Geometric Accuracy Verification of Heavy Ion Therapy Compensators Based on 3D Laser Scanner

Meng Li, Meng Wanbin, Ma Xiaoyun, Zhang Yanshan

(Gansu Wuwei Cancer Hospital Heavy Ion Center, Wuwei, Gansu 733000, China)

Abstract: Objective: To use a handheld 3D laser scanner to perform three-dimensional reconstruction of compensators used in heavy ion uniform scanning therapy, and then use the reconstructed data to verify the compensator machining accuracy. Methods：Using Geomagic Qualify software, the three-dimensional reconstruction data (test values) and the manufacturer's compensator data (reference values) were aligned and analyzed and compared. The 3D comparison report was used to inspect the compensator machining accuracy. Compensators meeting the machining accuracy requirements underwent absolute dose verification for the heavy ion plan, analyzed using Gamma analysis. Results：Under the conditions that the pass rate for compensator 3D geometric deviation within ±0.5 mm must be ≥95% and the pass rate for absolute dose deviation of the heavy ion plan (within the criteria) must be ≥90%, the pass rates for compensator 3D geometric deviation and heavy ion plan absolute dose deviation were (97.86 ± 0.83)% and (96.76 ± 3.24)%, respectively. Conclusion: Using a handheld 3D scanner for three-dimensional reconstruction of compensators used in heavy ion uniform scanning therapy and using Geomagic Qualify software for 3D comparison to verify compensator machining accuracy is a reasonable, accurate, simple, and feasible method for compensator verification.

Subject words: Heavy ions; Radiotherapy; Compensator; 3D laser scanner

Chinese Library Classification Number: R730.55

Document Code: A

Article ID: 1671-170X(YYYY)NN-NNNC-NN

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In December 2019, China's first heavy ion therapy system with independent intellectual property rights was officially put into clinical application at the Heavy Ion Center of Gansu Wuwei Cancer Hospital. This made China the fourth country in the world to achieve heavy ion therapy for tumors after Japan, Germany, and the United States, realizing a historic breakthrough in China's clinical application of large-scale medical equipment. Heavy ion therapy for tumors is a contemporary recognized advanced and effective radiotherapy method, internationally known as "the best radiotherapy ray in the 21st century". It is particularly suitable for malignant tumors in the skull base, head and neck, pelvis and other parts, primary and metastatic lung cancer, liver cancer, soft tissue sarcoma, skin malignant tumors, breast cancer, and locally recurrent colorectal cancer, pancreatic cancer, prostate and genitourinary system tumors, gynecological malignant tumors, etc. ^[1-6]. Compared with conventional radiotherapy rays, the unique physical and biological characteristics of heavy ions cause their energy deposition in the human body to be concentrated at the end of their range, forming a high-dose "Bragg Peak" at the distal end of the depth-dose curve ^[7-8], while the dose deposited along the path is small, lateral scattering is small, and the Relative Biological Effectiveness (RBE) is high. It also has strong killing power for radiation-resistant and hypoxic tumors ^[9-12].

Currently, the equipment used at the Gansu Wuwei Cancer Hospital Heavy Ion Center mainly has two treatment modes: passive scanning (i.e., uniform scanning) and active scanning (i.e., spot scanning) ^[13-14]. During uniform scanning, the beam is first expanded transversely by scanning magnets, then the Bragg Peak is spread out longitudinally to an appropriate size by a ridge filter. The treatment planning system (TPS) calculates how to accurately place the spread-out Bragg Peak (SOBP) onto the tumor, achieving passive treatment. Due to the uncertainty of the Relative Biological Effectiveness (RBE) in the tail of the heavy ion beam, during plan design, it is often required that normal tissues behind the tumor along the beam direction receive little or no irradiation. Therefore, a compensator needs to be manufactured according to the specific distribution of the target volume to remove the dose that falls beyond the tumor (distally) after spreading, through the modulation of the compensator ^[15-16]. The compensator is an important external accessory required in the heavy ion passive scanning (uniform scanning) treatment mode. It needs to be custom-manufactured according to the patient's treatment plan. After the compensator is processed and before it is officially used for radiotherapy, physicists must perform quality assurance (QA) and quality control (QC) on the compensator to ensure its processed shape is consistent with the planned value and the machining accuracy is within the specified range (error within ±0.5 mm) before it can be used for radiotherapy.

After multiple investigations, studies, and explorations, we found that a handheld 3D stereo scanner can achieve three-dimensional digital collection of object models and perform data preprocessing through object scanning, ultimately outputting a three-dimensional reconstructed model.

Using Geomagic Qualify analysis software, the three-dimensional reconstructed model can be compared and analyzed with the CAD model converted from the planning system output, obtaining a compensator machining error analysis report, thus achieving quality control of the compensator machining accuracy. A retrospective analysis was conducted on the verification reports of 98 compensators for 92 patients randomly selected who received treatment at the Heavy Ion Center of Gansu Wuwei Cancer Hospital from April 2020 to April 2021, to summarize the compensator verification method for referenc

1 Materials and Methods

1.1 General Materials

The Handyscan 700 handheld self-positioning 3D laser scanner (Creaform, Canada) and VXelements 3D preprocessing software were used as the compensator 3D digital acquisition system (Figure 1). Other equipment and software included: Geomagic Qualify 2013 analysis software, heavy ion accelerator (the first domestically produced heavy ion accelerator developed by the Institute of Modern Physics, Chinese Academy of Sciences), heavy ion planning system (the first domestic heavy ion radiotherapy planning system jointly developed by the Institute of Modern Physics, Chinese Academy of Sciences and Shanghai Datu Medical Technology Co., Ltd.), NX data conversion software, PTW OCTAVIUS Detector 1500XDR T10051 (PTW, Germany), Detector interface 4000 (PTW, Germany), MEPHYSTO software (PTW, Germany), solid water (RW3, PTW, Germany).

1.2 Methods

1.2.1 Reference Data Acquisition

After the carbon ion treatment planning system (ciPlan) completes the patient treatment plan, the patient's compensator data is exported. The NX data conversion software is used to convert the compensator data into a CAD model that the factory can process. This CAD model is the standard reference data for compensator verification.

1.2.2 Measurement Data Acquisition

A handheld 3D scanner can be used to detect and analyze the shape (geometric structure) and appearance data of objects or environments in the real world. It uses triangulation to construct 3D graphics, i.e., by emitting laser light from the handheld scanner onto the target object, measuring the distance from the object's surface to the scanner with two detectors (cameras), and converting it into a 3D model through computer software.

The handheld 3D laser scanner was used to scan the compensator. After setting appropriate scanner parameters, the compensator data information was collected. Data processing was then performed using VXelements 3D preprocessing software to remove impurities and splash points, ultimately obtaining the measurement data ^[17-18]. During scanning, attention must be paid to the scanning distance and angle to minimize blind spots.

1.2.3 Data Comparison

The 3D file output by the scanner was imported into Geomagic Qualify analysis software and set as the Test file. The compensator CAD model was imported into the software and set as the Reference file. The two files were then aligned using an appropriate alignment method, followed by a 3D comparison, generating a report. For 3D analysis, the maximum critical value, maximum nominal value, minimum nominal value, and minimum critical value were set to +2 mm, +0.5 mm, -0.5 mm, and -2 mm, respectively. When the percentage of deviations between ±0.5 mm was ≥95%, the compensator was considered to meet clinical requirements and the verification passed.

1.2.4 Plan Verification

Under the premise that the isocenter offset, field flatness, uniformity, symmetry, and stability of each heavy ion energy level met clinical treatment requirements, the patient's plan underwent planar dose verification using the PTW 1500XDR ionization chamber, solid water, and corresponding auxiliary equipment. PTW software used temperature and pressure correction. Error analysis used the Gamma analysis method, with the distance-to-agreement (DTA) and dose difference criteria set to 5 mm/3%. A measurement result ≥90% was considered as plan verification passed ^[19].

2 Results

Compensator qualification is the prerequisite for the absolute dose qualification of the radiation field. This study randomly analyzed and summarized the verification reports of 98 compensators from 92 patients. All compensators (100%, 98/98) had 3D geometric deviations meeting clinical requirements, i.e., the percentage of 3D geometric deviations within ±0.5 mm was ≥95%, with a specific percentage of (97.86 ± 0.83)%. Through the 3D comparison of Geomagic Qualify analysis software (Figure 2), the names of the Test and Reference files, the deviation chromatogram, and statistical analysis (such as 3D deviation, maximum distance, average distance, standard deviation, RMS) can be read visually. The deviation chromatogram is divided into 13 color segments. According to our defined maximum critical value = ±2 mm, maximum nominal value = ±0.5 mm, minimum nominal value = -0.5 mm, and minimum critical value = -2 mm, green indicates the deviation is within the allowable range, i.e., when the deviation is between ±0.5 mm, the machining accuracy of this compensator meets clinical requirements, and the compensator verification passes. A shift towards warmer colors indicates a deviation > +0.5 mm, meaning the Test file value at that point is greater than the Reference file value and the deviation exceeds +0.5 mm, i.e., the compensator material at that point is shallower than planned; a shift towards cooler colors indicates a deviation < -0.5 mm, meaning the Test file value is less than the Reference file value and the deviation exceeds -0.5 mm, i.e., the compensator material at that point is deeper than planned; if an area is gray, it indicates missing data acquisition for that area in the Test file, making comparison impossible. The 3D comparison report generated by Geomagic Qualify analysis software not only summarizes and displays the results of the above 3D comparison in text and images but also displays the specific number of points and percentage of the deviation distribution in the form of a table and histogram.

Analyzing the absolute dose verification reports for the radiation fields of the 98 compensators from 92 patients, 100% (98/98) of the planned field absolute dose pass rates were greater than 90%, with a specific absolute dose deviation pass rate of (96.76 ± 3.24)%.

3 Discussion

Due to various factors such as the accuracy of the milling machine, processing technology, and special material requirements, compensators are highly likely to have machining accuracy that does not meet clinical needs during the manufacturing process. Therefore, physicists need to detect their machining accuracy. However, using traditional distance measurement methods is not only cumbersome and labor-intensive but also cannot cover the entire radiation field range. The Handyscan 3D handheld self-positioning 3D laser scanner can accurately measure the actual situation of the compensator processing. Its resolution can reach 0.05 mm, volumetric accuracy is 0.02 mm + 0.06 mm/m, it can perform 480,000 measurements per second, and it is small in size, lightweight (only 0.85 kg), easy to operate and move, and has low environmental requirements (operating temperature range: 15–40 °C; operating humidity range (non-condensing): 10%–90%). It is simple and convenient to operate, can be performed independently by one physicist, can improve work efficiency, and provides reliable and accurate results.

Of course, the Handyscan 3D handheld self-positioning 3D laser scanner system itself may have some minor deviations. If the scanner is not calibrated in a timely manner before each use or after being moved or transported, and measurements are taken directly, this deviation can increase, ultimately affecting the compensator measurement results.

Due to the small size of compensators, the complexity of human tissue leading to sharp depth changes in small areas within the compensator, and the requirements for compensator machining accuracy, there might be situations where a very small area cannot be scanned by the scanner (The Handyscan 3D handheld self-positioning 3D laser scanner has high resolution and volumetric accuracy; only when an area has an extremely small volume and is extremely deep might the scanner fail to acquire data). Although the unscanned area would be very small, it could still affect the percentage of compensator error within ±0.5 mm to some extent. This requires us to analyze specific issues based on the actual situation. If it really cannot be resolved by various means or has no significant impact on the whole, it can be ignored.

Deformation of the compensator caused by surface oil stains or bumps can also cause changes in distance during scanning, increasing compensator error and affecting the final result. Therefore, before scanning the compensator, we need to keep its surface clean and handle it gently.

The setting of scanner parameters, scanning duration, movement of the compensator during scanning, and the operator's experience can also affect the scanner's acquisition of compensator data, thereby affecting the deviation between the Test file and the Reference file, and thus the final result.

Therefore, selecting reasonable scanning parameters, increasing the reflectivity of the compensator surface, relatively long-duration multi-angle scanning, keeping the compensator surface clean during scanning, handling it gently, keeping the compensator position relatively stationary, and establishing a standard compensator scanning procedure document (Standard Operating Procedure, SOP) will all help reduce unnecessary deviations caused by the above reasons and improve the accuracy and reliability of the scanning comparison results.

Due to the uncertainty of the Relative Biological Effectiveness (RBE) in the tail of the heavy ion beam, the dose falling beyond the tumor (distally) after spreading can be removed through the modulation of the compensator, protecting tissues behind the tumor along the beam direction from or reducing irradiation. Therefore, the accuracy of the patient's compensator greatly affects treatment precision and thus efficacy ^[16]. Compensator geometric accuracy verification is a very important verification item in heavy ion therapy.

Compensator qualification is the prerequisite for the absolute dose qualification of the radiation field. For a heavy ion uniform scanning therapy compensator, verification passing does not necessarily mean the absolute dose verification for that field will pass. However, if the absolute dose verification for a field passes, then the compensator used for that field absolutely meets clinical requirements. From the above results, when verified qualified compensators were used for absolute dose verification, the absolute dose of the planned field using that compensator was all ≥90%, meaning the field plan verification passed. This indicates that the method of using a handheld 3D stereo scanner for compensator verification is reasonable.

Heavy ion therapy technology has just been launched in China. There are no complete sets of equipment or methods for quality control in all links, requiring users to continuously explore and research before finally applying them clinically. The use of compensators domestically is the first case. There is no specialized equipment on the market for compensator quality control. Therefore, how to achieve precise quality control of compensators is one of the key projects for the clinical application of heavy ion uniform scanning radiotherapy technology. Using a handheld 3D self-positioning scanner can accurately reconstruct the three-dimensional data of the compensator. Applying Geomagic Qualify analysis software enables 3D comparative analysis between the three-dimensional reconstructed model and the CAD model converted from the planning system output, obtaining a compensator machining error analysis report and achieving quality control of the compensator machining accuracy. For now, this is an accurate, convenient, and reliable method for compensator verification.

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Preliminary Review: Ma Shuqian

Secondary Review: Liu Wenyu