Purposes Of Lymphoscintigraphy Preparations And Injection Technique

The wound edges following a wide excision do not necessarily drain to the same lymph node or even same basin as the original lesion [3,18,19]. The relevant cutaneous lymphatics may be disrupted. Studies have shown that if lymphatic mapping is performed after wide local excision, the mean number of sentinel lymph nodes removed is increased. The number of patients where two or more basins have to be dissected is also increased to suggest that more extensive surgery is performed when compared to mapping prior to wide local excision [19]. Because this risk is likely to be much smaller after diagnostic excision with a narrow margin, it is important to perform lymphoscintigraphy and sentinel lymphadenectomy preceding therapeutic wide excision.

The purpose of lymphoscintigraphy for lymphatic mapping is to demonstrate the lymphatic drainage pathway of the neoplasm; to be more precise: to indicate the drainage basin, to determine the number of lymph nodes that are on a direct drainage pathway, to differentiate these first-tier nodes from subsequent nodes, and to locate sentinel nodes outside the usual nodal basins. The request form for lymphoscintigraphy should describe the disease, its location, and prior management. Relevant parts of the medical history of the patient should be mentioned. For instance, prior inguinal hernia surgery may prevent drainage of a lower abdominal wall melanoma to that groin. The purpose of the study is stated on the form. The surgeon should inform the patient of the reason for the scintigra-phy and outline how it is done so that the patient knows what to expect.

Lymphoscintigraphy can be performed in any nuclear medicine department, using the standard equipment. The nuclear medicine physician asks the patient about allergic reactions in the past because anaphylaxis can occur, albeit rarely. A number of radiopharmaceuticals is available for lymphoscintigraphy. They all have the same radionuclide: technetium-99m (99mTc). Advantages and disadvantages of the well-known tracers are discussed under the next section. The amount of radioactivity is determined by the need for good quality images without exposing the patient to unnecessarily large doses of radioactivity. A dose of around 20 MBq (approximately 0.5 mCi) is sufficient. Intraoperative gamma-ray detection is often done utilizing the same dose of the tracer that was used for the lymphoscintigraphy. If the operation is to take place the next day, the dose should be increased taking into account the 6.2-h physical half-life of 99mTc. A dose of 40-80 MBq (approximately 1-2 mCi) allows reliable gamma probe detection the following day.

We are interested in the route of drainage of a cutaneous lesion. Therefore, the tracer is injected intradermally, raising a wheal. Subcutaneous administration is simpler to accomplish but may not delineate the route of drainage from an overlying cutaneous site. Also, drainage from the dermis is a lot faster than drainage from subcutaneous tissue. Intradermal injection of the tracer is painful. This is especially true for sulfur-containing agents because of the low pH. The injection site may be prepared with a local anesthetic in the form of an ointment. Alternatively, the tracer may be mixed with a local anesthetic. The volume of the tracer should be such that it can be evenly distributed around the skin lesion or the biopsy wound. A volume of 0.2-1.0 mL is sufficient. A thin needle is used. The tracer is administered in close proximity to the lesion or biopsy site. It is enticing to inject the entire volume in one deposit—at theenoofthe lesion nearest to the lymphatic field. However, a sentinel node may be missed with this approach. We have seen the tracer injected at opposite ends of the lesion go to different sentinel nodes. Uptake of the tracer by the lymphatic system usually happens instantly. Within 1 min, it may flow through lymphatic ducts to the drainage basin. Because this early flow needs to be observed, the tracer is injected with the patient on the scintigraphy table and imaging is started immediately.

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  • Crispus
    What to expect lymphoscintigraphy?
    8 years ago

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