Matsumura Akihiko, Yusa Ken, Kanai Tatsuaki, Mizota Manabu, Ohno Tatsuya, Nakano Takashi
Gunma University Heavy Ion Medical Center, Gunma University, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan.
Research Center of Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba 263-8555, Japan.
Med Phys. 2015 Sep;42(9):5188-94. doi: 10.1118/1.4928145.
A conventional broad beam method is applied to carbon ion radiotherapy at Gunma University Heavy Ion Medical Center. According to this method, accelerated carbon ions are scattered by various beam line devices to form 3D dose distribution. The physical dose per monitor unit (d/MU) at the isocenter, therefore, depends on beam line parameters and should be calibrated by a measurement in clinical practice. This study aims to develop a calculation algorithm for d/MU using beam line parameters.
Two major factors, the range shifter dependence and the field aperture effect, are measured via PinPoint chamber in a water phantom, which is an identical setup as that used for monitor calibration in clinical practice. An empirical monitor calibration method based on measurement results is developed using a simple algorithm utilizing a linear function and a double Gaussian pencil beam distribution to express the range shifter dependence and the field aperture effect.
The range shifter dependence and the field aperture effect are evaluated to have errors of 0.2% and 0.5%, respectively. The proposed method has successfully estimated d/MU with a difference of less than 1% with respect to the measurement results. Taking the measurement deviation of about 0.3% into account, this result is sufficiently accurate for clinical applications.
An empirical procedure to estimate d/MU with a simple algorithm is established in this research. This procedure allows them to use the beam time for more treatments, quality assurances, and other research endeavors.
群马大学重离子医疗中心将传统的宽束方法应用于碳离子放射治疗。根据该方法，加速后的碳离子被各种束流装置散射以形成三维剂量分布。因此，等中心处每个监测单位的物理剂量（d/MU）取决于束流装置参数，在临床实践中应通过测量进行校准。本研究旨在开发一种利用束流装置参数计算d/MU的算法。
通过PinPoint电离室在水模中测量两个主要因素，即射程移位器依赖性和射野孔径效应，其设置与临床实践中用于监测校准的设置相同。基于测量结果，利用一个简单算法开发了一种经验监测校准方法，该算法使用线性函数和双高斯笔形束分布来表示射程移位器依赖性和射野孔径效应。
评估得出射程移位器依赖性和射野孔径效应的误差分别为0.2%和0.5%。所提出的方法成功估算出d/MU，与测量结果的差异小于1%。考虑到约0.3%的测量偏差，该结果对于临床应用而言足够准确。
本研究建立了一种利用简单算法估算d/MU的经验方法。该方法使他们能够将束流时间用于更多的治疗、质量保证和其他研究工作。
