# How is the size of the pneumothorax estimated on chest radiograph?

Updated: Apr 28, 2020
• Author: Brian J Daley, MD, MBA, FACS, FCCP, CNSC; Chief Editor: Mary C Mancini, MD, PhD, MMM  more...
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In evaluating the chest radiograph, first impressions of pneumothorax size can be misleading. The following methods may be used to estimate the size of the pneumothorax:

• Calculate the ratio of the transverse radius of the pneumothorax (cubed) to the transverse radius of the hemithorax (cubed); to express the pneumothorax size as a percentage, multiply the fractional size by 100 (this formula assumes a constant shape of the lung when it collapses and is invalid if pleural adhesions are present); the ratio of lung size to hemithorax size to estimate pneumothorax size avoids the subjective underestimation of pneumothorax expressed as a percentage of previous lung volume
• A 2.5-cm margin of gas peripheral to the collapsing lung corresponds to a pneumothorax of about 30%; complete collapse of the lung is a 100% pneumothorax
• A simple approach involves measuring the distance from the apex of the lung to the top margin of the visceral pleura (thoracic cupola) on the upright chest radiograph, so that a small pneumothorax is a distance to the apex that measures less than 3 cm and large pneumothorax has greater than 3 cm distance to the apex

The cut point distinguishing small and large pneumothoraces varies somewhat among professional societies and experts. The British Thoracic Society uses 2 cm as the cutoff, [38] the American College of Chest Physicians uses 3 cm as the cut point, [39] and the Light Index uses 15% of the thoracic volume on the posterior-anterior film as the cut point. [40]