engleski

Evaluation of dose planes using 2D detector array– patient specific dosimetry or accelerator performance assessment?

Introduction The accuracy of dose calculation is one of the most important factors in the radiotherapy treatment process. In Monte Carlo (MC) based systems absorbed dose delivered by external photon beam can be reported either as dose-to-media (Dm) or dose-to-water (Dw). There is a common agreement that MC simulation is the most promising method for accurate calculation of absorbed dose1, 2. Nevertheless, there is still no general agreement regarding the choice of the calculation method1-3. Even so, such algorithms have their own inherent limitations. These limitations are particularly evident in high density media. The differences between Dm and Dw approaches in high-density tissues (e.g. bones) were significant and of opposite sign, when compared to measured values 2, 3. One of the methods that represent the current standard for patient specific dosimetry is the evaluation of dose planes measured with an ion chamber array inside a homogenous phantom using gamma method4, 5. Considering the fact that the beams pass only through homogenous water equivalent media in this case, this does not replicate the realistic conditions present when a patient is undergoing therapy. Therefore, to more accurately evaluate the capabilities of the TPS, gamma passing rate was examined for beams passing through inhomogenous phantoms in different geometries in addition to the ones obtained using the water equivalent homogenous phantom using Dm and Dw. Also, to better differentiate the underlying reasons for possible discrepancies, a selection of different plans were evaluated, ranging from simple rectangular fields to IMRT plans of different complexity. The results and analysis of this research will be presented in this presentation. Materials and methods In this study the research was performed using devices which are in clinical use at Radiotherapy Department of University Hospital Rijeka. Namely, 6MV beam of linear accelerator Siemens Oncor Expression with 160 multileaf collimator (MLC), Siemens Somatom Open CT simulator and Elekta Monaco (v.5.11.) TPS. Linear accelerator was commissioned and prepared for the clinical implementation of IMRT according to international standards. 2D detector array (IBA Matrixx IMRT) with 1020 ion chambers spaced at 0.7 cm distances one from another is used to evaluate TPS accuracy both in homogeneous phantom (IBA Cube Phantom) and antrophomorphic phantom (CIRS Thorax). Validation was based on gamma analysis with 3%/3mm and 2% /2mm criteria respectively. For the purpose of a thorough evaluation of the gamma results depending on inhomogenouus media, considering also different complexities of plans, several different phantom configurations in conjunction with the IBA Matrixx detector were used. All phantom combinations were scanned with 0.2cm slice thickness with appropriate relative electron densities tables assigned. In order of increased heterogenity complexity, the 2D detector was placed under different measuring conditions using homogeneous and semi-antrophomorphic phantoms: 1. MultiCube-IBA MultiCube homogenous phantom 2. PMMA plates (3 cm) and various thicknesses of the CIRS Thorax phantom placed perpendicular to the measuring plane with a. CIRS 5N=5 cm thickness b. CIRS 10N=10 cm thickness c. CIRS 15N=15 cm thickness 3. CIRS Thorax phantom positioned on the detector in regular manner. All calculations were performed using Elekta Monaco 5.11 TPS with Dw and Dm reporting modes, respectively. In order to achieve an appropriate level of dose calculation accuracy and consistency, all plans were calculated with 0.2cm grid size, 0.5% statistical uncertainty, and „per control point“ calculation mode. Different QA plans with all beams set to 0° were calculated using all above mentioned phantom configurations, ranging from simple square referent field (15×15 cm2) to real clinical IMRT plans: 1. IMRT1-prostate plan with 23 segments 2. IMRT2-CNS plan with 40 segments 3. IMRT3-H&N plan with 76 segments 4. IMRT4-H&N plan with 3 dose levels integrated boost (105 segments) Resuts and discussion The gamma analysis results for different measuring geometries and different levels of plan complexity as well as different reporting modes are presented in Table 1. and Figures 1 and 2. The results for IMRT plans are degrading depending on the thickness of non-water equivalent material of up to 9% for 3%/3mm, regardless of reporting mode- Dm or Dw. Adittionaly, the gamma passing rates were degrading depending on the level of complexity of plans, up to 15% when looking at 2%/2mm. These results together with the fact that 2%/2mm passing rates degrade more rapidly than 3%/3mm suggest that the resolution of the detector is one of the limiting factors of the analysis. Additionaly, there is a significant difference in the passing rates depending on Dw and Dm reporting modes. Our results raised question of possible limits of the gamma method in assesment of plan delivery quality. Consequently, good results obtained using standard patinet specific dosimetry metodology does not guarantee the accuracy of delivered dose distribution in real clinical cases. Reference 1. Reynaert N, Van der Marck S, Schaart D, Van der Zee W, Van Vliet-Vroegindeweij C, Tomsej M, et. al. Monte Carlo treatment planning for photon and electron beams. Radiation Physics and Chemistry (2007) ; 76: 643-686 2. Andreo P. Dose to 'water-like' media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate. Phys Med Biol. (2015) ; 21 60(6): 2619. 3. Ma C-M, Li J. Dose specification for radiation therapy: dose to water or dose to medium? Phys Med Biol. (2011) ; 21 ; 56(10): 3073-89. 4. Low D. A., Gamma Dose Distribution Evaluation Tool. Journal of Physics: Conference Series 250 (2010) 5. Son J., Baek T., Lee B., Shin D., Park S.Y., Park J., Lim Y.K., Lee S.B., Kim J. and Yoon M. A comparison of the quality assurance of four dosimetric tools for intensity modulated radiation therapy Radiol Oncol. (2015) ; 49(3): 307–313.

IMRT, PSD, 2D dosimetry, Dose to media, dose to water

nije evidentirano