Experimental measurements and Monte Carlo calculations of electron dose distribution for breast cancer patients and evaluation of the ADAC Pinnacle system

Abstract

Cancer is mainly treated with surgery, chemotherapy, radiotherapy, or sometimes a combination of these treatments. Among the above treatment methods, cancer patients are treated most frequently by radiotherapy, which deposits ionizing radiation to damage or destroy cancer cells. The goal of radiotherapy is to deliver a lethal dose to the tumor while avoid unnecessary damage to health tissues and critical structures. To accomplish this, radiotherapy is planned using treatment planning systems. Through radiation treatment planning, the best way of delivering the amount of radiation prescribed by the oncologist is decided. Even though the response of cancer cells and healthy cells are different, it is critical that the dose distribution in the patient should be tailored and accurate dose calculations are essential to radiation treatment planning. With the development of dose calculation algorithms and CT scanner, three-dimensional treatment planning systems are available commercially for electron beam dose distribution [1-3]. Calculating the dose distribution from an electron field is quite complicated because of the multiple scattering, especially in the presence of internal heterogeneities [4-6]. Since it is very important to calculate the dose accurately for radiotherapy, rigorous evaluation of the electron planning system is required to improve the treatment outcome. Many studies investigated the degree of discrepancies by comparing the dose distribution obtained from treatment planning system with measurements and Monte Carlo calculations. Many studies show that the Monte Carlo dose calculation method is more accurate than three-dimensional dose calculation algorithms, which have been implemented in several commercial treatment planning systems currently available. In this study, experimental measurements and Monte Carlo calculations of electron dose distributions for breast cancer patients were performed and the results were compared with those by the ADAC Pinnacle treatment planning system (Advanced Diagnosis, Automation, and Control laboratory by Philips), used at Ewha Womans University Medical Center. A total of 24 breast cancer patients treated with 6, 9, and 12 MeV electron beams in our institution were selected for this study. The first part is to compare monitor unit (MU) values obtained from experimental measurements and the ADAC Pinnacle system calculations. Adding real patients’ irregular blocks, monitor units at the dose maximum depth (d_(max)) were calculated from experimental measurements and the ADAC Pinnacle system. The second part is to compare electron dose distributions of measurements and Monte Carlo calculations in flat water phantom at gantry zero position and for a 10 x 10 ㎠ field size. Finally, dose distributions between the ADAC Pinnacle system calculations and Monte Carlo calculations using real patients’ CT were compared. The maximum differences of monitor unit values between experimental measurements and calculations by the ADAC Pinnacle system in flat water phantom at gantry zero position were 3% for 6 MeV, 2% for 9 MeV and 1% for 12 MeV. Comparison of depth doses and lateral dose profiles calculated by the ADAC Pinnacle system and Monte Carlo method, using water phantom, has generally shown good agreement. It was shown that the agreement between the measurements and Monte Carlo for central axis depth dose curves was better than 2% of local absorbed dose and within the geometrical edges of the field, the agreement between measured and Monte Carlo calculated dose profiles was better than 2%. In comparison of the dose distributions along the central beam axis calculated by the ADAC Pinnacle system and Monte Carlo method for real-patient cases, the maximum difference was 3% at most and almost within 2%. On the basis of the results presented in this study, we can conclude that the ADAC Pinnacle treatment planning system for electron beams is capable of giving results comparable to those of experimental measurements and Monte Carlo calculations.;암은 전 세계적으로 사망의 주요한 원인이 되는 질병으로서 매년 암으로 인해 사망하는 사람의 수가 늘어나고 있다. 암을 치료하기 위해 통상적으로 수술 요법, 화학 요법, 방사선 치료 등의 방법을 사용하지만, 그 중에서도 방사선 치료를 가장 많이 사용한다. 방사선치료분야는 방사선의 에너지를 높여서 체내 깊숙이 위치한 악성종양 등을 치료하는 분야로서 고 에너지 방사선을 발생시키는 최첨단 가속기의 개발과 치료기술, 방법, 치료 계획 등을 연구 수행하는 가장 효과적인 종양치료의 한 분야이다. 방사선 치료에 있어서 기본적인 목표는 방사선 조사를 통해 건강한 세포에 가해질 수 있는 피해를 최소화하면서 암세포를 가능한 한 많이 파괴하는 것이다. 이를 위해서 치료 계획 단계에서 환자에게 흡수될 선량이 최적화되도록 방사선을 조사하는 방법을 결정하는데 치료 계획이 효과적으로 이루어지기 위해서는 정확한 선량 계산이 필수적이다. 전자선 치료 계획에서의 선량 계산을 수행하기 위한 다양한 알고리즘들이 개발됨에 따라, 3차원 치료 계획 시스템 (3D RTP)이 전자선 선량 분포 계산에 많이 이용되고 있다. 현재까지 임상에 적용된 알고리즘들의 경우 방사선 빔을 기술하는 데에 있어서 물리의 부분을 무시함으로써 계산의 정확성을 충분히 확신할 수는 없다. 이러한 기존의 알고리즘들에 비해 가급적 빔 수송의 물리를 그대로 기술하는 알고리즘이 몬테카를로(Monte Carlo) 방법으로서, 국내외의 많은 연구를 통해 몬테카를로 방법에 의한 선량 계산은 다른 어떤 알고리즘보다 정확한 결과를 준다고 알려졌다. 본 논문에서는 실제 측정 그리고 몬테카를로 선량 계산 결과와 비교함으로써, ADAC Pinnacle 치료 계획 시스템의 표면조직에 있는 종양 치료 시 사용되고 있는 고 에너지 전자 선량 계산에 있어서의 정확성에 대해 연구해보았다.