and 99% [98-99], respectively. The PPV trended towards higher values as disease stage increased. At the 3-month scan, the majority of actions derived from positive findings were surgery or earlier expedition of the subsequent follow-up scan.

CONCLUSION

The high rate of recurrence in patients with high-risk MM treated with adjuvant immunotherapy emphasizes the need for follow-up. The potential harm by a moderately low specificity reflecting a high number of false-positive results must be weighed against the benefit of early detection of recurrence.

Development of Dual Receptor Enhanced Pre-Targeting Strategy-A Novel Promising Technology for Immuno-Positron Emission Tomography Imaging

PET imaging has become an important diagnostic tool in the era of precise medicine. Various pre-targeting systems have been reported to address limitations associated with traditional immuno-PET. However, the application of these mono-receptor based pre-targeting (MRPT) strategies is limited to non-internalizable antibodies, and the tumor uptake is usually much lower than that in the corresponding immuno-PET. To circumvent these limitations, we develop the first Dual-Receptor Pre-Targeting (DRPT) system through entrapping the tumor-receptor-specific radioligand by the pre-administered antibody. Besides the similar ligation pathway happens in MRPT, incorporation of a tumor-receptor-specific peptide into the radioligand in DRPT enhances both concentration and retention of the radioligand on tumor, promoting its ligation with pre-administered mAb on cell-surface and/or internalized into tumor-cells. In this study, 64Cu based DRPT shows superior performance over corresponding MRPT and immuno-PET using internalizable antibodies. Besides, the compatibility of DRPT with short-lived and generator-produced 68Ga is demonstrated, leveraging its advantage in reducing radio-dose exposure. Furthermore, the feasibility of reducing the amount of the pre-administered antibody is confirmed, indicating the cost saving potential of DRPT. In summary, synergizing advantages of dual-receptor targeting and pre-targeting, we expect that this DRPT strategy can become a breakthrough technology in the field of antibody-based molecular imaging.

Precision Nuclear Medicine: The Evolving Role of PET in Melanoma

The clinical management of melanoma patients has been rapidly evolving with the introduction of new targeted immuno-oncology (IO) therapeutics. The current diagnostic paradigms for melanoma patients begins with the histopathologic confirmation of melanoma, initial staging of disease burden with imaging and surgical approaches, treatment monitoring during systemic cytotoxic chemotherapy or IO therapeutics, restaging after completion of adjuvant systemic, surgical, and/or external radiation therapy, and the detection of recurrent malignancy/metastatic disease following therapy. New and evolving imaging approaches with positron-emission tomography (PET) imaging technologies, imaging methodologies, image reconstruction, and image analytics will likely continue to improve tumor detection, tumor characterization, and diagnostic confidence, enabling novel precision nuclear medicine practices for managing melanoma patients. This review will examine current concepts and challenges with existing PET imaging diagnostics for melanoma patients and introduce exciting new opportunities for PET in the current era of IO therapeutics.

PET Imaging of Melanoma Using Melanin-Targeted Probe

Melanin exists in the most of melanoma lesions. Melanin plays an important role in melanoma progression, metastasis, therapy response, and the overall survival of patients. Therefore, melanin is a critical target for melanoma diagnosis and therapy. Many melanin targeting probes, such as radioisotope-labeled benzamide analogs, have been developed for melanoma diagnosis using positron emission tomography (PET). The N-(2-(diethylamino)-ethyl)-¹⁸F-5-fluoropicolinamide (¹⁸F-P3BZA) probe is one of the benzamide analogs and has been preliminarily tested for clinical diagnosis of melanoma in our recent studies. It has shown high specificity and favorable in vivo performance for PET of melanoma. Herein, we describe the detailed synthesis protocol of ¹⁸F-P3BZA and PET/CT imaging procedure for animal models and patients.

Whole-Body versus Routine Skull Base to Mid-thigh 18F-Fluorodeoxyglucose Positron Emission Tomography/ Computed Tomography in Patients with Malignant Melanoma

Objectives:

The objectives of this study are to assess the utility of whole-body ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) (skull vertex to toes) imaging relative to the standard field of view (skull base to mid-thigh) in patients with primary melanoma site that is not located in the lower extremities.

Material and Methods:

The primary site of the melanoma and metastatic disease was determined based on ¹⁸F-FDG PET/CT findings in 26 patients. The FDG avid sites were tabulated as the primary site, lower extremity, brain, and other sites. The hypothesis is that routine skull base to mid-thigh versus whole-body ¹⁸F-FDG PET/CT in patients with malignant melanoma will not change management.

Results:

Patients (26) were divided into those with primary melanoma site in either the lower extremities (six patients) or other site (20 patients). Four of the six patients with the primary site in the lower extremities also had positive findings in the ipsilateral inguinal lymph nodes. One of the patients with a positive inguinal lymph node had metastatic sites in the external iliac region and lungs on the initial study. On follow-up imaging, this patient also exhibited diffuse metastatic disease, including a lower extremity. None of the remaining patients in this group had positive findings other than the primary site in the lower extremity. Of the remaining 20 patients with the primary site not in the lower extremity, one had diffuse metastatic disease that included a lower extremity. However, lower extremity involvement would not change patient management in this case. A second patient in this group had diffuse metastatic disease that also involved the brain. However, no metastatic disease was present in the lower extremities in this patient. None of the remaining 18 patients in this group had metastatic disease in a lower extremity. Two patients in the entire study group of 26 had brain metastasis on contrast-enhanced head CT, with one having multiple brain metastasis. PET failed to demonstrate some of the brain lesions. In the other patient with solitary brain metastasis detected on contrast-enhanced head CT, PET was negative.

Conclusion:

¹⁸F-FDG PET/CT imaging of the lower extremity may not be justified if the primary neoplasm is not located in the lower extremities. Elimination of lower extremity imaging will reduce scanning time and additional radiation exposure. Similarly, PET/CT imaging of the brain may not be justified if contrast-enhanced CT or magnetic resonance imaging of the head is already obtained since these are more sensitive.