Timothy J. Waldron

X-RAY FIELD COMPENSATION WITH MULTILEAF COLLIMATORS

PURPOSE It has been proposed that conformal therapy can be carried out with static ports that are each individually compensated to deliver an optimal total dose distribution. If this proposal is to be implemented, one must have a means of compensating or modulating the fluence distributions within the boundaries of individual treatment fields. A theory was developed and implemented to achieve this goal. METHODS AND MATERIALS The theory allowed creation of a leaf-setting sequence for a desired level of field-modulation precision. This method of beam modulation was experimentally verified using radiographic film to integrate the dose delivered by the sequence of discrete static multileaf collimator-defined subfields. RESULTS Beam profiles were generated that matched the planned beam profiles to within the specified degree of precision. CONCLUSION This methodology is a candidate for implementation of inverse planning for conformal therapy.

Realization and verification of three-dimensional conformal radiotherapy with modulated fields

PURPOSE We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields. METHODS AND MATERIALS Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multileaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patients outer contour. Seven therapy verification films were placed between the phantom slices. RESULTS Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning. CONCLUSIONS It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to invaginated and bifurcated target volumes using fields modulated by multileaf collimators.

Clinical dosimetry for implementation of a multileaf collimator

In order to initiate the use of a multileaf collimator (MLC) in the clinic, a set of technical procedures needs to be available sufficient to create MLC leaf settings and to deliver an accurate dose of radiation through the MLC-shaped field. Dosimetry data for clinical use of the MLC were measured. Dosimetric characteristics included central axis percent depth dose, output factors, and penumbra. In this paper, it has been concluded that a dose control monitor unit calculation procedure that has been applied to the use of conventional secondary field-shaping blocks can be applied to the multileaf collimator dosimetry. The multileaf collimator penumbra (20% to 80%) is only slightly wider (1-3 mm) than the penumbra of the conventional collimator jaws. Beams-eye-view comparisons made between the isodose curves in fields shaped by conventional Cerrobend blocks and isodose curves in fields shaped by the multileaf collimator demonstrated that the 50% isodose line at 10-cm depth exhibited the discrete steps of the multileaf collimator leaves, but that the 90% and 10% isodose curves of the multileaf were close to those shaped by Cerrobend blocks.

Quality assurance for nonradiographic radiotherapy localization and positioning systems: Report of Task Group 147

New technologies continue to be developed to improve the practice of radiation therapy. As several of these technologies have been implemented clinically, the Therapy Committee and the Quality Assurance and Outcomes Improvement Subcommittee of the American Association of Physicists in Medicine commissioned Task Group 147 to review the current nonradiographic technologies used for localization and tracking in radiotherapy. The specific charge of this task group was to make recommendations about the use of nonradiographic methods of localization, specifically; radiofrequency, infrared, laser, and video based patient localization and monitoring systems. The charge of this task group was to review the current use of these technologies and to write quality assurance guidelines for the use of these technologies in the clinical setting. Recommendations include testing of equipment for initial installation as well as ongoing quality assurance. As the equipment included in this task group continues to evolve, both in the type and sophistication of technology and in level of integration with treatment devices, some of the details of how one would conduct such testing will also continue to evolve. This task group, therefore, is focused on providing recommendations on the use of this equipment rather than on the equipment itself, and should be adaptable to each users situation in helping develop a comprehensive quality assurance program.

Radiation therapy plan checks in a paperless clinic

Traditional quality assurance checks of a patients radiation therapy plan involve printing out treatment parameters from the treatment planning system and the “record and verify” (R&V) system and visually checking the information for one‐to‐one correspondence. In a paperless environment, one can automate this process through independent software that can read the treatment planning data directly and compare it against the parameters in the R&V systems database. In addition to verifying the data integrity, it is necessary to check the logical consistency of the data and the accuracy of various calculations. The results are then imported into the patients electronic medical record. Appropriate workflows must be developed to ensure that no steps of the QA process are missed. This paper describes our electronic QA system (EQS), consisting of in‐house software and workflows. The EQS covers 3D conformal and intensity modulated radiation therapy, electrons, stereotactic radiosurgery, total body irradiation, and clinical set ups with and without virtual simulation. The planning systems handled by our EQS are ADAC Pinnacle and Varian FASTPLAN, while the R&V systems are LANTIS and VARIS. The improvement in our plan check process over the paperless system is described in terms of the types of detected errors. The potential problems with the implementation and use of the EQS, as well as workarounds for data that are not easily accessible through electronic means, are described. PACS numbers: 87.55.Qr, 87.55.tg, 87.55.tm

Validation of a new virtual wedge model

Results of a validation study of a commercial virtual wedge device recently installed at our institution are presented. The wedge simulation produces an energy fluence from the treatment head that is equivalent to the primary energy fluence attenuated through a wedge-shaped slab of water with the central axis fluence set to unity. A simple exponential formula used to compute off-axis wedge factors is compared to beam profiles measured in a water phantom. A fast Fourier transform (FFT) convolution dose calculation is compared to measured dose profiles. Measured and calculated central axis wedged/open field ratios as a function of depth are also compared.

SU-F-T-02: Estimation of Radiobiological Doses (BED and EQD2) of Single Fraction Electronic Brachytherapy That Equivalent to I-125 Eye Plaque: By Using Linear-Quadratic and Universal Survival Curve Models

PURPOSE To test the radiobiological impact of hypofractionated choroidal melanoma brachytherapy, we calculated single fraction equivalent doses (SFED) of the tumor that equivalent to 85 Gy of I125-BT for 20 patients. Corresponding organs-at-risks (OARs) doses were estimated. METHODS Twenty patients treated with I125-BT were retrospectively examined. The tumor SFED values were calculated from tumor BED using a conventional linear-quadratic (L-Q) model and an universal survival curve (USC). The opposite retina (α/β = 2.58), macula (2.58), optic disc (1.75), and lens (1.2) were examined. The % doses of OARs over tumor doses were assumed to be the same as for a single fraction delivery. The OAR SFED values were converted into BED and equivalent dose in 2 Gy fraction (EQD2) by using both L-Q and USC models, then compared to I125-BT. RESULTS The USC-based BED and EQD2 doses of the macula, optic disc, and the lens were on average 118 ± 46% (p < 0.0527), 126 ± 43% (p < 0.0354), and 112 ± 32% (p < 0.0265) higher than those of I125-BT, respectively. The BED and EQD2 doses of the opposite retina were 52 ± 9% lower than I125-BT. The tumor SFED values were 25.2 ± 3.3 Gy and 29.1 ± 2.5 Gy when using USC and LQ models which can be delivered within 1 hour. All BED and EQD2 values using L-Q model were significantly larger when compared to the USC model (p < 0.0274) due to its large single fraction size (> 14 Gy). CONCLUSION The estimated single fraction doses were feasible to be delivered within 1 hour using a high dose rate source such as electronic brachytherapy (eBT). However, the estimated OAR doses using eBT were 112 ∼ 118% higher than when using the I125-BT technique. Continued exploration of alternative dose rate or fractionation schedules should be followed.

Clinical validation of a real-time applicator position monitoring system for gynecologic intracavitary brachytherapy

Purpose: To validate the clinical feasibility and efficacy of a real-time applicator position monitoring system (RAPS) through a phantom study and a prospective clinical trial. Methods and materials: The RAPS measures the brachytherapy applicator displacement in real-time by computing the relative displacement between two infrared reflective targets, one attached to the applicator and the other to the patients skin. A phantom study was performed to compare RAPS measurements with the ground truth. Six cervical cancer patients were enrolled in the clinical trial using MRI-based high-dose-rate brachytherapy with a Tandem-and-Ovoids applicator. The results from the RAPS are compared with the clinical method. Results: In the phantom study, an average difference between RAPS measurements and known displacements was 0.02 ± 0.01 mm in the superior-inferior direction, 0.02 ± 0.02 mm in the lateral direction, and 0.11 ± 0.06 mm in the anterior-posterior direction. In the clinical trial, the absolute difference in applicator displacement between the RAPS and the clinical method was 1.46 ± 1.13 mm. In all patient cases, a maximum applicator displacement of 6.66 mm (2.0 ± 1.5 mm) was observed using the RAPS. Conclusions: This work demonstrates the clinical efficacy of RAPS to measure applicator displacement.

SU‐GG‐J‐193: Using Small‐Deformation Linear‐Elastic Registration to Quantifying Ventilation‐Competent Lung Imaging From Clinical 4DCT Datasets: Toward Selective Avoidance IMRT for Locally Advanced Non‐Small‐Cell Lung Cancer

Purpose: To verify the feasibility of a small‐deformation inverse‐consistent linear‐elastic (SICLE) registration algorithm in quantifying ventilation‐competent sublung images from 4D CT datasets. Materials/Method: SICLE, a deformable image registration algorithm developed at the University of Iowa, was utilized to register the 100% inhalation and 100% exhalation phases of the lung which were extracted from clinical 4D CT datasets. The SICLE method performs inverse consistent image registration in which the forward and reverse transformations are estimated jointly while minimizing the inverse consistency error. After deformable registration, ventilation was calculated using the equation Δ V/Vex = 1000 ( HUin − HUex )/( HUex (1000 + HUin )) on voxels of the inhalation and exhalation images, where HU is in Hounsfield Units and ΔV/Vex is the change in regional volume divided by the local volume. Results: SICLE was found to be efficient and consistent in deformable image registering lungCTimage datasets with different phases. 3D ventilation images were quantified and given in coronal, transverse, and sagittal view. Quantitative ventilation in a given region of interest was also determined. Ventilation‐competent sublung regions were constructed with different functionality levels of 90, 70, 50, and 30%. Conclusion: Utilizing a small‐deformation inverse‐consistent linear‐elastic registration algorithm, it was feasible to quantify ventilation competent subregions from clinical 4D CT datasets. Given the availability of 4D CT technology, this study opens a pathway to integrate functional lunginformation into radiotherapy for locally advanced non‐small‐cell lungcancer.