ExacTrac Snap Verification: a new tool for ensuring quality control for lung stereotactic body radiation therapy.

Purpose: The intrafraction verification provided by ExacTrac X-ray 6D Snap Verification (ET-SV) allows the tracking of potential isocenter displacements throughout patient position and treatment. The aims of this study were (1) to measure the intrafraction variations of the isocenter position (random errors); (2) to study the amplitude of the variation related to the fraction duration; and (3) to assess the impact of the table movement on positioning uncertainties.

Methods And Materials: ET-SV uses images acquired before or during treatment delivery or both to detect isocenter displacement. Twenty patients treated with stereotactic body radiation therapy (SBRT) for lung tumors underwent SV before or during each beam. Noncoplanar beams were sometimes necessary. The time between the setup of the patient and each SV was noted, and values of deviations were compiled for 3 SV time groups: SV performed at ≤10 min (group 1), between 11 and 20 min (group 2), and ≥21 min (group 3). Random errors in positioning during the use of noncoplanar fields were noted.

Results: The mean isocenter deviation±SD was 2±0.5 mm (range, 1-8 mm). The average deviations±SD increased significantly from 1.6±0.5 mm to 2.1±0.8 mm and 2.2±0.6 mm for groups 1, 2, and 3 (P=.002), respectively. Percentages of deviation≥3 mm were 7.06%, 22.83%, and 28.07% and 1.08%, 4.15%, and 8.4% for ≥5 mm (P<.0001). For 11 patients, table rotation was necessary. The mean isocenter deviation±SD increased significantly from 1.9±0.5 mm before table rotation to 2.7±0.5 mm (P=.001) for the first beam treated after rotation.

Conclusions: SV detects isocenter deviations, which increase in amplitude and frequency with the fraction duration, and enables intrafraction verification for SBRT (taking into account clinical condition and technical issues). SV gives accurate targeting at any time during irradiation and may raise confidence to escalate the dose. SV appears to be an important tool for ensuring the quality control of SBRT.