Response assessment on systemic therapy

RECIST 1.1 and RECIST committee clarification; iRECIST 2017; PERCIST; NICE CG151

A measurement study, not a detection study. The protocol is dictated by the baseline: same modality, same phase, same coverage, comparable slice thickness. A technically excellent scan that differs from the baseline produces numbers that cannot be compared with it.

Patient on systemic anticancer therapy attending for interval imaging to decide whether treatment continues, changes or stops.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CT Abdomen and Pelvis — Portal Venous Phase

CT Abdomen and Pelvis

What we'd amend, and why

Contrast-enhanced portal-venous CT reproduces the phase in which the baseline target lesions were measured. Phase matters to measurement, not only to detection: a liver deposit measured on an arterial acquisition and remeasured on a venous one changes apparent size because its margins are defined by different enhancement gradients. Matching the phase is what makes the difference in millimetres attributable to the treatment rather than to the protocol.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwaystandard — Adults — interval response CT
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Worth asking the referrer (1)

None of these hold the request up. They sharpen the protocol or the plan that follows.

What study and protocol served as the baseline, and when was it done?
Response is a comparison. Matching the baseline protocol is the requirement; the choice is downstream of it.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Febrile on treatment — a sepsis search, not a response scan

Role	Study & protocol	Why this answers the question
First line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase) usually appropriate	Read the request again before protocolling it: fever in a patient on cytotoxic therapy is neutropenic sepsis until the count says otherwise, and neutropenic sepsis has a mortality that is measured in the hours to the first dose of antibiotic. So the first thing this answer has to say is what imaging does NOT do. Empirical broad-spectrum antibiotics are given within an hour of presentation and are never held for a scan; a CT that delays the first dose is a harm, not a work-up. What imaging then contributes is a source, and the thorax is where the highest-yield occult source sits — pneumonia, and invasive pulmonary fungal disease in the patient neutropenic beyond about ten days. Thin-section reconstruction is what makes that second question answerable, because the halo and reverse-halo signs, and small nodules with a ground-glass rim, are the early findings and they are lost on thick sections. The intravenous contrast earns its place for pleural, mediastinal and vascular-access-related sources rather than for the lung parenchyma, and where it cannot be given an unenhanced thin-section acquisition still answers the fungal question. One reading instruction dominates: with no neutrophils there is often no consolidation, no abscess rim and no fat stranding to see, so a normal or near-normal CT does not exclude infection and must not be reported as if it did.
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	Added in the same sitting wherever there are abdominal symptoms, an unexplained rising inflammatory response, or no thoracic source. The specific things being looked for are neutropenic enterocolitis — caecal and right colonic wall thickening, which is a medical diagnosis with a surgical trapdoor — perianal and perineal sepsis, biliary and line-related collections, and in the recovering neutropenic patient the microabscesses of hepatosplenic candidiasis. Contrast is what shows bowel wall enhancement and a non-enhancing collection centre, so this is the one place in the sepsis work-up where dropping it costs the answer outright. Because the acquisition matches the routine restaging protocol, the response measurement can usually be taken from it as a secondary output — but the report is written for the sepsis question and the slot is booked for the sepsis question.

Entered from fever or sepsis recorded on the request. Leave it unset and this card behaves exactly as it did before — the arm sharpens the answer when the referrer tells us and never withholds one when they do not.
Antibiotics are not gated on this scan and neither is the neutrophil count. A request that reads "please scan before we start antibiotics" is the one to push back on.
The measurement is not lost. Where the acquisition matches the response baseline, report the sepsis question first and the interval measurement second; where it does not, say so and treat the next study as the response timepoint rather than pretending continuity.
Immunosuppression alone does not enter this arm, deliberately. Every patient on cytotoxic therapy is immunosuppressed, and routing all of them down a sepsis pathway would be exactly the over-flagging that trains people to ignore the tool. Fever is the discriminator.

Adults — interval response CT

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Contrast-enhanced portal-venous CT reproduces the phase in which the baseline target lesions were measured. Phase matters to measurement, not only to detection: a liver deposit measured on an arterial acquisition and remeasured on a venous one changes apparent size because its margins are defined by different enhancement gradients. Matching the phase is what makes the difference in millimetres attributable to the treatment rather than to the protocol.
First line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase)	Thoracic target lesions are remeasured in the same venous acquisition as the baseline. Where all thoracic targets are pulmonary nodules measured on lung windows, an unenhanced chest study can be adequate — but only if that is what the baseline was.
Problem solving	FDG PET-CT FDG PET-CT — skull base to mid-thigh	Metabolic response criteria are the right tool where anatomical size lags behind biological effect — treated lymphoma, gastrointestinal stromal tumour, and residual masses after chemotherapy. It answers a different question from RECIST and should be requested as such, not substituted silently for a CT the oncologist intends to measure.

Pitfalls

Vetting an interval scan without knowing what the baseline protocol was, and defaulting to the local house protocol that happens to differ from it.
Changing coverage between timepoints — a chest-only follow-up after a chest/abdomen/pelvis baseline cannot exclude progression outside the chest, however normal it looks.
Comparing a thin-section volumetric study against a thick-section prior, which manufactures apparent growth or stability.
Reading enlargement on the first scan after starting immunotherapy as failure. Pseudoprogression is expected, and iRECIST calls that scan unconfirmed progression and requires a confirmatory study at 4-8 weeks before treatment is stopped.
Continuing three-monthly contrast-enhanced CT indefinitely without asking whether the result would change management — surveillance intensity is a vetting decision too.
Running a febrile patient on chemotherapy through the routine restaging queue because the request said "interval CT". The measurement can wait; the source of sepsis cannot, and empirical antibiotics must not wait for the scanner.
Reporting a normal CT in a neutropenic patient as excluding infection. With no neutrophils there is often no consolidation, no abscess rim and no fat stranding to see.

Priors — what to pull first

Load the baseline, not just the most recent scan. Response is measured against baseline and progression against the smallest previous sum, and the two comparators are often different studies.
Where the baseline was performed elsewhere, obtaining the actual images rather than the report is what makes measurement possible.

What makes a good request

The single most useful line on a response request is what the baseline was and when. Without it the protocol is a guess and the report has to hedge.
RECIST 1.1 requires target lesions of at least 10 mm in long axis (15 mm short axis for nodes) measured on CT reconstructed at 5 mm or thinner, a maximum of five target lesions and two per organ, and explicitly requires that the same imaging technique be used throughout follow-up. Those are technical constraints on the vetter as much as on the reporter.
A patient on a checkpoint inhibitor is assessed with iRECIST, not RECIST 1.1, and the difference is not academic. Pseudoprogression means tumour burden that increases before it falls, so iRECIST records the first progression as UNCONFIRMED (iUPD) and requires confirmation on a repeat study at 4-8 weeks before progression is declared (iCPD); a clinically stable patient continues treatment in the interval. Reading a single enlarging scan as failure under RECIST 1.1 in that patient stops an effective drug.
Metabolic response has its own criteria and its own technical preconditions. PERCIST measures SUL-peak in a fixed small volume within the most active lesion and is only interpretable when uptake time, fasting state, glucose and ideally the scanner are matched to the baseline; Deauville is the five-point scale for lymphoma. A PET requested "to assess response" without those matched conditions returns a number nobody can compare with anything.
A patient who becomes febrile between cycles is not attending for a response scan, whatever the request says. That is the fork below, and it changes the tempo, the report and the slot rather than the protocol.

Scoring this once it is done

The classification and risk tools this question ends in.

RECIST 1.1 Tumor Response iRECIST (Immune-related RECIST)PERCIST 1.0 PET Response mRECIST for Hepatocellular Carcinoma Choi Criteria for GIST Response Nodule Volume Doubling Time (VDT)

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

CT Chest — Contrast-Enhanced (Venous Phase)CT Abdomen and Pelvis — Portal Venous Phase FDG PET-CT — skull base to mid-thigh

Confirm locally

CT Abdomen and Pelvis — Portal Venous Phase: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.