TSRA Content:
Author: Abigail Fong, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Leroy Woodson, MD
The backbone of thoracic imaging studies is the chest radiograph (CXR), computed tomography (CT) scan, and positron emission tomography-computed tomography scan (PET-CT). These studies help to identify, diagnose, and localize pulmonary lesions.
A CXR is often the starting point for thoracic imaging, as CXRs are easy to obtain, inexpensive, and have low radiation exposure. However, CXRs provide a two-dimensional image, and due to limitations of the modality, can miss smaller lesions or those obscured by the heart, great vessels, or spine. Though unlikely to be diagnostic alone, CXR can identify pulmonary lesions and the sequela of those lesions if bronchial obstruction or pleural effusion is present. Review of a CXR should be systematic and evaluate the airway, skeletal elements, cardiac silhouette, diaphragm, pleural effusions, pulmonary fields, great vessels and mediastinum, and any central lines, hardware, or drains.
CT scans are useful in screening for lung cancer and diagnosing and localizing pulmonary abnormalities. The US Preventative Services Task Force recommends a low-dose CT scan yearly for patients with at least a 20-pack-year smoking history who are either still smoking or quit in the last 15 years. Generally, non-contrast scans are used to investigate the chest unless soft tissue or vascular issues are suspected. An MRI can be used to assess soft tissue or nerve involvement (for example, in superior sulcus tumors) or evaluate metastatic disease but is not widely used in thoracic surgery.
Review of a chest CT scan should: 1) confirm the presence, size, and location of the lesion, 2) characterize the nature of the lesion (solid versus ground-glass opacity, spiculated versus smooth), 3) evaluate for any other pleural or parenchymal diseases (additional nodules, effusions, obstructions), 4) assess the mediastinum for lymphadenopathy, and 5) examine for metastatic disease (liver, adrenal gland).
A PET-CT is useful in identifying mediastinal disease and distant metastasis and has been shown to decrease the rate of futile operations. PET-CT can also be used in conjunction with systemic therapies to track disease response.
Mediastinal Lymph Nodes
The mediastinum should be visualized using abdominal or bone windows to look for enlarged lymph nodes (>1 cm in diameter). Identifiable mediastinal adenopathy will usually justify nodal sampling via cervical mediastinoscopy or transbronchial EBUS biopsy before lung resection.
Evaluating for Resectability
When considering a lesion’s resectability, both anatomic and patient factors must be considered. Anatomically, resectability is driven by the lesion’s relationship to the bronchus and/or artery of the pulmonary segment or lobe in question. The relationship of the lesion to structures of the pulmonary hilum needs to be evaluated to determine if lobectomy will be feasible or if pneumonectomy, bilobectomy, or sleeve resection will be needed to achieve an R0 resection. CT with contrast may be of assistance if evaluating a hilar lesion to better delineate vasculature and anatomic planes. Additionally, any hilar lymphadenopathy should be noted as this can make lobectomy more difficult. If considering a segmentectomy, examine the relationship of the lesion to the segmental bronchi to ensure that resection is technically feasible with adequate resection margins.
Patient factors may also drive resectability as they dictate the patient’s ability to tolerate a given pulmonary resection. It is helpful to assess the degree of pulmonary parenchymal disease and the functionality of the lung tissue. Centrilobular emphysema is characteristically heterogeneous in distribution, with the upper lobes bearing the majority of the disease. Because of this asymmetry, resection of a severely diseased lobe will often be better tolerated than preoperative pulmonary function tests might suggest (see below for preoperative risk stratification).
Operative Localization
In reviewing the CT scan, using a combination of axial, sagittal, and coronal imaging can help identify the lesion’s location in relation to anatomic landmarks. Small nodules can be differentiated from pulmonary vasculature on CT by their lack of continuity for multiple adjacent slices of the CT scan. Ensure that the images are available intraoperatively for review. Keep in mind the patient is laterally positioned; therefore, lateral distance becomes vertical in the operative chest. Utilize anatomic landmarks’ relationship to the lesion seen on CT, such as the fissures, bronchial divisions, carina, spine, and aortic arch, as clues to localize the lesion in the lung. Estimate the cranial-to-caudal location of a nodule by determining how much of each lobe is present at the slice of the CT scan where the nodule is found. Keep in mind that the lung when decompressed may change its relationship to the chest wall and mediastinal anatomy, but should maintain relatively consistent ratios of lung parenchyma to total lung mass and consistent relationship to bronchi and pulmonary vasculature.
Finding parenchymal lesions identified on CT scan in the OR, especially smaller and deeper lesions, can be challenging. This is especially true when utilizing minimally invasive techniques where tactile feedback may be limited in the case of VATs procedures or nonexistent in the case of robotic procedures. Patient systematic examination of the parenchyma for tactile or visual clues of the lesion should be performed. Additionally, the entire lung parenchyma should also be examined for any additional small nodules that may have been overlooked.
Pulmonary Function Testing - FEV1 and DLCO
Spirometry is essential for any patient considered for parenchymal resection and is useful in estimating the postoperative risk of morbidity and mortality. The two most important tests are forced expiratory volume in 1 second (FEV1) and diffusion capacity for carbon monoxide (DLCO). Patients with predicted preoperative FEV1 and DLCO >80% require no additional testing for sublobar resections, lobectomy, or pneumonectomy and are generally considered low-risk surgical candidates. Patients with preoperative FEV1 or DLCO <80% should undergo basic prediction of postoperative function and additional preoperative cardiopulmonary testing.
To estimate predicted postoperative FEV1 and DLCO (ppo FEV1/DLCO), multiply the preoperative values by the ratio of the number of remaining pulmonary segments after resection to the total number of segments. There are 10 segments in each lung. On the right, there are 3 segments in the upper lobe, 2 in the middle lobe, and 5 in the lower lobe. On the left, there are 4 in the upper lobe and 5 in the lower lobe. Therefore, a RUL lobectomy in a patient with preoperative FEV1 of 70% of expected, would have a ppoFEV1 of 70% x 17/20 =59.5%. Because this anatomic method assumes homogenous lung function, it underestimates postoperative function if the resection includes more diseased (ie. emphysematous) segments. Obstructed, destroyed, or otherwise nonfunctional segments of the lung may be discounted from the postoperative estimate if they are to be resected, as these areas do not effectively contribute to lung function.
Alternatively, quantitative methods can be used to calculate more precise postoperative lung function estimates. This is especially important in patients with marginal ppoFEV1/DLCO or severely heterogeneous pulmonary disease. Ventilation or perfusion scintigraphy can be used to calculate postoperative residual lung function by measuring the fraction of total function present in each region of the lung and subtracting this fraction from the total function.
A ppoFEV1 and ppoDLCO of at least 30% (though some providers prefer 40%) is the general threshold permitting lobectomy or pneumonectomy. Patients with ppoFEV1 and ppoDLCO >60% are considered low-risk surgical candidates for pulmonary resection. Patients with ppoFEV1 and ppoDLCO >30% but <60% are recommended for further cardiopulmonary testing. Patients with ppoFEV1 and ppoDLCO <30% are considered high-risk surgical candidates.
Low technology exercise tests or more formal cardiopulmonary exercise testing (CPET) can be performed to assess postoperative risk. Stair climbing or shuttle walking may also estimate risk; the ability to climb >22 meters or walk >400 meters predicts acceptable risk. CPET measures oxygen consumption during exercise (i.e. VO2max) and is the most accurate way to estimate postoperative morbidity in these high-risk patients. Preoperative VO2max of >15 mL/kg/min predicts successful recovery from pulmonary resection with acceptable morbidity and mortality, while VO2max <10 mL/kg/min generally indicates prohibitive surgical risk. Patients with a VO2max of 10-15 mL/kg/min have higher surgical risk and should be counseled as such before any intervention.