Comparison of patient setup accuracy for optical surface-guided and X-ray-guided imaging with respect to the impact on intracranial stereotactic radiotherapy.

PURPOSE: The objective of this work is to estimate the patient positioning accuracy of a surface-guided radiation therapy (SGRT) system using an optical surface scanner compared to an X­ray-based imaging system (IGRT) with respect to their impact on intracranial stereotactic radiotherapy (SRT) and intracranial stereotactic radiosurgery (SRS). METHODS: Patient positioning data, both acquired with SGRT and IGRT systems at the same linacs, serve as a basis for determination of positioning accuracy. A total of 35 patients with two different open face masks (578 datasets) were positioned using X­ray stereoscopic imaging and the patient position inside the open face mask was recorded using SGRT. The measurement accuracy of the SGRT system (in a "standard" and an SRS mode with higher resolution) was evaluated using both IGRT and SGRT patient positioning datasets taking into account the measurement errors of the X­ray system. Based on these clinically measured datasets, the positioning accuracy was estimated using Monte Carlo (MC) simulations. The relevant evaluation criterion, as standard of practice in cranial SRT, was the 95th percentile. RESULTS: The interfractional measurement displacement vector of the SGRT system, σSGRT, in high resolution mode was estimated at 2.5 mm (68th percentile) and 5 mm (95th percentile). If the standard resolution was used, σSGRT increased by about 20%. The standard deviation of the axis-related σSGRT of the SGRT system ranged between 1.5 and 1.8 mm interfractionally and 0.5 and 1.0 mm intrafractionally. The magnitude of σSGRT is mainly due to the principle of patient surface scanning and not due to technical limitations or vendor-specific issues in software or hardware. Based on the resulting σSGRT, MC simulations served as a measure for the positioning accuracy for non-coplanar couch rotations. If an SGRT system is used as the only patient positioning device in non-coplanar fields, interfractional positioning errors of up to 6 mm and intrafractional errors of up to 5 mm cannot be ruled out. In contrast, MC simulations resulted in a positioning error of 1.6 mm (95th percentile) using the IGRT system. The cause of positioning errors in the SGRT system is mainly a change in the facial surface relative to a defined point in the brain. CONCLUSION: In order to achieve the necessary geometric accuracy in cranial stereotactic radiotherapy, use of an X­ray-based IGRT system, especially when treating with non-coplanar couch angles, is highly recommended.

Sweeping-beam technique with electrons for large treatment areas as total skin irradiation : Dosimetric and technical aspects of a modified Stanford technique.

PURPOSE: Total skin electron beam therapy (TSEBT) is still a technical and therapeutic challenge today. Thus, we developed TSEBT using a sweeping-beam technique. METHODS: For treatment delivery, a linear accelerator Versa HD (ELEKTA, Stockholm, Sweden) with high-dose-rate electrons (HDRE) was used with a dose rate of 9000 MU/min. Dosimetry quality assurance was performed by multiple measurements with film dosimetry, 2D array, and Roos chamber. RESULTS: Clinical experience shows that treatment durations of 75 to 90 min are usual for the Stanford technique without using HDRE. With this new sweeping-beam irradiation technique, the total treatment time of a daily fraction could be reduced to 20 min while keeping over- and underdosing low. The treatment area is about 60 cmâ€¯× 200 cm and the dose distribution is uniform within 2% and 5% in vertical and horizontal directions, respectively. Initially, the electron energy of 6 MeV is reduced to 3.2 MeV by 1­cm polymethylmethacrylat (PMMA) scatter and the irradiation conditions of a source-surface distance (SSD) of 350 cm. The photon contamination drops to under 1%. CONCLUSION: These results show that the mean dose to total skin varies between 1.3 and 1.8 Gy. The sweeping-beam technique with electrons has a homogeneous dose distribution in connection with a short treatment time.