Investigation of Radiation Isocenter Accuracy of Linear Accelerator – New Approach of Comprehensive Evaluation of Gantry, Couch, and Collimator Isocenters

Abstract

Objective:

For stereotactic machines, recommendations demand stringent localization and precise delivery, as reflected by isocenter verification. Determining and analyzing the radiation isocenter for all clinical combinations of gantry, collimator, and couch are crucial for the quality of radiation therapy delivered. Although several radiation isocenter verification devices exist, there is no time-efficient system available for verifying the combined radiation isocenter. This paper presents a newly patented approach for the combined accuracy verification of the gantry, collimator, and couch radiation isocenter.

Methods:

The novel combined radiation isocenter verification tool is used to expose a star test pattern on the dosimetric film at various angular combinations of the gantry, collimator, and couch. The exposed dosimetric film was evaluated using Radiochromic.com’s standard third-party film analysis software.

Results:

According to the results, the combined radiation isocenter for unit 1 is 0.74 mm dia (0.37 mm radius), and for unit 2, it is 0.62 mm dia (0.31 mm radius). These results are in good agreement with the requirements for three-dimensional conformal radiotherapy, intensity-modulated radiation therapy, volumetric-modulated arc therapy, stereotactic radiosurgery, and stereotactic body radiotherapy techniques.

Conclusion:

This combined radiation isocenter verification tool aids in comprehensive evaluation of gantry, couch and collimator isocenters using radiographic film. It is easier to implement and faster to analyse compared to existing techniques, using a PC-based program that minimizes human error and increases measurement accuracy. This new approach can be used for routine quality assurance checks.

INTRODUCTION

Modern radiotherapy treatment techniques such as three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiation therapy (IMRT), volumetric-modulated arc therapy (VMAT), stereotactic body radiotherapy (SBRT), and stereotactic radiosurgery (SRS) aim to deliver maximum radiation dose to the cancer cells and the minimum radiation dose to the surrounding critical structures and normal tissues. Therefore, various quality assurance (QA) tests should be conducted on linear accelerator for ensuring accurate dose delivery.[1]

The first step in measuring QA parameters pertaining to the accuracy of the accelerator is to ensure that the reference frame precisely specifies the isocenter position to be within the desired tolerance. The determination and analysis of radiation isocenter is crucial in the whole process and has a very important role on the quality of radiation therapy delivered.

Modern radiotherapy treatment delivery equipment optimize the dose distributions often by making use of the gantry, collimator, and couch rotations. Special techniques involving high dose per fraction, such as SRS/SBRT, require high spatial accuracy. A point in space at which the collimator axis, gantry axis, and couch axis intersect is called the radiation isocenter. Each component that either moves the patient during treatment, changes the position of the radiation source, or collimates the radiation field will affect how accurately the machine irradiates the target.[2] Any notable discrepancies in the accuracy can lead to geographical miss.

Understanding the mechanical precision of the linear accelerator and the potential ramification for targeting accuracy is essential while navigating clinical decision-making, such as prescribing treatment margins or implementing procedures that require a high degree of accuracy.[3]

For stereotactic machines, recommendations demand the stringent localization accuracy for all clinical combinations of gantry, collimator, and couch. The isocenter is technically a spatial point but in reality it is a small circle or sphere, the size of which needs to be minimized. Minimizing the radiation isocenter improves the accuracy of stereotactic treatments. The AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS/SBRT recommends that the radiation isocenter should not exceed 1 mm dia for SRS and 1.5 mm dia for SBRT.

Hence, any treatment delivery system‘s isocenter accuracy should be within a diameter of ≤1 mm, to be fully compliant with the TG 142,[4] MPPG 9A,[5] and MPPG.8A[6] guidelines. In addition, the determination of the system’s radiation isocenter is crucial in its commissioning and for ensuring accuracy in treating a target volume.

Methods to measure isocenter accuracy using radiographic film, called “star shots”, have been in use for decades.[7] This 2D star shot lacks the ability to relate all three axes to each other. Tests such as the Winston–Lutz analyze how the center of the radiation field changes with different gantry, collimator, and couch positions with respect to a stationary target.[8] Although multiple methods have been proposed to measure the isocenter location in three dimensions,[9] there is need for a simple time efficient system to verify the combined isocenter of gantry, collimator, and couch exhibiting high levels of accuracy, efficacy, and precision. This paper presents a new, patent filed, approach for the combined accuracy verification of gantry, collimator, and couch radiation isocenter. Compared with the conventional star shot technique, this new approach can verify isocenter accuracy by relating all three axes to each other. This new system aids in the generation of combined star shot pattern on a single dosimetric film of simultaneous changes of rotation angles of gantry, collimator, and the patient couch. This combined accuracy verification exhibits enhanced efficacy, high precision, and streamlines verification.

MATERIALS AND METHODS

Combined radiation isocenter was verified for two units of Siddharth II Superia-Linear Accelerator (Panacea Medical Technologies Pvt Ltd, India). Siddharth II is designed to deliver advanced treatment techniques. It supports a range of treatment modalities, including 3DCRT, IMRT, VMAT, and SBRT. The system provides precise treatment with stereotactic imaging and utilizes 6MV photon beams with both flattening filter and flattening filter-free modes for precise treatment. These treatment modalities are delivered with image-guided radiotherapy (IGRT) using on-board dual kV imaging for generating cone beam computed tomography (CBCT).

The system consists of a gantry that encloses the entire MV beam device and kV beam device with wide bore dimension of 150 cm and rotational range of ±180°. Collimation is performed at three levels to shape the radiation beam to conform to the shape of a user-defined treatment volume and to prevent healthy tissues and critical organs from radiation. The system comprises of a multileaf collimator with 46 pairs of leaves on both sides. The collimator rotation motion ranges from ±100° with an accuracy of ±0.5°. Patient support system has an isocentric rotation of ±30º with a maximum patient load of 185 kg.

The combined isocenter accuracy was measured using the star pattern image. A Gafchromic EBT4 dosimetric film was sandwiched between two phantom slabs of thickness 1.5 cm above the film and 1 cm below the film. The slab phantom was placed at 100 cm SSD. The film-slab phantom set-up was aligned to the patient positioning lasers using markings on the film.

The films were exposed to a narrow collimated field of 0.2 cm × 30 cm at different combinations of gantry, collimator, and couch. A new patent filed Panacea-made isocenter verification tool was used to expose the films at different angles. It was placed on the treatment couch guided by the lasers as shown in Figure 1. The gantry and the collimator were set at 0° and the film was irradiated at couch angles of 0°, 25°, and 335°. Subsequently the gantry and couch were set at 0° and the film was irradiated at collimator angles 60°, 90°, and 300°. Later, the collimator and couch were set at 0° and the film was irradiated at gantry angles of 45°, 105°, and 255°, respectively.

Figure 1.

Combined isocentric tool placed on treatment couch based on laser position. (a) Combined radiation isocentric tool film setup. (b) Combined radiation isocentric tool setup on machine

In addition to above, conventional spoke tests of the gantry, collimator, and couch were performed to identify radiation isocenter in each axis separately.

The exposed film was digitized with Epson (EPSON 12000XL) flatbed scanner following recommendations given in TG-235[10] film scanning procedures. The scanner was kept ON for 40 min before scanning for warming-up and five empty scans were performed to stabilize the scanner lamp. Transmission scanning mode was selected to scan the film. A scanning resolution of 75 dpi was selected to avoid noise in the scanning process. The image type was chosen as 48-bit RGB. The star test pattern exposed in the dosimetric film was evaluated using Radiochromic.com’s standard third-party film analysis software.

RESULTS

The results of the verification of dosimetric film of combined radiation isocenters for two consecutive units are analyzed as shown in Figure 2. As per the result, for the unit 1 the combined radiation isocenter size was 0.74 mm dia (0.37 mm radius) and for the unit 2 it was 0.62 mm dia (0.31 mm radius). The results are in good agreement for 3DCRT, IMRT, VMAT, SRS, and SBRT techniques. The results of the isocentric evaluation by the conventional method and by the isocentric verification tool are shown in Figures 3 and 4 and Table 1.

Figure 2.

Dosimetric film analysis results combined isocenter

Figure 3.

Dosimetric film analysis result - Spoke test (conventional method)

Figure 4.

Unit 1 – Dosimetric film analysis result - Spoke test (isocenter verification tool)

Table 1.

Unit 1 – Comparison of Conventional Spoke test results relative to Results with Combined Isocenter Tool

Isocenter Radius in mm
	Results from conventional method	Expected results in Combined Isocenter Tool	Results with Combined Isocenter Tool
Gantry	0.21	0.30	0.43
Collimator	0.08	0.13	0.19
Couch	0.45	0.35	0.49

DISCUSSION

A novel technique has been introduced to streamline the verification of combined radiation isocenter in medical linear accelerators, presenting a more efficient alternative to the current methods. This technique leverages a specialized isocenter verification tool, which simplifies the verification process, enhancing both speed and accuracy. In addition to its ease of use, this tool allows for the simultaneous verification of radiation isocenter accuracy using radiographic film.

Compared to traditional methods, this new approach offers significant advantages in terms of implementation and data analysis. By utilizing a PC-based program, the results can be quickly analyzed and quantified, reducing the time and complexity typically associated with these measurements. Moreover, system’s ease of use minimizes the potential for human error, thus enhancing overall measurement accuracy.

As this approach, using a combined isocenter tool, involves exposing it to narrow collimated fields at specific axes of rotation for the gantry, collimator, and couch, it enables the user to identify which axis is out of tolerance by analyzing the star shot pattern on the film.

This method has the potential to improve routine QA checks, as it enables more accurate assessments of combined isocenter accuracy. The Siddharth-II ring gantry, which employs this technique, ensures highly precise radiation beam delivery. The system guarantees isocenter accuracy within a diameter of ≤1 mm, in full compliance with the TG-142, MPPG9A, and MPPG.8A guidelines. This ensures adherence to stringent quality standards, making the technique a valuable addition to medical linear accelerator QA protocols.

CONCLUSION

The novel isocenter verification technique represents a significant advancement in the field of medical linear accelerator QA. By combining ease of implementation, fast and accurate data analysis, and reduced human error, this method enhances the precision of radiation isocenter verification. This approach holds great potential for improving routine QA checks and advancing the reliability of radiation therapy.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.