What are the indications, procedure details, and contraindications for a 99mTc (technetium‑99m) sulfur colloid scan?

99mTc Sulfur Colloid Scan

99mTc sulfur colloid is a versatile radiotracer used primarily for sentinel lymph node mapping in breast cancer and melanoma, imaging reticuloendothelial cells in the liver/spleen/bone marrow, and evaluating gastrointestinal transit and aspiration.

Primary Indications

Oncologic Applications

For sentinel lymph node (SLN) localization, 99mTc sulfur colloid is the standard radiopharmaceutical in the United States 1. The FDA-approved indications include:

• Lymph node mapping in breast cancer and malignant melanoma patients when used with a hand-held gamma counter 1
• Peritoneo-venous (LeVeen) shunt patency evaluation in adults 1

In breast cancer specifically, SLN biopsy using 99mTc sulfur colloid is appropriate for patients with unifocal tumors <4 cm, clinically negative lymph nodes, and no prior breast surgery that disrupted lymphatic flow 2. The technique demonstrates 99.8% identification rates when properly performed 3.

Reticuloendothelial Imaging

Both adults and pediatric patients can undergo imaging of functioning reticuloendothelial cells in the liver, spleen, and bone marrow 1. This application is particularly useful for diagnosing conditions like extramedullary hematopoiesis in myelofibrosis 4 and identifying accessory splenic tissue 5.

Gastrointestinal Studies

The agent is indicated for esophageal transit studies, gastroesophageal reflux evaluation, and detection of pulmonary aspiration of gastric contents in both adults and children 1.

Procedure Details

Dosing by Indication

Lymph node mapping (subcutaneous injection):

• 3.7 to 37 MBq (0.1 to 1 mCi) in volumes of 0.1 to 1 mL 1
• Most centers use 18.5-92.5 MBq for optimal visualization 6, 7

Liver/spleen imaging (intravenous):

• Adults: 37 to 296 MBq (1 to 8 mCi) 1
• Newborns: 7.4 to 18.5 MBq (0.2 to 0.5 mCi) 1
• Children: 0.56 to 2.78 MBq (0.015 to 0.075 mCi) per kg body weight 1

Bone marrow imaging (intravenous):

• Adults: 111 to 444 MBq (3 to 12 mCi) 1
• Pediatric: 1.11 to 5.55 MBq (0.03 to 0.15 mCi) per kg body weight 1

Timing Considerations

For SLN mapping, injection timing significantly impacts visualization rates:

• Day-before-surgery injection (6-18 hours prior) with high-dose 99mTc sulfur colloid (≥37 MBq) detects more nodes than early imaging and allows for preoperative lymphoscintigraphy 8, 7
• Same-day injection (2-4 hours preoperatively) is commonly used but may result in lower detection rates 2
• Intraoperative injection is feasible and convenient, achieving 99.8% identification rates without requiring nuclear medicine visits 3

Delayed imaging generally detects more nodes than early imaging, though the intraoperative gamma probe remains more sensitive than gamma camera imaging 8.

Preparation Factors

Particle size critically affects lymphatic migration:

• Filtered 99mTc sulfur colloid (particles <0.1 micron) demonstrates faster transport to lymph nodes and is preferred for lymphoscintigraphy 9
• Unfiltered preparations have bimodal particle distribution but remain effective 10
• Heating protocol affects particle size: reduced heating time with older 99mTc-pertechnetate elution produces the highest percentage of small particles (<0.03 microns) 10

The preparation method (dry heat block vs. water bath), amount of 99mTc-pertechnetate added, and time between calibration and injection do not significantly affect clinical outcomes 11.

Injection Technique for Breast Cancer

Optimal injection technique remains somewhat controversial 8:

• Peritumoral injection is most common but has higher nonvisualization rates
• Subdermal or periareolar injection shows better visualization rates
• Combined injection sites (peritumoral + periareolar) are used at some centers
• Four-point injection technique (at 2,5,7, and 10 o'clock positions) is recommended when using blue dye 2

Critical technical point: Perform SLN mapping before excising the primary tumor to avoid disrupting the lymphatic network 2.

Imaging Protocol

For lymphoscintigraphy:

• Gamma camera imaging at 60-90% visualization rate in most centers 8
• Internal mammary node visualization ranges from <10% to 40% depending on tumor location and technique 8
• Medial breast tumors more likely drain to internal mammary nodes, though any location can drain there 8

Intraoperative detection:

• Gamma probe more sensitive than gamma camera for detecting axillary SLNs 8
• Even with negative lymphoscintigraphy, probe-based detection should be attempted 8

Contraindications and Precautions

Absolute Contraindications

There are no absolute contraindications listed in the FDA label 1.

Relative Contraindications and Special Populations

Pregnancy:

• Breast lymphoscintigraphy during pregnancy presents very low fetal risk with maximum absorbed dose of 4.3 mGy (well below the 50 mGy threshold for adverse effects) 6
• However, sentinel node biopsy should not be offered to pregnant women under 30 weeks gestation 12
• Limited data exist on radioactive tracer use in pregnancy; isosulfan blue or methylene blue dye is discouraged 12

Lactation:

• Advise lactating women to pump and discard breast milk for 24 hours and minimize close infant contact for 6 hours after receiving 99mTc sulfur colloid 1

Pediatric patients:

• Approved for use in children for reticuloendothelial and gastrointestinal imaging 1
• For oral/oropharyngeal cancer, most trial protocols exclude pediatric patients 13

Other considerations:

• Known allergy to albumin colloid is a relative contraindication 13
• Primary tumor treatment by external beam radiotherapy may affect lymphatic drainage 13

Patient-Specific Factors Affecting Outcomes

Obese and elderly patients have higher rates of false-negative lymphoscintigraphy compared to thinner, younger patients 8.

Safety Profile

Radiation Exposure

The radiation burden is remarkably low:

• Effective dose approximately 0.048-0.2 mSv 13
• Comparable to 1 day of natural background radiation 13
• Approximately 4% of the dose administered for a conventional bone scan 8
• No isolation, precautions, or special radiation monitoring required 8

Fetal exposure with 18.5 MBq injection: maximum 0.013 mSv, orders of magnitude below the 100-200 mSv threshold for deterministic effects 13.

Adverse Reactions

Anaphylactic reactions including rare fatalities have occurred following intravenous administration 1. Have resuscitation equipment and personnel immediately available.

The most frequently reported adverse reactions include:

• Rash
• Urticaria
• Anaphylactic shock
• Hypotension 1

Allergic reactions to blue dye (when used concurrently) occur in 1-2% of patients, with true anaphylaxis in 0.25-0.5% 8.

Clinical Pearls

For optimal SLN detection in breast cancer:

• Use both radiocolloid and blue dye for increased sensitivity 2
• High-volume SLN surgeons exhibit improved detection rates 2
• If no SLN identified intraoperatively, complete axillary lymph node dissection should be considered 8
• Nonvisualization on gamma camera plus nondetection by probe associates with higher frequency of nodal involvement 8

Preparation optimization:

• Use filtered 99mTc sulfur colloid for lymphoscintigraphy when rapid nodal migration is desired 9
• Both filtered and unfiltered preparations are effective in the United States 8