SBRT Lung: Optimal PTV Margins and Local Control

SGRT-based stereotactic body radiotherapy for lung cancer setup accuracy and margin of the PTV

OBJECTIVE

Surface-guided radiation therapy (SGRT, AlignRT) was used to analyze motion during stereotactic body radiotherapy (SBRT) in lung cancer patients and to explore the margin of the planning target volume (PTV).

METHODS

The residual errors of the AlignRT were evaluated based on grayscale cone-beam computed tomography registration results before each treatment. AlignRT log file was used to analyze the correlation between the frequency and longest duration of errors larger than 2 mm and lasting longer than 2 s and maximum error with age and treatment duration. The displacement value at the end of treatment, the average displacement value, and the 95% probability density displacement interval were defined as intrafraction errors, and PTV1, PTV2, PTV3 were calculated by Van Herk formula or Z score analysis. Organ dosimetric differences were compared after the experience-based margin was replaced with PTV3.

RESULTS

The interfraction residual errors were Vrt0 , 0.06 ± 0.18 cm; Lng0 , -0.03 ± 0.19 cm; Lat0 , 0.02 ± 0.15 cm; Pitch0 , 0.23 ± 0.7°; Roll0 , 0.1 ± 0.69°; Rtn0 , -0.02 ± 0.79°. The frequency, longest duration and maximum error in vertical direction were correlated with treatment duration (r = 0.404, 0.353, 0.283, p < 0.05, respectively). In the longitudinal direction, the frequency was correlated with age and treatment duration (r = 0.376, 0.283, p < 0.05, respectively), maximum error was correlated with age (r = 0.4, P < 0.05). Vertical, longitudinal, lateral margins of PTV1, PTV2, PTV3 were 2 mm, 4 mm, 2 mm; 2 mm, 2 mm, 2 mm, 3 mm, 5 mm, 3 mm, respectively. After replacing the original PTV, mean lung dose (MLD), 2-cm3 chest wall dose (CD), lung V20 decreased by 0.2 Gy, 2.1 Gy, 0.5%, respectively (p < 0.05).

CONCLUSION

AlignRT can be used for interfraction setup and monitoring intrafraction motion. It is more reasonable to use upper and lower limits of the 95% probability density interval as an intrafraction error.

Electromagnetic Transponder Based Tracking and Gating in the Radiotherapeutic Treatment of Thoracic Malignancies

PURPOSE

This report details our institutional workflow and technique for use of the Calypso electromagnetic transponder system with respiratory gating for localization and tracking of lung tumors during stereotactic radiation therapy for early stage thoracic malignancies.

METHODS AND MATERIALS

Sixteen patients underwent bronchoscopic fiducial placement of 3 transponders in small airways in proximity to the primary tumor. Transponders were placed <19 cm from the most anterior skin location of the patient for appropriate tracking functionality. Patients underwent simulation with 4-dimensional assessment and were treated with transponder based positional gating if tumors moved >5 mm in any direction. Tumor motion <5 mm was not gated and treated using an internal target volume approach. A 5 mm uniform planning target volume was used. Before treatment, fiducial placement and tumor location were verified by daily kilovoltage (kV) and cone beam computed tomography image guidance. Tracking limits were placed based on the movement of the transponders from the centroid of the structures on the maximum intensity projection image. The Calypso treatment system paused treatment automatically if beacons shifted beyond the predefined tracking limits.

RESULTS

All 16 patients underwent successful implantation of the electromagnetic transponders. Eight patients exhibited tumor motion sufficient to require respiratory gating, and the other 8 patients were treated using a free breathing internal target volume technique. Difficulty with transponder sensing was experienced in 3 patients as a result of anatomic interference with the placement of the sensing arrays; each of these cases was successfully treated after making setup modifications. Triggered imaging of fiducials during treatment was consistent with real-time positioning determined by the Calypso tracking system.

CONCLUSIONS

Respiratory gated electromagnetic based transponder guided stereotactic body radiation therapy using the workflow described is feasible and well tolerated in selected patients with early stage lung malignancies.