Volume 30, Issue 9, September 2003
Index of content:
- PH. D. THESES ABSTRACTS
30(2003); http://dx.doi.org/10.1118/1.1595871View Description Hide Description
This dissertation explores beta dose profiles of microspheres packed in arteries, various source geometries of that can be used for therapeutic purposes, and dosebackscatter factors for selected beta sources. A novel treatment method by injecting microspheres into feeding arteries of arteriovenous malformations (AVM) is under preclinical investigation. To optimize radiationdose to the clinically important area, i.e., arterial wall, preliminary dosimetric studies were needed. Monte Carlo calculations were performed for several geometries simulating arteries filled with microspheres packed by random packing methods. Arterial radii used in the simulation varied from 50 μm to 3 mm; microsphere radii varied from 10 μm to 0.7 mm. Dose varied significantly as a function of microsphere size, for constant arterial sizes. For the same sizes of arteries, significant dose increase was observed because of inter-artery exposure for large arteries (>0.1 cm radius) filled with large microspheres (>0.03 cm radius). Dose increase between small arteries (<0.03 cm radius) was less significant. The dose profiles of prototype beta brachytherapy sources were calculated using MCNP 4C Monte Carlo code as well as dose point kernel (DPK) for selected cases. Dose profiles were similar to those for beta sources currently used indicating that can substitute for current sources for certain cases. The DPK and MCNP results matched closely. Backscattering of electrons is a prominent secondary effect in beta dosimetry. The backscattering is closely correlated with factors such as geometry of source and scattering material, and composition of scattering material. The backscattering factors were calculated for selected beta sources that are currently used as well as for other potentially useful sources. The factors were calculated as a function of distance from the interface between water and scatterers. These factors were fit by a simple function for future incorporation into a DPK code. Backscattering effect was significant for short distances from interfaces between water and scattering material.
30(2003); http://dx.doi.org/10.1118/1.1596144View Description Hide Description
Respiration affects the instantaneous position of almost all thoracic and abdominal structures (lung, breast, liver, pancreas, etc.), posing significant problems in the radiotherapy of tumors located at these sites. The diaphragm, for example, has been shown to move approximately 1.5 cm in the superior-inferior direction during normal breathing. During radiotherapy, margin expansion around the tumor, based on an estimate of the expected range of tumor motion, is commonly employed to ensure adequate dose coverage. Such a margin estimate may or may not encompass the “current” extent of motion exhibited by the tumor, resulting in either a higher dose to the surrounding normal tissue or a cold spot in the tumor volume, leading to poor prognosis. Accounting for respiratory motion by active management during radiotherapy can, however, potentiate a reduction in the amount of high dose to normal tissue. Active management of respiratory motion forms the primary theme of this dissertation. Among the various techniques available to manage respiratory motion, our research focused on respiratory gated and respiration synchronized radiotherapy, with an external marker to monitor respiratory motion. Multiple session recordings of diaphragm and external marker motion revealed a consistent linear relationship, validating the use of external marker motion as a “surrogate” for diaphragm motion. The predictability of diaphragm motion based on such external marker motion both within and between treatment sessions was also determined to be of the order of 0.1 cm. Gating during exhalation was found to be more reproducible than gating during inhalation. Although, a reduction in the “gate” width achieved a modest reduction in the margins added around the tumor further reduction was limited by setup error. A motion phantom study of the potential gains from respiratory gating indicated margin reduction of 0.2–1.1 cm while employing gating. In addition, gating also improved the quality of images obtained during simulation by reducing the motion artifacts typically seen during CTimaging. An analysis of several patient breathing patterns with (audio instructions and visual feedback) and without training, indicated that breathing training improved the reproducibility of amplitude and/or frequency of patient breathing cycles. A phantom based study by superposition of sinusoidal motion of a “simulated” tumor onto the initial beam aperture as formed by the multileaf collimator revealed that target dose measurements obtained with such a motion synchronized setup were equivalent to those delivered to a static target by a static beam. An attempt to acquire respiration synchronized (4D) CTimages of a motion phantom and a patient also yielded a 4D CT data set with reduced motion artifacts. Respiratory gated and respiration synchronized radiotherapy are both viable approaches to account for respiratory motion during radiotherapy. While respiratory gated radiotherapy has been successfully implemented in some centers, several technical advances are required for clinical implementation of respiration synchronized radiotherapy. Future applicability of either of the above approaches as routine treatment procedures will be determined by their potential clinical gains over currently available methods.
30(2003); http://dx.doi.org/10.1118/1.1598671View Description Hide Description
The aim of the thesis is to develop methods for improving accuracy in dose calculations for photonbrachytherapytreatment planning. This is achieved by separating the total dose into its primary and scatter components and through the use of three-dimensional integration methods to calculate the scatterdose. The collapsed-cone kernel-superposition algorithm, used clinically for treatment planning with external photon beams, was adapted for scatter-dose calculations in brachytherapy. A successive-scattering approach was developed to minimize the artifacts from the method’s angular discretization of energy transport, which are otherwise a problem for the steep fluence gradients around brachytherapy sources. Methods for scaling kernels for heterogeneities, accounting for both photoelectric absorption and the emission of characteristic x rays, were derived. It is shown how data from a source-characterization formalism, that separates the total dose into its primary and scatter components, can support the collapsed cone algorithm with the input required to model clinical sources. With the methods developed in this work, the collapsed cone algorithm can be used for three-dimensional scatterdose calculations over the brachytherapy energy range, handle all heterogeneous materials and model clinical sources. An application in which this might be of particular importance is in designing individualized patient shields for intermediate energy isotopes such as and In this energy range (60–100 keV), thin foils of high atomic number effectively protect sensitive organs, however without three-dimensional scatter-dose calculations, errors as large as 10–20 % can occur within distances of therapeutic interest on the side targeted for treatment. Through use of the collapsed cone algorithm for scatterdose calculations, these dose reductions are predicted within 3% compared to the results of full-scale Monte Carlo simulations.
30(2003); http://dx.doi.org/10.1118/1.1603966View Description Hide Description
Patient repositioning and organ motion lead to uncertainty in targeting the tumor in radiation therapy. This decreases the dose to the tumor and increases the dose to healthy tissues. Ultimately, tumor control is reduced and complications are increased. This work investigates the hypothesis that modeling geometric uncertainties can accurately estimate the dose distribution delivered when uncertainties are present. These modeling results provide a more accurate representation of the delivered dose distribution than present approaches. These uncertainties are conventionally addressed by adding a margin to the clinical target volume to define a planning target volume (PTV). Despite widespread use, some PTV implementation details have not been addressed. A mathematical model is developed to investigate these details and leads to recommendations for clinical implementation. However, limitations remain when using a PTV. A superior method of accounting for geometric uncertainties incorporates their effect into the dose calculation. This can be achieved by convolution of the planned dose distribution with a probability density function describing the uncertainty. Two assumptions in this model are that the dose distribution is shift invariant and that treatment extends over an infinite number of fractions. The errors resulting from each assumption are quantified. Assuming shift invariance leads to large errors near the patient surface. A “Corrected Convolution” method that reduces these errors was developed. Errors due to finite fractionation are large for very few fractions (hypofractionation), but are unlikely to impact treatment plan evaluation. The impact of geometric uncertainties for hypofractionated prostate cancer treatment is further explored. Hypofractionated treatments were simulated to quantify the impact of geometric uncertainties. The results suggest geometric uncertainties will not limit the clinical effectiveness of prostate hypofractionation. Convolution can assess the sensitivity of different techniques to geometric uncertainties. Simplified intensity modulated arc therapy plans were compared to conventional techniques. The magnitude of the change caused by geometric uncertainties varied by technique. Contrary to common assumptions, geometric uncertainties do not always result in a worse treatment than planned. In conclusion, existing methods to account for geometric uncertainties are limited. Modeling geometric uncertainties with convolution has the potential to improve clinical treatment decisions .