Oncology Trials: How is Tumor Size Defined?

 

What is Tumor Size Based On in Oncology Trials?

In oncology clinical trials, tumor size is typically measured using radiological imaging techniques and is based on standardized criteria, such as RECIST (Response Evaluation Criteria in Solid Tumors). The tumor size used in waterfall plots and treatment response assessments is derived from longest diameters or volumetric measurements of target lesions.

1. Measurement of Tumor Size

Tumor size is usually assessed through radiologic imaging techniques, including:

  • CT Scan (Computed Tomography)
  • MRI (Magnetic Resonance Imaging)
  • PET Scan (Positron Emission Tomography)
  • Ultrasound (for specific tumors)

These imaging methods help oncologists measure the tumor’s longest diameter and monitor changes over time.

2. RECIST Criteria for Tumor Size Assessment

RECIST (Response Evaluation Criteria in Solid Tumors) is the most widely used standard for measuring tumor size and treatment response. It defines tumor size as:

  • Sum of the longest diameters (SLD) of target lesions (up to 5 measurable lesions across organs).
  • For lymph nodes, the shortest diameter is used.
  • Non-measurable lesions (e.g., ascites, bone metastases) are tracked qualitatively.

3. How Tumor Size Change is Calculated

The percentage change in tumor size is calculated as:

Tumor Size Change (%)=New Tumor Size−Baseline Tumor SizeBaseline Tumor Size×100\text{Tumor Size Change (\%)} = \frac{\text{New Tumor Size} - \text{Baseline Tumor Size}}{\text{Baseline Tumor Size}} \times 100

Where:

  • Baseline Tumor Size = Initial measurement before treatment.
  • New Tumor Size = Measured size after treatment.

4. Tumor Response Categories Based on Size Change

According to RECIST 1.1, tumor response is classified as:

Response CategoryDefinition
Complete Response (CR)     Tumor completely disappears (100% reduction).
Partial Response (PR)      ≥ 30% decrease in the sum of target lesion diameters.
Stable Disease (SD)     Between -30% and +20% change (no significant growth/shrinkage).
Progressive Disease (PD)     ≥ 20% increase in tumor size or new lesions appear.




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