Reviewing Prior Comparisons

1. Are prior studies available in PACS? (Default: Search all linked PACS systems)
2. What is the most recent comparable study? (Default: Same modality, same body region)
3. Are outside studies available (CD, external PACS)? (Default: Check for imported studies)
4. What is the clinical question requiring comparison? (Default: Disease progression, treatment response, post-operative change)
5. Is this patient on a treatment protocol requiring interval measurement? (Default: No — if yes, apply RECIST or relevant criteria)
6. Are prior reports available (even without images)? (Default: Check report archive)

Documents to Request

• All prior imaging studies of the same body region (ideally same modality)
• Prior radiology reports with findings and measurements
• Outside imaging records (CDs, imported studies)
• Clinical notes documenting interim events (surgery, treatment, injury)
• Treatment timeline (chemotherapy cycles, radiation dates)
• Prior measurement tables (for oncologic tracking)

Step 1: Identify and Prioritize Relevant Prior Studies

Comparison Study Selection Hierarchy

Multi-Timepoint Comparison

For findings tracked over time, reference multiple timepoints:

• Most recent prior — for interval change
• Baseline — for overall trend and response assessment
• Nadir — for oncologic measurement (smallest recorded size)

Document: "Comparison: CT chest dated MM/DD/YYYY (most recent), CT chest dated MM/DD/YYYY (baseline). Additional reference: PET/CT dated MM/DD/YYYY."

Step 2: Technique Comparison Assessment

Before comparing findings, document any technique differences that affect interpretation.

Step 3: Standardized Interval Change Assessment

Change Descriptors — Use Precise Language

Language to Avoid

Step 4: Structured Comparison Documentation

Per-Finding Comparison Format

For each significant finding, document:

[Finding]: [Current description], measuring [X mm], 
[change descriptor] compared to [prior study date] 
when it measured [Y mm] ([percentage change or absolute change]).

Examples:

Right upper lobe pulmonary nodule: Solid nodule measuring 12 mm, 
increased from 8 mm on CT dated 01/15/2025 (50% increase, 
interval 6 months). Per Fleischner Society criteria, 
recommend PET/CT or tissue sampling.

Hepatic cyst in segment 7: Simple cyst measuring 3.2 cm, 
stable compared to CT dated 06/10/2024 (3.1 cm). 
No follow-up required.

Comparison Summary Table (for Multiple Findings)

Step 5: Trending and Longitudinal Assessment

For findings tracked over multiple timepoints, provide a trend summary:

Growth Rate Calculation (Pulmonary Nodules)

• Volume doubling time (VDT) is the gold standard for growth assessment
• VDT <400 days = suspicious for malignancy
• VDT >600 days = likely benign
• Formula: VDT = (t × ln2) / (3 × ln(d₂/d₁)), where t = time interval, d₁ = initial diameter, d₂ = current diameter

Measurement Variability Thresholds

Not all size changes are clinically meaningful:

If a size change falls within measurement variability, state: "Within expected measurement variability; recommend continued follow-up to establish trend."

Checkpoint B: Post-Draft Alignment (Mandatory)

1. Are all comparison studies identified with specific dates and modalities?
2. Are technique differences documented that may affect comparison?
3. Does every significant finding have an explicit change descriptor with measurements?
4. Are measurements provided for both current and prior studies?
5. Is the comparison language specific and unambiguous?

Quality Audit

• All reasonably available prior studies are identified and referenced with dates
• Most recent comparable study is used as the primary comparison
• Technique differences between studies are documented
• Every significant finding has an explicit interval change descriptor
• Measurements are provided for both current and prior timepoints
• Percentage and/or absolute change is calculated for measurable findings
• Standardized change language is used (not vague terms like "grossly unchanged")
• Multiple timepoints are referenced for trending when available
• New findings are confirmed absent on prior studies
• Resolved findings are confirmed absent on current study
• Measurement variability thresholds are applied (not over-interpreting small changes)
• Outside studies or reports are referenced when institutional priors are unavailable
• Comparison section of the report lists all comparison studies with dates and modalities
• Growth rate or volume doubling time is considered for nodule surveillance

Guidelines

1. Always reference prior comparison studies with specific dates and modalities — "compared to prior" without a date is medicolegally insufficient.
2. Provide measurements for both the current and prior study when describing interval change — "increased" without numbers is not actionable.
3. Account for technique differences before concluding interval change; a lesion may appear larger simply due to different contrast timing or slice thickness.
4. Use standardized interval-change descriptors consistently throughout the report.
5. When priors are unavailable, state "No prior comparison available" and recommend comparison on follow-up; do not guess at stability.
6. For oncologic surveillance, reference both the most recent prior (for interval change) and the baseline (for overall response assessment).
7. Apply measurement-variability thresholds — a 1 mm change in a 6 mm nodule is within measurement error, not true growth.
8. When outside imaging is referenced by report only (no images available), state "Per prior report dated [date] from [institution]" and note that direct image comparison was not possible.