Rol of 18F-FDG PET CT in Cutaneous Melanoma:
The PET-CT 18F-FDG study in melanoma is the most sensitive for the search for distant metastasis (12).
It is applied in early stages with sentinel node invasion and stages III and IV according to the TNM classification 8(11).
Fig 12-14. Their contribution to the management of the patient is related to the assessment of the response to treatment,
re-staging and/or monitoring of the disease.
The 18F-FDG PET-CT is more sensitive than other tracers in the detection of distant metastases of cutaneous malignant melanoma,
due to tumor viability and cell proliferation(5).
Indications of PET CT in cutaneous malignant melanoma(13):
- Not indicated as a diagnostic tool.
- Not indicated as the first choice to assess brain Mets.
In this case,
the best study is magnetic resonance.
- Indicated in stages I and II with sentinel node invasion.
Fig.
15-16.
- Indicated in stages III and IV.
Fig.
17-19.
- Indicated in re-staging to define a surgical treatment in stage III,
with local aggressive disease and negative conventional studies of extension.
- To assess treatment response,
especially in advanced stages.
- To assess recurrence in high-risk patients.
Our protocol of 18F-FDG PET CT in cutaneous malignant melanoma:
The patient should not make physical efforts from 24 hours before to avoid that the increase of muscular activity increases the concentration of the tracer in those areas and prevent physiological uptake.
The protocol used for the scans was fasting for at least 4 hours and glucose levels below 150 mg/dl.
Followed by one hour of rest for complete biodistribution of 18F-FDG (10-20mCi or 150 uCi/kg)(14) in the body,
after which they switched to the PET-CT hybrid scanner.
The scan was performed in a full body,
supine position,
followed by a low dose of radiation in tomography without intravenous or oral contrast,
whose objective was to fuse the images and have a location of the lesions.
All non-physiological uptake was categorized as a lesion when its maximum standard uptake value (SUVmax) was> 2.4,
a semiquantitative measure that shows the relationship between the average activity of the body and the intensity of activity in the selected area.
PET-CT Images:
PET-CT has a specificity of 43 to 97%,
this variation in the different studies is due to false positives caused by inflammation,
myocardial metabolism,
cerebral metabolism,
the presence of brown adipose tissue,
the urinary excretory system,
normal intestinal activity or by attenuation artifacts(15).
The use of SUVmax of> 2.4 shows a sensitivity of 91% and specificity of 89% in lymph nodes ≥8 to 10 mm,
values applicable to any lesion observed in the scan.
Nodes smaller than 10 mm with an SUVmax of <1.4 are considered benign(16).
One of the limitations of the PET-CT study in the assessment of melanoma is the detection of distant metastases at the cerebral level,
with a sensitivity close to 61%; converting magnetic resonance imaging as the ideal study for suspecting this type of metastasis due to its sensitivity close to 99%(17,
18).