PET probe-guided surgery: applications and clinical protocol

Gamma probes have been used in medicine and surgery since the mid 1940s. There have been significant technical improvements in probe technology, including a number of clinical applications that have now become standard procedures in contemporary oncologic surgery [1‐9]. The introduction of FDG-PET in functional imaging in recent years has markedly improved cancer detection and management. The oncological applications of PET are rapidly expanding with development of new positron-emitting radiopharmaceuticals. Parallel to the advances in PET imaging, and availability of new systemic treatment options, the treatment paradigm in oncology has also shifted towards more aggressive therapeutic interventions including cytoreductive techniques and metastasectomies. This new strategy applies to a number of malignancies including, but not limited to thyroid cancer, neuroendocrine tumors, colorectal cancer (CRC), and melanoma [11‐15].

Intraoperative localization of PET-positive recurrent/metastatic lesions can be facilitated using a hand-held PET probe. PET probe essentially is a high energy gamma probe that is designed to process the 511 keV photons of PET tracers. Intraoperative gamma probe performance, as a general rule, is a function of radiopharmaceutical uptake, clearance kinetics, and probe engineering, all ultimately determining the target to background ratio (TBR) and detection threshold. A minimum TBR of 1.5:1 is needed in the operative field for the operating surgeon to be comfortable that the differences between tumor tissue and normal adjacent tissue are real [10]. Due to the high energy photon fluxes, achieving a satisfactory TBR intraoperatively is highly challenging. As such, the clinical use of PET probes has been limited to clinical trial settings, and no standard PET probe-guided surgery protocol has been developed.

PET probe is used routinely in the surgical oncology practice at the Center for Cancer Care at Goshen Health System. This paper discusses the indications and the clinical utility of the PET probe technique, and a PET probe-guided surgery protocol.

The success of a PET probe-guided surgery depends on numerous factors including the FDG avidity of the tumor, timing of surgical exploration in reference to injection of FDG, anatomic location of the lesion, its relative proximity to main sites of physiologic uptake/accumulation, and technical properties of the probe. The current study was performed to review the performance of the PET probe in surgical practice, and to validate/establish the optimal surgical protocol. Surgical performance was evaluated based on the probe's the ability to identify lesions seen on diagnostic imaging, and more importantly, actual contribution to the surgical exploration.

All the lesions identified on imaging studies were detected by the PET probe. The size of the lesions varied from 0.8 cm to 4 cm. The PET probe, however, was clearly most useful in detection of surgically occult lesions. Occult lesion was defined as a lesion that was not easily seen or palpated during routine surgical exploration. Some occult lesions included tumor masses measuring 2 cm or above. These were located in regions where the immediate surgical access was difficult due to a barrier of adhesions or scar tissue. A posterior mediastinal exploration and two abdominal re-explorations for recurrent nodal disease involving periportal and celiac regions that were complicated with severe scarring were successfully completed using the PET probe. PET probe effectively directed the surgeon to the lesion(s) which were not found on initial visual and manual exploration. In a patient with metastatic gastric cancer, a N2 node was located using the probe and included in the resection which otherwise could have been retained following the planned gastrectomy. Locating a lesion that is expected to be masked under a scar tissue is probably the strongest indication for PET probe-guided exploration. In a diagnostic setting, we have found PET probe to be very useful in patients with lymphoma who require restaging or regrading of their disease in follow-up. The patients most suitable for this approach are those presenting with non-palpable but FDG-PET positive lymph nodes in axilla, where a surgical dissection is relatively more challenging than that of the groin or neck.

The design of a probe that has the ability to process 511 keV photons, and its intraoperative use against significant background activity is technically challenging. We have previously demonstrated the efficacy and feasibility of PET probe-guided surgical exploration in different clinical settings in a phase II diagnostic study [10]. The technical performance of a probe system is determined by detector sensitivity, spectral resolution, scatter rejection electronics, and shielding [11]. Optimum lesion detectability requires high counting efficiency, an adequate shielding method, and electronics capable of discriminating target signal from radiation noise. Our review of data revealed that there was a trend of TBR improving over time. Improved TBRs (although they were not statistically significant) were observed six hours after FDG administration. There was satisfactory count rate and TBR for surgical detection up to 6 hours post-administration of 15 mCi activity, which makes this technique clinically feasible.

Detector material for the probe plays an important role in the sensitivity of the system. A GSO crystal is known to have a better stopping power than the NaI or CsI crystals and much better efficiency than the semiconductors used in medium energy gamma probes [16]. The PET probe also has to have heavier shielding. The profile, size and the weight of the probe and the ergonomics of the probe have to be non-restrictive to surgical exploration. The analyzer of the probe system allows exclusion of scattered radiation which determines the specificity of the system. Therefore, analyzer electronics are equally (if not more) important in the overall performance of the probe system. Poor electronic suppression of scattered photons seriously degrades the TBR. This can cause small target lesions to be missed in the background. Effective electronic suppression of scattered photons requires a precisely placed photon energy acceptance window set atop a scatter rejecting threshold.

FDG avidity is determined by glycolytic activity of the tumor and the viable tumor concentration in a given lesion. This feature contributes to the success of probe detection; however, it has nothing to do with the specificity. Individual cancer types may also show significant variability in terms of FDG avidity. Even in the same patient, different lesions may have different degrees of FDG uptake. The probe detects any FDG avid lesion whether it is malignant or inflammatory.

FDG metabolism and clearance occurs at a much faster rate in normal tissues than tumor tissue, and thus TBR improves with time resulting in better lesion detection when imaging is delayed [17]. We have observed that tumor-to-non-tumor and tumor-to-organ rations were higher for the delayed images than for the 1.5-h routine images, and lesion detectability was improved in nodal and hepatic metastases. Our current study indicated that longer intervals accentuated the TBR, and resulted better lesion detection. The background radiation tends to decrease while the tumor uptake is retained. The in-situ TBR is also strongly affected by the areas of physiologic uptake or accumulation. The brain uptake in the head and neck region, cardiac uptake in the chest, kidney uptake and the accumulation inside the bladder in the abdomen and pelvis affect the in-situ TBR. Areas of physiologic uptake show attenuation over time, and use of an intraoperative bladder catheterization minimizes interference from bladder accumulation of FDG.

The role of surgery in recurrent/metastatic cancer is being redefined. With the advance of tools or early diagnosis, and the improvements in systemic therapy, metastasectomy and cytoreductive surgical techniques in selected cases are considered viable management options. The clinical indications for metastasectomy and/or cytoreductive resections are beyond the scope of this discussion. Parallel to the development of techniques to identify favorable tumor biology, and having the ability to select patients who are expected to have a more protracted disease course (such as in differentiated thyroid cancer and neuroendocrine tumors) or tumors which are responsive to systemic therapies (such as colorectal cancer and breast cancer) more patients will be considered for metastasectomy. PET probe, with further refinements in the design and technical performance, might prove to be a very useful tool in surgical management of recurrent/metastatic disease. This study demonstrates the technical ability and feasibility of FDG-positive lesion detection using a PET probe.

Currently, with all recognized limitations, FDG imaging represents a new standard for functional/biologic imaging in oncology. Many more new PET radiopharmaceuticals are being developed for distinct tumor types and tumor phenotypes [18, 19]. Peptide-based agents, kinetically and diagnostically, possess the most favorable radiochemistry. Cu-64 and Ga-68 labeled octreotide are being increasingly utilized in detection of occult neuroendocrine tumors [20]. All PET radiopharmaceuticals have unique biodistribution characteristics defining different in-situ TBR profiles, some clearly yielding much better TBR than that of FDG in a given lesion.

The clinical indications of the PET probe will individually be determined based on the tumor and patient characteristics as well as the biodistribution patterns of the selected PET radiopharmaceuticals. In carefully selected indications, PET probe can be considered as a useful adjunct in surgical practice.