On the automated definition of mobile target volumes from 4D‐CT images for stereotactic body radiotherapy

Abstract

Stereotactic body radiotherapy (SBRT) can be used to treat small lesions in the chest. A vacuum-based immobilization system is used in our clinic for SBRT, and a motion envelope is used in treatment planning. The purpose of this study is to automatically derive motion envelopes using deformable image registration of 4D-CT images, and to assess the effect of abdominal pressure on the motion envelopes. 4D-CT scans at ten phases were acquired prior to treatment for both free and restricted breathing using a vacuum-based immobilization system that includes an abdominal pressure pillow. To study the stability of the motion envelope over the course of treatment, a mid-treatment 4D-CT scan was obtained after delivery of the third fraction for two patients. The planning target volume excluding breathing motion (PTV(ex)) was defined on the image set at full exhalation phase and transformed into all other phases using displacement maps from deformable image registration. The motion envelope was obtained as the union of PTV(ex) masks of all phases. The ratios of the motion envelope to PTV(ex) volume ranged from 1.3 to 2.5. When pressure was applied, the ratios were reduced by as much as 29% compared to free breathing for some patients, but increased by up to 9% for others. The abdominal pressure pillow has more motion restriction effects on the anterior/inferior region of the lung. For one of the two patients for whom the 4D-CT scan was repeated at mid-treatment, the motion envelope was reproducible. However, for the other patient the tumor location and lung motion pattern significantly changed due to changes in the anatomy surrounding the tumor during the course of treatment, indicating that an image-guided approach to SBRT may increase the efficacy of this treatment.

Quality assurance device for four‐dimensional IMRT or SBRT and respiratory gating using patient‐specific intrafraction motion kernels

“…The multiphase 4D image sets were transferred into a research module of the Pinnacle 3 treatment planning system (Philips Radiation Oncology Systems, Madison, WI). The 3D motion kernels were designed using a previously published method, (7) and each ITV was created using the derived organ motion kernels (7) . To account for setup error and other possible residual errors during therapy, each PTV was generated by expanding the respective ITV by a margin of 5 mm (8) …”

Estimating the 4D respiratory lung motion by spatiotemporal registration and super‐resolution image reconstruction

“…For example, one of the most important applications of 4D-CT deformable registration is to automatically determine the motion envelope of tumor. 3,6,7 Thus, by using the estimated motion envelope to account for breathing motion, instead of simply expanding based on experience, the size of ITV can be greatly reduced. Other applications of 4D-CT deformable registration include ventilation imaging, 8 motioncompensated cone-beam CT reconstruction, 9 and respiratory motion modeling.…”

“…This helps the expansion of CTV in a more accurate and patientspecific way. [3][4][5] However, 4D-CT does not provide respiratory motion information between consecutive phases explicitly, which boosts the investigation of deformable registration methods for estimating the respiratory motion from 4D-CT images. For example, one of the most important applications of 4D-CT deformable registration is to automatically determine the motion envelope of tumor.…”

MLC tracking for lung SABR reduces planning target volumes and dose to organs at risk

“…The GTV was defined on the end-of-exhale phase, namely the GTVTRACK, to assure a proper localization and delineation of the tumour [7,[15][16][17]. Collimators were angled to have the leaf trajectory parallel to the superior-inferior motion (85/95°) [18].…”