Evaluation of mechanical and geometric accuracy of two different image guidance systems in radiotherapy

Development of a new coordinate calibration phantom for a light-section-based optical surface monitoring system

A calibration phantom made of Derlin requires manual translational and rotational adjustments when calibrating a light-section-based optical surface monitoring system (VOXELAN) with a phantom material that insufficiently reflects the red-slit laser of the system. This study aimed to develop a new calibration phantom using different materials and to propose a procedure that minimizes setup errors. The new phantom, primarily made of PET100, which exhibits good reflectivity without scattering or attenuating the red-slit laser at the phantom surface, was shaped in a manner similar to that of previous designs. The detection accuracy and stability were evaluated using six different regions of interest (ROIs) and compared with previous phantom designs. The coordinate coincidence between the machine and VOXELAN was compared for both phantom designs. The detection accuracy and stability of the new phantom in the reference ROI setting were found to be better than those of previous phantoms. In the lateral, longitudinal, and vertical directions, the coordinate coincidences in translational directions for the previous phantom were obtained at 1.07 ± 0.66, 1.46 ± 0.47, and 0.26 ± 0.83 mm, whereas those for the new phantom were obtained at 0.28 ± 0.21, 0.18 ± 0.30, and - 0.30 ± 0.29 mm, respectively. The rotational errors of the two phantoms were identical. The new phantom exhibited improved detection stability because of its good reflectivity. Additionally, the new placement procedure was linked to the six-degrees-of-freedom couch. A combination of the new phantom and its new placement procedure is suitable for coordinate calibration of VOXELAN.

The Usefulness of Adaptative Radiotherapy in Prostate Cancer: How, When, and Who?

The aim of this study was to develop a deformable image registration (DIR)-based offline ART protocol capable of identifying significant dosimetric changes in the first treatment fractions to determine when adaptive replanning is needed. A total of 240 images (24 planning CT (pCT) and 216 kilovoltage cone-beam CT (CBCT)) were prospectively acquired from 24 patients with prostate adenocarcinoma during the first three weeks of their treatment (76 Gy in 38 fractions). This set of images was used to plan a hypofractionated virtual treatment (57.3 Gy in 15 fractions); correlation with the DIR of pCT and each CBCT allowed to translate planned doses to each CBCT, and finally mapped back to the pCT to compare with those actually administered. In 37.5% of patients, doses administered in 50% of the rectum (D50) would have exceeded the dose limitation to 50% of the rectum (R50). We first observed a significant variation of the planned rectal volume in the CBCTs of fractions 1, 3, and 5. Then, we found a significant relationship between the D50 accumulated in fractions 1, 3, and 5 and the lack of compliance with the R50. Finally, we found that a D50 variation rate [100 × (administered D50 − planned D50/planned D50)] > 1% in fraction three can reliably identify variations in administered doses that will lead to exceeding rectal dose constraint.

Optimization of brain tumours irradiation determining the set-up margin

The aim of this work was to evaluate whether the excising margin of the clinical tumor volume and planning target volume correspond with calculated radiation margin based on systematic errors, and definition of radiation margins of individual brain lobes. This research was a retrospective cross-sectional study. We checked the systematic errors and calculated their average and the size of radiation margins. The average systematic errors were calculated in four directions: lateral, longitudinal, vertical, and rotation. The largest average systematic error was calculated in the lateral direction in the cerebellar area, and the error was also statistically significant(p < 0.05). In rotational direction we notice the statistically significant difference in frontal lopbe (p = 0.037), and cerebellar area (p = 0.002). The largest safety margin, as measured by the apverage systematic errors, is requirped for irradiation of the cerebellum. The safety margin size of 6.94 mm was calculated according to the formula of Van Herk. However, the smallest safety margin can be used for irradiation of the occipital lobe of the brain, namely 4.85 mm. The linear regression results that only cerebellar lesions affect lateral displacements. Based on our calculation of the mean systematic errors, we estimate that the clinical target volume - planning target volume safety margin can't be reduced further from the current 5 mm to a size of 3 mm without the use of image guided radiotherapy.

Independent 6D quality assurance of stereotactic radiotherapy repositioning on linacs

PURPOSE

A high level of accuracy while positioning the patient is mandatory for frameless stereotactic radiotherapy (SRT), as large doses in multiple fractions can be delivered near organs at risk. The objective of this study is to propose an end-to-end quality assurance method to verify that submillimetre alignment can be achieved with stereotactic conventional linacs.

METHODS

We used a TrueBeam® linear accelerator equipped with a 6DOF robotic couch. The "ISO Cube" phantom was used with a homemade stand designed to generate known translational and rotational offsets. A reference CT scan was performed with straight alignment of the phantom. The procedure introduced 1.6° angular offset for the couch pitch and roll, at various gantry angles. The couch base was also moved between 0° and 270°. We compared the results with the daily machine performance check tests (MPC, Varian).

RESULTS

The mean isocentre size, MV and kV imager offsets were found to agree to within 0.1mm, 0.1mm and 0.3mm respectively, and were in close agreement between the methods. For a total four months data collection period, the mean deviation between requested and measured 6DOF couch shifts was 0.6mm and 0.2°. Errors on field size were smaller than 1mm for 97.7% of the 324 data points.

CONCLUSION

Results demonstrate that the linac equipped with a 6DOF robotic positioner and CBCT imaging satisfies requirements for SRT. Our methodology, based on a modified Winston-Lutz quality control, allowed us to quantitatively assess end-to-end accuracy of a linac in order to safely deliver SRT.

Registration methods in radiotherapy

Purpose

The aim of this study is to present a short and comprehensive review of the methods of medical image registration, their conditions and applications in radiotherapy. A particular focus was placed on the methods of deformable image registration.

Methods

To structure and deepen the knowledge on medical image registration in radiotherapy, a medical literature analysis was made using the Google Scholar browser and the medical database of the PubMed library.

Results

Chronological review of image registration methods in radiotherapy based on 34 selected articles. A particular attention was given to show: (i) potential regions of the application of different methods of registration, (ii) mathematical basis of the deformable methods and (iii) the methods of quality control for the registration process.

Conclusions

The primary aim of the medical image registration process is to connect the contents of images. What we want to achieve is a complementary or extended knowledge that can be used for more precise localisation of pathogenic lesions and continuous improvement of patient treatment. Therefore, the choice of imaging mode is dependent on the type of clinical study. It is impossible to visualise all anatomical details or functional changes using a single modality machine. Therefore, fusion of various modality images is of great clinical relevance. A natural problem in analysing the fusion of medical images is geographical errors related to displacement. The registered images are performed not at the same time and, very often, at different respiratory phases.