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Neurology · Neuro-oncology

Neuro-Oncology PET (Amino-Acid Tracers)

Snapshot

For brain tumours, amino-acid PET (¹⁸F-FET, ¹⁸F-FDOPA, ¹¹C-methionine) outperforms FDG because normal cortex has low amino-acid uptake, giving high tumour-to-background contrast. It aids glioma grading and delineation, biopsy and radiotherapy planning, and — most usefully — distinguishing recurrent/residual tumour from treatment-related change (pseudoprogression, radionecrosis). FDG PET is limited by high cortical background.

Read the full article →In-depth, fully referenced version

Gliomas over-express L-type amino-acid transporters (LAT1), so amino-acid analogues accumulate in tumour with little uptake in surrounding brain. This contrast underpins their advantage over FDG for delineation and recurrence assessment.

Amino-acid > FDGLow cortical background
Recurrence vs necrosisKey use
RT/biopsyTarget definition
Simulated MRI, FET PET and FDG PET slices after treatment of a left frontal glioma, with schematic dynamic FET curves: recurrence shows intense FET uptake and an early-peaking, declining curve; treatment change shows a faint rim and a steadily rising curve; FDG uptake in the lesion is no higher than cortex.
Figure. Simulated images. When new MRI enhancement follows glioma treatment, amino-acid PET separates recurrence (high tumour-to-brain uptake; the dynamic ¹⁸F-FET curve peaks early, then declines) from treatment-related change (low uptake; a continuously increasing curve); both FET images share one colour scale. FDG is less useful because high cortical uptake hides the lesion (after PET-RANO/EANO, Albert et al., 2016, and the EANM/EANO/RANO/SNMMI guideline, Law et al., 2019).

When to image

  • Distinguishing tumour recurrence/residual disease from radionecrosis or pseudoprogression.
  • Delineating tumour extent for biopsy and radiotherapy planning.
  • Supporting glioma grading and prognostication.
  • Assessing treatment response.

How to read it

  • Increased amino-acid uptake with high tumour-to-brain ratio suggests active tumour.
  • Dynamic ¹⁸F-FET time–activity curves help separate high-grade tumour from benign change.
  • Treatment-related change tends to show lower tumour-to-brain ratios and continuously increasing FET time–activity curves (an early peak with a declining curve suggests high-grade tumour) — correlate with MRI.

Protocol

  • Static ± dynamic amino-acid PET (FET/FDOPA/MET) with tumour-to-background ratios.
  • Co-register with contrast-enhanced MRI for anatomical correlation.

Diagnostic performance

  • Amino-acid PET separates recurrence from treatment change more accurately than FDG.
  • High tumour-to-background contrast improves delineation for planning.
  • Some uptake occurs in inflammation — interpret with MRI.

Pitfalls

  • FDG’s high cortical uptake obscures many gliomas — prefer amino-acid tracers.
  • Inflammatory and post-treatment changes can take up amino-acid tracers.
  • Availability of amino-acid tracers is limited in some centres.
Evidence & guidelines
  • PET-RANO/EANO recommendations (Albert et al., 2016) and the joint EANM/EANO/RANO/SNMMI guideline (Law et al., 2019) support amino-acid PET in glioma management.
  • Amino-acid PET is complementary to advanced MRI (perfusion/spectroscopy).
In depth
  • Amino-acid and receptor tracers exploit high tumour-to-brain contrast; NK-1 receptor (substance P) is overexpressed in 55/58 gliomas (WHO II–IV) and on tumour neovasculature.
  • Targeted alpha therapy delivers ²¹³Bi- or ²²⁵Ac-DOTA-substance P locoregionally through stereotactically placed intratumoral or intracavitary catheters, with co-injected ⁶⁸Ga-DOTA-substance P PET/CT used to check the distribution.
  • Alpha particles have a very short tissue range (<100 µm) and high LET (roughly 100 keV/µm), causing complex DNA double-strand breaks whose effect depends little on oxygenation or cell-cycle phase, which may help overcome radioresistance.
  • ²¹³Bi (half-life 46 min) decays mainly via the alpha emitter ²¹³Po; ²²⁵Ac (half-life 9.9 days) releases four alpha particles through its decay chain, and its longer half-life was introduced to improve dose distribution within the tumour.
  • The ²¹³Bi alpha emission has a mean tissue range of about 81 µm, far shorter than the several-millimetre range of the beta emitter ⁹⁰Y, favouring treatment of gliomas in eloquent (critically located) brain.
  • ⁹⁰Y-DOTATOC targets somatostatin receptor subtype 2, which is expressed mainly in lower-grade (WHO grade II–III) gliomas; somatostatin-receptor expression is inversely related to substance P receptor expression in glioblastoma, limiting its use there.
  • In a 2008 pilot study, resection-cavity ¹³¹I-81C6 (anti-tenascin) plus radiotherapy and temozolomide gave a median overall survival of about 21 months in glioblastoma, and ²¹¹At-ch81C6 in the resection cavity was feasible and safe in recurrent brain tumours.

Sources: Kneifel et al. Clin Cancer Res 2006 · 12:3843-50 (PMID 16778112) · Hennig et al. Int J Cancer 1995 (PMID 7790112) · Królicki et al. EJNMMI 2021 · 48:3595-605 (PMID 33860346) · Królicki et al. EJNMMI 2021 (PMID 33860346) · Królicki et al. Clin Nucl Med 2023 (PMID 36854309) · Cordier et al. EJNMMI 2010

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