Volume 28, Issue 3, March 2001
Index of content:
- PH. D. THESES ABSTRACTS
28(2001); http://dx.doi.org/10.1118/1.1350681View Description Hide Description
Investigations on computer optimization of radiotherapytreatment planning (inverse planning) have demonstrated that dose distributions can often be conformed tightly to a target volume by customizing the beam intensity profiles within the treatment field. The aim of this study was the development and clinical implementation of static and dynamic intensity modulated treatment techniques using a multileaf collimator. Using static beam intensity modulation, a technique for penumbra enhancement at the superior and inferior field edges of axial coplanar treatment plans was developed. Due to penumbra enhancement, the length of all treatment fields could be reduced by typically 1.5 cm, while still achieving an adequate dose in the planning target volume. As a result, the dose delivery to critical structures could often be reduced with respect to our (previous) standard treatment without intensity modulation. A dosimetrical study showed that application of this technique in lung treatments could also compensate for the increased lateral secondary electrontransport in lungtissue. For calculation of the required leaf motions to generate optimized intensity modulated beam profiles by means of dynamic multileaf collimation an algorithm was developed that fully avoids tongue-and-groove underdosage effects. Dose measurements showed that, using these leaf motions, the accuracy and stability of intensity modulated profiles was generally within 2%. For individual patients, a fast and accurate method for pretreatment verification of each optimized beam was implemented, based on absolutedose measurements with an electronic portal imaging device. Static and dynamic beam intensity modulation is presently applied routinely in our institution for treatment of head and neck cancer patients and prostate cancer patients.
28(2001); http://dx.doi.org/10.1118/1.1350683View Description Hide Description
In radiotherapy simple procedures for the calculation of dose or monitor units (MU) are required, for example, for direct (fixed SSD) beam irradiation techniques. It can also be used as independent checks of the calculations performed by the more sophisticated treatment planning (TP) systems. The algorithms used in MU calculations must comply with accuracy requirements comparable with those for TP systems. This thesis deals with the accuracy achievable in general in dose calculations for megavoltage photon beams. In the first section, the dose calculation procedure is discussed as part of the full dosimetry chain, starting with beam calibration up to the clinical dose delivery. The different aspects of a MU calculation program are discussed from the viewpoint of a separation of the dose variation with field size into a head scatter and a phantom scatter component. For this purpose, the parameters are defined at the reference depth in the phantom of 10 cm, while for the measurements a mini-phantom is used. A consistent formalism for MU calculation is described. In the second section several aspects of a calculation procedure, such as the concept of equivalent squares, the dose variation due to the presence of blocking trays, and the influence of asymmetric field setting on the head scatter component are dealt with. The third section of the thesis deals with the quality assurance of treatment planning systems. An extension of the AAPM TG 53 test set is proposed, while the results of its application to seven commercially available TP systems are presented. Finally, tolerances to be used with a comparison of dose calculations with measurements in different clinical situations are discussed.
Use of a convolution/superposition-based treatment planning system for dose calculations in the kilovoltage energy range28(2001); http://dx.doi.org/10.1118/1.1350684View Description Hide Description
A number of procedures in diagnostic radiology and cardiology make use of long exposures to x-rays from fluoroscopy units. Although numerous studies have been performed to measure or calculate skindose from these procedures, there have only been a handful of studies to determine the dose to the other organs. This thesis was focused on devising a method to calculate the absorbed dose to underlying tissues and organs. The work was performed in several stages. First, a commercial convolution/superposition-based treatment planning system used in radiation oncology was modified and complemented to make it usable with the low energies of x-rays used in diagnostic radiology. This required generation of energy deposition kernels in the kilovoltage energy range. The kernels were generated using the EGS4 Monte Carlo system of codes and added to the treatment planning system. The treatment planning system was then evaluated for its accuracy of calculations at low energies within homogeneous and heterogeneous materials. Next, the system was used to determine the dose distribution and lungdose in a humanoid phantom and to determine the lungdose from a sample cardiac procedure. The results were subsequently compared to other methods and studies. These dose distributions can also be used to create dose-volume histograms for internal organs irradiated by low energy beams. Using these data and the concept of normal tissue complication probability developed for radiation therapy, the risk of future complications in a particular organ can be estimated.