Anthony K. Ho

Evaluation of water-equivalent plastics as phantom material for electron-beam dosimetry

This investigation evaluated samples of three phantom materials designed as substitutes for water for electron beam calibration and depth ionization measurements. Two of the materials are commercially available (photon-electron Solid Water and Plastic Water), while the third (Homat) is not. Applying the values for water for all factors used in the calibration protocol of the American Association of Physicists in Medicine [Task Group 21, Med. Phys. 10, 741-771 (1983)] results in a discrepancy in calculated peak dose rates. Eliminating this discrepancy requires the additional inclusion of a multiplicative correction factor of approximately 1.015 for beam energies below 10 MeV, 1.01 for beam energies between 10 and 12 MeV, and 1.005 for beam energies above 12 MeV. Measurements for R50 and extrapolated range may be made in these materials with no corrections. Some improvement can be made in the performance of the phantom material by optimizing the match to water specifically for electron beams without regard for photon beam response. As with all radiation oncology apparatus, calibration phantoms need acceptance testing before routine use.

Accuracy of the point source approximation to high dose-rate Ir-192 sources

The accuracy of the point source approximation used in dose calculations for an implant comprised of multiple high dose rate (HDR) Ir-192 source dwell positions is investigated. First, a single dwell position implant is modeled. The exposure rate about the source is calculated using both the point source approximation and the more rigorous line source formalism. A comparison of these calculated exposure rates is made. It is found that for each HDR Ir-192 source dwell position, the point source approximation results in a dose overestimation of 1% at a distance of 1 cm on the source transverse axis, while dose underestimations of more than 2% can be found at a distance of 1 cm on the source longitudinal axis. Even larger errors occur closer to the source. The results of this academic study are then extended to two clinical cases--an endobronchial treatment and a tandem and ovoids setup, both involving multiple source dwell positions. Since clinical HDR Ir-192 implants are comprised of many individual source dwell positions, there will be inaccuracy in the calculated overall dose distribution leading to dose delivery errors. For example, the dose delivered to a prescription point located 0.5 cm from an endobronchial applicator will be 3% lower than prescribed. Similar errors are produced in gynecologic implants. To decrease below 0.5% the dose delivery error resulting from the point source approximation, prescription points should be at a distance of at least 1 cm from any applicator. Since the dosimetry error is a direct result of the choice of model used to describe the source, the use of anisotropy factors accounting for the variation of photon fluence around the HDR Ir-192 source will not completely correct the calculation.

Evaluation of a well-type ionization chamber for calibration of HDL and LDR brachytherapy sources

The Atomlab 44D well-type ionization chamber is being evaluated for calibration of high dose rate (HDR) Ir-192 and low dose rate (LDR) Cs-137 sources. The chamber has a flat response (sweet spot) of +/- 0.5% along approximately 3.5 cm for an Ir-192 HDR linear source and 3 cm for a Cs-137 LDR spherical source. The short-term stability of the chamber was determined using a cylindrical Cs-137 source positioned in the sweet spot region. The chamber response over a range of 17.4 mCi (644 MBq, Cs-137) and 8.82 Ci (326 GBq, Ir-192) is evaluated. The chamber may be used for calibrating the activities of both HDR Ir-192 and LDR brachytherapy sources.

Electron energy constancy check using a five-chamber detector array.

Two methods are shown here to determine the 50% depth ionization (d50) using buildup materials of different thickness placed on top of a five-chamber detector array. In the first method, two sets of different thickness buildup material are required to perform the check, one set for checking 6 and 9 MeV, while another set is used for 12, 16, and 20 MeV electron beam from a Varian Clinac 2100C. The second method only requires two data points to determine the d50 depth for each energy. The d50 depths determined were compared with the d50 depth obtained using a water phantom with ionization chamber measurements. The method is simple to use especially for departments that use a similar detector to perform quality assurance tests such as output/symmetry/flatness check.

Comparison of nucletron and rocs brachytherapy treatment planning systems for LDR and HDR applications

The Nucletron Plato system is being used in our Institute for both high dose rate (HDR) and low dose rate (LDR) treatment planning, while the radiation oncology computer system (ROCS) is used for external beam planning. This paper compares both systems, using a gynecological application for the LDR case and an esophageal application for the HDR. It is shown that ROCS may be used as a backup to the Nucletron treatment planning system for LDR and simple HDR cases. The Nucletron planning system is a better system than ROCS for both LDR and HDR applications.

Neural network object recognition for inspection of patient setup in radiation therapy using portal images

This paper presents a method to improve the performance of inspection of patient setup in radiation therapy using portal images. The proposed algorithm is used to match real-time portal and simulator images which contain the information of patient position relative to the beam position. Two approaches are proposed for this purpose. One approach is to use a multi-scale (multiresolution) analysis of the acquired image where the image is examined at several levels of detail simultaneously. Another approach is to use a more robust optimization method to find the global minimum of a cost function which is generalized from matching both anatomical structures and radiation treatment fields. The optimization method used here is a concurrent (coarse-and-fine) multiresolution model-based object recognition technique using a multi-layer Hopfield neural network. The performance of the algorithm is demonstrated in a clinical radiotherapy application.

A simple backup for a radiosurgery treatment planning system

To calculate the dose distribution and the number of monitor unit (MU) per arc, all radiosurgery systems utilize some sort of computer. These computers are, of course, subject to equipment malfunction such as problems with the magnetic tape drive, keyboard, mouse, etc. Since most radiosurgery procedures are quite invasive and time consuming, it is important to have a reliable and reasonably accurate backup system for planning the treatment. This paper will show that a simple PC based system, along with a digitizer, may be used as a backup for a commercial, VAX based radiosurgery system. A complete radiosurgery planning procedure was carried out on a head phantom with a target imbedded inside. The treatment planning and verification using the PC based system is also compared with that using the VAX based system.

Evaluation of dose delivery based on a comparison of dosimetry calculations using open beam and wedged beam depth dose data

The purpose of this study is to evaluate the magnitude of the error in dose delivery caused by the use of open beam depth dose data in dosimetry calculations for wedged photon beams. Isodose plans were calculated for treatments given in a 3-field isocentric prostate or rectal setup using an open AP beam with two lateral wedged beams. The dose distributions were first calculated using open beam depth dose data for all three fields. Next, the open beam data was used only for the AP field and true wedged beam depth dose data was substituted for the two lateral wedged fields. The magnitude of the depth dose variations for wedged vs open beams depends on the nominal beam energy, the wedge angle, and the depth of measurement. Consequently, isodose distributions calculated for wedged fields were found to be different when true wedged beam depth dose data was used instead of open beam data as is commonly done. Monitor unit calculations using a field size specific wedge factor show that dose delivery errors up to 4% can result from the use of open beam depth dose data in wedged beam dose distribution calculations for a 6-MV photon beam. Accurate treatment planning for wedged fields requires the use of wedged beam depth dose data specific to each wedge. Simply using open beam depth dose data in dose calculations for wedged beams will result in dose delivery errors, the magnitude of which depends on the combination of wedge angle, field size, and nominal beam energy.

Application of the “bioeffects” algorithm of a treatment planning system

In this study, both a four-field box and two-field AP/PA treatment plan are combined with two insertions of Cs-137 in a tandem and ovoids setup, to evaluate the bioeffects program of a treatment planning system. External beam energies studied are 18 and 6 MV. It is shown that there is a slight difference in the 50-70 time dose fractionation (TDF) isolines when comparing 6 MV and 18 MV, for the AP/PA setup. There is practically no difference for TDF isoline values larger than 80 for both energies with either the four-field or the two-field setup. This is because the brachytherapy contributed the majority of the dose to the regions near the applicator and the TDF values reflect the higher dose delivered by the brachytherapy relative to the external beams in that region. For this simple evaluation of the bioeffects program, the combination of the external beam plan and the brachytherapy plan does not give us enhanced information on the effectiveness of the plan.