Stereotactic body radiation therapy (SBRT), or stereotactic ablative radiotherapy (SABR), has been emerging as an excellent alternative for medically inoperable early stage NSCLC patients. Conceptually derived from cranial stereotactic radiosurgery, the planning and delivery of SBRT is characterized by highly target-conformal dose distributions with steep dose gradients towards normal tissues, allowing the administration of potent tumor-ablative radiation doses. In lung SBRT, a total of 45-50 Gy of radiation is delivered in 3-5 fractions over a 10-20 days’ duration. Calculated by LQ with α/β =10 Gy, a less than 5 cm tumor is generally treated with higher than 100 biologically equivalent dose (BED). Available data demonstrated an impressive 80-95% local tumor control at 2-5 years and good lung function preservation (3-6). The recently published RTOG 0236 phase II study demonstrated 3-years 98% local tumor control and 56% survival (7). This result is quite comparable to the reported 53% 5-year survival with surgical resection, based on thousands of patients in the International Association for the Study of Lung Cancer Staging Project (8). Lung SBRT is comparably superior than radiofrequency ablation (RFA), an alternative invasive procedure with a moderate 60% tumor control rate for less than 3 cm tumors, but is also associated with much higher procedure-related morbidities, mainly caused by pneumothorax and hemorrhage (9).

How to accurately locate small pulmonary targets for lung SBRT is the subject of one article published in this issue of Journal of Thoracic Disease. In their study, Shen et al. investigated the application of double CT imaging to measure the respiratory movement of small pulmonary tumors during SBRT (10). A total of 122 small pulmonary tumors in 45 patients were measured. Four-slice spiral CT scans were conducted twice in all patientsonce each at the end of quiet inhalation and of exhalation, and three times in 17 patients - with one additional free breathing image. The displacement of the tumor center in three directions was measured. The study showed an overall 3D motion of 10.10±7.16 mm in 122 tumors, with 1.96±2.03, 5.19±4.69 and 7.38±6.48 mm in the X, Y and Z directions, respectively. The extent of tumor motion was influenced by the pulmonary location of individual tumor: greater motion was noted in tumors in the lower, left and anterior locations than in the upper, right and posterior locations. In contrast to 4-dimentional CT, this is a relatively less expansive, yet practical method for target localization. Their conclusions are in general agreement with published results (11).

The success of lung SBRT relies largely on accurate target localization, which enables precise ablative radiotherapy to target while maximizing the spared surrounding normal tissues from treatment-related side effects. It is an eminent observation that small-sized lung tumors are moving targets, which changes not only their locations, but also their shapes and volumes as the lung inflates and deflates. In addition, respiratory-induced motion can cause severe geometrical distortion of tumors and normal tissues in free breathing CT scanning. A variety of methods and techniques have been used to determine the exact location of a moving target inside of lung. Voluntary breathholding during imaging and treatment represents one simple, but often problematic approach in many lung cancer patients due to poor lung function and anxiety issues. Four-dimensional CT (4D-CT) is currently considered a standard methodology to reduce motion artifact and allow accurate determination of internal target volume (ITV; Figure 1). In 4D-CT, an oversampled spiral CT scan with continuous slices is acquired simultaneously, while the respiratory motion (arbitrarily divided into 10 phases) is recorded by an infrared camera-based motiontracking system (12). 4D-CT is capable of accurately defining the location and volume of the tumor and its surrounding organs over time during breathing cycles.