John W. Wong

A software tool for the quantitative evaluation of 3D dose calculation algorithms

Current methods for evaluating modern radiation therapy treatment planning (RTP) systems include the manual superposition of calculated and measured isodose curves and the comparison of a limited number of calculated and measured point doses. Both techniques have significant limitations in providing quantitative evaluations of the large number of dose data generated by modern RTP systems. More sophisticated comparison techniques have been presented in the literature, including dose-difference and distance-to-agreement (DTA) analyses. A software tool has been developed that uses superimposed isodose plots, dose-difference, and DTA distributions to quantify errors in computed dose distributions. Dose-difference and DTA analyses are overly sensitive in regions of high- and low-dose gradient, respectively. The logical union of locations that fail both dose-difference and DTA acceptance criteria, termed the composite evaluation, is calculated and displayed. The composite evaluation provides a method for the physicist to efficiently identify regions that fail both the dose-difference and DTA acceptance criteria. The tool provides a computer platform for the quantitative comparison of calculated and measured dose distributions.

On methods of inhomogeneity corrections for photon transport

Eight methods of photon inhomogeneity correction were examined for their photon transport approximations. The methods were categorized according to the different approaches used to model scatter photon dose contribution. They were the ratio of TAR (RTAR) and the modified Batho power law which utilized only the 1-D density information along the primary photon path; the equivalent TAR (ETAR) and the FFT convolution methods which incorporated the 3-D density information of the medium for empirical scatter dose calculation; the differential SAR (DSAR), the delta volume (DV), dose spread array (DSA), and differential pencil beam (DPB) methods which employed explicit 3-D scatter ray-trace calculation. Cobalt-60 measurements in horizontal slab phantoms were used to allow simpler data analysis. RTAR consistently overestimated lung corrections by approximately 10%. The scatter ray-trace approach was not always better as the DSAR calculations were inferior to those using the Batho method. The ray-tracing DV, DPB, and DSA methods agreed with measurements mostly to within 2%, at the expense of long computation time. The nonscatter ray-tracing ETAR and FFT convolution calculations were only slightly inferior in the same geometries. These methods improve on the current 1-D methods and should be seriously considered for fast optimization purposes in practical 3-D treatment planning.

Verification data for electron beam dose algorithms

The Collaborative Working Group (CWG) of the National Cancer Institute (NCI) electron beam treatment planning contract has performed a set of 14 experiments that measured dose distributions for 28 unique beam-phantom configurations that simulated various patient anatomic structures and beam geometries. Multiple dose distributions were measured with film or diode detectors for each configuration, resulting in 78, 2-D planar dose distributions and one, 1-D depth-dose distribution. Measurements were made for 9- and 20-MeV electron beams, using primarily 6 x 6- and 15 x 15-cm applicators at several SSDs. Dose distributions were measured for shaped fields, irregular surfaces, and inhomogeneities (1-D, 2-D, and 3-D), which were designed to simulate many clinical electron treatments. The data were corrected for asymmetries, and normalized in an absolute manner. This set of measured data can be used for verification of electron beam dose algorithms and is available to others for that purpose.

Portal dose images. II, patient dose estimation

Due to the many sources of uncertainties in radiotherapy, conventional treatment planning can only provide a nominal presentation of the dose delivered to the patient. Provided that large setup errors can be detected and corrected, we propose that measured and calculated portal dose images can be used to improve estimation of patient dose. The iterative approach described in this paper requires an accurate method of dose calculations, 3-dimensional CT data that closely represents the patient and the measured portal dose image. From the CT data, a portal dose image is calculated for comparison with the measured one. The differences are then used to modify the original CT data so that a new image can be calculated. The process is repeated until the calculated and measured images agree to satisfaction. At that point, the internal dose is calculated using the modified CT data. If the method is successful, daily portal dose images could be used to cumulatively estimate patient dose throughout the course of treatment. This manuscript describes 60Co simulation results to demonstrate the feasibility of the approach.

A METHOD TO ANALYZE 2-DIMENSIONAL DAILY RADIOTHERAPY PORTAL IMAGES FROM AN ON-LINE FIBER-OPTIC IMAGING SYSTEM

On-line radiotherapy imaging systems allow convenient treatment verification and generate a wealth of data. Quantitative analysis of data will provide important information about the nature of treatment variations. Using an inhouse fiber-optic imaging system to acquire daily portal images for five patients, we have developed a method to analyze the cumulative positional variation of blocks in the 2-dimensional images. For each beam arrangement used to treat a particular patient, a reference portal image was established. All other images for that patient were registered with respect to the anatomical landmarks visible on the reference image. Two-dimensional frequency distributions describing the overlap of the blocks during the course of treatment were then calculated and superimposed on the reference image. Results of the analysis show positional and quantitative information about the daily variation in block placement, and appeared to be site-dependent. Long term verification studies using on-line imaging systems will be important in the understanding of treatment uncertainties.

Systematic verification of a three-dimensional electron beam dose calculation algorithm.

A three-dimensional electron beam dose calculation algorithm implemented on a commercial radiotherapy treatment planning system is described. The calculation is based on the M. D. Anderson Hospital (M.D.A.H.) pencil beam model, which uses the Fermi-Eyges theory of thick-target multiple Coulomb scattering. To establish the calculation algorithms accuracy as well as its limitations, it was systematically and extensively tested and evaluated against a set of benchmark measurements. Various levels of dose and spatial tolerances were used to validate the calculation quantitatively. Results are presented in terms of the percentage of data points meeting a specific tolerance level. The algorithms ability to accurately simulate commonly used clinical setup geometries, including standard or extended SSDs, blocked fields, irregular surfaces, and heterogeneities, is demonstrated. Regions of disagreement between calculations and measurements are also shown. The clinical implication of such disagreements is addressed, and the algorithmic assumptions involved are discussed.

Study of treatment variation in the radiotherapy of head and neck tumors using a fiber-optic on-line radiotherapy imaging system

On-line radiotherapy imaging systems allow convenient daily acquisition of portal images for treatment verification. The information can also be used to study treatment variability. Using a prototype fiber-optic imaging system, we have measured the treatment variation of 17 head and neck patients. Daily digital portal images were acquired for the on-cord left and right lateral fields. Treatment variations were quantified using the Cumulative Verification Image Analysis (CVIA) method developed at our institute. In the CVIA method, daily portal images were aligned according to three anatomical points predefined on a digitized simulation, or prescription, image. After each image alignment, the block position was cumulated in a bit-map and superimposed on the prescription image to give a cumulative verification summary image. Iso-frequency distributions, or contours, of the block overlap were calculated and examined with respect to the prescription treatment area. The range of the treatment variation was large for the 17 patients. On average, separation of the 0% to 100% block overlap contours was about 10 mm, and the 20% to 80%, 5 mm. The block overlap contours were also used to calculate the frequency with which the prescription area as defined on the simulation film had been treated. The fraction of the prescription area treated depended on the accuracy of the treatment setup and patient repositioning, as expected. At best, approximately 95% of the prescribed area was irradiated 100% of the time during the entire course of radiotherapy. At worst, approximately 70% of the prescribed area was irradiated 100% of the time. These results demonstrate that despite immobilization, large setup variation can still occur. Presenting treatment variation data as population averages does not reflect on the large variation that may be observed in the individual patient.

Photon dose calculation incorporating explicit electron transport

Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation effects near the interfaces of different media, where charged particle equilibrium is not established, remain unsolved. Furthermore, changes in atomic number, which affect the multiple Coulomb scattering of the secondary electrons, are not accounted for by current photon dose calculation algorithms. As local interface effects are mainly due to the perturbation of secondary electrons, a photon-electron cascade model is proposed which incorporates explicit electron transport in the calculation of the primary photon dose component in heterogeneous media. The primary photon beam is treated as the source of many electron pencil beams. The latter are transported using the Fermi-Eyges theory. The scattered photon dose contribution is calculated with the dose spread array [T.R. Mackie, J.W. Scrimger, and J.J. Battista, Med. Phys. 12, 188-196 (1985)] approach. Comparisons of the calculation with Monte Carlo simulation and TLD measurements show good agreement for positions near the polystyrene-aluminum interfaces.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet : linearity, signal-to-noise ratio, and energy response characteristics

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Interinstitutional experience in verification of external photon dose calculations

Under the auspices of NCI contracts, four institutions have collaborated to assess the accuracy of the pixel-based dose calculation methods they employ for external photon treatment planning. The approach relied on comparing calculations using each groups algorithm with measurements in phantoms of increasing complexity. The first set of measurements consisted of ionization chamber measurements in water phantoms in normally incident square fields, an elongated field, a wedged field, a blocked field, and an obliquely incident beam. The second group of measurements was carried out using thermoluminescent dosimeters in phantoms designed to investigate the effects of surface curvature, high density heterogeneities, and low density heterogeneities. The final study tested the entire treatment planning system, including CT data conversion, in an anthropomorphic phantom. Overall, good agreement between calculation and measurements was found for all algorithms. Regions in which discrepancies were observed are pointed out, areas for algorithm improvement are identified and the clinical import of algorithm accuracy is discussed.