Dosimetric verification of lung phantom calculated by collapsed cone convolution: A Monte Carlo and experimental evaluation

Abstract

OBJECTIVE: To evaluate the dose calculation accuracy in the Prowess Panther treatment planning system (TPS) using the collapsed cone convolution (CCC) algorithm. METHODS: The BEAMnrc Monte Carlo (MC) package was used to predict the dose distribution of photon beams produced by the Oncor® linear accelerator (linac). The MC model of an 18 MV photon beam was verified by measurement using a p-type diode dosimeter. Percent depth dose (PDD) and dose profiles were used for comparison based on three field sizes: 5×5, 10×10, and 20×20cm2. The accuracy of the CCC dosimetry was also evaluated using a plan composed of a simple parallel-opposed field (11×16cm2) in a lung phantom comprised of four tissue simulating media namely, lung, soft tissue, bone and spinal cord. The CCC dose calculation accuracy was evaluated by MC simulation and measurements according to the dose difference and 3D gamma analysis. Gamma analysis was carried out through comparison of the Monte Carlo simulation and the TPS calculated dose. RESULTS: Compared to the dosimetric results measured by the Farmer chamber, the CCC algorithm underestimated dose in the planning target volume (PTV), right lung and lung-tissue interface regions by about –0.11%, –1.6 %, and –2.9%, respectively. Moreover, the CCC algorithm underestimated the dose at the PTV, right lung and lung-tissue interface regions in the order of –0.34%, –0.4% and –3.5%, respectively, when compared to the MC simulation. Gamma analysis results showed that the passing rates within the PTV and heterogeneous region were above 59% and 76%. For the right lung and spinal cord, the passing rates were above 80% for all gamma criteria. CONCLUSIONS: This study demonstrates that the CCC algorithm has potential to calculate dose with sufficient accuracy for 3D conformal radiotherapy within the thorax where a significant amount of tissue heterogeneity exists.

Dose calculation accuracy for photon small fields in treatment planning systems with comparison by Monte Carlo simulations

“…CCC algorithm had a very large difference in the buildup region which is due to the fact that this algorithm does not model the buildup region when entering from a low density to a high-density region. 42 All evaluated algorithms had better results in bigger fields and with a decrease of field size, deviations with MC simulation increased, as reported in previous studies. 6,40,42 The source of these differences in small fields is reported to be the loss of lateral charged particle equilibrium.…”

“…42 All evaluated algorithms had better results in bigger fields and with a decrease of field size, deviations with MC simulation increased, as reported in previous studies. 6,40,42 The source of these differences in small fields is reported to be the loss of lateral charged particle equilibrium. 42,43 As reported by different authors most deviations appear in fields smaller than 1×1 cm 2 .…”

“…This algorithm accurately models primary photons from source and also scattered photon and electron fluence, for this job it solves the linear Boltzmann transform equation. 42 All model-based algorithms that investigated had an agreement with MC simulation in small photon fields in a homogenous media concerning this that CCC algorithm from Monaco TPS had a superior accuracy toward the others. Clarkson algorithm as we discussed as a correction based algorithm had good accuracy in larger fields and homogenous media, but with decrease of field size and appearing of loss of charged particle equilibrium its accuracy drops dramatically.…”

“…6,40,42 The source of these differences in small fields is reported to be the loss of lateral charged particle equilibrium. 42,43 As reported by different authors most deviations appear in fields smaller than 1×1 cm 2 . Palmans et al have stated that partial occlusion of primary photons from source happens when the field size is comparable with focal spot size, also they mentioned that in medical accelerators focal spot size is about 0.5 cm.…”

Comparison of central, peripheral, and weighted size-specific dose in CT

“…An understanding of the dose distribution for different phantom sizes may be useful for calculating the dose to an organ at a specific position within a patient of a certain size [24]. Organ dose estimation has indeed been proposed using the SSDE concept [25], but it generally obtained using Monte Carlo simulations on various phantom models [26][27][28] or using anthropomorphic phantoms [29]. Direct organ dose calculation using the patient image is undergoing development and faces many challenges that need to be addressed [30].…”