Factors influencing intrafractional target shifts in lung stereotactic body radiation therapy.

PURPOSE: The objective of this study is to investigate the influence of tumor size, location, and patient characteristics on the intrafractional target shift in lung stereotactic body radiation therapy (SBRT). METHODS AND MATERIALS: Sixty-nine stage I lung cancer patients with partial upper body fixation who received SBRT in our center were included in this study. The patients had pre- and post-radiation therapy cone-beam computed tomography (CBCT) at each fraction for target alignment during radiation therapy setup. The 3-dimensional (3D), intrafractional shift and on-couch time for each fraction were recorded with soft tissue matching. Statistical correlations of these shifts with the characteristics of the tumor (volume, location in upper vs lower, central vs peripheral, right vs left, and internal motion) and the patient (age, sex, performance status, pulmonary function, body mass index, and on-couch time) were determined. RESULTS: Overall, 470 intrafractional shifts were measured on the 69 patients. The median 3D shift was 2.4 mm (range, 0-15 mm) and the median on-couch time was 17 minutes (range, 10-55 minutes). On 55 occasions (11.7%), 3D shifts larger than 5 mm were observed. The tumor volume, tumor location (upper vs lower thorax), and the patient motion tendency were significantly associated with the probability of larger intrafractional shifts (3D shift >5 mm). Linear regression model analysis indicated male sex (P = .039), larger tumor volume (P = .019), and a motion tendency (P = .0006) are predictors for patients with a large intrafractional shift. No other factors were good predictors of large intrafractional target shifts in lung SBRT. CONCLUSIONS: In lung SBRT patients with large tumor size, being male and large motion tendency are prone to having a >5 mm intrafractional target shift determined from the CBCT soft tissue matching system. Prior knowledge of these factors would be beneficial in selecting a customized immobilization system for the lung SBRT patients with the specific conditions.

The influence of target and patient characteristics on the volume obtained from cone beam CT in lung stereotactic body radiation therapy.

PURPOSE: To investigate the influence of tumor and patient characteristics on the target volume obtained from cone beam CT (CBCT) in lung stereotactic body radiation therapy (SBRT). MATERIALS AND METHODS: For a given cohort of 71 patients, the internal target volume (ITV) in CBCT obtained from four different datasets was compared with a reference ITV drawn on a four-dimensional CT (4DCT). The significance of the tumor size, location, relative target motion (RM) and patient's body mass index (BMI) and gender on the adequacy of ITV obtained from CBCT was determined. RESULTS: The median ITV-CBCT was found to be smaller than the ITV-4DCT by 11.8% (range: -49.8 to +24.3%, P<0.001). Small tumors located in the lower lung were found to have a larger RM than large tumors in the upper lung. Tumors located near the central lung had high CT background which reduced the target contrast near the edges. Tumor location close to center vs. periphery was the only significant factor (P=0.046) causing underestimation of ITV in CBCT, rather than RM (P=0.323) and other factors. CONCLUSIONS: The current clinical study has identified that the location of tumor is a major source of discrepancy between ITV-CBCT and ITV-4DCT for lung SBRT.

Implementation of Lung Stereotactic Ablative Radiotherapy at a Regional Cancer Centre.

BACKGROUND: For patients with inoperable Stage I (T1-T2, N0, M0) non-small-cell lung cancer, stereotactic ablative radiotherapy (SABR), also known as stereotactic body radiation therapy (SBRT), has demonstrated survival outcomes similar to surgery. Lung SABR is a technically challenging means of delivering precise, high-dose radiation to a small tumor volume. At many cancer centres, the widespread use of SABR is impeded by the complexity of the implementation process. This study will aim to provide a detailed guide to the steps involved in delivering lung SABR in a reliable and efficient manner. PROCESS: The execution of this intricate treatment program at our cancer centre required the collaboration of a multidisciplinary team. Input from several professionals within radiation oncology was necessary, including medical physicists, dosimetrists, radiation therapists, nurses, and radiation oncologists. Expert guidelines have been developed which give careful consideration to each step of the process, including 1) reliable and reproducible patient immobilization, 2) a method to account for tumor and organ motion, 3) the use of multiple treatment fields to deliver highly conformal radiation dose with a rapid dose fall off, 4) daily imaging that allows for repositioning from simulation to treatment, 5) accurate and precise dose-calculation algorithms, and 6) a vigorous quality assurance program. Lung SABR was introduced at our centre in 2007 and thus far 92 patients have been treated. There are currently three treatment machines capable of performing this procedure. BENEFITS/CHALLENGES: Patient immobilization through the use of body cushions, accurate tumor and organ delineation via the use of four-dimensional computed tomography simulation, development of firm treatment planning guidelines, treatment verification using cone beam computed tomography, and a robust quality assurance program have all been instrumental in ensuring the safe and effective delivery of lung SABR. However, the process was laden with challenges, from delineating the optimum immobilization technique that balances patient comfort and motion, to introducing ways of making novice staff comfortable with a new method for treatment verification. CONCLUSIONS: There are outcome and toxicity data being collected on patients undergoing lung SABR at our cancer centre. This will serve as a self-assessment tool for our implementation process. Moreover, as future indications for SABR change, this initial implementation step will serve as a framework on which to continue building comprehensive guidelines.