The so-called gamma probe is used to count radioactivity in the surgical field intraoperatively, usually without producing any scintigraphic image but yielding both a numerical readout and an audible signal proportional to the counting rate. Recently, intraoperative imaging probes have become ­commercially available. The handheld probe (or detector component) is connected to a counting unit either with a cable or with a wireless system. The detector is usually of limited size, basically a long narrow cylinder with diameter varying from a minimum of about 12 mm up to a maximum of 16–18 mm. It may be slightly angled in order to allow for easier access in specific applications and easier manipulation within the surgical field. The gamma probe can be utilized in the surgical field because its outer surface is made of a material that can be sterilized (usually metal), or it can simply be covered with a sterilized wrapping (such as those used for intraoperative ultrasound probes). Through the digital readout and acoustic signal, the gamma probe enables the surgeon to precisely localize areas of maximum radioactivity accumulation, thus guiding identification and removal of the target tissue.

Different types of gamma probes are commercially available. They can be divided into two main categories: (a) crystal scintillation probes and (b) semiconductor probes (see also Chap. 3 of this book). Choice between these two main types of probes is based on several key operating parameters which may differ depending on the procedure [17–20]. For example, SLNB in patients with breast cancer or melanoma requires different parameters from those that are optimal for radioguided surgery of parathyroid adenomas. In addition, some technical features of the probe vary depending upon whether radiopharmaceuticals are labeled with ^99mTc or other radionuclides, including positron-emitting radiopharmaceuticals [21–23]. The probe is connected to a small control unit, usually a laptop PC, with a flexible cable that may also be covered with sterilized wrapping, although Bluetooth-like connections have now become available, permitting the entire instrument’s easy use in the operating room. Besides setting the most effective energy window for detection/counting depending on the radionuclide employed, the control unit can adjust several acquisition parameters such as the time and mode of radioactivity counting and can display the radioactivity counting rate in various digital readout formats. At the same time, the unit usually emits an audible signal, the pitch/tone of which varies with the varying counting rate. The acoustic signal helps the surgeon to explore the surgical field without looking at the control unit display.

The main operating parameters characterizing a gamma probe are (a) sensitivity, (b) energy resolution, (c) spatial resolution, and (d) linearity of counting. Sensitivity is defined as the counting rate per unit of radioactivity and is usually expressed in counts per second (cps) per unit of radioactivity in kBq. KBq is used usually because of the small amount of radioactivity generally detected in radioguided surgery. This parameter is intrinsically linked to the efficiency of the probe in detecting radiation. Energy resolution is instead intrinsically linked to the counting statistics and to the ability to detect “true” counts arising in the target versus secondary scattered radiation.

Spatial resolution can be defined as the ability to identify very close radioactive sources as distinct from each other and depends critically on the collimation of the probe. Linearity of counting relates to the dead time and varies therefore for different detectors. A good gamma probe exhibits a counting linearity up to 4,000–5,000 cps.

As mentioned above, intraoperative probes specifically designed to detect the high-energy gamma rays originated by the annihilation process have become commercially available, thus making it possible to use radioguidance for resection of tumor lesions with high uptake of, for example, [¹⁸F]FDG or other tumor-seeking radiopharmaceuticals.

Nevertheless, major advantages in both preoperative and intraoperative localization of target tissue to be surgically resected (either it be the sentinel lymph node or tumor tissue previously labeled by local or systemic administration of suitable radiopharmaceuticals) have been made possible by the use of SPECT/CT and/or of intraoperative imaging probes. In fact, hybrid imaging with SPECT/CT supplies to the surgeon a set of anatomotopographic information that guides resection through the optimal surgical access according to the principle of least invasive surgery. As reviewed recently [24, 25] and as exemplified in Fig. 28.2, this approach is especially useful when planning surgery in complex anatomical regions such as the head and neck or the pelvis [26–33].

Evidence is also growing on the usefulness of intraoperative imaging probes for radioguided surgery, employing such instrumentation either during conventional open surgery or during laparoscopic surgery [34–36]. Real-time intraoperative imaging provided by this instrumentation improves monitoring of surgical excision of the target tissues (Fig. 28.3).

A most recent interesting development in radioguided surgery is the system so-called freehand SPECT, in which a continuous positioning system installed in the operating room is based on a fixed pointing device, on the patient’s body, and respectively, on the handheld gamma counting probe. Position of the gamma probe relative to the fix device is tracked by infrared positioning technology, and the output of the intraoperative gamma probe is spatially coregistered in the surgical field (depicted by a video camera) and displayed on a monitor in which the surgeon can easily check location and depth of the foci of radioactivity accumulation to be resected [37]. An example of application of this novel approach is given in Fig. 28.4.

Lymphoscintigraphy is critical to most cases of radioguided SLNB. Imaging agents employed for lymphoscintigraphy are radiocolloids that, after their interstitial administration, allow the scintigraphic visualization of the lymphatic vessels draining the injection site. These colloids are preferentially retained by the first lymph node encountered along the course of the draining lymphatic vessel(s), by definition the sentinel node(s).

The general term “colloid” indicates a class of macromolecules of micellar size with similar physicochemical and biological behaviors. They are particles varying between about 5 and about 1,000 nm (0.005–1 μm) in size. Physiologic endogenous colloids are heterogeneous in composition, and their presence in circulating fluids can be constituted by fragments of bacteria, or lipidic micellae absorbed in the gastrointestinal tract, or cell debris produced during normal cellular tissue renewal. Endogenous colloids circulating in the lymph or blood share a common fate. In fact, they are removed from the circulation by macrophages, a ubiquitous clearing/purification system located in the bone marrow, spleen, liver, and in the sinusoidal spaces of lymph nodes.

Once within the lymph node through the afferent lymphatic channels draining the injection site, radiocolloids are phagocytized by macrophages lining the lymph node sinusoids. Such retention of radiocolloids in the sentinel lymph node is a physiological process, and it does not indicate presence or absence of metastases in the lymph node.

The speed of lymphatic drainage from the interstitial injection site and the amount retained in the sentinel lymph node depend mainly on the size of the radiocolloids, which may be either an inorganic substance (¹⁹⁸Au-colloid, ^99mTc-antimony sulfur, ^99mTc-colloidal sulfur, ^99mTc-stannous fluoride, ^99mTc-rhenium sulfur) or derived from biological substances (nano- or microcolloidal human serum albumin). In particular, small size radiocolloids (less than about 100 nm) migrate quite fast from the injection site through the lymphatic system. They allow lymphoscintigraphy to be performed in a relatively short time usually (within 45–60 min after injection). However, they are not efficiently retained in the sentinel node, so that more than one lymph node, often as many as to 3–4, can typically be visualized along a single lymphatic draining channel. This may complicate the identification of the sentinel lymph node unless there are sequential images acquired at short time intervals after interstitial injection. Larger size radiocolloids are retained more efficiently in the sentinel lymph node, so that scintigraphy does not generally visualize more than 1–2 nodes along a single lymphatic draining vessel. The migration of larger particles from the interstitial administration site is very slow, so that a longer time, even 2 h or more, can be required to complete lymphoscintigraphy. The nuclear medicine physicians’ experience and the agent’s commercial availability affect choice of the radiopharmaceutical. ^99mTc-albumin nanocolloid (that has a quite narrow range of particle size, with over 90% of the particles being smaller than 80 nm) is commercially available and most widely employed in Europe, while ^99mTc-sulfur colloid (with a wide range of particle size between about 20 and 400 nm) is widely employed in the USA. To overcome the problem of heterogeneous particle size distribution, in some nuclear medicine centers, ­^99mTc-sulfur colloid is filtered immediately after labeling and prior to administration, in order to remove particles with a diameter greater than a certain size, usually 220 nm or other size [38, 39].

The sentinel lymph node hypothesis (see Fig. 28.1) is that lymph and tumor cells entering the lymphatic circulation migrate through the lymphatic vessels in an orderly manner to the first lymph node draining the anatomic region. From the breast, especially for superficial and outer quadrant tumors, drainage is toward the axilla but occasionally is toward the internal mammary chain, especially from deeply seated tumors in the inner quadrants. The breast embryologically originates from ectodermal tissue, as a skin appendage, and therefore shares a pattern of lymphatic drainage with the overlying skin. The mammary gland is interposed between the superficial (subdermal) and the deep (subcutaneous) lymphatic plexuses, the two systems being interconnected by a dense network of lymphatic vessels (Fig. 28.5a). Lymphatic vessels surrounding the mammary lobules predominantly drain to the subareolar Sappey’s plexus, which is part of the superficial plexus of the skin. Most of the lymph produced in the breast is therefore drained to the subareolar region, progressing then toward the ipsilateral axillary nodes (Fig. 28.5b). A small fraction of the lymph produced in the breast (about 3%) is instead drained to lymph nodes of internal mammary chain, while an even smaller amount drains to other lymph nodes such as the intercostal, pectoral muscles, contralateral breast, or even abdominal nodes [40].

By following these routes of drainage, lymph and cancer cells entering the lymph space spread initially to the first node, which is the sentinel lymph node. If the sentinel node is free of metastasis, the probability of tumor cells skipping the sentinel node and metastasizing to second- or third-echelon lymph nodes is an exceptionally rare event [41–49].

SLNB has been an important step toward the adoption of less invasive surgical techniques. When the sentinel node is free of metastasis, the patient can be spared an axillary lymph node dissection that until a few years ago was considered the standard staging procedure for breast cancer. Axillary lymph node dissection itself has minimal impact, if any, on survival and therefore has little or no curative purpose; the main goal of this procedure is staging the N parameter in the axilla [50]. Along with other prognostic factors such as hormone receptor status, proliferative index, etc., the N stage is used to stratify patients for optimal postsurgical management. Because of low accuracy of the methods currently available for preoperative axillary nodal assessment, including ultrasound and even [¹⁸F]FDG-PET, surgical staging is necessary in most cases [51]. In early breast cancer, the fraction of patients with axillary lymph node metastases is only 20–30%; as many as 70–80% of axillary dissections reveal no metastasis and, therefore, could have been avoided. Axillary lymph node dissection results in a high incidence, about 25–30%, of postoperative complications that can reduce quality of life, such as delayed wound healing, lymphedema, peripheral nerve injury, or even brachial plexus injury. SLNB greatly decreases the number of patients who may experience such complications [52–55], by limiting node dissection to those patients in whom the sentinel node was found to harbor metastasis. Metastasis in sentinel lymph node(s) is usually treated with completion axillary dissection not because of curative intent but mainly to accurately stage the cancer and determine the number of involved lymph nodes and extent of their involvement.

Optimal conditions for deriving maximum benefit from successful SLNB are to be found in early breast cancer patients with clinically negative axilla and negative ­ultrasound examination. Mammographic screening can detect tumors with a diameter of about 1 cm (T1a-b), where the probability of metastasis in axillary lymph nodes is even <20%. The use of SLNB is also well established for patients with T1c tumors (diameter up to 2 cm), where the probability of lymph node metastasis is about 20–30%. Nevertheless, the procedure can be beneficial also in patients with larger tumors; therefore, contraindications to SLNB in patients with breast cancer are progressively decreasing, and also patients with larger tumors should be considered candidates for SLNB [56–59].

In addition to reducing the need for axillary lymph node dissection in patients with low probability of axillary metastasis (T1 tumors), SLNB is more accurate than conventional axillary dissection in detecting nodal disease [60]. With SLNB, histopathologic examination is limited to only one or very few lymph nodes, on average no more than two per patient, and the pathologist can more carefully examine the entire lymph node(s) using multiple sections and enhanced staining methods. In this manner, it is possible to detect very small metastatic foci <2 mm in size, or micrometastasis, and even smaller, isolated tumor cell clusters that could have been neglected in a conventional histopathologic examination of an entire axillary dissection, when about 15–25 nodes must be analyzed. Such greater accuracy in staging allows optimal planning of postsurgical adjuvant treatment.

Lymphatic circulation within solid tumors (including breast cancer) is generally grossly abnormal, disrupted, and inefficient. This has relevance for the modality of interstitial radiocolloid injection technique for lymphoscintigraphy, in that intratumoral administration as was originally proposed is less logical than peritumoral injection. In fact, lymphatic circulation in the peritumoral area is normal and actually represents the entrance site to lymph vessels of cancer cells detached from the growing edge of the tumor which eventually give rise to lymph nodal metastasis. Nevertheless, additional radiocolloid injection modalities have been proposed and widely adopted in clinical practice [38].

Three main parameters define an optimal radiocolloid administration technique for lymphoscintigraphy and subsequent radioguided SLNB: injection site, injected volume, and injected activity. A fourth parameter to be taken into account is the time elapsed between injection (with subsequent lymphoscintigraphy) and surgery, as it specifically influences the amount of radioactivity to be injected.

Since the intratumoral space is almost devoid of an efficient lymphatic circulation, intratumoral radiocolloid administration requires the injection of high activities (up to 370 MBq or 10 mCi) and large volumes of the radiocolloid suspension (up to 4 mL). Injection of such large volumes can substantially increase interstitial pressure at the injection site, thus possibly altering the pattern of lymphatic drainage and forcing the radiocolloids to enter drainage routes different from those prevailing in baseline conditions. Furthermore, most of the injected radioactivity is retained at the injection site, often causing interference—the so-called “shine-through” effect—in scintigraphic and intraoperative sentinel lymph node detection. Finally, the slow drainage of radiocolloids from the tumor can cause poor scintigraphic visualization of the lymphatic channels and possibly lead to failure of lymphoscintigraphic imaging and of intraoperative identification of the sentinel lymph node.

All the above considerations explain why, at present, most nuclear medicine centers prefer the peritumoral injection (ultrasound-guided) or intra/subdermal injection over the intratumoral route of radiocolloid administration. The technique of periareolar injection is also increasing in popularity [61–63].

The rationale of intra/subdermal administration stems from the fact that lymph is drained from the intra/subdermal space to the subcutaneous plexus, which is the merging point for lymph originating from the underlying breast parenchyma (see Fig. 28.5a). Thus, a radiocolloid injected intra/subdermally displays the same pathways of lymphatic drainage as the underlying breast gland and of cancer cells entering the lymphatic space. An important advantage of intra/subdermal injection is that radiocolloids migrate much faster from the administration site to lymph nodes compared to peritumoral injection and far more rapidly than the extremely slow migration after intratumoral injection. Furthermore, ultrasound guidance is not required for injection, as is often required for performing peritumoral injection. Similar considerations apply to the periareolar route of radiocolloid administration. The rationale for this technique is the fact that the lymph produced in the breast flows to the periareolar Sappey’s plexus, before draining to axillary lymph nodes (see Fig. 28.5b). An important advantage of this modality of radiocolloid injection is that it can be easily performed even in cases of nonpalpable or multifocal tumors [61, 63].

Peritumoral radiocolloid administration is usually performed by depositing two aliquots on each side of the tumor, each one of 0.1–0.5 mL containing 37–74 MBq of the radiopharmaceutical. Tumor location and injection sites may be identified by ultrasound and/or x-ray stereotactic guidance.

Intra/subdermal radiocolloid administration is performed on the skin overlying the tumor, in such a way so as to produce a small wheal. The injected volume is approximately 0.1–0.3 mL, containing about 37–74 MBq of radiocolloid, and in some nuclear medicine centers, more than one injection is performed in adjacent sites. Such intra/subdermal route of radiocolloid administration is generally preferred in the case of superficial, easily palpable tumors, especially if located in the outer quadrants of the breast.

Periareolar radiocolloid administration is generally performed with four injections of 25–30 MBq in 0.1–0.3 mL, each one at the edge of the areola at Sappey’s plexus. In some nuclear medicine centers, only two aliquots are injected, approximately on each side of the areola. Periareolar injection is often preferred for nonpalpable tumors.

Regardless of the injection site, after injection, the patient is asked to lightly massage the breast to facilitate lymphatic drainage of radiocolloids.

After intra/subdermal or periareolar radiocolloids administration, lymphatic drainage and subsequent lymph nodes visualization is usually quicker than after peritumoral injection (20–30 min compared to 2–3 h on average). After 15–18 h, during surgery, radiocolloids migrated to lymph nodes represent about 1% of the injected activity after intra/subdermal administration, while it is about 0.1% after peritumoral administration.

From the technical point of view, the easiest modality of radiocolloid injection is either the intra/subdermal or the periareolar route, after which scintigraphic visualization of lymphatic drainage is readily feasible and efficient, including dynamic acquisitions. In the published literature, there are no reports of significant differences in outcome between the intra/subdermal, the periareolar technique, and the peritumoral technique, except for differences in the frequency of visualization of lymph drainage to the internal mammary chain [64, 65]. After peritumoral administration, lymphoscintigraphy shows drainage to the internal mammary chain in 20–30% of the cases, while this fraction is much lower (3–5%) after intra/subdermal or periareolar administration. In some centers, the two different techniques are employed together in the same patient, while in other centers the intra/subdermal or periareolar route injection is preferred for superficial tumors and the peritumoral route for deeply seated tumors.

Cutaneous melanoma can manifest differently from patient to patient. It is often highly aggressive and usually fatal if diagnosed in advanced stages. Better prognosis is observed in the early stages, when the probability of distant metastasis is low and resection of the primary tumor and regional lymph nodes may be curative. Five-year mortality is approximately 20% in stages I–II but higher in stage III (35%) and stage IV (90%) [102, 103].

In the very early stages of disease (with Breslow thickness of 0.76–1.5 mm), the incidence of metastatic involvement of the sentinel lymph node(s) is only about 5%, but it increases up to 20% in melanomas with thickness between 1.5 and 4 mm. With Breslow thickness >4 mm, the incidence of metastases in sentinel lymph node rises to about 50% [104]. The 5-year mortality is around 70–75% with nodal metastasis, while it is much lower (about 30%) for patients without regional metastasis [104]. Radioguided SLNB probably has maximum clinical impact in the early-to-­intermediate stages of disease (Breslow thickness <4 mm) [105–108].

While in breast cancer axillary lymphadenectomy performed in case of positive sentinel node is mainly used to better stratify patients for prognostic purposes, in cutaneous melanoma locoregional lymphadenectomy represents immediately an effective therapy for patients with metastases in sentinel lymph nodes. In fact, lymphadenectomy reduces mortality from about 70% to about 50% in these patients. The status of the sentinel lymph node is included in the latest version of TNM staging for cutaneous melanoma reflecting its importance for staging and treatment [109, 110].

Similarly as for breast cancer patients, for patients with melanoma, the presence of a sentinel lymph node free from metastasis obviates the need for lymphadenectomy, while the presence of metastasis mandates it [111, 112].