IntroductionAdvanced, Monte Carlo (MC) based dose calculation algorithms, determine absorbed dose as dose to medium-in-medium (Dm,m) or dose to water-in-medium (Dw,m). Some earlier studies identified the differences in the absorbed doses related to the calculation mode, especially in the bone density equivalent (BDE) media. Since the calculation algorithms built in the treatment planning systems (TPS) should be dosimetrically verified before their use, we analyzed dose differences between two calculation modes for the Elekta Monaco TPS. We compared them with experimentally determined values, aiming to define a supplement to the existing TPS verification methodology.Materials and methodsIn our study, we used a 6 MV photon beam from a linear accelerator. To evaluate the accuracy of the TPS calculation approaches, measurements with a Farmer type chamber in a semi-anthropomorphic phantom were compared to those obtained by two calculation options. The comparison was made for three parts of the phantom having different densities, with a focus on the BDE part.ResultsMeasured and calculated doses were in agreement for water and lung equivalent density materials, regardless of the calculation mode. However, in the BDE part of the phantom, mean dose differences between the calculation options ranged from 5.7 to 8.3%, depending on the method used. In the BDE part of the phantom, neither of the two calculation options were consistent with experimentally determined absorbed doses.ConclusionsBased on our findings, we proposed a supplement to the current methodology for the verification of commercial MC based TPS by performing additional measurements in BDE material.
BackgroundThe accuracy of dose calculation is crucial for success of the radiotherapy treatment. One of the methods that represent the current standard for patient-specific dosimetry is the evaluation of dose distributions measured with an ionization chamber array inside a homogeneous phantom using gamma method. Nevertheless, this method does not replicate the realistic conditions present when a patient is undergoing therapy. Therefore, to more accurately evaluate the treatment planning system (TPS) capabilities, gamma passing rates were examined for beams of different complexity passing through inhomogeneous phantoms.Materials and methodsThe research was performed using Siemens Oncor Expression linear accelerator, Siemens Somatom Open CT simulator and Elekta Monaco TPS. A 2D detector array was used to evaluate dose distribution accuracy in homogeneous, semi-anthropomorphic and anthropomorphic phantoms. Validation was based on gamma analysis with 3%/3mm and 2%/2mm criteria, respectively.ResultsPassing rates of the complex dose distributions degrade depending on the thickness of non-water equivalent material. They also depend on dose reporting mode used. It is observed that the passing rate decreases with plan complexity. Comparison of the data for all set-ups of semi-anthropomorphic and anthropomorphic phantoms shows that passing rates are higher in the anthropomorphic phantom.ConclusionsPresented results raise a question of possible limits of dose distribution verification in assessment of plan delivery quality. Consequently, good results obtained using standard patient specific dosimetry methodology do not guarantee the accuracy of delivered dose distribution in real clinical cases.
Advances of radiation delivery devices have increased the complexity of the
radiation oncology treatments. Herewith, outcome of the treatment, as well
as patient safety, strongly depend on the consistency of absorbed dose
delivery. Both can be ensured by comprehensive system of verification of
calculated absorbed dose distributions. Standard method is evaluation of
calculated absorbed dose distribution according to gamma method, using a 2-D
detector and a homogeneous phantom, to obtain measured dose distribution.
Purpose of this research was to investigate the influence of tolerance
criteria on gamma passing rate. Additionally, the agreement in heterogeneous
phantom was analysed. Absorbed dose calculations were performed using systems Monaco and XiO. Detector with 1020 ionization chambers in homogeneous
phantom and semi-anthropomorphic phantom was used for measurements. Absorbed
dose distributions of around 3500 patients were analysed using gamma method.
In homogeneous phantom, average gamma passing rates were within tolerance
for 3 %/2 mm. For measurements in heterogeneous media, the highest average
gamma passing rate was obtained for small volumes of medium treatment
complexity (??=93.84%), while large volumes of treatment with low
complexity yielded the lowest gamma passing rates (??= 83.22%).
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