11.3 Oncology Imaging Procedures
Key Takeaways
- Oncology NM includes Ga-67 tumor imaging, receptor/peptide agents, molecular breast imaging, lymphoscintigraphy/sentinel node mapping, and PET tumor tracers
- Sentinel node mapping uses filtered Tc-99m sulfur colloid or Tc-99m tilmanocept (Lymphoseek®, CD206 binding) with intradermal/interstitial injection and sequential imaging ± probe
- FDG PET/CT is the workhorse: whole-body or skull-base-to-mid-thigh FOV; melanoma often needs true vertex-to-toes coverage
- Specialized PET: PSMA agents for prostate cancer and Ga-68/Cu-64 Dotatate for somatostatin-receptor neuroendocrine tumors
- FDG patient prep emphasizes fasting (~4–6 h), controlled glucose, rest after injection, and timed uptake (~60 min typical) to avoid muscle and brown-fat artifacts
11.3 Oncology Imaging Procedures
Quick Answer: Know Ga-67 tumor imaging, peptide/receptor scans, molecular breast imaging, sentinel node (filtered SC / tilmanocept), and PET (FDG, PSMA, Dotatate) with correct FOV, prep, and timing.
Ga-67 Tumor Imaging
Ga-67 citrate binds transferrin and localizes in some tumors (classic teaching: lymphoma, selected other malignancies) and infection/inflammation. Physical t½ ≈ 78 h; principal gammas include ~93, 185, and 300 keV—use appropriate multi-energy windows and medium-energy collimation.
| Item | Teaching practice |
|---|---|
| Adult activity | Often ~5–10 mCi IV (protocol-dependent) |
| Imaging times | Commonly 48–72 h (sometimes earlier/later sets) |
| Biodistribution | Liver, spleen, bone marrow, bowel, lacrimal/nasopharynx—bowel prep may be used |
| Modern context | Largely replaced by FDG PET for many tumors, but still testable |
Bowel activity is a classic pitfall—delayed imaging, SPECT, and clinical correlation help.
Peptide / Receptor Imaging (Overview)
Receptor-targeted agents map tumors expressing specific surface targets:
| Target / class | Example agents (teaching) | Typical use |
|---|---|---|
| Somatostatin receptors (SSTR) | In-111 pentetreotide (Octreoscan®); Ga-68/Cu-64 DOTATATE PET | Neuroendocrine tumors |
| PSMA | F-18/Ga-68 PSMA PET agents | Prostate cancer staging/recurrence |
| Other peptides | Protocol-specific investigational/clinical agents | Selected solid tumors |
Tech points: verify correct agent and energy settings; hydrate; time imaging to package insert/SOP (In-111 pentetreotide often 4 and 24 h class; Dotatate PET ~45–90 min class). Know that cold somatostatin analogs (octreotide therapy) may need holding per protocol before SSTR imaging.
Molecular Breast Imaging (MBI)
Molecular breast imaging (also related historically to BSGI) uses a breast-specific gamma camera and typically Tc-99m sestamibi. The patient is positioned with gentle compression; each breast is imaged in standard projections. Indications include dense breasts, problem-solving after mammography, and selected high-risk evaluations—not a replacement for screening mammography in average-risk populations.
| Item | Notes |
|---|---|
| Agent | Tc-99m sestamibi (typical adult ~20–30 mCi class—follow protocol) |
| Injection | Preferably contralateral arm to the breast of concern when possible |
| Timing | Begin imaging ~5–10+ min after injection per SOP |
| Trap | Infiltration or wrong-side injection confounds interpretation |
Lymphoscintigraphy and Sentinel Node Mapping
Goal: identify the first draining lymph node(s) for surgical probe-guided biopsy (breast, melanoma, other).
| Tracer | Mechanism / notes |
|---|---|
| Filtered Tc-99m sulfur colloid | Smaller particles enter lymphatics from interstitial injection |
| Tc-99m tilmanocept (Lymphoseek®) | Binds mannose receptors (CD206) on reticuloendothelial cells in nodes |
Injection technique: intradermal, subdermal, peritumoral, or areolar protocols per surgeon/lab—not standard IV. Activity is low (often ~0.5–1 mCi class total, protocol-specific). Image injection site and drainage basins dynamically/static; mark skin as ordered; provide images/probe counts for the OR. Contamination at the skin and shine-through from the injection site are major pitfalls—use shielding, transmission markers, and careful positioning.
PET Tumor Imaging
FDG PET/CT — General Principles
F-18 FDG is a glucose analog trapped in cells with high glycolysis (many malignancies, infection, inflammation, brain, heart under some conditions).
| FOV choice | Typical use |
|---|---|
| Skull base to mid-thigh | Standard oncology staging for many solid tumors |
| True whole body (vertex to toes) | Melanoma, cutaneous malignancies, myeloma surveys, search for unknown primary when ordered |
| Limited regional | Therapy response or known single-site problem |
Adult activity is often ~10–20 mCi class (weight-based common). Uptake period typically ~60 minutes (45–90 min range) in a warm, quiet room; patient rests, avoids talking/chewing to reduce muscle and laryngeal uptake.
FDG Patient Prep (High Yield)
| Rule | Rationale |
|---|---|
| Fast ~4–6 hours (water OK per protocol) | Lower insulin/glucose competition |
| Check fingerstick glucose | High glucose decreases tumor FDG uptake; extreme hyperglycemia may require reschedule per policy |
| No strenuous exercise 24 h | Prevents muscle uptake |
| Warm waiting area | Reduces brown fat FDG uptake |
| Diabetes management | Follow lab SOP for insulin timing—never improvises doses without protocol |
| Remove metal; empty bladder | Artifact and pelvic clarity |
Brown fat (supraclavicular, paravertebral) and tense muscle uptake are classic false “nodes.” Document recent chemo/radiation, growth-factor therapy, and infection—they alter patterns.
PSMA PET
Prostate-specific membrane antigen ligands (Ga-68 or F-18 labeled) image prostate cancer soft-tissue and bone disease with high target contrast. Prep is agent-specific (often less stringent fasting than FDG); hydrate and void. Know it is not FDG—different biology and biodistribution (salivary, lacrimal, kidneys, bladder, bowel).
Neuroendocrine (Dotatate) PET
Ga-68 DOTATATE (and related SSTR agonists) maps somatostatin receptor–positive NETs. Imaging after appropriate uptake time; correlate with prior octreotide therapy holds. Normal intense uptake in pituitary, spleen, kidneys, adrenals, and uncinate process of pancreas can trap newcomers—know expected biodistribution at a tech level.
Oncology Procedure Comparison
| Study | Primary agent class | Tech focus |
|---|---|---|
| Ga-67 tumor | Ga-67 citrate | ME collimator, 48–72 h, bowel activity |
| MBI | Tc-99m sestamibi | Breast-specific camera positioning |
| Sentinel node | Filtered SC or tilmanocept | Interstitial injection, mapping, probe |
| FDG PET | F-18 FDG | Prep, glucose, FOV, 60-min uptake |
| PSMA PET | PSMA ligand | Prostate cancer pathway, hydration |
| Dotatate PET | SSTR agonist | NET pathway, med holds |
Match cancer type → tracer → FOV → prep. That tetrad wins Domain V oncology items.
Documentation and Coordination
Oncology days often combine injection, long uptake, PET/CT acquisition, and surgeon coordination for sentinel-node cases. Document glucose, last meal time, injection site and residual syringe activity, uptake duration, and any insulin or steroid use. For receptor studies, record recent cold-analog therapy. Clear handoff notes prevent repeat injections and protect quantitative comparisons on therapy-response scans.
For staging cutaneous melanoma with FDG PET/CT, which field of view is most appropriate when a true whole-body survey is ordered?
Which pair correctly matches a sentinel lymph node radiopharmaceutical with its localization concept?
Which preparation instruction is most important before F-18 FDG PET oncology imaging?