Intraoperative Use Of A Gamma Probe

Although definite blue staining of a sentinel node with identification of at least one blue-stained afferent lymphatic channel entering the node currently remains the gold standard for assessment of whether a lymph node is or is not a true sentinel node, intraoperative use of a gamma detection probe can provide valuable confirmatory evidence.

A number of compact, lightweight gamma probes are now available which can be used intraoperatively to assist with the sentinel lymphadenectomy procedure. All these devices are fundamentally similar in design and function, and their relevant features are described in detail in Chapter 5. They comprise a handheld probe containing a gamma detecting crystal connected to an easily portable control box (Fig. 6). The probe and the cable connecting it to the control box can be inserted into a sterile plastic sleeve for intraoperative use.

A gamma probe can be used for three distinctly separate purposes during a sentinel lymphadenectomy procedure: (1) to assist in locating each sentinel node, (2) to confirm that a node is ''hot'' both in vivo and after it has been removed, and (3) to check that there are no residual hot nodes in a lymph node field after removal of the presumed sentinel node or nodes.

Sentinel Node Location

Although the estimated position of each sentinel node will ideally have been marked on the overlying skin at the time of preoperative lymphoscintigraphy, it is useful to confirm its location with a gamma probe before making a skin incision. This is particularly valuable when the position of the patient on the operating table differs from the position they were in when the lymphoscintigram was obtained. The surgeon may occasionally decide, for example, to perform an axillary sentinel lymphadenectomy with the patient in a lateral position so that the subsequent wide excision of a primary melanoma site on the back can be performed

Figure 6 Hand-held gamma probe and control box (Neoprobe 1000, Neoprobe Corporation, Dublin, OH, U.S.A.).

Intraoperative Gamma Probe

Figure 6 Hand-held gamma probe and control box (Neoprobe 1000, Neoprobe Corporation, Dublin, OH, U.S.A.).

without having to reposition and redrape the patient, whereas the preoperative lymphoscintigram was performed in routine fashion with the patient in the supine position, with the arm extended at a 90° angle to the body. Checking the position of the sentinel node with a gamma probe prior to skin incision can also be very useful in situations where anatomical considerations make it difficult for the nuclear medicine physician to mark the position of the sentinel node(s) accurately, for example, in the submandibular region or in the groin of a grossly obese patient with an overlying fatty apron. Having made the skin incision, if the sentinel node cannot be located readily by its blue dye staining, the tip of the gamma probe, clad in its sterile sheath, can be inserted into the wound and moved around, with adjustment of the angle of the probe to indicate the direction of the hot sentinel node. During this maneuver, care must be taken, however, to ensure that residual activity at the primary melanoma site does not provide misleading information (e.g., when the axilla is being explored and the primary melanoma site is on the upper back).

Confirming Sentinel Node Identity

Even 24 h after intradermal radionuclide injection at a primary melanoma site, the sentinel node invariably remains the hottest node in the draining lymph node field [5,18]. It is valuable to use the gamma probe to confirm the identity of any node suspected to be a sentinel node (Table 1; Fig. 7). The probe can be used for the purpose of confirmation both in vivo and after removal of the node. Sometimes the sentinel node is not obviously blue stained [11,12]. It is very rare, however, for a true sentinel node not to contain radioactive colloid, provided the intradermal isotope is injected at the primary melanoma site at least 1 or 2 h preparatively. If the isotope is injected immediately preoperatively, either alone or in combination with blue dye (as is the practice in some centers), the reliability of sentinel node confirmation with the gamma probe is diminished. Nevertheless,

Table 1 Gamma Probe Recordings for a Woman Aged 24 Years Undergoing Sentinel Node Biopsy

Site of nodes removed at operation

Blue staining?

Indicated by scintigraphy?

Node counts ex vivo (per 10 s)

Field count following removal of node(s) (per 10 s)

Ratio of node to lymphatic field count

Micro-metastases identified?

Left axilla

Yes

Yes

43,598

7,391

5.9

Yes

(superficial)

Left axilla

Yes

Yes

28,105

7,391

3.8

Yes

(deep)

Right axilla

Yes

Yes

10,604

2,563

4.1

No

Sentinal Node Scan Pictures

Figure 7 Preoperative lymphoscintigram (delayed scan) showing two sentinel nodes (partly superimposed in this view) in the left axilla and a single sentinel node in the right axilla. (Gamma probe counts for the sentinel nodes are listed in Table 1.) Excision-biopsy of a melanoma 1.5 mm in thickness on her left upper back had been performed 3 weeks earlier. Preoperative lymphoscintigraphy had revealed that there appeared to be two sentinel nodes in the left axilla lying one immediately deep to the other, and on delayed imaging, a single fainter sentinel node in the right axilla. All three sentinel nodes were identified without difficulty at the time of surgery and removed. Subsequent histological examination of the left axillary sentinel nodes revealed micrometastatic disease in both of them, and a full regional node dissection of the left axilla was therefore performed. No additional positive nodes were present.

Figure 7 Preoperative lymphoscintigram (delayed scan) showing two sentinel nodes (partly superimposed in this view) in the left axilla and a single sentinel node in the right axilla. (Gamma probe counts for the sentinel nodes are listed in Table 1.) Excision-biopsy of a melanoma 1.5 mm in thickness on her left upper back had been performed 3 weeks earlier. Preoperative lymphoscintigraphy had revealed that there appeared to be two sentinel nodes in the left axilla lying one immediately deep to the other, and on delayed imaging, a single fainter sentinel node in the right axilla. All three sentinel nodes were identified without difficulty at the time of surgery and removed. Subsequent histological examination of the left axillary sentinel nodes revealed micrometastatic disease in both of them, and a full regional node dissection of the left axilla was therefore performed. No additional positive nodes were present.

the objectivity of the technique is still very useful. Detection of gamma radiation, measured in counts per second with a gamma probe, is completely objective, whereas confident detection of blue staining of a node can sometimes be difficult and operator dependent (and thus less reliable). The relatively less objective nature of lymphatic mapping and sentinel node identification with blue dye has led to suggestions that its use is unnecessary. However, it is readily demonstrable that second-tier nodes in the regional lymph node field (i.e., nodes which are not sentinel nodes) can become hot quite quickly after isotope injection [9,10,18]. Thus, any technique that relies on the gamma probe alone to identify sentinel nodes is likely to result in the removal of additional, nonsentinel nodes [18], defeating the main purpose of the sentinel lymphadenectomy procedure which is to be superselective and to avoid unnecessary disturbance of nonsentinel nodes and their afferent and efferent lymphatics.

Checking for Residual "Hot" Nodes

After removal of the presumed sentinel node or nodes, the residual count in the regional lymph node field can be checked with a gamma probe, and any residual hot areas can be explored. Usually, such hot spots are due to activity in second-tier nodes [19], which when exposed are not blue stained and do not have blue-stained afferent lymphatics draining directly from the primary melanoma site. The degree of residual activity in a node field must be assessed in relation to the activity present in the sentinel node(s) and is determined by the time elapsed since isotope injection at the primary melanoma site. Absolute levels of radioactivity are clearly meaningless. What is important is the ratio of counts in a given node to the counts in the remainder of the node field [12,20].

It has been suggested, for example, that to be identified as a sentinel node on the basis of gamma probe assessment the ratio of counts in sentinel and non-sentinel nodes should be over 3: 1 in vivo and 10:1 ex vivo [20]. However, this is not a useful guide if attempts are made, in the true spirit of a selective ''sentinel node only'' biopsy policy, not to remove nonsentinel nodes. Others have suggested comparing sentinel node activity with "background" activity, but without specifying how background levels are to be determined. After analysis of the data from a large number of patients undergoing sentinel node biopsy, we have proposed that the ratio of sentinel node activity to residual activity in the node field after removal of the sentinel node should be over 3:1 [12]. In most cases, this ratio is greatly exceeded, but it appears to provide a useful lower level below which the likelihood of a node being a true sentinel node is very low. If, on checking a node field after removal of all presumed sentinel nodes, a node with activity of more than three times the activity elsewhere in the node field is found, it should thus be regarded as a possible sentinel node and removed for histological examination, even if it is not blue stained.

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Responses

  • giselda piccio
    What does a hot node mean gamma probe sentinel node?
    6 months ago
  • Sebastian Cole
    Can you identify radioactive sentinel nodes without gamma probe?
    2 months ago
  • leah
    Is there a code for use of gamma probe when excision of sentinel lymph node?
    11 days ago

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