The Octavius Detector 1000 SRS is an accurate, precise, and reliable detector, very useful for the daily performance of the patient specific quality assurance of radiotherapy treatment plans.

Purpose: The aim for the study was to investigate characteristics of the OCTAVIUS Detector 1000 SRS for verification of the small field size IMRT and SRS beams. Methods: Several different treatments sites have been chosen. High modulation steep gradient small field size beams were delivered in two different setup settings. In the first setup virtual water phantom has been placed on the detector and irradiated. In the same setup EDR film has been irradiated. Measured data has been analyzed. In the second setup detector has been placed in the PTW Octavius phantom and irradiated. In the same setup EDR film and PTW 729 have been irradiated. Measured data has been analyzed. Results: In the first setup detector has been irradiated with virtual water phantom and measured Result showed excellent agreement with the film measurements. Along the compared profiles, absolute dose measured was in ±1 % of the film measurements in the high dose region and ±3 % in low dose region. Measured results showed also very good agreement in region of high gradients. Second set of measurements showed excellent agreement with the film measurements in high dose region with absolute dose difference within ±1% and ±3% in low dose region. Agreement with PTW 729 was in ±3 % in high and low dose region along the compared profiles. Region of high gradient showed better agreement with the film than with PTW 729 due to the inferior detector resolution. Conclusion: Broad range of measurements performed in the investigation showed that OCTAVIUS Detector 1000 SRS is a dosimetrically accurate and sensitive device and it can be used for clinical verification of the highly modulated small field size beams used in the IMRT and SRS treatments.

IMRT places a higher requirement on dose grid resolution than conventional radiation therapy. While 0.30-0.40 cm grid was assumed adequate for conformal treatment planning, smaller dose grid is required at least in the areas of high dose. In the cases where steep dose gradients exist smaller grid size should be used while calculating and evaluating treatment plans, as the choice of the calculation grid size may in certain cases even influence clinical results.

Purpose: To evaluate the performance of a new commercially available device for patient specific SBRT pretreatment QA.Materials and Methods: The PTW OCTAVIUS1000SRS was used in this study for patient specific QA measurements for patient undergoing SBRT treatments. Forty five (n=45) individual field and five (n=5) composite plans were measured. All plans were optimized and calculated using Eclipse v8.9. All measurements were performed with the gantry static at the upright orientation. The detector array was placed inside the PTW Octavius II phantom and the source to detector plane distance was set to 100cm. The gamma index was the metric of choice in order to quantify the agreement between measurements and calculations. The tolerance criteria used for the evaluation were 3% dose difference and 3mm distance to agreement and also 2%/2mm. Results: The results of the comparison using 3%/3mm were inconclusive. The number of detectors included in the comparison and the rather loose criteria used caused most comparisons to yield a gamma index of 100%. Comparing the measured and calculated planar dose distributions using 2%/2mm as gamma index tolerances, the average gamma index was 95.9% (min:90.9, max:99.2%). The agreement was also reflected while comparing profiles and isodose distributions. Conclusion: The new PTW Octavius 1000SRS device is able to provide a reliable solution for the patient specific QA of SBRT and SRS plans where small fields are commonly used. Project was partially funded by PTW, NY

Purpose: To estimate the dose distributions delivered to the patient in each treatment fraction using deformable image registration (DIR) and assess the radiobiological impact of the inter‐fraction variations due to patient deformation and setup. Methods: The work is based on the cone beam CT (CBCT) images and treatment plans of two lung cancer patients. Both patients were treated with intensity modulated radiation therapy (IMRT) to 66Gy in 2Gy/fraction. The treatment plans were exported from the treatment planning system (TPS) to the Velocity AI where DIR was performed and the same deformation matrix was used for the deformation of the planned dose distribution and organ contours to each CBCT dataset. A radiobiological analysis was performed based on the radiobiological parameters of the involved organs at risk (OARs) and planning target volume (PTV). Using the complication free tumor control probability (P+) index, differences in P+ were observed between each CBCT as well as between CBCT and planning dose distributions. Results: The optimal CBCT P? values ranged from 91.6 % to 94.8 % for patient #1 and from 88.8 % to 90.6 % for patient #2. At the dose level of the clinical prescription, the CBCT P+ values ranged from 80.3% to 80.7% for patient #1 and from 80.7% to 81.0% for the patient #2. The planning CT P+ values were 81.0% and 80.7% for the two patients, respectively. These differences emphasize the significance of using the radiobiological analysis when assessing changes in the dose distribution due to the tumor motion and lung deformations. Conclusion: Daily setup variations yield to differences in the actual dose delivered versus the planned one. The observed differences were rather small when only looking at the dosimetric comparison of the dose distributions, however the radiobiology analysis was able to detect clinically relevant differences among the studied dose distributions.