Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of dynamic intensity modulated radiation therapy (IMRT) delivery were investigated. The EPID was utilized with continuous frame-averaging during the beam delivery. Properties studied included effect of buildup, dose linearity, field size response, sampling of rapid multileaf collimator (MLC) leaf speeds, response to dose-rate fluctuations, memory effect, and reproducibility. The dependence of response on EPID calibration and a dead time in image frame acquisition occurring every 64 frames were measured. EPID measurements were also compared to ion chamber and film for open and wedged static fields and IMRT fields. The EPID was linear with dose and dose rate, and response to MLC leaf speeds up to 2.5 cm s(-1) was found to be linear. A field size dependent response of up to 5% relative to dmax ion-chamber measurement was found. Reproducibility was within 0.8% (1 standard deviation) for an IMRT delivery recorded at intervals over a period of one month. The dead time in frame acquisition resulted in errors in the EPID that increased with leaf speed and were over 20% for a 1 cm leaf gap moving at 1.0 cm s(-1). The EPID measurements were also found to depend on the input beam profile utilized for EPID flood-field calibration. The EPID shows promise as a device for verification of IMRT, the major limitation currently being due to dead-time in frame acquisition.

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The purpose of this study was to compare the dosimetry of CG‐Darc with three‐dimensional conformal radiation therapy (3D CRT) and volumetric‐modulated arc therapy (RapidArc) in the treatment of breast cancer with APBI. CG‐Darc plans were generated using two tangential couch arcs combined with a simultaneous noncoplanar gantry arc. The dynamic couch arc was modeled by consecutive IMRT fields at 10° intervals. RapidArc plans used a single partial arc with an avoidance sector, preventing direct beam exit into the thorax. CG‐Darc and RapidArc plans were compared with 3D CRT in 20 patients previously treated with 3D CRT (group A), and in 15 additional patients who failed the dosimetric constraints of the Canadian trial and of NSABP B‐39/RTOG 0413 for APBI (group B). CG‐Darc resulted in superior target coverage compared to 3D CRT and RapidArc (V95%: 98.2% vs. 97.1% and 95.7%). For outer breast lesions, CG‐Darc and RapidArc significantly reduced the ipsilateral breast V50% by 8% in group A and 15% in group B (p<0.05) as compared with 3D CRT. For inner and centrally located lesions, CG‐Darc resulted in significant ipsilateral lung V10% reduction when compared to 3D CRT and RapidArc (10.7% vs. 12.6% and 20.7% for group A, and 15.1% vs. 25.2% and 27.3% for group B). Similar advantage was observed in the dosimetry of contralateral breast where the percent maximum dose for CG‐Darc, 3D CRT, and RapidArc were 3.9%, 6.3%, and 5.8% for group A and 4.3%, 9.2%, and 6.3% for group B, respectively (p<0.05). CG‐Darc achieved superior target coverage while decreasing normal tissue dose even in patients failing APBI dose constraints. Consequently, this technique has the potential of expanding the use of APBI to patients currently ineligible for such treatment. Modification of the RapidArc algorithm will be necessary to link couch and gantry rotation with variable dose rate and, therefore, enable the use of CG‐Darc in clinical practice.PACS number: 80

125I brachytherapy sources are being used with increasing frequency for interstitial implants in tumor sites, especially the prostate. Recently, a new 125I source design has become commercially available for clinical applications. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [S(k), 99std]. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. The measured data were used to validate the Monte Carlo simulations that were performed in Solid Water using the PTRAN code. The Monte Carlo calculations were then performed in liquid water to obtain the dosimetric information for clinical applications in accordance with TG-43 recommendations. The results indicated that the dose rate constant, lambda, of the Pharma Seed model BT-125-I 125I source was 0.90 +/- 0.06 cGy h(-1) U(-1) using thermoluminescent dosimeter (TLD) measurements and 0.92 +/- 0.03 cGy h(-1) U(-1) using Monte Carlo simulations in Solid Water. The calculated value in liquid water was found to be 0.95 +/- 0.03 cGy h(-1) U(-1). The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in Solid Water phantom material. The Monte Carlo simulations were performed for distances ranging from 0.1 to 10 cm from the source center in Solid Water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. The anisotropy constant, phi(an), of the Pharma Seed source in water was found to be 0.975. Complete dosimetric data are described in this manuscript. Per TG-43, the values reported in water should be used for clinical treatment planning systems.