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New Lesions

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New Lesions

title: “New Lesions” type: descriptor category: recist/descriptor tags: [recist, new-lesions, progressive-disease, fdg-pet] sources:

  • file: “recist_eisenhauer_2009.pdf” page: “237-239”
  • file: “recist_schwartz_2016.pdf” page: “132-137” date_created: 2025-04-15 date_modified: 2025-04-15 confidence: high anki_deck: “Radiology::RECIST” weak_area: false

Definition

A new lesion is tumor involvement identified on follow-up imaging at a site that was either:

  • Not present at baseline, OR
  • Not imaged at baseline

The appearance of any new unequivocal malignant lesion triggers overall PD — regardless of target lesion response, non-target lesion status, or any other factor. This is the most powerful override rule in RECIST.

New Lesion — Unequivocal Standard

New lesions must be unequivocal:

  • Clearly attributable to malignancy
  • Not attributable to differences in scanning technique, patient positioning, or benign findings
  • If equivocal: repeat imaging at the next scheduled assessment for confirmation

Equivocal New Lesions

If a new lesion is equivocal, do NOT declare PD immediately. Repeat the scan at the next scheduled timepoint. If confirmed → PD from the time the equivocal lesion was first seen. If not confirmed → no PD from that finding.

Lesions in Unscanned Anatomical Locations

Any lesion identified in a location not imaged at baseline is automatically considered a new lesion.

Examples:

  • Baseline chest/abdomen CT, no brain imaging → new brain lesion on follow-up brain MRI → new lesion = PD
  • Baseline CT without bone windows, no dedicated bone imaging → new sclerotic rib lesion on follow-up bone scan → new lesion = PD

This prevents gaming by selectively omitting scans of high-risk anatomical sites.

Modality-Specific Rules

CT and MRI

  • New lesions identified on CT or MRI are evaluated by standard anatomical criteria
  • New lesions can be of any size — a new 5mm lung nodule counts as PD even though it is below the 10mm measurability threshold

FDG-PET — Special Role in New Lesion Detection

FDG-PET is the only modality that can detect new lesions before they are structurally visible on CT. PET detects metabolic activity (glucose uptake), which precedes morphological change.

PET has three roles in new lesion detection:

Scenario 1: Baseline PET Positive → Follow-up PET New Focus

  • Baseline PET showed FDG-avid disease
  • Follow-up PET shows a new FDG-avid focus in a new anatomical location
  • → New lesion = PD (confirmed without needing CT correlation)

Scenario 2: Baseline PET Negative → Follow-up PET New Focus

  • Baseline PET was negative (no FDG-avid disease)
  • Follow-up PET shows a new FDG-avid focus
  • → PD based on new lesion (negative baseline → any new positive focus = new disease)

Scenario 3: No Baseline PET → Follow-up PET New Focus

  • No baseline PET was performed
  • Follow-up PET shows a suspicious focus not visible on CT
  • → Cannot declare PD from PET alone
  • Must obtain CT or MRI confirmation of a corresponding anatomical lesion
  • If CT confirms a new anatomical lesion → PD
  • If CT does not confirm → no PD from that finding

PET Does Not Measure Target Lesions

Increased FDG uptake in an existing target lesion — even if markedly more avid than baseline — does NOT trigger PD unless the lesion also meets the ≥20% + 5mm anatomical size threshold on CT. PET is for new lesion detection only, not for measuring response in existing lesions.

Ultrasound

  • New lesions identified on ultrasound require confirmation by CT or MRI
  • Ultrasound alone is insufficient for declaring a new lesion or PD

New Lesion Detection Flowchart

Follow-up scan shows suspicious new finding
           │
           ▼
Is it clearly malignant (unequivocal)?
      Yes           No (equivocal)
       │                 │
       ▼                 ▼
  New lesion = PD    Repeat scan next visit
                        │          │
                        ▼          ▼
                   Confirmed    Not confirmed
                     = PD          = no PD
              (backdated to 1st appearance)

Pseudolesions — Not New Lesions

Certain imaging findings are NOT new lesions and should not trigger PD:

  • Treatment-induced necrosis or hemorrhage: Lesions that become visible on CT after therapy because they were previously isodense to surrounding tissue (e.g., tumor becoming hypodense after imatinib, revealing an underlying cystic change that was always there). These are not truly new — FDG-PET can help distinguish metabolically active tumor from necrotic debris.
  • Post-treatment inflammatory changes
  • Artifacts from scanning technique differences

Reappearance After CR

If a lesion that disappeared (patient had CR) reappears on follow-up:

  • If the prior overall response was definitively CR → reappearance of any disease = PD
  • If the prior overall response was not CR (e.g., SD, PR) → reappearance alone does not automatically equal PD — evaluate against other criteria

The Reappearance Rule

“Prior patient status” refers to overall response, not individual lesion assessment. A lesion that vanished when overall = CR and reappeared = PD. But a lesion that vanished when overall was never CR (e.g., patient had SD in targets + non-targets present) — reappearance of that lesion alone is not automatically PD.

New Lesions and the Overall Response Table

See RECIST 1.1 Overview. Every row with “New Lesions: Yes” = Overall PD. There are no exceptions.

Graph View

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