Optimization of the Dose Rate Effect in Tetrazolium Gellan Gel Dosimeters.

Tetrazolium salts provide an appealing candidate for 3D gel dosimeters as they exhibit a low intrinsic color, no signal diffusion and excellent chemical stability. However, a previously developed commercial product (the ClearView 3D Dosimeter) based on a tetrazolium salt dispersed within a gellan gum matrix presented a noticeable dose rate effect. The goal of this study was to find out whether ClearView could be reformulated in order to minimize the dose rate effect by optimizing of the tetrazolium salt and gellan gum concentrations and by the addition a thickening agent, ionic crosslinkers, and radical scavengers. To that goal, a multifactorial design of experiments (DOE) was conducted in small-volume samples (4-mL cuvettes). It showed that the dose rate could be effectively minimized without sacrificing the integrity, chemical stability, or dose sensitivity of the dosimeter. The results from the DOE were used to prepare candidate formulations for larger-scale testing in 1-L samples to allow for fine-tuning the dosimeter formulation and conducting more detailed studies. Finally, an optimized formulation was scaled-up to a clinically relevant volume of 2.7 L and tested against a simulated arc treatment delivery with three spherical targets (diameter 3.0 cm), requiring different doses and dose rates. The results showed excellent geometric and dosimetric registration, with a gamma passing rate (at 10% minimum dose threshold) of 99.3% for dose difference and distance to agreement criteria of 3%/2 mm, compared to 95.7% in the previous formulation. This difference may be of clinical importance, as the new formulation may allow the quality assurance of complex treatment plans, relying on a variety of doses and dose rates; thus, expanding the potential practical application of the dosimeter.

Experimental assessments of intrafractional prostate motion on sequential and simultaneous boost to a dominant intraprostatic lesion.

PURPOSE: To investigate experimentally the impact of intrafractional prostate motion on the delivered dose to a dominant intraprostatic lesion (DIL) using volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) with sequential and simultaneous boost. METHODS: A series of six IMRT and VMAT treatment plans were generated, evaluated, and compared for two patient CT scans with dissimilar anatomies. Plans were generated for the prostate with and without the DIL. Plans were delivered using a Varian CLINAC and 2D dose distributions were measured using mapcheck(TM)-mapphan(TM) system. The effect of the prostate intrafractional motion on the delivery of the plans was studied by delivering the plans to the mapcheck(TM)-mapphan(TM) system on a programmable motion platform. Prostate intrafractional motion was simulated based on six different motion patterns from the literature obtained on Calypso system (Calypso System, Calypso Medical, Seattle, WA, USA) in a clinical study that provided continuous, real-time localization, and monitoring of the prostate. Absolute dose differences and Gamma analysis were used to assess the quality of a total of 42 plans with motion and without motion. RESULTS: Dose escalation to the whole prostate from 76 to 86 Gy caused the rectum and bladder to exceed normal tissue tolerances in both patients. All the DIL boost plans satisfied the planning criteria and delivery quality assurance when motion was not present. For a single fraction, the motion pattern with large constant shift caused the largest dose delivery discrepancy with mean Gamma value (1.14-1.44) and the lowest plan passing percentage (18.9%-35.7%), while the motion pattern with continuous random changes during treatment had the least impact on dose delivery with mean Gamma value (0.33-0.55) and the highest passing percentage (81.9%-100%) for all the investigated plans. For dose escalation to DIL in the presence of intrafractional prostate motion, a significant difference was observed between the different motion patterns (p < 0.05), but no significant difference in the sensitivity to motion between the various plans was observed (p = 0.30). Based on Gamma analysis, treatment courses in which 15% of the fractions are dominated by severe motion proved to be significantly different from those dominated by random motion (p < 0.05). CONCLUSIONS: The impact of intrafractional prostate motion on dose delivery is sensitive to different motion patterns but not to different delivery techniques. Dose escalation to DIL using either sequential or simultaneous boost plans with 7 mm PTV margin is achievable in the presence of intrafractional prostate motion, even if the severe motion comprised 8.6% (3 out of the 35) treatment fractions.

Commissioning a fast Monte Carlo dose calculation algorithm for lung cancer treatment planning.

A commercial Monte Carlo simulation package, NXEGS 1.12 (NumeriX LLC, New York, NY), was commissioned for photon-beam dose calculations. The same sets of measured data from 6-MV and 18-MV beams were used to commission NXEGS and Pinnacle 6.2b (Philips Medical Systems, Andover, MA). Accuracy and efficiency were compared against the collapsed cone convolution algorithm implemented in Pinnacle 6.2b, together with BEAM simulation (BEAMnrc 2001: National Research Council of Canada, Ottawa, ON). We investigated a number of options in NXEGS: the accuracy of fast Monte Carlo, the re-implementation of EGS4, post-processing technique (dose de-noising algorithm), and dose calculation time. Dose distributions were calculated with NXEGS, Pinnacle, and BEAM in water, lung-slab, and air-cylinder phantoms and in a lung patient plan. We compared the dose distributions calculated by NXEGS, Pinnacle, and BEAM. In a selected region of interest (7725 voxels) in the lung phantom, all but 1 voxel had a gamma (3% and 3 mm thresholds) of 1 or less for the dose difference between the NXEGS re-implementation of EGS4 and BEAM, and 99% of the voxels had a gamma of 1 or less for the dose difference between NXEGS fast Monte Carlo and BEAM. Fast Monte Carlo with post-processing was up to 100 times faster than the NXEGS re-implementation of EGS4, while maintaining +/- 2% statistical uncertainty. With air inhomogeneities larger than 1 cm, post-processing preserves the dose perturbations from the air cylinder. When 3 or more beams were used, fast Monte Carlo with post-processing was comparable to or faster than Pinnacle 6.2b collapsed cone convolution.

The response of prototype plane-parallel ionization chambers in small megavoltage x-ray fields.

Accurate small-field dosimetry has become important with the use of multiple small fields in modern radiotherapy treatments such as IMRT and stereotactic radiosurgery. In this study, we investigate the response of a set of prototype plane-parallel ionization chambers, based upon the Exradin T11 chamber, with active volume diameters of 2, 4, 10, and 20 mm, exposed to 6 MV stereotactic radiotherapy x-ray fields. Our goal was to assess their usefulness for accurate small x-ray field dose measurements. The relative ionization response was measured in circular fields (0.5 to 4 cm diameter) as compared to a 10 x 10 cm2 reference field. A large discrepancy (approximately 40%) was found between the relative response in the smallest plane-parallel chamber and other small volume dosimeters (radiochromic film, micro-metal-oxide-semiconductor field-effect transistor and diode) used for comparison. Monte Carlo BEAMnrc simulations were used to simulate the experimental setup in order to investigate the cause of the under-response and to calculate appropriate correction factors that could be applied to experimental measurements. It was found that in small fields, the air cavity of these custom-made research chambers perturbed the secondary electron fluence profile significantly, resulting in decreased fluence within the active volume, which in turn produces a chamber under-response. It is demonstrated that a large correction to the p(fl) correction factor would be required to improve dosimetric accuracy in small fields, and that these factors could be derived using Monte Carlo simulations.

Intensity-modulated arc therapy for treatment of high-risk endometrial malignancies.

PURPOSE: We developed an intensity-modulated arc therapy (IMAT) technique for the treatment of women with high-risk endometrial malignancies. In the context of multimodality therapy, nodal and tumor bed irradiation was delivered while respecting tolerance doses for critical structures. METHODS AND MATERIALS: Five patients were planned and treated with the IMAT technique after hysterectomy. Computed tomographic (CT) scans for treatment planning were acquired with the tumor bed contoured as the clinical target volume (CTV(tumor_bed)) and the iliac and presacral vessels contoured as the gross tumor volume (GTV). In 2 patients the lower para-aortic nodes were included into the GTV. The small bowel, iliac crests, femoral heads, bladder, and rectum were contoured as critical organs. For the nodes, a CTV(nodes) was generated with a 7-10-mm margin around the vessels, and the planning target volume (PTV(nodes)) was generated by a further 5-mm expansion. For the tumor bed, the PTV(tumor_bed) was generated with a margin of 7-10 mm around CTV(tumor_bed). Planning constraints included adequate coverage of the tumor bed (>95% receiving > or =45 Gy) and nodes (> or =95% receiving > or =40 Gy). Arc combinations with different extents were tested, and the final plan was generated based on the balance between complexity (number of arcs), PTV coverage, and critical structure sparing. Conventional and 8-field intensity-modulated radiation therapy (IMRT) plans were generated for each patient for comparison purposes. All patients were treated with IMAT. RESULTS: We found that two anterior intensity-modulated arcs (300 degrees to 30 degrees and 330 degrees to 60 degrees ) adequately treated the PTVs. Furthermore, this IMAT technique allowed sparing of small bowel and the iliac crests (marrow space) to a similar degree as the 8-field IMRT. The 8-field IMRT yielded better dose uniformity than IMAT in the target volumes; however, neither technique was as uniform as the conventional plan. In the 5 patients, IMAT treatment was well tolerated and completed as planned. CONCLUSIONS: We successfully piloted an optimized intensity-modulated arc technique to treat 5 high-risk endometrial cancer patients undergoing multimodality treatment. This allowed a significant reduction in dose to bone marrow and small bowel compared with conventional techniques and was simpler to deliver than multifield IMRT.

Reducing the weight of the EC-L cassette.

PURPOSE: To model the EC-L portal film cassette to understand how its weight could be reduced without compromising image quality. METHODS AND MATERIALS: The BEAM99 Monte Carlo code was used to simulate a 6-MV X-ray beam impinging on a water phantom 15 or 40 cm thick and subsequently reaching image receptors of different designs. The image receptor model included the front cassette wall, lead or copper plates 0-1.2 g/cm(2) in thickness, a Lanex Fast screen pair, and the rear cassette wall. The signal generated in the phosphor screen from primary and scattered photons and charged particles was calculated for all image receptors and for both a water phantom and a water phantom plus bone object. Subject contract was calculated using the formalism of Motz and Danos, and the detective quantum efficiency was calculated using the formalism of Swank. Experimental cassettes with copper plate thicknesses of 0.5, 0.25, and 0.0 mm were used to image patients at several anatomic sites. RESULTS: Multiple scatter, especially at large field sizes, generates a low-energy X-ray component that can overrespond in the image receptor. The lead plate is more effective in reducing this X-ray scatter component than the copper plate. Filtering of the X-ray beam by the patient hardens the X-ray spectrum of a 6-MV X-ray beam, reducing subject contrast for thick patients. The front wall of the cassette plays an important role in contributing to the signals generated in the image receptor when the thickness of the metal plate is reduced. Over a wide variety of field sizes and patient thicknesses, subject contrast and detective quantum efficiency are relatively independent of metal plate thickness. CONCLUSIONS: The results suggest that a redesign of the EC-L cassette where the front wall of the cassette becomes part of the image receptor, and where the metal plate is changed to lead and the thickness reduced to 0.2 g/cm(2), would generate images of quality comparable to those of the existing EC-L cassette when used with a 6-MV X-ray beam. This change would reduce the weight of the EC-L cassette by 1040 g.