Stereotactic body radiation therapy patient specific quality assurance using a two-dimensional array at extended source to surface distance.

METHODS: Five patients with 38 fields have been analyzed in this study. The plans were optimized for the following clinical sites: one liver, one lung, one brain, one prostate and one spine. The detector array used for the measurements was the PTW Seven29 array. All the plans were optimized and calculated using Eclipse v8.9. The center of the array was setup at 215 cm from the source and all the fields were measured and analyzed one by one. All the 30 measurements were performed on a NovalisTX linear accelerator equipped with a high definition multileaf collimator. The evaluation was based mainly on gamma index passing rates using 2 mm distance to agreement (DTA) and 2% dose difference. RESULTS: The accuracy of the Eclipse Treatment Planning System (TPS) at extended Source to Surface Distances (SSDs) using an ionization chamber was measured to be within 1.0%. All the field measurements were performed and analyzed 35 individually. The percent of the points that had a gamma index of less than 1 using 3%/3 mm was >99% for all the measurements. In order to better evaluate our process and distinguish smaller differences a new set of results was obtained by applying gamma index tolerances of 2%/2mm. In this case, the gamma index passing rates ranged from 90.8 to 100% (95.5%±3%). The profile comparison showed that the detector array measurements followed closely the calculated 40 profiles, even for fields optimized with multiple peaks and valleys. CONCLUSION: The choice of the IMRT QA device has an important role in the results of the patient specific QA of the delivered dose to the patient in the case of small targets as in the treatment of spinal targets. In this study, we demonstrated that an extended SSD measurement can improve the sampling resolution of a two-dimensional (2D) detector array, in our case the PTW 45 Seven29 array. This method was shown to be accurate and efficient for measuring highly modulated small fields for pre-treatment patient specific QA.

Clinical Evaluation of a Two-dimensional Liquid-Filled Ion chamber Detector Array for Verification of High Modulation Small Fields in Radiotherapy.

INTRODUCTION: Clinical evaluation of a two-dimensional (2D) liquid-filled ion chamber detector array used in the verification of highly modulated small beams of stereotactic body radiation therapy (SBRT) has been conducted. MATERIALS AND METHODS: Measurements with the Octavius 1000 SRS (PTW, Freiburg, Germany) detector with 977 liquid-filled ion chambers were compared against EDR2 film and PTW Octavius Seven29. The performance of detector array has been evaluated on ten SBRT patient plans. Dose profiles of individual and composite fields' calculated using Pinnacle3 treatment planning system were compared against measurements with Octavius 1000 SRS detector array, EDR2 film, and Octavius Seven29 detector. Gamma index and profile comparison were used in the evaluation and assessment of the detector's performance. RESULTS: The Gamma index measurements show agreement between Pinnacle3 computations and Octavius 1000 SRS array, PTW Octavius Seven29, and EDR2 film for >90% of the points using 2%, 2 mm tolerance criteria. Profiles obtained with the Octavius 1000 SRS were in agreement with the EDR2 film profiles, demonstrating the detector's superior sampling rate. CONCLUSIONS: The Octavius 1000 SRS is a dosimetrically accurate device to perform quality assurance checks in SBRT treatments. The broad range of measurements performed in this study quantified the dosimetric accuracy of Octavius 1000 SRS detector in the clinical setup of the small fields in radiotherapy.

Characterization of a two-dimensional liquid-filled ion chamber detector array used for verification of the treatments in radiotherapy.

PURPOSE: The purpose of the study is to investigate the characteristics of a two-dimensional (2D) liquid-filled ion chamber detector array, which is used for the verification of radiotherapy treatment plans that use small field sizes of up to 10 × 10 cm. METHODS: The device used in this study was Octavius 1000 SRS model (PTW, Freiburg, Germany). Its 2D array of detectors consists of 977 liquid-filled ion chambers arranged over an area of 11 × 11 cm. The size of the detectors is 2.3 × 2.3 × 0.5 mm (volume of 0.003 cm(3)) and their spacing in the inner area of 5.5 × 5.5 cm is 2.5 mm center-to-center, whereas in the outer area it is 5 mm center-to-center. The detector reproducibility, dose linearity, and sensitivity to positional changes of the collimator were tested. Also, the output factors of field sizes ranging from 0.5 × 0.5 to 10 × 10 cm(2) both for open and wedged fields have been measured and compared against those measured by a pin-point ionization chamber, liquid filled microchamber, SRS diode, and EDR2 film. RESULTS: Its short-term reproducibility was within 0.2% and its medium and long-term reproducibility was within 0.5% (verified with air ionization chamber absolute dose measurements), which is an excellent result taking into account the daily fluctuation of the linear accelerator and the errors in the device setup reproducibility. The dose linearity and dose rate dependence were measured in the range of 0.5-85 Gy and 0.5-10 Gy min(-1), respectively, and were verified with air ionization chamber absolute dose measurements was within 3%. The measurements of the sensitivity showed that the 2D Array could detect millimetric collimator positional changes. The measured output factors showed an agreement of better than 0.3% with the pinpoint chamber and microliquid filled chamber for the field sizes between 3 × 3 and 10 × 10 cm(2). For field sizes down to 1 × 1 cm(2), the agreement with SRS diode and microliquid filled chamber is better than 2%. The measurements of open and wedge-modulated field profiles were compared to the film and ionization chamber in water measurements. CONCLUSIONS: 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.