This calculator determines the Volume Doubling Time (VDT) of a pulmonary nodule based on two sequential CT scans. VDT is a critical metric used by radiologists and pulmonologists to assess the probability of malignancy in solid nodules.
Mean diameter (average of long and short axis)
Must be from the same imaging modality
Time Interval:–
Growth in Diameter:–
Volume Doubling Time (VDT):–
function calculateGrowth() {
// Get inputs
var date1Str = document.getElementById('date1').value;
var date2Str = document.getElementById('date2').value;
var d1 = parseFloat(document.getElementById('dia1').value);
var d2 = parseFloat(document.getElementById('dia2').value);
var errorDiv = document.getElementById('inputError');
var resultDiv = document.getElementById('results');
// Reset UI
errorDiv.style.display = 'none';
resultDiv.style.display = 'none';
// Validation
if (!date1Str || !date2Str || isNaN(d1) || isNaN(d2)) {
errorDiv.innerHTML = "Please fill in all fields with valid dates and measurements.";
errorDiv.style.display = 'block';
return;
}
if (d1 <= 0 || d2 <= 0) {
errorDiv.innerHTML = "Nodule diameter must be greater than zero.";
errorDiv.style.display = 'block';
return;
}
var date1 = new Date(date1Str);
var date2 = new Date(date2Str);
// Calculate time difference in days
var timeDiff = date2.getTime() – date1.getTime();
var days = timeDiff / (1000 * 3600 * 24);
if (days <= 0) {
errorDiv.innerHTML = "Follow-up scan date must be after the initial scan date.";
errorDiv.style.display = 'block';
return;
}
// Calculation Logic
// VDT = (ln(2) * T) / (ln(V2/V1))
// Since Volume is proportional to Diameter cubed: V = (4/3)*pi*(D/2)^3
// The ratio V2/V1 simplifies to (D2/D1)^3
// Formula becomes: VDT = (ln(2) * days) / (3 * ln(d2/d1))
var intervalText = Math.round(days) + " days";
var growthMm = (d2 – d1).toFixed(1);
var growthPct = ((d2 – d1) / d1 * 100).toFixed(1);
var growthText = growthMm + " mm (" + growthPct + "%)";
var vdt = 0;
var vdtText = "";
var interpretText = "";
if (d2 <= d1) {
vdtText = "No Growth / Shrinking";
interpretText = "Stable or Regression: The nodule has either remained the same size or decreased in volume. Stability over 2 years typically suggests a benign etiology, though subsolid nodules may require longer follow-up.";
} else {
// Main Formula
vdt = (Math.log(2) * days) / (3 * Math.log(d2 / d1));
vdt = Math.round(vdt);
vdtText = vdt + " days";
// Interpretation Guidelines
if (vdt < 30) {
interpretText = "Rapid Growth (< 30 days): Typically suggests an infectious or inflammatory process rather than malignancy, though rapidly growing metastases are possible.";
} else if (vdt >= 30 && vdt <= 400) {
interpretText = "Intermediate Growth (30 – 400 days): This range is generally considered suspicious for malignancy (e.g., non-small cell lung cancer). Clinical correlation required.";
} else {
interpretText = "Slow Growth (> 400 days): Suggests a slow-growing lesion. While often benign, certain low-grade malignancies (like carcinoid tumors or lepidic growth adenocarcinomas) can exhibit slow growth.";
}
}
// Output
document.getElementById('resInterval').innerText = intervalText;
document.getElementById('resGrowth').innerText = growthText;
document.getElementById('resVDT').innerText = vdtText;
document.getElementById('resInterpretation').innerHTML = interpretText;
resultDiv.style.display = 'block';
}
Understanding Nodule Growth and VDT
The assessment of pulmonary nodule growth is a cornerstone of thoracic radiology. While size is a static measurement, growth rate adds a dynamic dimension to the diagnosis, helping differentiate between benign and malignant lesions.
What is Volume Doubling Time?
Volume Doubling Time (VDT) is the time, measured in days, required for a nodule to double its volume. Since pulmonary nodules are 3D structures, a small increase in diameter represents a significant increase in volume. For example, a 26% increase in diameter roughly equals a doubling of volume.
Why use VDT instead of just diameter change?
Measuring simple diameter changes can be misleading due to measurement error and the non-linear relationship between diameter and volume. The VDT formula standardizes the growth rate, allowing physicians to categorize nodules based on biological behavior:
Infectious/Inflammatory: Often grow very rapidly (VDT < 20-30 days).
Malignant: Typically grow at a steady rate (VDT usually between 30 and 400 days).
Benign: Often stable or grow very slowly (VDT > 400-500 days).
How to Measure Diameter Correctly
To ensure accurate results using this calculator, inputs should follow standard radiological guidelines:
Consistency: Both measurements should be taken from the same imaging modality (usually CT) and window settings (Lung Window).
Measurement Axis: Use the mean diameter. Measure the long axis and the short axis on the same transverse slice and calculate the average.
Slice Thickness: Thinner slice reconstructions (e.g., 1mm) provide more accurate volumetric data than thick slices.
The Math Behind the Calculator
This calculator uses the modified Schwartz formula which assumes the nodule is roughly spherical. While modern software can calculate exact volume segmentation, the diameter-based formula remains the clinical standard for manual calculation.
The formula used is:
VDT = [ln(2) × Δt] / [3 × ln(D2/D1)]
Where:
Δt: Time interval in days.
D1: Initial diameter.
D2: Follow-up diameter.
MEDICAL DISCLAIMER: This tool is intended for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Radiographic measurements are subject to inter-observer variability. Clinical decisions should never be based solely on this calculator. Always consult with a qualified healthcare provider regarding lung nodule management.