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A Robust Conditional Value-at-Risk (C-VaR) Optimization Approach for Direct Optimization of Beam Directions and Apertures in Intensity Modulated Radiation Therapy Method

Sarjoughian, Tina | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55693 (01)
  4. University: Sharif University of Technology
  5. Department: Industrial Engineering
  6. Advisor(s): Eshghi, Kourosh
  7. Abstract:
  8. Breast cancer has been one of the main causes of women's deaths in recent years. Today, various methods are used to treat this disease, among which intensity-modulated radiation therapy is one of the most common methods. The goal of radiation therapy is to destroy cancer cells while protecting the surrounding healthy tissues. In left breast cancer, due to the proximity of the heart and breast tissue, we are at risk of delivering an excessive dose to the heart and increasing cardiac complications after the treatment. In addition, due to the irregular and unpredictable breathing movements of the patient during the treatment and the resulting uncertainty, we face more challenges. In order to include the uncertainty caused by breathing movements, the patient's breathing cycle is divided into five phases according to the patient’s inhale and exhale. Each breathing pattern can be interpreted as a probability mass function (PMF), where the probability of each phase is the fraction of time spent in that phase. Also, the dose-influence matrix for the clinical target volume and heart tissue in each phase is different. On the other hand, A treatment plan in this technique requires decision-making for three main problems: selection of beam angles, intensity map calculation, and leaf sequencing. These problems have a sequential relationship; the output of each problem is used as the input of the next problem. It is noteworthy that solving each of these problems separately may reduce the quality of the treatment plan. Also, this discretization may cause a significant reduction in the quality of the treatment plan, an increase in the number of apertures and a prolongation of the treatment time. Therefore, the necessity of designing radiation therapy plannings which are compatible with treatment equipments is evident. In this research, we present a new integrated framework for simultaneous decision-making of directions, intensities, and apertures shape, called direct angle and aperture optimization, and develop a mixed-integer linear mathematical model for the problem. In this model, breathing motion uncertainty is taken into account by using optimization framework that combines robust optimization with a conditional value-at-risk (CVaR) representation of clinical dose-volume criteria. For solving this problem, a heuristc algorithm is used, which requires three steps to solve the problem and produce high-quality treatment plans. The output of this algorithm is used either as input to the larger model, or as a stand-alone result if the output happens to be clinically acceptable.
  9. Keywords:
  10. Intensity Modulated Radiation Therapy (IMRT) ; Robust Optimization ; Conditional Value at Risk ; Heuristic Algorithm ; Direct Angle and Aperture Optimization ; Breast Cancer

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