In-vivo quality assurance of dynamic tumor tracking (DTT) for liver SABR using EPID images.

PURPOSE: To assess dynamic tumor tracking (DTT) target localization uncertainty for in-vivo marker-based stereotactic ablative radiotherapy (SABR) treatments of the liver using electronic-portal-imaging-device (EPID) images. The Planning Target Volume (PTV) margin contribution for DTT is estimated. METHODS: Phantom and patient EPID images were acquired during non-coplanar 3DCRT-DTT delivered on a Vero4DRT linac. A chain-code algorithm was applied to detect Multileaf Collimator (MLC)-defined radiation field edges. Gold-seed markers were detected using a connected neighbor algorithm. For each EPID image, the absolute differences between the measured center-of-mass (COM) of the markers relative to the aperture-center (Tracking Error, (ET )) was reported in pan, tilt, and 2D-vector directions at the isocenter-plane. PHANTOM STUDY: An acrylic cube phantom implanted with gold-seed markers was irradiated with non-coplanar 3DCRT-DTT beams and EPID images collected. Patient Study: Eight liver SABR patients were treated with non-coplanar 3DCRT-DTT beams. All patients had three to four implanted gold-markers. In-vivo EPID images were analyzed. RESULTS: Phantom Study: On the 125 EPID images collected, 100% of the markers were identified. The average ± SD of ET were 0.24 ± 0.21, 0.47 ± 0.38, and 0.58 ± 0.37 mm in pan, tilt and 2D directions, respectively. Patient Study: Of the 1430 EPID patient images acquired, 78% had detectable markers. Over all patients, the average ± SD of ET was 0.33 ± 0.41 mm in pan, 0.63 ± 0.75 mm in tilt and 0.77 ± 0.80 mm in 2D directions The random 2D-error, σ, for all patients was 0.79 mm and the systematic 2D-error, Σ, was 0.20 mm. Using the Van Herk margin formula 1.1 mm planning target margin can represent the marker based DTT uncertainty. CONCLUSIONS: Marker-based DTT uncertainty can be evaluated in-vivo on a field-by-field basis using EPID images. This information can contribute to PTV margin calculations for DTT.

Factors predictive of symptomatic radiation injury after linear accelerator-based stereotactic radiosurgery for intracerebral arteriovenous malformations.

PURPOSE: To investigate predictive factors in the development of symptomatic radiation injury after treatment with linear accelerator-based stereotactic radiosurgery for intracerebral arteriovenous malformations and relate the findings to the conclusions drawn by Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC). METHODS AND MATERIALS: Archived plans for 73 patients who were treated at the British Columbia Cancer Agency were studied. Actuarial estimates of freedom from radiation injury were calculated using the Kaplan-Meier method. Univariate and multivariate Cox proportional hazards models were used for analysis of incidence of radiation injury. Log-rank test was used to search for dosimetric parameters associated with freedom from radiation injury. RESULTS: Symptomatic radiation injury was exhibited by 14 of 73 patients (19.2%). Actuarial rate of symptomatic radiation injury was 23.0% at 4 years. Most patients (78.5%) had mild to moderate deficits according to Common Terminology Criteria for Adverse Events, version 4.0. On univariate analysis, lesion volume and diameter, dose to isocenter, and a V(x) for doses ≥8 Gy showed statistical significance. Only lesion diameter showed statistical significance (p < 0.05) in a multivariate model. According to the log-rank test, AVM volumes >5 cm(3) and diameters >30 mm were significantly associated with the risk of radiation injury (p < 0.01). The V(12) also showed strong association with the incidence of radiation injury. Actuarial incidence of radiation injury was 16.8% if V(12) was <28 cm(3) and 53.2% if >28 cm(3) (log-rank test, p = 0.001). CONCLUSIONS: This study confirms that the risk of developing symptomatic radiation injury after radiosurgery is related to lesion diameter and volume and irradiated volume. Results suggest a higher tolerance than proposed by QUANTEC. The widely differing findings reported in the literature, however, raise considerable uncertainties.

An investigation of a new amorphous silicon electronic portal imaging device for transit dosimetry.

The relationship between the pixel value and exit dose was investigated for a new commercially available amorphous silicon electronic portal imaging device. The pixel to dose mapping function was established to be linear for detector distances between 116.5 cm to 150 cm from the source, radiation field sizes from 5 x 5 cm2 to 20 x 20 cm2 and beam energies of 6 to 18 MV. Coefficients in the mapping function were found to be dependent on beam energy and field size. Open and wedged field profiles measured with the device showed agreement to a maximum of 5% and 8%, respectively, as compared to film. A comparison of relative transmission measurements between the EPID and ion chamber indicate a maximum deviation of 6% and 2% at 6 and 18 MV, respectively, for an attenuator thickness of 21 cm and SDD > or = 130 cm. It was found that accuracies of better than 1% could be obtained if detector position and field size specific fitting parameters were used to generate unique mapping functions for each configuration.