Influence of Cy berKnife Prescription Isodose Line on the Discrepancy of Dose Results Calculated by the Ray Tracing and Monte Carlo Algorithms for Head and Lung Plans: A Phantom Study

Incorporation of the Monte Carlo(MC)algorithm in optimizing CyberKnife(CK)plans is cumbersome,and early models unconfigured MC calculations,therefore,this study investigated algorithm-based dose calculation discrepancies by selecting different prescription isodose lines(PIDLs)in head and lung CK plans.CK plans were based on anthropomorphic phantoms.Four shells were set at 2-60 mm from the target,and the constraint doses were adjusted according to the design stratcgy.After optimization,30%-90%PIDL plans were generated by ray tracing(RT).In the evaluation module,CK plans were recalculated using the MC algorithm.Therefore,the dosimetric parameters of different PIDL plans based on the RT and MC algorithms were obtained and analyzed.The discrepancies(mean+SD)were 3.72%+0.31%,3.40%+0.11%,3.47%+0.32%,0.17%+0.11%,0.64%+3.60%,7.73%+1.60%,14.62%+3.21%and 10.10%+1.57%for Djs,Dmeam),Dys,and coverage of the PTV,DGI,V,,V;and V,in the head plans and-6.32%+1.15%,-13.46%+0.98%,-20.63%+2.25%,-34.78%+25.03%,12248%+175.60%,-12.92%+5.41%,3.19%+4.67%and 7.13%+1.56%in the lung plans,respectively.The following parameters were significantly correlated with PIDL:dp98%at the 0.05 level and dpal,dys and dv3 at the 0.01 level for the head plans;dp98e%at the 0.05 level and do1e%,dpmeam,Ccoweange,dool,dvs and dv;at the 0.01 level for the lung plans.RT may be used to calculate the dose in CK head plans,but when the dose of organs at risk is close to the limit,it is necessary to refer to the MC results or to further optimize the CK plan to reduce the dose.For lung plans,the MC algorithm is recommended.For early models without the MC algorithm,a lower PIDL plan is recommended;otherwise,a large PIDL plan risks serious underdosage in the target area.

Clinical Comparison of Pencil Beam Convolution and Clarkson Algorithms for Dose Calculation

Purpose: The purpose of this work is to study and quantify the differences in calculated dose computed with two algorithms available in treatment planning systems: Pencil Beam Convolution and Clarkson. Material and Methods: Four different types of treatment cases were analyzed: lung, head and neck, brain and prostate. For each case, the volume definition was based on a clinical CT-scan acquisition. The patients were treated with 3-dimensional radiation therapy. For each patient, 2 treatment plans were generated using exactly the same configuration of beams. In plan 1 and plan 2, the dose was calculated using the Clarkson and Pencil Beam Convolution algorithms, respectively, without heterogeneity correction. To evaluate the treatment plans, the monitor units, isodose curves, dose volume histograms and quality index were compared. A statistical analysis was carried out using Wilcoxon signed rank test. Results: The difference observed for monitor unites was 1.2% for lung and less than 1% for head and neck, brain and prostate. Wilcoxon test showed that there was “no statically significant difference, (p > 0.05)”. The dosimetric parameters derived from dose volume histograms were higher for organs at risks using Clarkson compared to Pencil Beam Convolution algorithm inviting clinician to make “safer” prescriptions. For quality index there was no statistically significant difference between both algorithms for all quality indexes, (p > 0.05). Conclusion: The clinical evaluation of a treatment plan should be made regarding the calculation algorithm which, in turn, is linked to the experience of the clinician.

Study of the Photon Flux and the Dose Rate in the Vicinity of a 60Co Gamma Irradiator

The present work presents an overview of the study of some dosimetric quantities in the vicinity of the Tunisian Gamma Irradiation Facility. Firstly, we have confirmed our previous calculation of the photon flux and the dose rates, using a simulation with GEANT 4. A good agreement between calculation and simulation was obtained, which well confirmed the modeling of the CNSTN extended source by a pencil-like source. Secondly we have determined the isodose curves in the vicinity of the irradiator using a straightforward calculation. Finally, we have presented many comments for some published work concerning the methods used to determine these dosimetric quantities.