Chemical Pathology 3: Tumour Markers Flashcards
Tumour markers
Substance produced by tumour / An effect of tumour on healthy tissue:
1. Enzyme
- ∵ High metabolic demand of proliferative cell —> nonspecifically elevated in tumours
—> LDH —> in haematologic malignancy (high cell turnover)
—> Alkaline phosphatase —> in metastatic carcinoma of bone, HCC
—> Neuro-specific enolase —> in neuroendocrine tumours
- Level tend to correlate with no. of tumour cells —> useful for monitoring success of therapy
- Specific protein
- β2 microglobulin
—> on surface of all nucleated cells
—> nonspecific marker of high cell turnover
- Immunoglobulin
—> specific measure of plasma cell production of monoclonal proteins in Multiple Myeloma (e.g. one type of Ig elevated, other suppressed) - Hormone
- e.g. ACTH, GH
- diagnosing Endocrine tumours - Oncofetal antigen
- e.g. AFP, CEA (carcinoembryonic antigen)
- expressed transiently in normal fetal development —> turned on again in tumour formation - Carbohydrate antigen
- e.g. CA15-3 in Breast, CA125 in Ovarian, CA19-9 in GI (pancreas, stomach)
- monoclonal defined antigens directly identified from tumour extracts / cell lines
- use for **monitoring treatment but **NOT diagnosis - Receptor
- used to classify tumours for therapy
- e.g. Estrogen / Progesterone receptors +ve in breast cancer —> Tamoxifen therapy - Metabolite
- Urine metabolomics —> promising approach for bladder cancer detection: metabolites released from bladder cancer cells may be enriched in urine samples
Ideal tumour marker criteria
- Produced by tumour cells —> enter circulation
- Present at low level in normal individuals / benign disease but ***increase substantially in cancer (preferably only in 1 type of cancer —> more specific)
- Easily quantifiable with an inexpensive assay (readily available to measure)
- Present in detectable quantities at early / preclinical stage
- Quantitative levels proportional to tumour burden
- High diagnostic sensitivity (few false negatives) and specificity (few false positives)
Clinically all available tumour markers do not fit criteria
—> must combine with history, clinical signs and symptoms, histology to facilitate decision making
Use of tumour markers
Descending order of current usefulness:
- Prediction of therapeutic response
- very few markers have predictive power
- except **Steroid receptors, **HER2 receptors in breast cancer - Monitoring effectiveness / response to therapy
- current biomarkers can already be readily interpretable and more economical than imaging modalities - Population screening
- current biomarkers suffer from too low diagnostic sensitivity / specificity
- current biomarkers only elevated at late stages of disease except ***PSA - Diagnosis
- current biomarkers suffer from too low diagnostic sensitivity / specificity - Determine prognosis
- most cancer markers have some prognostic value —> but prediction accuracy is rather poor —> cannot determine specific therapeutic interventions - Tumour staging
- poor accuracy of markers except ***AFP, HCG - Detect early recurrence (controversial)
- lead time (between detection and recurrence) is short —> not affect outcome
- clinical relapse could occur without biomarker elevation
- biomarker elevation can be nonspecific
Screening
Most tumour markers are found in normal / benign conditions as well
—> PSA in BPH
—> false positive result
—> unnecessary treatment / biopsy
Low prevalence
—> decrease ***positive predictive value (∵ less true positive)
—> even tumour markers that are highly sensitive and specific are not useful for screening
—> only AFP, PSA are used in screening (∵ high incidence)
Cancer susceptibility testing
Using molecular diagnostic
—> identify germline mutations in patients with family history of:
1. Breast / Ovarian cancer: BRCA1, BRCA2
2. Familial colon cancers: APC
Diagnosis
- Some tumour markers can be used to aid diagnosis
- Significant elevations of HCG, AFP
—> suggest **Extragonadal germ cell tumour
—> should be obtained in all patients with **poorly differentiated tumour of unknown primary sites
***Prognosis
- Tumour marker concentration generally ***increase with tumour progression
- Tumour marker levels at diagnosis can reflect ***aggressiveness of tumour
—> help predict outcome for patients
Monitor effectiveness of therapy, disease recurrence
After surgery / chemotherapy
—> usually tumour makers decrease serially
If initial treatment is effective
—> tumour markers can be used as a highly sensitive marker of recurrence (level will ↑)
Sometimes tumour markers (e.g. CEA) have a lead time of ***several months before disease can be detected by other modalities
—> tumour markers can pick up early
—> allow earlier treatment during relapse
Laboratory considerations for tumour marker measurement
- Wide variability in tumour marker concentration between different ***manufacturers
- due to differences in Ab specificity, Analyte heterogeneity, Assay design etc.
—> Not directly comparable —> need to use same methodology to monitor tumour marker concentration in patient - Wide concentration range of tumour markers
- Immunoassays = most commonly used to measure tumour markers - Extremely high tumour marker concentration
- analyte / antigen concentration exceed working range
—> Capture / Label Ab can be saturated by antigen
—> lack of “sandwich” formation
—> Antigen excess / Hook effect
—> falsely low measurement (not proportional to tumour burden)
—> Underestimate actual tumour marker concentration
—> High dose hook shape can be mistaken as responding (以為Tumour marker跌左其實只係Hook effect)
—> can resolve problem by ***Dilution to within analytical range
—> repeat testing
—> yield higher / more accurate values - Heterophile Ab
- circulating Ab in an individual which is against human / animal immunoglobulin reagents (e.g. patients exposed to mice develop anti-mouse Ab (HAMAs))
—> significant interference in immunoassays
—> false-positive / false-negative
—> can resolve problem by investigate results that are inconsistent with history and clinical scenario
—> dilute samples —> analyse linearity of dilutions: samples with heterophile Ab do not give linear results upon dilution
OR
—> use commercial blocking reagents to remove HAMAs
High-performance liquid chromatography (HPLC)
Extraction process separates analytes of interest from plasma / urine —> applied to a column: separated by their physical characteristics (charge, size, polarity)
—> used for detection of small molecules
—> e.g. endocrine (***Catecholamine) metabolites in plasma / urine
—> help diagnose carcinoid tumours, pheochromocytoma, neuroblastoma
Advantages:
- Not subject to Hook effect
- Not subject to Heterophile Ab
Disadvantages:
- More labour-intensive —> expensive
- Require more experience and skills than automated immunoassay
Alpha-fetoprotein (AFP)
- Glycoprotein related to Albumin
- Transport protein —> Regulate oncotic pressure in ***fetus
- Infants initially have high serum AFP values —> decline to adult levels at age 2
- Measured using “sandwich” immunoassays —> subject to high dose ***Hook effect
- Display heterogeneity
—> certain isoforms preferentially produced by malignant cells
—> increase specificity of AFP by analysis of ***Lectin-binding AFP (tumour specific)
—> increase only in HCC but not in benign liver conditions (use ELISA)
- Diagnosis, Staging, Prognosis, Treatment monitoring of ***HCC
- used to detect HCC in area with high prevalence e.g. China
- conjunction with Ultrasound every 6 months in patients at high risk of HCC (e.g. Hep B / Hep C-induced cirrhosis)
- evaluation of patients with liver mass
—> very high levels in high risk individual
—> diagnostic of HCC
—> but 20% HCC patient have normal AFP - if normal —> can be metastatic tumour
- Classification, Monitoring therapy for Testicular cancer (Yolk sac tumour, Embryonal carcinoma)
- Yolk sac tumour: always present in high amount, proportional to volume - AFP is not completely specific for HCC, Testicular cancer
- AFP can increase in:
—> **Pregnancy, Liver regeneration, Hepatitis, Alcoholic liver disease, Cirrhosis, Biliary tract obstruction
—> Other types of malignancy e.g. **Gastric adenocarcinoma
Case 1 and 2
See notes
Human Chorionic Gonadotropin (HCG)
Glycoprotein, Dimeric hormone (Alpha + Beta subunits)
- degrade into multiple fragments
1. **Intact hCG molecule (detected by most immunoassays)
2. Nicked hCG
3. **Free beta subunit (detected by most immunoassays) —> sensitive, specific for aggressive neoplasms
4. Hyperglycosylated intact form
Elevated in:
- Pregnancy (by placenta)
-
Gestational trophoblastic diseases (invariably elevated)
- Placental site **trophoblastic tumours
- **Choriocarcinoma
- Hydatidiform mole
- Persistent / Invasive gestational trophoblastic neoplasia - ***Germ cell tumours (diagnostic marker):
- Testicular cancer (for classification)
- Ovarian cancer (prognostic indicator) - Healthy peri-menopausal / post-menopausal women
- Pituitary gland can produce persistently low levels of hCG
- how to differentiate —> Pituitary hCG production can be suppressed by a 3-week oral contraceptive pills
Prostate-specific antigen (PSA)
Serine protease present in **Prostatic epithelial cells, **Seminal fluid
- Short t1/2 (2-3 days)
- 2 major forms circulating:
1. Free
2. Bound to α1-antichymotrypsin / α2-microglobulin (mostly)
—> normal: Free + Bound < 4 ng/ml
- PSA levels ***increase with age
- Prostate cancer release ***30x PSA than normal
—> used in screening + early detection
—> PPV = 50% for PSA >10 μg/L (4-10: Diagnostic grey zone)
—> biopsy to confirm presence
Prostate biopsy:
- costly
- complications
Controversy:
- some RCT shown that PSA screening reduces prostate cancer mortality, another large trial showed no benefit
- ~20% men with prostate cancer have PSA levels within normal ranges
- also elevated in:
1. **Prostate infections
2. **BPH
3. Digital rectal exam
4. Recent ejaculation etc. - concern of over-diagnosis since prostate cancer has low mortality rate
- screening utility decrease with age —> not appropriate for men with <10 year life expectancy
In addition to standard cutoff values of **Total PSA
1. PSA velocity (rate of rise over time)
2. PSA density
3. Free PSA / Total PSA (%fPSA: free to total PSA ratio) —> help differentiate between benign condition and malignancy
—> BPH patients showed **significant increase in %fPSA
—> Prostate cancer patients showed ***reduced %fPSA
—> %fPSA <25% detects 95% prostate cancer
After radical prostatectomy
—> serum PSA should become undetectable
—> increase in PSA strongly suggest recurrence
—> lead time (delay in clinical symptoms) 1-5 years
[-2]proPSA (p2PSA) and PHI
Both have larger AUC in sensitivity / 100-specificity curve
[-2]proPSA (p2PSA):
- significantly elevated in prostate cancer patients (more consistent compared to other proPSA)
- elevated in almost all of peripheral zone cancer but not in transitional zone tissue (BPH)
—> more prostate cancer specific
**Beckman Coulter Prostate Health Index (PHI)
(p2PSA / fPSA) x (√PSA)
- PHI can stratify risk of prostate cancer in men with PSA **4-10
- PHI 35: commonly used cutoff
