A new type of radiographic film, Kodak EDR2 film, was evaluated for dose verification of intensity modulated radiation therapy (IMRT) delivered by a static multileaf collimator (SMLC). A sensitometric curve of EDR2 film irradiated by a 6 MV x-ray beam was compared with that of Kodak X-OMAT V (XV) film. The effects of field size, depth and dose rate on the sensitometric curve were also studied. It is found that EDR2 film is much less sensitive than XV film. In high-energy x-ray beams, the double hit process is the dominant mechanism that renders the grains on EDR2 films developable. As a result, in the dose range that is commonly used for film dosimetry for IMRT and conventional external beam therapy, the sensitometric curves of EDR2 films cannot be approximated as a linear function, OD = c * D. Within experimental uncertainty, the film sensitivity does not depend on the dose rate (50 vs 300 MU/min) or dose per pulse (from 1.0 x 10(-4) to 4.21 x 10(-4) Gy/pulse). Field sizes and depths (up to field size of 10 x 10 cm2 and depth = 10 cm) have little effect on the sensitometric curves. Percent depth doses (PDDs) for both 6 and 23 MV x rays were measured with both EDR2 and XV films and compared with ion chamber data. Film data are within 2.5% of the ion chamber results. Dose profiles measured with EDR2 film are consistent with those measured with an ion chamber. Examples of measured IMRT isodose distributions versus calculated isodoses are presented. We have used EDR2 films for verification of all IMRT patients treated by SMLC in our clinic. In most cases, with EDR2 film, actual clinical daily fraction doses can be used for verification of composite isodose distributions of SMLC-based IMRT.

A new type of radiographic film, EDR (extended dose range) film, has been recently become available for film dosimetry. It is particularly attractive for composite isodose verification of intensity modulated radiation therapy because of its low sensitivity relative to the more common Kodak XV film. For XV film, the relationship between optical density and dose, commonly known as the sensitometric curve, depends linearly on the dose at low densities. Unlike XV film, the sensitometric curve of EDR film irradiated by megavoltage x rays is not linearly dependent on the dose at low densities. In this work, to understand the mechanisms governing the shape of the sensitometric curves, EDR film was studied with kilovoltage x rays, 60Co gamma rays, megavoltage x rays, and electron beams. As a comparison, XV film was also studied with the same beams mentioned above. The model originally developed by Silberstein [J. Opt. Soc. Am. 35, 93-107, 1945)] is used to fit experimental data. It is found that the single hit model can be used to predict the sensitometric curve for XV films irradiated by all beams used in this work and for EDR films exposed to kilovoltage x rays. For EDR film irradiated by 60Co gamma rays, megavoltage x rays, and electron beams, the double hit model is used to fit the sensitometric curves. For doses less than 100 cGy, a systematic difference between measured densities and that predicted by the double hit model is observed. Possible causes of the observed differences are discussed. The results of this work provide a theoretical explanation of the sensitometric behavior of EDR film.

Virtual Wedge (VW) is a Siemens treatment modality which generates wedge-shaped dose distributions by moving a collimator jaw from closed to open at a constant speed while varying the dose rate in every 2 mm jaw position. In this work, the implementation and verification of VW in a radiotherapy treatment planning (RTP) system is presented. The VW implementation models the dose delivered by VW using the Siemens monitor units (MU) analytic formalism which determines the number of MU required to generate a wedge-fluence profile at points across the VW beam. For any set of treatment parameters, the VW algorithm generates an "intensity map" that is used to model the modification of fluence emanating from the collimator. The intensity map is calculated as the ratio of MU delivered on an axis point, divided by the monitor units delivered on the central-axis MU(0). The dose calculation is then performed using either the Clarkson or Convolution/ Superposition algorithms. The VW implementation also models the operational constraints for the delivery of VW due to dose rate and jaw speed limits. Dose verifications with measured profiles were performed using both the Clarkson and Convolution/Superposition algorithms for three photon beams; Siemens Primus 6 and 23 MV, and Mevatron MD 15 MV. Agreement within 2% or 2 mm was found between calculated and measured doses, over a large set of test cases, for 15, 30, 45, and 60 degree symmetric and asymmetric VW fields, using the manufacturer's supplied mu and c values for each beam.

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 and 23 MV photon beams from a Siemens Primus linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses and surface doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron "contamination." The present study of a 60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5 X 5, 20 X 20, and 35 X 35-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a "clear" 60Co gamma-ray beam of 1-cm diameter.