A technical application using a stereotactic navigation system with fusion images of [18F]-2-fluorodeoxyglucose (FDG) positron tomography and computed tomography (PET-CT) in the case of a metastatic melanoma of unknown primary site is described. A 50-year-old woman presented with a slow, growing level V neck lump which was cytologically proved to be a metastatic melanoma despite the absence of prior or existing history of skin malignancy. Whilst detailed physical examination failed to yield the site of the primary lesion, full body FDG-PET images isolated FDG-avid subclinical scalp lesions. Fused PET-CT data provided the navigation system with accurate localisation of the subclinical metastatic lesion. Histopathological examination of the navigation-guided resection specimen confirmed that the lesion was excised with acceptable margins. This case illustrates the feasibility of navigation-assisted resection of a subclinical malignant melanoma lesion and may have a role to play in the management of the melanoma of unknown primary.
Positron emitted tomography (PET) with glucose analogue 18-fluorodeoxyglucose (FDG) is a non-invasive, metabolic imaging modality . Tumour localisation is based on the avid FDG uptake by malignant cells, which have a much higher glucose utilisation rate than normal cells. It is often used to detect the unknown primary with occult cervical metastasis , including malignant melanoma of unknown primary . When FDG-PET images are used alone, it is extremely difficult to establish the exact three-dimensional (3D) position of the FDG avid area, limiting its value in disease staging and treatment . In relation to malignant melanoma, fusion of FDG-PET and computed tomography (CT) images is more precise than conventional FDG-PET alone when used in diagnosis and staging .
The goal of intraoperative localisation of a pre-determined area on imaging is now achievable with a degree of accuracy through navigation systems. The authors describe a technique utilising navigation-assisted surgery based on fused FDG PET-CT images to localise a subclinical metastatic melanoma lesion, detected only on imaging.
A 50-year-old woman presented with a slowly enlarging 3 cm lump in the left level 5 neck area. The resultant ultrasound guided, fine needle aspiration cytology indicated metastatic melanoma despite the absence of any current or previous malignant cutaneous lesions. A whole body FDG PET-CT was requested when systematic examination of the skin, oral, otolaryngological, urogenital system and eyes were found to be normal.
PET-CT data acquisition
The whole body PET-CT was performed using a hybrid PET-CT system (Biograph 64 TruePoint PET-CT, Siemens, Germany) with an 18-FDG radiotracer. The patient fasted 6 h prior to the intravenous injection of 18-FDG titred to body weight and data acquisition commenced 1 h post-injection. The CT scanning protocol used was: slice thickness 5 mm; pitch 0.8 mm; rotation 0.5 s; and field of view 700 mm. The PET scanner had a field of view that matched the CT scan and the total acquisition time for PET-CT head and neck data was 7.5 min. The PET-CT image showed several FDG-avid lesions ( Fig. 1 ). A scalp lesion was seen in the left parietal region and the other represented the positive node in the level 5 region of the left neck.
The navigation system (VoNavix, IVS Technology GmbH, Chemnitz, Germany) used was an optical-based system, specialised for craniofacial stereotactic navigation. The hardware consisted of a 3D camera with an infrared light source, passive markers (located on the patient and the instrument pointer) and a computer workstation. The system software (Voxim, IVS Technology GmbH, Chemnitz, Germany) constructs a virtual 3D dataset using acquired CT or MR images. The patient’s PET-CT images were exported as diacom files and image fusion was performed using the navigation software. Using segmentation, the area of enhanced radiotracer activity was isolated ( Fig. 2 ), allowing its identification on the virtual 3D dataset model.
Following standard skin preparation and draping, three passive markers were fixed onto the contralateral skull. Patient-to-image registration was performed using three match points on fixed soft tissue landmarks (tragus, alar and medial canthus). The target area was identified purely by stereotactic navigation and this was marked out with surgical marking ink ( Fig. 3 ). Resection of the marked area (diameter 18 mm) was performed allowing a 1 cm margin around the target area and this was sent for histopathological examination. The patient underwent left modified radical neck dissection and the postoperative recovery period was uneventful.