Changes in electron beam dosimetry with a new scattering foil-applicator system on a CL2100C.

PURPOSE: The optimization of clinical electron beams is a challenge to accelerator manufacturers. There are numerous variations and reports of scattering-foil and applicator configurations. The accelerator at our facility was recently updated with new foils and applicators. We conducted many dosimetric tests to critically evaluate dosimetric changes and their clinical effects. METHODS AND MATERIALS: The new dual foil systems are thicker and have shaped disks seated on the lower foils. The 12 MeV beam no longer shares a common foil used for 6 and 9 MeV. The applicators now have denser collimating plates, and Fiberglas no longer connects the plates. The new applicator set includes a rectangular 10 x 6 cm applicator that uses one photon jaw setting for all energies. After the electron beam energies were tuned to previous specifications (energy according to ionization depths, symmetry to +/- 2%, and flatness to +/- 6%), recommissioning took place. Electron beam output checks at various source-to-skin distances (SSD) were conducted for all energies and applicators. Computer-driven water scanning provided percent depth dose, profile, isodose, and Bremsstrahlung data. Surface doses, in-air electron dispersion, effective SSDs, and leakage were also measured. All results compared the previous and updated systems. RESULTS: We found little change in relative percent depth doses for 100 cm SSD between the two systems. The differences in PDD due to increasing SSD, however, decreased with the updated system. Surface doses decreased in most cases, while Bremsstrahlung increased in all cases (typically by a factor of two). Beam uniformity indices increased significantly, while penumbra widths decreased. Diagonal profiles are now quite flat for large fields. For a 20 MeV beam, the 90% width along the diagonal axis for a 25 x 25 applicator at dmax depth has increased from 25 to 32 cm. There was little or no change in 'effective SSD' or in-air dispersion. Leakage outside the applicators was reduced by a factor of two to three. The flatness characteristics of the 10 x 6 cm applicator were poor in comparison to the improved flatness of the new square applicators. CONCLUSIONS: The updated scattering foil-applicator electron beam system has yielded many dosimetric changes. Major improvements have been made in beam flatness and leakage. These positive changes have not been accompanied by any clinically significant dosimetric deficiencies.

Dosimetry and clinical implementation of dynamic wedge.

PURPOSE: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. METHODS AND MATERIALS: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. RESULTS: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. CONCLUSIONS: Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.

Interleaf leakage for 5 and 10 mm dynamic multileaf collimation systems incorporating patient motion.

Use of dynamic multileaf collimation (DMLC) for intensity modulated radiation therapy (IMRT) is accelerating. Delivery systems have the ailment of interleaf leakage (IL). This is compounded by the inefficiency of IMRT delivery, estimated to be a factor of 5 for DMLC. With IL on the order of 4%, it is possible to deliver as much as 20% of the prescribed dose to nonprescribed regions. However, IL is characterized by narrow Gaussian peaks of approximately 0.5-1.0 mm full-width-half-maximum (FWHM). We performed a leakage study for 5 and 10 mm leaf systems, accounting for intratreatment and intertreatment motions. In solid phantoms, film was placed perpendicular to beams. DMLC patterns delivered step-wedged distributions. The same field was duplicated using a collimating jaw in a segmented fashion to obtain baseline data of primary and scatter contributions. Longitudinal shifts up to 4 mm and angulations up to 4 degrees were introduced during beam delivery by running multiple patterns, to arrive at a composite delivery. The intent of these rigid body motion experiments was to replicate patient motion. Clinical IMRT fields using segmented MLC were also tested. Films were scanned and converted to dose. A microionization chamber confirmed film data at discrete points. In all cases shifts diminished IL peak values. In the step-wedge case, the net 18 MV IL peaks diminished from 3.6% to 3.2% for the 10 mm system. The 5 mm system IL values decreased from 4.0% to 3.2% with a 2 mm shift but increased to 4.0% with 4 mm shifts. The clinical field data followed the same pattern with a washing out of peak values, but the overall transmission to shielded regions slightly increased. Therefore nonprescribed regions are influenced by an effective transmission value rather than discrete peak IL values. The 5 mm leaf system does not introduce increased IL and is an appropriate system for IMRT.

Noise in polymer gel measurements using MRI.

With the development of conformal radiotherapy, particularly intensity modulated radiation therapy (IMRT), there is a clear need for multidimensional dosimeters. A commercial polymerizing gel, BANG-2 gel (MGS Research, Inc., Guilford, CT), has recently been developed that shows potential as a multi-dimensional dosimeter. This study investigates and characterizes the noise and magnetic resonance (MR) artifacts from imaging BANG-2 gels. Seven cylindrical vials (4 cm diam, 20 cm length) were irradiated end on in a water bath and read using MRI (B0=1.5 T, TE=20 ms/100 ms, TR=3000 ms). The gel calibration compared the measured depth-dose distributions in water against the change in solvent-proton R2 relaxivity of the gel. A larger vial (13 cm diam, 14 cm length) was also irradiated to test the calibration accuracy in a vial of sufficient volume for dose distribution measurements. The calibration curve proved accurate to within 1.3% in determining the depth dose measured by the larger vial. An investigation of the voxel-to-voxel (IXIX 3 mm3) noise and sensitivity response curve showed that the voxel-to-voxel variation dominated the dose measurement uncertainty. The voxel-to-voxel standard deviation ranged from 0.2 Gy for the unirradiated gel to 0.7 Gy at 20 Gy. Slice-to-slice R2 magnitude deviations were also observed corresponding to 0.2 Gy. These variations limited the overall accuracy of the gel dose measurements and warrant an investigation of more accurate MR readout sequences.

Evaluation of polymer gels and MRI as a 3-D dosimeter for intensity-modulated radiation therapy.

BANG gel (MGS Research, Inc., Guilford, CT) has been evaluated for measuring intensity-modulated radiation therapy (IMRT) dose distributions. Treatment plans with target doses of 1500 cGy were generated by the Peacock IMRT system (NOMOS Corp., Sewickley, PA) using test target volumes. The gels were enclosed in 13 cm outer diameter cylindrical glass vessels. Dose calibration was conducted using seven smaller (4 cm diameter) cylindrical glass vessels irradiated to 0-1800 cGy in 300 cGy increments. Three-dimensional maps of the proton relaxation rate R2 were obtained using a 1.5 T magnetic resonance imaging (MRI) system (Siemens Medical Systems, Erlangen, Germany) and correlated with dose. A Hahn spin echo sequence was used with TR = 3 s, TE = 20 and 100 ms, NEX = 1, using 1 x 1 x 3 mm3 voxels. The MRI measurements were repeated weekly to identify the gel-aging characteristics. Ionization chamber, thermoluminescent dosimetry (TLD), and film dosimetry measurements of the IMRT dose distributions were obtained to compare against the gel results. The other dosimeters were used in a phantom with the same external cross-section as the gel phantom. The irradiated R2 values of the large vessels did not precisely track the smaller vessels, so the ionization chamber measurements were used to normalize the gel dose distributions. The point-to-point standard deviation of the gel dose measurements was 7.0 cGy. When compared with the ionization chamber measurements averaged over the chamber volume, 1% agreement was obtained. Comparisons against radiographic film dose distribution measurements and the treatment planning dose distribution calculation were used to determine the spatial localization accuracy of the gel and MRI. Spatial localization was better than 2 mm, and the dose was accurately determined by the gel both within and outside the target. The TLD chips were placed throughout the phantom to determine gel measurement precision in high- and low-dose regions. A multidimensional dose comparison tool that simultaneously examines the dose-difference and distance-to-agreement was used to evaluate the gel in both low-and high-dose gradient regions. When 3% and 3 mm criteria were used for the comparisons, more than 90% of the TLD measurements agreed with the gel, with the worst of 309 TLD chip measurements disagreeing by 40% of the criteria. All four MRI measurement session gel-measured dose distributions were compared to evaluate the time behavior of the gel. The low-dose regions were evaluated by comparison with TLD measurements at selected points, while high-dose regions were evaluated by directly comparing measured dose distributions. Tests using the multidimensional comparison tool showed detectable degradation beyond one week postirradiation, but all low-dose measurements passed relative to the test criteria and the dose distributions showed few regions that failed.