Index of content:
Volume 31, Issue 10, October 2004
- PH. D. THESES ABSTRACTS
Evaluation of radiobiological effects in intensity modulated proton therapy: New strategies for inverse treatment planning31(2004); http://dx.doi.org/10.1118/1.1786191View Description Hide Description
Currently, treatment planning for intensity modulated proton therapy (IMPT) usually disregards variations of the relative biological effectiveness (RBE). To investigate the potential clinical relevance of a variable RBE for beam scanning techniques, new strategies for the evaluation of radiobiological effects and for the incorporation of the RBE into the inverse planning process are presented. These strategies are based on a fast algorithm for three-dimensional calculations of the dose averaged linear energy transfer (LET) as a measure of the local radiation quality, and on a simple phenomenological approach for the RBE as a function of dose, LET and tissue type. It was found that the biological effect depended strongly on the type of scanning technique used, mainly due to differences in the LET distributions. New objective functions that account for LET and RBE were integrated into an inverse planning software, which now allows simultaneous multifield optimization of the biological effect in a reasonable time. With these methods, unfavorable RBE effects can be identified and compensated for by direct optimization of the product of RBE and dose, which is demonstrated for several clinical examples. The proposed strategies are therefore valuable tools to evaluate and improve the quality of treatment plans in IMPT.
Development and clinical introduction of an inverse planning dose optimization by simulated annealing (IPSA) for high dose rate brachytherapy31(2004); http://dx.doi.org/10.1118/1.1796111View Description Hide Description
High dose rate brachytherapy is a promising radiation treatment modality that uses temporarily implanted catheters to deliver the curative dose directly in the tumor. A programmable robotic device (the afterloader) moves a single tiny radioactive source (192Ir) along the catheters using a flexible cable attached to the source. With this flexible system, a wide variety of dose distributions can be generated from a given implant simply by adjusting the length of time (dwell time) that the source dwells at any location within the implanted catheters (dwell position). The challenge is to select the optimal sequence of dwell times related to the unique clinical situation of each patient. This treatment-planning problem can be formalized as a combinatorial optimization problem. The optimization algorithm presented in this thesis is conceived to perform this task. An inverse planning (IP) approach has been adopted to guide the optimization process. This means that the optimization is guided by clinical objectives described by means of dose constraints specified to each digitized anatomical structure. A simulated annealing (SA) optimization engine has been designed to solve this particular problem in a short time for clinical applications (about ). This inverse planning by simulated annealing (IPSA) algorithm has been successfully implanted in four institutions: UCSF(1), CHUQ(2), NIH(3), CAV(4). At the moment of writing this thesis, more than 300 patients have been treated at these institutions for a wide variety of anatomical sites. Clinical studies performed by clinicians using IPSA demonstrated that the algorithm produces superior treatment plans from a dosimetric point of view than the conventional method using geometrical optimization. IPSA improves the target dose coverage while minimizing the dose delivered to organs at risk and provides consistent results from one patient to another. Both dosimetric indices and overall procedure time were improved with the clinical introduction of IPSA.