Contact : +27 (0) 861 NUCLEAR [682 5327] or +27 (0)12 942 0845

SPECT-CT and PET-CT in Skin Cancer

Issuu Medical Chronicle Dec Jan 2020

Skin cancer and more specifically malignant melanoma rates are on the rise the world over – and currently accounts for 1% of the world’s cancer incidence. South Africa, with its sunny climate and poor personal ultraviolet protection measures, has one of the highest incidences of malignant melanoma in the world, comparable that to front-runner Australia. More than 90% of cases of skin cancer are caused by exposure to ultraviolet radiation from the Sun. This exposure increases the risk of all three common types of skin cancer, namely malignant melanoma, squamous cell carcinoma and basal cell carcinoma of which malignant melanoma followed by SCC are the most aggressive forms.

Initial evaluation:

Upon excision of any suspicious lesions, if confirmed to be melanoma, histology determines the depth, which is used to stage the tumour itself. The greater the tumour depth the higher the increased likelihood that the tumour may in fact already have spread. Nuclear medicine is intimately involved in the nodal and metastatic staging of both squamous cell carcinoma and malignant melanoma:

Three-dimensional hybrid SPECT-CT for sentinel lymph node mapping versus two-dimensional planar imaging:

The sentinel lymph node is the first node draining a tumour – so named after the Roman sentinels that stood vigil at night and would be the first to encounter the enemy – and it has been demonstrated to confer good patient prognosis when negative for tumour histologically. SLN mapping using peri-tumoral injection of a radio-active colloid allows us to image the likely lymphatic drainage basin of the tumour and excision of that single node offers a less invasive technique for nodal staging with fewer complications and is considered the only reliable method for identifying micrometastatic disease. Where intra-operative frozen section histology confers sentinel lymph node tumour involvement the surgeon can move to full nodal block dissection.

Block dissection of the regional lymph nodes, whereby all the lymph nodes draining the tumour (regardless of metastatic involvement) are dissected, e.g. axilla or inguinal region does offer the opportunity for curative surgery especially in small tumours. This approach, however, is fraught with complications for example, seroma, hematoma, lymphoedema of the affected limb drained by the excised nodes, neurological complications (e.g. numbness or paraesthesias), motion restriction due to scarring, axillary web syndrome, wound dehiscence (uncoupling) and infection. In fact, it has been demonstrated that there is no improved survival benefit in elective whole nodal basin excision, with a concomitant increased level of morbidity. It would therefore be preferable to avoid dissection of lymph nodes where there is clearly no metastatic involvement, by means of sentinel node mapping.

Traditionally a series of two-dimensional images are taken after the injection that both tracks the flow of tracer from the injection site and localized collection in the sentinel lymph node. Skin markings are then applied to the area in question using the vertical and horizontal vector coordinates of the node, which the surgeon then uses as a guide to locate the node intraoperatively by using a radiation detector called a gamma-probe (as it detects the gamma radiation emitted).

Single-photon emission computed tomography-computed tomography, a three-dimensional version of the traditional gamma scintigraphy, fused to anatomical imaging e.g. CT, has been found to be of additional value in sentinel lymph node localization. It has been found to determine the exact anatomical location due to better anatomical resolution of the CT component; is more sensitive for finding lymph nodes on inconclusive or negative planar studies; detected additional nodes over and above those found on planar images and has helped to reduce the number of false positive findings.

Moreover, and probably most importantly surgical approach can be better planned prior to excision to limit complications.

Currently sentinel lymph node mapping is deemed clinically appropriate in early stage malignancy with low Breslow depth or Clark score. Should nodal involvement intraoperatively be demonstrated then block dissection of the remaining nodes will usually be followed by whole body staging for metastases using 18FDG PET-CT.

PET-CT for metastatic staging and follow-up:

Stage T3 or T4 malignancy (using the AJCC TNM staging criteria) depending on tumour depth (specifically in malignant melanoma) usually qualifies for whole body PET-CT staging imaging as there is an increased risk of metastases. These patients, and certainly patient with stage T4 may altogether skip sentinel lymph node mapping in favour of the highly sensitive PET-CT scan. Lower stage disease may also require PET-CT staging depending on whether sentinel lymph node histology demonstrated positive nodal involvement.

Regardless, F18-flurodeoxyglucose or FDG, a glucose analogue is injected into the system after suitable preparation – a diet of high proteins and fat for 24 hour prior to imaging. 60-90 minutes after injection, in the case of melanoma, the patient is imaged from head-to-toe as melanoma is notorious for metastasizing superficially and typical cancer spread mechanisms – the so-called “skip lesions”. With skin cancer typically being aggressive and having a higher metabolic rate than normal tissue it takes up the “copy-cat” sugar moiety FDG with gusto.

The nodal and metastatic involvement is then used to accurately stage the patient and guide further treatment whether that be local excision, radiation or chemotherapy. With metastatic skin cancer, previously considered incurable, in the case of malignant melanoma staging PET-CT is becoming more important with the advent of melanoma-specific immunotherapy – the so-called check-point inhibitors. 

The immune cells express surface proteins called check-points that prevent it attacking cancer cells which they ordinarily scavenge and destroy if not for these “check-points”. These inhibiting antibodies have demonstrated fantastic results in terms of treatment response – at great cost though. As such PET-CT may prove to better decide which patients may benefit initially from cheaper traditional regimes and ultimately again be the cost-effectiveness determinant as it has proven so many times before.

Both SPECT-CT and PET-CT are used similarly in squamous cell carcinoma. This form of cancer, however, has a lower propensity for metastases and more predictable pattern of spread. Nevertheless, it is still regarded as one of the more aggressive types and proper staging should be appropriately sought.

“Wear sunscreen”

Baz Luhrmann

Visit Us On FacebookVisit Us On TwitterVisit Us On LinkedinVisit Us On InstagramCheck Our Feed