Cancer Monitoring: Tests, Imaging, and Liquid Biopsy

Cancer monitoring is the systematic, ongoing process of evaluating disease status in patients who have been diagnosed with cancer — whether during active treatment, after completing curative therapy, during intentional observation of a low-risk tumor, or while managing advanced disease over time. Understanding what cancer monitoring involves, and why each tool is used, helps patients make sense of what can otherwise feel like an endless series of blood draws, scans, and appointments.

Cancer monitoring is not the same as cancer screening, which detects cancer in people with no prior diagnosis. It is also distinct from cancer follow-up care, which focuses on survivorship issues like late effects and psychosocial support. Cancer monitoring is specifically about tracking what the cancer is doing — whether it is responding to treatment, remaining stable, growing, or returning after appearing to have been eliminated.

8–10 mo Median lead time by which liquid biopsy (ctDNA) detects cancer recurrence before conventional imaging in breast cancer studies — offering an earlier window for intervention

RECIST 1.1 International standard for measuring solid tumor response on CT/MRI: ≥30% decrease = partial response; ≥20% increase or new lesions = progressive disease

~30% Proportion of low-risk prostate cancer patients on active surveillance who convert to active treatment within 2 years — often driven by monitoring anxiety rather than true pathologic grade progression

≥5 CTCs CellSearch threshold for circulating tumor cells per 7.5 mL blood associated with shorter PFS and OS in metastatic breast, colorectal, and prostate cancer (Cristofanilli et al., NEJM 2004)

What Is Cancer Monitoring?

Modern cancer monitoring encompasses four distinct contexts:

On-treatment monitoring: Tracking response to chemotherapy, targeted therapy, immunotherapy, or radiation — assessing whether the cancer is shrinking, stable, or growing while on active therapy
Post-treatment surveillance: Periodic evaluation after curative-intent treatment to detect recurrence at its earliest, most treatable stage
Active surveillance: Intentional, structured monitoring of a low-risk tumor without immediate active treatment — curative therapy is deferred but not abandoned; a clear protocol triggers intervention if the cancer changes
Metastatic disease monitoring: Ongoing assessment to determine whether a treatment regimen is still working and when to change strategies

For more on the survivorship context of monitoring — including late effects, care coordination, and survivorship care plans — see the cancer follow-up care guide. For clinical trials testing next-generation monitoring technologies, see cancer clinical trials.

Tumor Markers — What Blood Tests Can (and Cannot) Tell You

Tumor markers are proteins, antigens, or other substances measurable in blood or body fluids that are produced by cancer cells or by the body in response to cancer. Used serially over time, they provide a trend line your physician can act on — even when a single value in isolation means little.

Marker	Cancer Type	Primary Monitoring Use
CEA	Colorectal (primary); breast, lung, gastric	Post-surgical CRC surveillance every 3–6 months × 5 years; rising CEA can identify potentially curable hepatic/pulmonary metastases
PSA	Prostate cancer	Active surveillance; biochemical recurrence post-RP (threshold >0.2 ng/mL × 2); post-RT Phoenix criterion (nadir + 2 ng/mL); metastatic monitoring
CA-125	Ovarian cancer	Treatment response; recurrence surveillance; ~80–90% sensitivity for advanced disease; rising CA-125 prompts CT imaging
AFP	HCC; non-seminomatous germ cell tumors	HCC surveillance every 6 months (with ultrasound per AASLD); testicular cancer treatment response
Beta-hCG	Choriocarcinoma; non-seminoma testicular	One of oncology’s most sensitive markers — even minor post-treatment elevations are clinically significant
Thyroglobulin (Tg)	Differentiated thyroid cancer	Post-thyroidectomy; suppressed Tg <0.2 ng/mL = excellent response; rising Tg triggers neck ultrasound ± whole-body scan
Calcitonin	Medullary thyroid carcinoma	Primary biomarker for this tumor type; highly sensitive and specific
Chromogranin A	Neuroendocrine tumors (NETs)	Disease burden and treatment response monitoring
CA 19-9	Pancreatic; biliary tract cancer	Treatment response monitoring (not useful for screening or early detection)
LDH	Lymphoma; melanoma; germ cell tumors	Non-specific disease burden marker; incorporated into staging systems
CA 15-3 / CA 27-29	Breast cancer	Metastatic disease monitoring only — ASCO explicitly does NOT recommend for asymptomatic early-stage surveillance; no survival benefit shown

Tumor Markers Cannot Stand Alone

A rising marker is a signal — not a diagnosis. It prompts confirmatory imaging, not an immediate treatment change. A normal marker does not rule out recurrence in most cancer types. Never adjust your cancer management based solely on a tumor marker result without imaging confirmation and discussion with your oncology team.

Imaging in Cancer Monitoring

CT Scan and RECIST 1.1

CT scanning is the workhorse of cancer response assessment for solid tumors. Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 provides the international standard for measuring tumor response — used in clinical trials and clinical practice alike. Radiologists identify up to 5 measurable target lesions and track the sum of their longest diameters across scans.

Complete Response (CR)

Disappearance of all target lesions — no measurable disease remaining on imaging

Partial Response (PR)

≥30% decrease in the sum of longest diameters — significant shrinkage, cancer still present

Stable Disease (SD)

Neither PR nor PD criteria met — cancer not growing enough to progress or shrinking enough to respond

Progressive Disease (PD)

≥20% increase in sum of diameters, or any new lesions — treatment is not controlling disease

In most metastatic solid tumors, CT imaging is repeated every 8–12 weeks (every 2–3 treatment cycles) during first-line treatment. When your oncologist says your cancer has “partially responded” or “progressed,” they are usually applying RECIST criteria even if they do not use that term. Stable disease — which might sound like bad news — can represent an excellent outcome for many advanced cancers where the goal is disease control rather than cure.

PET-CT and the Deauville Scale

PET-CT exploits the Warburg effect — cancer cells consume glucose at dramatically higher rates than normal tissue. FDG (18F-fluorodeoxyglucose), a radioactive glucose analog, accumulates in metabolically active tumor tissue and appears as bright “hot spots” on imaging. The Lugano classification and Deauville 5-point scale are standard for lymphoma response: Deauville 1–2 = complete metabolic response; Deauville 3 = borderline (context-dependent); Deauville 4–5 = residual metabolic disease. The RATHL trial (Johnson PWM, NEJM 2016) established that Hodgkin lymphoma patients with a negative interim PET at 2 cycles can safely omit bleomycin, reducing pulmonary toxicity without compromising cure rates.

PSMA-PET (piflufolastat/Pylarify; flotufolastat) has transformed prostate cancer monitoring — identifying metastatic disease at PSA levels below 1 ng/mL where conventional CT and bone scan are essentially blind.

MRI and the Pseudoprogression Challenge

MRI is essential for brain tumor monitoring, liver characterization, pelvic tumors, and bone marrow assessment. In patients with glioblastoma who undergo concurrent chemoradiation with temozolomide, 20–30% show apparent tumor enlargement on the first post-treatment MRI (typically 6–12 weeks after radiation) — representing inflammatory pseudoprogression rather than true cancer growth. Multiparametric MRI (mpMRI) with PI-RADS scoring is now standard in prostate cancer active surveillance, with a negative predictive value approaching 90% for ruling out clinically significant cancer in PI-RADS 1–2 lesions.

Liquid Biopsy — The Future of Cancer Monitoring

Liquid biopsy is one of the most rapidly evolving areas in oncology. Circulating tumor DNA (ctDNA) — fragments of cancer DNA shed into the bloodstream during cell death — has a plasma half-life of approximately 90 minutes, meaning ctDNA levels reflect current tumor burden almost in real time. This contrasts sharply with imaging, which can only detect anatomically visible disease that has already grown to a detectable size.

Resistance mutation detection. The EGFR T790M plasma test is now a standard alternative to repeat tissue biopsy in patients with EGFR-mutant NSCLC who progress on first-line EGFR TKI therapy. Plasma T790M sensitivity is approximately 70% versus ~80% for tissue, but avoids an invasive procedure. T790M-positive patients (plasma or tissue) are eligible for osimertinib.

Post-surgical minimal residual disease (MRD) detection. The landmark DYNAMIC trial established ctDNA-guided adjuvant chemotherapy in stage II colorectal cancer:

DYNAMIC Trial — NEJM 2022 (Tie J et al.): ctDNA Guides Adjuvant Chemo in Stage II Colorectal Cancer

86%2-yr DFS: ctDNA+ patients who received adjuvant chemo

68%2-yr DFS: ctDNA+ patients who skipped adjuvant chemo

93%2-yr DFS: ctDNA− patients (similar whether or not given chemo — safely avoided)

First prospectively validated biomarker for guiding adjuvant chemotherapy in early-stage colorectal cancer — ctDNA-negative patients achieved excellent outcomes without chemotherapy and its associated toxicity.

Early recurrence detection. In breast cancer, ctDNA becomes detectable a median of 8–10 months before recurrence is visible on conventional CT imaging — and months earlier in several other solid tumor types as well. Whether acting on ctDNA-positive results before imaging-detectable disease improves survival is under active investigation in multiple trials (MERMAID-2 for lung cancer; INTERCEPT for various solid tumors).

cancer monitoring liquid biopsy ctDNA blood test detects minimal residual disease — Liquid biopsy detects circulating tumor DNA (ctDNA) from a simple blood draw — enabling cancer monitoring at the molecular level, with ctDNA becoming positive months before recurrence is visible on conventional CT imaging.

Circulating tumor cells (CTCs) — intact cancer cells shed from tumors into the bloodstream — are measured by the FDA-cleared CellSearch system. In metastatic breast cancer, ≥5 CTCs per 7.5 mL is associated with significantly shorter progression-free and overall survival (Cristofanilli M et al., NEJM 2004). Multiple FDA-cleared and breakthrough-designated ctDNA MRD tests are available: Signatera (Natera), Guardant Reveal, and Foundation One Tracker. Liquid biopsy is a powerful complement — not yet a replacement — for tissue biopsy, which remains essential for initial diagnosis and comprehensive molecular profiling.

Active Surveillance — When Monitoring Is the Treatment

Active surveillance is a structured, curative-intent monitoring program for low-risk cancers — curative treatment is deferred but not abandoned. It is fundamentally different from watchful waiting (expectant management), which is palliative: the patient is observed with no plan for curative intervention, and therapy starts only when symptoms require it.

Prostate cancer active surveillance is the NCCN-preferred option for low-risk localized prostate cancer (Grade Group 1; some favorable intermediate-risk patients):

PSA every 3–6 months for year 1; then every 6–12 months if stable
Multiparametric MRI at baseline; repeated for PSA changes or at 1-year intervals
Confirmatory biopsy within 6–12 months; repeat systematic ± MRI-targeted biopsy every 1–4 years
Triggers to convert to active treatment: grade reclassification (GG≥2); PSA doubling time <3 months; PI-RADS upgrade to 4–5; patient preference

The PROTECT trial at 15-year follow-up (Hamdy FC et al., NEJM 2023) found prostate cancer-specific mortality of 3.1% in the active monitoring arm versus 2.4% surgery and 3.0% radiation — not statistically significant (p=0.53). However, the active monitoring arm had more distant metastases (9.4% vs. 4.7% for surgery, p=0.008) — confirming that vigilant monitoring adherence is what makes active surveillance safe. Other cancers on active surveillance include low-risk papillary thyroid microcarcinomas, asymptomatic follicular lymphoma, small renal masses, and selected early-stage CLL.

~30% Convert to Active Treatment — Often Without True Pathologic Progression

CANARY consortium data show approximately 30% of prostate cancer AS patients convert to active treatment within 2 years. Critically, only 40–60% of conversions are driven by true grade reclassification on biopsy — the rest are driven by PSA changes or, in 20–30% of cases, anxiety about the surveillance approach itself. Psychological support alongside medical monitoring significantly improves AS adherence.

Monitoring Challenges: Immunotherapy, Scanxiety, and Hematology MRD

iRECIST and Pseudoprogression in Immunotherapy

Immunotherapy creates monitoring challenges that standard RECIST 1.1 was not designed to handle. Immune cell infiltration into tumors can transiently increase tumor size — making cancer appear to worsen even when it is responding. Pseudoprogression affects approximately 5–10% of immunotherapy-treated patients and can lead to premature treatment discontinuation.

The iRECIST criteria allow “unconfirmed progressive disease” (iUPD): if a patient appears to progress but is clinically stable, treatment can continue and confirmatory imaging performed 4–8 weeks later. If progression is confirmed, it becomes “immune confirmed progressive disease” (iCPD), driving a treatment change. The opposite phenomenon — hyperprogression (paradoxical tumor acceleration in ~10–15% of patients) — is associated with MDM2/MDM4 amplification and remains an active research area.

MRD in Hematologic Malignancies

ALL: MRD <0.01% (10⁻⁴) after induction drives treatment intensity; NGS-based assays detecting 10⁻⁶ are now available
Multiple myeloma: MRD negativity by NGS (ClonoSEQ, FDA-cleared) or PET-CT is an emerging regulatory endpoint; sustained MRD negativity predicts significantly prolonged PFS
CLL: Undetectable MRD (uMRD) after fixed-duration venetoclax-based therapy predicts prolonged disease-free intervals without continuous therapy
AML: MRD by flow cytometry or NPM1/FLT3-ITD PCR at day 28 predicts relapse risk and guides consolidation intensity

Scanxiety and Monitoring Interval Optimization

Scanxiety — anticipatory anxiety surrounding scheduled scans and waiting for results — affects 30–40% of cancer patients at clinically significant levels. Evidence shows extending surveillance intervals in patients who remain recurrence-free reduces scanxiety without increasing the risk of missing recurrence. If monitoring-related anxiety is affecting your quality of life, ask your oncologist whether your surveillance schedule can be safely extended. For comprehensive psychosocial survivorship support, see the cancer follow-up care article. For innovative monitoring technologies in trials, see cancer clinical trials.

Frequently Asked Questions

What is cancer monitoring and why does it continue after treatment ends?

Cancer monitoring is the systematic tracking of your cancer’s status — whether responding to treatment, stable, recurring, or progressing. It continues after active treatment because most cancer recurrences happen within the first 2–5 years after completing therapy, and catching a recurrence early — when it is smaller and potentially still curable — leads to substantially better outcomes than detecting it after symptoms develop. Monitoring tools range from blood tests and tumor markers to advanced imaging and liquid biopsy. Your specific monitoring schedule should be outlined in your survivorship care plan — ask your oncologist for one if you have not received it.

What do tumor markers actually tell my doctor?

Tumor markers are substances cancer cells produce that can be measured in blood. Over time, they provide a trend: a rising PSA after prostate surgery signals cancer cells may be active; a falling CA-125 during ovarian cancer chemotherapy indicates treatment is working. However, markers have critical limitations — they can be elevated by benign conditions (false positives) and some cancers never elevate their marker even when recurrence is present (false negatives). A rising marker typically triggers confirmatory imaging rather than a treatment change. For breast cancer specifically, ASCO does not recommend routine CA 15-3 or CA 27-29 for asymptomatic early-stage surveillance — no survival benefit has been demonstrated.

What is RECIST and how does it measure if treatment is working?

RECIST (Response Evaluation Criteria in Solid Tumors) version 1.1 is the international standard for measuring tumor response on CT or MRI. Radiologists measure up to 5 target lesions and sum their longest diameters. At each follow-up scan: ≥30% decrease = partial response; disappearance of all lesions = complete response; ≥20% increase or new lesions = progressive disease; everything in between = stable disease. When your oncologist says your cancer “partially responded” or “progressed,” they are usually applying RECIST even without using the term. Stable disease can represent an excellent treatment outcome for many advanced cancers where disease control is the goal.

What is a liquid biopsy and is it ready to replace tissue biopsy?

A liquid biopsy detects circulating tumor DNA (ctDNA) in your bloodstream using highly sensitive molecular tests. For monitoring, it has proven uses: detecting resistance mutations (EGFR T790M in lung cancer) without surgery; identifying minimal residual disease after curative surgery before CT scans can detect anything; and detecting recurrence months earlier than conventional imaging in several solid tumor types. The DYNAMIC trial (NEJM 2022) showed ctDNA could guide adjuvant chemotherapy in stage II colorectal cancer with superior outcomes compared to standard care. However, liquid biopsy is not ready to replace all tissue biopsy — initial diagnosis still requires tissue for histology and full molecular profiling, and not all tumors shed DNA equally. Think of it as a powerful complement to, not a replacement for, imaging and tissue analysis.

What is active surveillance and how is it different from watchful waiting?

Active surveillance is a structured, curative-intent monitoring program — curative treatment is deferred, not abandoned — with a defined schedule and clear triggers for switching to active treatment if the cancer changes. Watchful waiting (expectant management) is palliative in intent: the patient is observed with no plan for curative therapy, and treatment starts only when symptoms require it. The PROTECT trial at 15 years showed no significant difference in prostate cancer-specific mortality between active surveillance, surgery, and radiation for low-risk disease — confirming that carefully monitored AS is a valid option. If your doctor recommends active surveillance, it means your tumor characteristics are favorable enough that the risks of immediate treatment currently outweigh the benefits — and monitoring ensures intervention happens at the right time.

What is pseudoprogression and why does it matter for immunotherapy patients?

Pseudoprogression occurs when immune cells flood into a tumor in response to immunotherapy, temporarily making it appear larger on imaging — even when the cancer is actually responding to treatment. It affects approximately 5–10% of immunotherapy-treated patients and can lead to stopping effective treatment based on a misleading scan. The iRECIST criteria address this: if you appear to progress on immunotherapy but are clinically stable (no new symptoms, no functional decline), your oncologist may continue treatment and repeat imaging in 4–8 weeks before confirming progression. Do not stop immunotherapy for apparent radiographic worsening without discussing confirmation imaging with your oncology team. For patients exploring immunotherapy trials and their monitoring protocols, see the cancer clinical trials guide.

Tie J et al. — DYNAMIC trial ctDNA-guided adjuvant chemotherapy stage II CRC; NEJM 2022
Hamdy FC, Lane JA et al. — PROTECT trial 15-year AS vs. treatment prostate cancer; NEJM 2023
Johnson PWM et al. — RATHL trial interim PET Hodgkin lymphoma; NEJM 2016
Cristofanilli M et al. — CTC prognostic value metastatic breast cancer; NEJM 2004
Eisenhauer EA et al. — RECIST 1.1 criteria; Eur J Cancer 2009
Newcomb LF et al. — CANARY AS conversion data; JCO 2016
Champiat S et al. — Hyperprogression definition and incidence; Clin Cancer Res 2017
ASCO — Colorectal cancer surveillance guideline; 2013/updated
AASLD — HCC surveillance in cirrhosis; Hepatology 2018
AUA — Prostate cancer active surveillance guideline; 2023
NCI — Cancer diagnosis and staging resources: cancer.gov/about-cancer/diagnosis-staging
NCCN Guidelines — Disease-specific monitoring: nccn.org/guidelines

This article is for educational purposes only and does not constitute medical advice. Discuss all cancer monitoring decisions with your oncology care team.