NI-RADS ACR Neck Imaging Reporting and Data System

{
  "categories": [
    {
      "outcome_code": "0",
      "label": "incomplete",
      "defining_state": "new_baseline_without_required_prior_imaging_that_is_known_or_expected_to_become_available"
    },
    {
      "outcome_code": "1",
      "label": "no_evidence_of_recurrence",
      "defining_state": "expected_posttreatment_findings_without_suspicious_primary_site_or_neck_abnormality"
    },
    {
      "outcome_code": "2",
      "label": "low_suspicion",
      "primary_site_subcategories": [
        "2a_mucosal",
        "2b_deep"
      ],
      "neck_subcategory": "N2"
    },
    {
      "outcome_code": "3",
      "label": "high_suspicion",
      "defining_state": "discrete_or_progressive_abnormality_highly_suspicious_for_viable_tumor"
    },
    {
      "outcome_code": "4",
      "label": "known_or_definite_recurrence",
      "defining_state": "pathologically_proven_disease_or_definite_radiologic_and_clinical_progression"
    }
  ],
  "applicability": {
    "intended_use": "Standardized posttreatment imaging surveillance after definitive or curative-intent treatment of head and neck cancer.",
    "classification_unit": "one_examination_with_separate_primary_site_P_and_neck_N_assessments",
    "required_context": [
      "tumor_primary_site_and_histology",
      "treatment_type_and_completion_date",
      "baseline_and_prior_imaging",
      "clinical_examination_or_endoscopy_when_relevant",
      "modality_and_protocol",
      "separate_primary_site_and_neck_findings"
    ],
    "outside_scope": [
      "during_active_treatment_response_scoring",
      "untreated_initial_staging",
      "pathologic_diagnosis_from_imaging_category",
      "single_combined_P_and_N_category",
      "automatic_treatment_selection"
    ]
  },
  "separate_axes_and_qualifiers": {
    "primary_site_axis": "Assign a P category to the treated primary site and use the primary-specific 2a versus 2b branch when category 2 applies.",
    "neck_axis": "Assign an N category to treated cervical nodes and the neck independently; the category may legitimately differ from P.",
    "unassessable": "Use P-x or N-x when the relevant region cannot be assessed because of technique, artifact, coverage or anatomy. The x qualifier is not category 0 and is not an ordinal risk score.",
    "unknown_primary": "Use the declared unknown-primary convention rather than inventing a primary-site category when no treated primary exists.",
    "no_collapse_rule": "Never return one overall NI-RADS number by taking the higher or lower of P and N unless an explicitly named local implementation requires a separate summary outside the ACR axes."
  },
  "category_logic": {
    "category_0": {
      "gate": "Use only when a new baseline examination lacks a prior study that is known or expected to become available for comparison.",
      "action": "Issue an incomplete comparison state and add an addendum after the prior examination becomes available.",
      "distinction": "Technical nonassessability is P-x or N-x, not category 0; absence of any prior when none will be available is handled by clinical judgment rather than perpetual category 0."
    },
    "category_1": {
      "primary_and_neck": "Expected posttreatment anatomy, fibrosis, edema or other stable benign change without a suspicious focus.",
      "action": "Continue routine surveillance according to the cancer, treatment, timepoint and institutional pathway."
    },
    "category_2": {
      "P_2a_mucosal": "Focal non-mass-like mucosal enhancement or focal reduced diffusion that is low suspicion and accessible to direct clinical inspection.",
      "P_2a_action": "Recommend direct visual inspection or endoscopic correlation rather than treating 2a as a biopsy-proven recurrence.",
      "P_2b_deep": "Deep ill-defined nonnodular soft tissue or another equivocal deep posttreatment abnormality without a discrete high-suspicion mass.",
      "P_2b_action": "Recommend short-interval imaging; MRI is preferred for suspected perineural spread or skull-base evaluation when appropriate.",
      "N_2": "Residual heterogeneous enhancement, mild-to-moderate FDG uptake, a new or enlarging node without definitively malignant morphology, or meaningful PET/MRI discordance.",
      "N_2_action": "Recommend short-interval imaging and integrate morphology, diffusion, metabolic activity, trajectory and clinical findings."
    },
    "category_3": {
      "any_of": [
        "discrete_enhancing_mass_matching_the_tumor_bed",
        "intense_focal_FDG_uptake_compatible_with_tumor",
        "progressive_or_increasing_perineural_abnormality",
        "node_with_highly_suspicious_morphology_or_concordant_progression"
      ],
      "action": "Recommend image-guided or clinical biopsy when clinically indicated and technically feasible; multidisciplinary review can resolve contraindication, inaccessible site or overwhelming concordant evidence."
    },
    "category_4": {
      "any_of": [
        "pathologically_proven_residual_or_recurrent_tumor",
        "definite_radiologic_progression_with_concordant_clinical_evidence_when_tissue_is_unnecessary_or_not_feasible"
      ],
      "action": "Route to multidisciplinary clinical management while preserving site, extent and proof basis; category 4 is not a treatment regimen."
    }
  },
  "modality_and_version_boundary": {
    "CT_and_PET_CT": "Use the 2018 ACR white-paper framework and its modality-specific descriptors.",
    "MRI": "Use the current MRI v2025 release and its assessment-category table; do not assume every CT/PET descriptor has identical MRI wording.",
    "longitudinal_consistency": "State the release and modality used whenever comparison or automation could mix implementations.",
    "PET_limitation": "Inflammation, physiologic uptake and treatment timing can mimic disease; metabolic intensity must be integrated with anatomy and trajectory."
  },
  "decisive_boundaries": {
    "zero_versus_x": "Category 0 means comparison is incomplete because a required prior is expected; x means the anatomy is unassessable on the current examination.",
    "one_versus_two": "Expected stable posttreatment change without a focal concern is 1; a focal low-suspicion mucosal, deep or nodal abnormality is 2.",
    "P_2a_versus_P_2b": "A mucosal focus suitable for direct inspection is 2a; a deep ill-defined nonnodular abnormality requiring imaging surveillance is 2b.",
    "two_versus_three": "Equivocal low-suspicion change without a discrete convincing tumor pattern is 2; a discrete mass, intense concordant uptake or convincing progression is 3.",
    "three_versus_four": "High imaging suspicion awaiting confirmation is 3; pathology or definite radiologic-plus-clinical progression is 4."
  },
  "risk_interpretation": {
    "qualitative_only": "The ordinal category communicates suspicion and linked workup, not a universal recurrence percentage.",
    "why_probabilities_vary": [
      "primary_versus_neck_axis",
      "CT_PET_CT_or_MRI_release",
      "tumor_site_and_histology",
      "time_since_treatment",
      "prevalence_and_referral_setting",
      "reader_and_followup_reference_standard"
    ],
    "prohibited_inferences": [
      "category_1_equals_zero_recurrence_risk",
      "category_2_has_one_portable_percentage",
      "category_3_is_pathology",
      "category_4_selects_a_regimen"
    ]
  },
  "agent_output_contract": [
    "Return treatment type, completion date, modality, protocol, current release and comparison adequacy.",
    "Return P and N separately and use 2a or 2b only on the primary-site axis.",
    "Describe the exact mucosal, deep, nodal, diffusion, enhancement, FDG and trajectory evidence before the category.",
    "Return category-linked next step as a recommendation with clinical and technical qualifiers.",
    "Distinguish 0, P-x or N-x, unknown primary, suspicion, proof and treatment selection."
  ],
  "missing_input_behavior": [
    "If a known prior will become available, return category 0 and an addendum pathway rather than a false definitive category.",
    "If the primary site or neck is technically unassessable, return P-x or N-x for that axis and still classify the other axis when possible.",
    "If treatment status is not post-definitive surveillance, withhold NI-RADS and route to the appropriate staging or response framework.",
    "If P-2a versus P-2b cannot be resolved, expose the ambiguity and the finding location rather than collapsing it to a generic 2 without explanation."
  ],
  "supporting_sources": [
    {
      "role": "CT_PET_framework",
      "citation": "Aiken et al. J Am Coll Radiol. 2018;15:1097-1108",
      "doi": "10.1016/j.jacr.2018.05.006"
    },
    {
      "role": "current_MRI_release",
      "citation": "Aiken et al. J Am Coll Radiol. 2025",
      "doi": "10.1016/j.jacr.2025.07.023"
    },
    {
      "role": "official_release_index",
      "citation": "ACR NI-RADS current-release page",
      "url": "https://www.acr.org/Clinical-Resources/Clinical-Tools-and-Reference/Reporting-and-Data-Systems/NI-RADS"
    }
  ],
  "source_locator": "ACR NI-RADS MRI v2025 Assessment Categories table for 0, 1, primary 2a/2b, neck 2, 3, 4 and x qualifiers; Aiken et al. 2018, DOI 10.1016/j.jacr.2018.05.006, CT/PET white paper; ACR current-release index."
}