Biomedical subjects

Alexei Trofimov

Publications and source records attributed to Alexei Trofimov.

5 recordsLinked to original sources

PubMed2006

A unified approach for inversion problems in intensity-modulated radiation therapy.

We propose and study a unified model for handling dose constraints (physical dose, equivalent uniform dose (EUD), etc) and radiation source constraints in a single mathematical framework based on the split feasibility problem. The model does not impose on the constraints an exogenous objective (merit) function. The optimization algorithm minimizes a weighted proximity function that measures the sum of the squares of the distances to the constraint sets. This guarantees convergence to a feasible solution point if the split feasibility problem is consistent (i.e., has a solution), or, otherwise, convergence to a solution that minimally violates the physical dose constraints and EUD constraints. We present computational results that demonstrate the validity of the model and the power of the proposed algorithmic scheme.

Algorithms↗

PubMed2005

Temporo-spatial IMRT optimization: concepts, implementation and initial results.

With the recent availability of 4D-CT, the accuracy of information on internal organ motion during respiration has improved significantly. We investigate the utility of organ motion information in IMRT treatment planning, using an in-house prototype optimization system. Four approaches are compared: (1) planning with optimized margins, based on motion information; (2) the 'motion kernel' approach, in which a more accurate description of the dose deposit from a pencil beam to a moving target is achieved either through time-weighted averaging of influence matrices, calculated for different instances of anatomy (subsets of 4D-CT data, corresponding to various phases of motion) or through convolution of the pencil beam kernel with the probability density function describing the target motion; (3) optimal gating, or tracking with beam intensity maps optimized independently for each instance of anatomy; and (4) optimal tracking with beam intensity maps optimized simultaneously for all instances of anatomy. The optimization is based on a gradient technique and can handle both physical (dose-volume) and equivalent uniform dose constraints. Optimization requires voxel mapping from phase to phase in order to score the dose in individual voxels as they move. The results show that, compared to the other approaches, margin expansion has a significant disadvantage by substantially increasing the integral dose to patient. While gating or tracking result in the best dose conformation to the target, the former elongates treatment time, and the latter significantly complicates the delivery procedure. The 'motion kernel' approach does not provide a dosimetric advantage, compared to optimal tracking or gating, but might lead to more efficient delivery. A combination of gating with the 'motion kernel' or margin expansion approach will increase the duty cycle and may provide one with the most efficient solution, in terms of complexity of the delivery procedure and dose conformality to the target.

Algorithms↗

PubMed2003

Beam delivery sequencing for intensity modulated proton therapy.

Methods of beam fluence sequencing for intensity modulated proton therapy (IMPT) using the beam scanning technique are presented. Proton beam weight maps optimized by the treatment planning system (TPS) for a discrete set of regularly spaced narrow pencil beams were interpolated, using convolution with various kernel functions, to simulate continuous beam scanning on a raster pattern. Expected dose distributions at the proton Bragg peak range were then calculated and compared to those planned by the TPS, to evaluate the discrepancy due to the differences between the treatment planning and delivery approaches. An iteratively optimized adjustment was applied to the simulated continuous beam fluence profiles to reduce such discrepancy. Calculation showed that by accounting for the specifics of the scanning method, the planned dose distribution on the target may be reproduced to within 0.5% of the maximum target dose, given the pencil beam spacing smaller or equal to the beam sigma is used for treatment planning. For the beam weight maps generated using a spot spacing larger than sigma, a substantial reduction in the calculated dose discrepancy may be attained by applying an iterative adjustment of fluence profiles obtained by interpolating over artificially expanded set of beam spots.

Biophysical Phenomena↗

PubMed2003

Proton beams to replace photon beams in radical dose treatments.

With proton beam radiation therapy a smaller volume of normal tissues is irradiated at high dose levels for most anatomic sites than is feasible with any photon technique. This is due to the Laws of Physics, which determine the absorption of energy from photons and protons. In other words, the dose from a photon beam decreases exponentially with depth in the irradiated material. In contrast, protons have a finite range and that range is energy dependent. Accordingly, by appropriate distribution of proton energies, the dose can be uniform across the target and essentially zero deep to the target and the atomic composition of the irradiated material. The dose proximal to the target is lower compared with that in photon techniques, for all except superficial targets This resultant closer approximation of the planning treatment volume (PTV) to the CTV/GTV (grossly evident tumor volume/subclinical tumor extensions) constitutes a clinical gain by definition; i.e. a smaller treatment volume that covers the target three dimensionally for the entirety of each treatment session provides a clinical advantage. Several illustrative clinical dose distributions are presented and the clinical outcome results are reviewed briefly. An important technical advance will be the use of intensity modulated proton radiation therapy, which achieves contouring of the proximal edge of the SOBP (spread out Bragg peak) as well as the distal edge. This technique uses pencil beam scanning. To permit further progressive reductions of the PTV, 4-D treatment planning and delivery is required. The fourth dimension is time, as the position and contours of the tumor and the adjacent critical normal tissues are not constant. A potentially valuable new method for assessing the clinical merits of each of a large number of treatment plans is the evaluation of multidimensional plots of the complication probabilities for each of 'n' critical normal tissues/ structures for a specified tumor control probability. The cost of proton therapy compared with that of very high technology photon therapy is estimated and evaluated. The differential is estimated to be approximately 1.5 provided there were to be no charge for the original facility and that there were sufficient patients for operating on an extended schedule (6-7 days of 14-16 h) with > or = two gantries and one fixed horizontal beam.

Dose-Response Relationship, Radiation↗

PubMed2003

Optimization of beam parameters and treatment planning for intensity modulated proton therapy.

One of the objectives of the ongoing research and development work at the Northeast Proton Therapy Center (NPTC) in Boston is to perform optimized intensity modulated proton therapy (IMPT) treatments. Such treatments may be delivered by magnetically scanning a narrow proton pencil beam across the target volume, while both the scanning speed and the intensity of the beam are modulated. Localization of the proton dose in space allows one to yield dose distributions that are highly conformal to the target volume, thus minimizing the dose delivered to the surrounding healthy tissue. The aim of the current research is to determine technically optimal and clinically relevant specifications for the scanned beam delivery system, which is being developed in collaboration with Ion Beam Applications (IBA); and to create a link between the treatment planning and the beam delivery. IMPT treatment planning is performed for patient cases treated at the NPTC, with KonRad Pro software developed at the German Cancer Research Center (DKFZ). For the IMPT delivery, the proton intensity maps, optimized for discrete pencil beam spots, need to be translated into continuous scanning patterns. At the same time it is necessary to minimize the discrepancy between the planned and delivered doses which may result from such conversion, as well as from the technical limitations of the delivery system. Possibilities have been investigated for improving the proton dose conformity by optimizing the beam and scanning nozzle parameters, and by taking the specifics and limitations of the system into account in the treatment planning stage.

Chordoma↗