Measurement of Water Equivalent Coefficients of Materials Used in Heavy Ion Therapy Terminal

Measurement of Water Equivalent Coefficients of Materials Used in Heavy Ion Therapy Terminal

Meng Wanbin, Qi Ying, Ma Xiaoyun, Li Wanguo, Wang Kaiping, Zhu Fangfang, Zhang Fuyuan, Kang Kaili, Yang Xiaodong
(Radiotherapy Center, Gansu Wuwei Cancer Hospital, Wuwei, Gansu 7331000)

Abstract:Objective: To measure the water equivalent coefficients of equipment materials used in the heavy ion therapy terminal.Methods: Place materials of certain thicknesses uniformly in the measured heavy ion beam path, and measure the change in the Bragg peak position before and after placing the materials.Results: The water equivalent coefficient of the compensator material (polyethylene) is k = 1.0156, and that of the ridge filter material (aluminum) is k = 2.2342.Discussion: The measured water equivalent coefficients of the materials may vary under different thicknesses and energies. After comprehensively analyzing the impact of these errors, the average value measured under different conditions can be used for the processing of uniform scanning therapy terminal equipment.

Keywords: heavy ion, Bragg, water equivalent

1 Introduction

Heavy ion therapy technology is currently internationally recognized as the most advanced radiotherapy technology. Due to the existence of the Bragg peak, this unique physical characteristic allows it to deliver powerful irradiation to solid tumors while avoiding irradiation of normal tissues, thereby maximizing therapeutic efficacy^[1]. At present, China's first fully independent intellectual property heavy ion cancer treatment system has been established in Wuwei, Gansu, and is about to enter the clinical research stage, achieving the localization of large-scale medical equipment^[2]. This marks a critical step in the development of China's independent intellectual property heavy ion cancer treatment system^[3].

Currently, the main treatment mode of this equipment is uniform scanning. The beam is introduced from the synchrotron into the treatment room and must pass through a series of equipment in the therapy terminal before reaching the human body. Equipment in the therapy terminal, such as the ridge filter, range shifter, and compensator, can all affect the Bragg peak of the heavy ion beam. Therefore, the materials used in the design and processing of these devices must meet strict requirements. In addition to conventional performance requirements, their water equivalent coefficients must be measured before processing^[4].

2 Materials and Methods

2.1 Materials and Equipment

The materials measured were those used for processing the compensator and ridge filter, namely polyethylene and aluminum. Both materials were processed into uniform thickness and regular shapes in advance. The polyethylene material was a rectangular block of 8 cm × 8 cm with thicknesses of 20.14 mm, 30.06 mm, and 48.14 mm. The aluminum block was a cylindrical body with a diameter of 5 cm and thicknesses of 10.08 mm and 10.12 mm.

The equipment used included the horizontal therapy terminal of the heavy ion accelerator in Treatment Room No. 1 of Gansu Heavy Ion Hospital (China’s first heavy ion accelerator for radiotherapy developed by the Institute of Modern Physics, Chinese Academy of Sciences), a German PTW MP3-P 3D water tank, and matching ionization chambers and software.

2.2 Measurement Method

During measurement, the Bragg peak position of the beam was first measured without placing the material sample. Then, the material sample was placed in front of the 3D water tank, with two ionization chambers placed parallel to it. The reference ionization chamber was placed in front of the sample, and the ionization chamber for measuring the Bragg peak position was placed inside the water tank. All devices were placed on the treatment bed, and the orientation of all samples was consistent with the CT scanning direction. During measurement, the ionization chamber in the water tank moved along the beam direction with a measurement accuracy of 0.1 mm. To save time, only the dose distribution in the Bragg peak region was measured.

The formula for calculating the water equivalent coefficient of the material is:

k = (PN - PS) / dS

where P_N is the Bragg peak position without the material sample, P_S is the Bragg peak position with the material sample, and dS is the thickness of the sample in the beam measurement direction.

3 Results

The water equivalent coefficient of each material is the average of all measured values:

Compensator material (polyethylene): k = 1.0156

Ridge filter material (aluminum): k = 2.2042

4 Discussion

From the measurement data, it can be observed that for the same material, the water equivalent coefficient varies to some extent (approximately 2%) under different energies. However, under the same energy, the variation is minimal when the thickness changes^[5]. Currently, this heavy ion accelerator uniform scanning terminal is designed with five commonly used energy levels. The ridge filter and range shifter cannot be used exclusively for each energy level. Therefore, when processing these two devices, the average value of the coefficients for various energies must be used^[6]. However, when collecting data for these two devices in the TPS system, actual data under different energies can be input to reduce this error^[7]. As for the compensator, since the commonly used compensator thickness is 5 cm, most of the processing thickness within the field is far less than 5 cm. Therefore, based on the measurement results, the average water equivalent coefficient results in an error of less than 1 mm in the Bragg peak position under different energies. Compensators processed with this material can be used within the scope of clinical treatment.

When processing heavy ion therapy terminal equipment such as compensators, ridge filters, and range shifters, if the material specifications differ from previous ones, this data test must be performed on the new material to ensure the accuracy of patient treatment.

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