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IMAGING & ONCOLOGY
Figure 5. Fused SPECT-CT images of sentinel nodes. Sentinel node mapping using 3D SPECT/CT. The yellow arrows indicate the nodes: subpectoral; parasternal; interpectoral. Reprinted with permission from Ibusuki et al. Surg Oncol. 2009. [Epub ahead of print.]26
Low-resolution SPECT/CT When performing a low-resolution CT, the tube current is often fixed (1-2.5mA), allowing for no adjustment; but it does allow the operator to select the appropriate scan field of view. As with all acquisitions, this should include the entire lymphatic chain of interest, encompassing all foci of uptake identified on SPECT. A low-resolution CT provides an attenuation map for AC and a traditional CT image; and, despite its low quality, it can still be used to aid pre-surgical localisation23
.
Diagnostic quality SPECT/CT Scanning patients at diagnostic quality is a more complicated scenario. Protocols vary
greatly between both manufacturers and imaging departments and may also require adapting to suit patient needs. Due consideration should be given to each individual’s clinical need24
. A wide range of exposure parameters can be adjusted to optimise
protocols for each patient (kV, mA, pitch, detector configuration (slice thickness), rotation time and image reconstruction). Axillary and internal mammary lymph nodes have an average size of 1cm and consequently a 5mm slice thickness is adequate for detection and localisation. For staging of breast cancer, the whole chest and abdomen are frequently scanned to assess for metastases within the lung and liver; narrower slice widths (0.5mm to 2mm on contemporary multi-slice CT) have been shown to increase detection rates for pulmonary lesions <1cm in size25
.
Low-resolution or diagnostic quality SPECT/CT? In most centres, this choice would be limited by equipment availability. Buck et al
. Localising sentinel nodes and pulmonary lesions associated with breast cancer is also more acceptable with a low dose technique due to the low attenuating potential of
considered the merits of each technique and favoured using a low-resolution technique for accurate localisation, unless there was a clinical need to perform a diagnostic quality scan24
the thorax and the high inherent contrast this provides. Figure 5 illustrates the benefit of a fused image for the localisation of sentinel nodes in a patient with breast cancer.26
Future developments Promising results are beginning to emerge from pioneering work exploring the use of
‘freehand SPECT’27 . Using a higher dose than previously quoted (60-80MBq), an intra-
operative handheld device yielded discovery of at least one node in 49/50 patients, sensitivity 98 per cent, whereas planar imaging achieved a sensitivity of only 84 per cent for the same 91 lymph nodes27
with accurate 3D localisation detail for axillary nodes28
. It was concluded that ‘freehand SPECT’ aided the surgeon . Whilst more studies are needed to
support the validity of this technique, these initial results bode well for future developments.
CONCLUSION Histological techniques play a critical role in lymph node evaluation for the planning of minimally invasive surgical procedures for the resection of cancer. For this to occur, the sentinel node(s) needs to be identified and assessed; if necessary, the node(s) may require accurate localisation and removal. Various combinations of pre-surgical and intra- operative localisation are available to surgeons, including blue-dye, ultrasound, hand-held gamma probes, and nuclear medicine imaging. Imaging in nuclear medicine has advanced dramatically in recent times and SPECT/CT now allows accurate fusion of physiological (radionuclide uptake) and anatomical data to aid the localisation process. SPECT/CT is thought to allow a more accurate pre-operative biopsy, reducing morbidity, while it is also reducing operating times by allowing the surgeon to localise nodes of interest more quickly.
For further reading, please see The role and value of nuclear medicine in the imaging of sentinel nodes and breast cancer.
eRADIMAGING.com website. Available at: http://
www.eradimaging.com/site/article.cfm?ID=748&mode=ce. Published 10 February 2010.
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