Volume 27, Issue 2, February 2000
Index of content:
- PH. D. THESES ABSTRACTS
27(2000); http://dx.doi.org/10.1118/1.598849View Description Hide Description
This work investigates the finite-size pencil beam (FSPB) model for calculating dose deposition from photon beams generated by a clinical linear accelerator. The FSPB model, first published by Bourland and Chaney, uses the superposition of preconvolved “finite-size pencil beams” of small cross-sectional area to determine the dose deposition in a uniform water phantom. In the dose computation, FSPBs for a range of energy bins are pieced together like mosaic tiles to collectively form the cross section of the full beam. Depending on the full beam resolution, the superposition calculation can be much faster than full convolution. The results stress the importance of a knowledge of the photon spectrum of the beam for accurate dose calculations. However, published methods of indirect spectral measurements using transmission measurements through beam attenuators require mathematical fits with a large number of parameters and constraints. A simple strategy is presented for fitting transmission data that models important physical characteristics of photon beams produced in linear accelerators. The fitting equation has these advantages over previous methods: (1) the equation describes the shape of a bremsstrahlung spectrum based on physical expectations; (2) only three fit parameters are required with a single constraint. Results are presented for 4 and 6 MV photon beams. Comparisons of calculated and measured TMRs and output factors of open fields show excellent agreement. Results include discussions of FSPB generation, the method of spectral measurement, the effect of beam softening across the field cross section, and the method for modeling this effect using different FSPB weighting factors as a function of energy and location.
27(2000); http://dx.doi.org/10.1118/1.598850View Description Hide Description
Techniques for dosimetric verification of radiotherapy treatments using a CCDcamera based fluoroscopic electronic portal imaging device(EPID) are described. The dosimetric characteristics of the EPID were investigated and a method was developed to derive portal doseimages (PDIs) from measured EPIDimages.EPID and ionization chamber measurements agreed to within 1% (1σ). Subsequently, an algorithm was developed to predict these PDIs using the planning CT data of the patient and the irradiation geometry as determined in the treatment planning process. Furthermore, a method was developed to derive the on-axis patient dose from an EPID measurement, which was then compared with the intended dose. The method allows the discrimination of errors that are due to changes in patient anatomy and errors due to a deviating cGy/MU value. For 115 prostate cancer patients the differences between the average on-axis measured portal dose and the predicted portal dose for the three open beams were small: −0.3±2.3% (1σ). However, large (up to 15%) off-axis differences between measured and predicted PDIs were found, which were caused by frequently occurring gas pockets inside the rectum of the patients during treatment or during acquisition of the planning CT scan. The detected gas pockets did sometimes extend into the tumor volume area as outlined in the CT scan, implying internal organ motion. Finally, methods were developed for pretreatment verification of intensity modulated fields produced with compensators or dynamic multileaf collimation (DMLC). EPID measurements of dose profiles generated with DMLC agreed within 2% (1σ) with predictions and ionization chamber measurements.