Response to therapy typically is monitored by radiographic methods, such as radioiodine full-body scanning, fluorodeoxyglucose PET scan, CT scan, and MRI. These methods rely on visual determination of disease burden and often can be difficult to quantify, particularly when disease is extensive. Inspiratory and expiratory chest CT scan classically have been used to detect the presence of focal air trapping in other bronchiolar diseases such as noxious or toxic inhalation, including tobacco use, bronchiolitis obliterans (BOOP), diffuse pan bronchiolitis, asthma, and chronic granulomatous disease.10-14 It has been postulated that for BOOP, asthma, and smoking-related lung disease, particularly for very early or localized airway disease, chest CT imaging indicating air trapping may be the most sensitive method for detection.10,15,16 Nevertheless, there has been debate on how to accurately score air trapping on chest CT scan because the exact CT scan protocols used in the studies examining these diseases often differ, and there is variable interobserver variability. Editorials on this subject note that pulmonary function tests are more likely to miss focal areas of air trapping because they reflect a more global function of the lung. When the lung disease is more diffuse, such as in BOOP, there is a significant correlation between degree of air trapping and FEV1.13 Even though our patient had diffuse disease, he did not undergo inspiratory and expiratory chest CT imaging, but rather, his disease regression and progression were effectively monitored using pulmonary function tests. Initial spirometric values indicating airflow obstruction correlated with chest CT images, showing innumerable enlarging pulmonary nodules and the gross pathology of small airway infiltration by metastatic tumors. The airflow obstruction and degree of hypoxemia determined by serial measurements of ambulatory oximetry were significantly improved after treatment with 131I and sorafenib, suggesting that the initial hypoxia was not related to radioiodine toxicity. When the disease progressed, the degree of airflow obstruction and hypoxemia worsened. Pulmonary function tests were used as a noninvasive method of disease monitoring, and enabled early recognition of disease progression in this patient. He ultimately was referred for sorafenib treatment, which currently is ongoing. Repeat pulmonary function tests performed 2 months after treatment already have shown improvement in the degree of airflow obstruction. We do not suggest that patients be routinely monitored with pulmonary function testing, but spirometry was useful in the case presented. Chest CT imaging instead remains the standard method for monitoring metastatic disease progression and response to therapy. In this context, CT imaging may identify metastatic nodules, effusions, and endobronchial or peribronchial lesions. Chest imaging also may be necessary to evaluate pulmonary emboli, unrecognized emphysema, and infiltrates that may suggest infection or pneumonitis.