Generation and verification of QFix kVue Calypso-compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams

Implementation of the Calypso system: a commissioning experience

Background

The aim of this study was to describe the clinical implementation of the Calypso system with its potential impact on the treatment delivery.

Materials and methods

The influence of the electromagnetic array was investigated on the kilovoltage cone beam computed tomography (kV-CBCT) image quality using the CATPHAN 504 CBCT images. Then, the QFix kVue Calypso couch top and the array attenuation, and their dosimetric influence on the Volumetric modulated arc therapy (VMAT) treatments of prostate was evaluated.

Results

Regarding the image quality, a significant increase of noise (p < 0.01) was detected with the array in place, resulting in a significant decrease in signal noise ratio (SNR) (p < 0.01). No difference in absolute contrast was observed. Finally, there was a significant decrease in contrast noise ratio (CNR) (p < 0.01) even if the deviation was only of 2.5%. For the dosimetric evaluation, the maximum attenuation of the couch was 12.02% and 13.19% for X6 and X6 flattening filter free (FFF), respectively (configuration of rails out). Besides, the mean attenuation of the array was 1.15% and 1.67% for X6 and X6 FFF, respectively. For the VMAT treatment plans, the mean dose was reduced by 0.61% for X6 and by 0.31% for X6 FFF beams when using the electromagnetic array.

Conclusions

The Calypso system does not affect significantly the kV-CBCT image quality and the VMAT plan dose distribution.

Dosimetric Evaluation of the QFix kVueTM Calypso Couch Top

PURPOSE

To evaluate the dosimetric accuracy of the default couch model of the QFix kVueTM Calypso couch top in the treatment planning system.

METHODS

With the gantry 180°, field size 20 × 20 cm, 6 MV, we measured the depth dose, off-axis dose, and dose plane of different depths in the phantom with the couch rails in and out, respectively. Isocenter doses at different angles were also obtained. The results were compared to the doses calculated using the default couch top model and the real scanned couch top model. Then we revised the default model according to the measured results.

RESULTS

With "Rails In," the depth dose, off-axis dose, and dose plane of the default couch top model had a big difference with the dose of the real scanned couch top model and the measured result. The dose of the real scanned couch top model was much closer to the measured result, but in the region of the rail edge, the difference was still significant. With "Rails Out," there was a minor difference between the measured result, the dose of the default couch top model and the real scanned couch top model. The difference between the measurement and the default couch top model became very small after being revised.

CONCLUSIONS

It is better to avoid the beam angle passing through the couch rails in treatment plans, or you should revise the parameter of the QFix kVueTM Calypso couch top model based on the measured results, and verify the treatment plan before clinical practice.

Couch modeling optimization for tomotherapy planning and delivery

We sought to validate new couch modeling optimization for tomotherapy planning and delivery. We constructed simplified virtual structures just above a default setting couch through a planning support system (MIM Maestro, version 8.2, MIM Software Inc, Cleveland, OH, USA). Based on ionization chamber measurements, we performed interactive optimization and determined the most appropriate physical density of these virtual structures in a treatment planning system (TPS). To validate this couch optimization, Gamma analysis and these statistical analyses between a three‐dimensional diode array QA system (ArcCHECK, Sun Nuclear, Melbourne, FL, USA) results and calculations from ionization chamber measurements were performed at 3%/2 mm criteria with a threshold of 10% in clinical QA plans. Using a virtual model consisting of a center slab density of 4.2 g/cm³ and both side slabs density of 1.9 g/cm³, we demonstrated close agreement between measured dose and the TPS calculated dose. Agreement was within 1% for all gantry angles at the isocenter and within 2% in off‐axis plans. In validation of the couch modeling in a clinical QA plan, the average gamma passing rate improved approximately 0.6%–5.1%. It was statistically significant (P < 0.05) for all treatment sites. We successfully generated an accurate couch model for a TomoTherapy TPS by interactively optimizing the physical density of the couch using a planning support system. This modeling proved to be an efficient way of correcting the dosimetric effects of the treatment couch in tomotherapy planning and delivery.

Technical Note: Evaluation of the systematic accuracy of a frameless, multiple image modality guided, linear accelerator based stereotactic radiosurgery system

PURPOSE

To evaluate the total systematic accuracy of a frameless, image guided stereotactic radiosurgery system.

METHODS

The localization accuracy and intermodality difference was determined by delivering radiation to an end-to-end prototype phantom, in which the targets were localized using optical surface monitoring system (OSMS), electromagnetic beacon-based tracking (Calypso®), cone-beam CT, "snap-shot" planar x-ray imaging, and a robotic couch. Six IMRT plans with jaw tracking and a flattening filter free beam were used to study the dosimetric accuracy for intracranial and spinal stereotactic radiosurgery treatment.

RESULTS

End-to-end localization accuracy of the system evaluated with the end-to-end phantom was 0.5 ± 0.2 mm with a maximum deviation of 0.9 mm over 90 measurements (including jaw, MLC, and cone measurements for both auto and manual fusion) for single isocenter, single target treatment, 0.6 ± 0.4 mm for multitarget treatment with shared isocenter. Residual setup errors were within 0.1 mm for OSMS, and 0.3 mm for Calypso. Dosimetric evaluation based on absolute film dosimetry showed greater than 90% pass rate for all cases using a gamma criteria of 3%/1 mm.

CONCLUSIONS

The authors' experience demonstrates that the localization accuracy of the frameless image-guided system is comparable to robotic or invasive frame based radiosurgery systems.

Precise film dosimetry for stereotactic radiosurgery and stereotactic body radiotherapy quality assurance using Gafchromic™ EBT3 films

Purpose

The purpose of this study is to evaluate the dosimetric uncertainty associated with Gafchromic™ (EBT3) films and establish a practical and efficient film dosimetry protocol for Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT).

Method and materials

EBT3 films were irradiated at each of seven different dose levels between 1 and 15 Gy with open fields and standard deviations of dose maps were calculated at each color channel for evaluation. A scanner non-uniform response correction map was built by registering and comparing film doses to the reference ion chamber array-based dose map delivered with the same doses. To determine the temporal dependence of EBT3 films, the average correction factors of different dose levels as a function of time were evaluated up to 4 days after irradiation.

An integrated film dosimetry protocol was developed for dose calibration, calibration curve fitting, dose mapping, and profile/gamma analysis. Patient specific quality assurance (PSQA) was performed for 83 SRS/SBRT treatment plans, and analysis of the measurements and calculations are presented here.

Results

The scanner response varied within 1 % for the field sizes less than 5 × 5 cm², and up to 5 % for the field sizes of 10 × 10 cm² for all color channels. The scanner correction method was able to remove visually evident, irregular detector responses for larger field sizes. The dose response of the film changed rapidly (~10 %) in the first two hours and became smooth plateaued afterwards, ~3 % change between 2 and 24 h. The uncertainties were approximately 1.5, 1.7 and 4.8 % over the dose range of 3~15 Gy for the red, green and blue channels. The green channel showed very high sensitivity and low uncertainty in the dose range between 10 and 15 Gy, which is suitable for SRS/SBRT commissioning and PSQA. The difference between the calculated dose and measured dose of ion chamber measurement at isocenter was −0.64 ± 2.02 for all plans, corresponding to a 95 % confidence interval of (−1.09, −0.26). The percentage of points passing the 3 %/1 mm gamma criteria in absolute dose, averaged over all tests was 95.0 ± 4.2.

Conclusion

We have developed the EBT3 films based dosimetry protocol to obtain absolute dose values. The overall uncertainty has been established to be 1.5 % for SRS and SBRT PSQA.