Assessment for recurrence by radio-labelled F-18 FDG is gaining acceptance.3
Although radio-guided surgery was initially proposed in 1949,4
it was not until 1994 when in vitro and in vivo properties of F-18 FDG probe detection was extensively described in phantom and rat models.5 6
The benefits of F-18 FDG in intraoperative probe detection include high glucose avidity for use in the anaerobic pathway in many tumour cells, in conjunction with established PET/CT imaging without requirements for additional injections, lack of a host immunological response, safe radiation burden to patient and personnel, and a short half-life of approximately 110 min.6 7
The fluoride molecule prevents the tumour cell from metabolising the marker.
Limitations include high background glucose affinity in the brain, myocardium, kidney and bladder.8
Tumours near these organs would not provide the adequate TBR to alert the surgeon. Bladder catheterisation and diuretics are commonly used to decrease background activity in the pelvis and retroperitoneum, and ceramic composite shielding has also been proposed.9
The handheld probe also cannot differentiate inflamed tissue from malignancy. Another issue is tissue depth and sensitivity of hand-probe detection. Thin normal tissue overlying the tumour may mask a high TBR.10
Fortunately the continuing advancement of new surgical probes which combine multiple detectors attempt to minimise background radiation while increasing sensitivity.10
The use of handheld F-18 FDG detection probes have been studied in cancer patients with melanoma,7 11
non-small cell lung cancer,11
iodide-negative thyroid cancer14
and ovarian cancer.15
In a retrospective review of five melanoma patients, Franc et al described the handheld PET probe having a sensitivity of 89% and specificity of 100% in detecting metastatic tissue.7
Three of the five patients had non-visualised and non-palpable metastases that were detected by the probe. Numerous authors have used a TBR of 1.5 as a suggested cut-off such that the surgeon would find it comfortable enough to resect the tissue deemed as likely malignant.7 10 16 13–15
Other studies used a cut-off of three SDs higher than background activity12
and one study11
used a TBR of 3.
Here we describe two patients with a previous diagnosis of melanoma and status postinguinal dissection prior to presenting at our institution. In the first case the patient had extensive scar tissue from her previous operations. The use of the PET probe guided the direction of adhesiolysis and uncovered the small 2 cm mass later confirmed to be malignant. The use of the probe to locate a lesion hidden under scar tissue is consistent with the conclusion of one review article on PET-probe guided surgery.17
In the second case the PET probe detected additional ‘hot’ lymph nodes not noted on preoperative pelvic PET imaging secondary to relatively low spatial resolution. The size and direct manipulation of the probe on the operating table offers the surgeon more information during exploration that complements the preoperative imaging. At our cutaneous oncology centre the use of the handheld PET probe is limited to certain cases of recurrent melanoma resection. A recent randomised trial on adjuvant therapy for melanoma patients who had undergone complete metastectomy did not show a survival benefit, but the researchers recognised that only select patients with a minimal number of metastasis can stand to gain any potential benefit of metastectomy combined with adjuvant therapy.18
Both patients described in this case report had regional metastasis confined to one anatomic region.
- F-18 FDG is a commonly used marker in surveillance of recurrence or metastasis in cancer patients.
- In carefully selected individuals, the use of a handheld PET probe can aid surgeons in detecting occult lesions in the surgical bed.
- Whether metastectomy in melanoma patients confers a survival benefit remains to be elucidated.