ABSTRACT
Objective To research the effect of cavity under Bolus to anisotropic analytical algorithm (AAA) on calculation precision of dose in shallow tissue based on Monte Carlo method;Methods A 30 cm × 30 cm × 30 cm water phantom with the upper surface was constructed which was located at the source-axis distance (SAD) of the medical linear accelerator and the center as well as coincided with the central axis of the radiation field in Eclipse treatment panning system. Above the water phantom, a water film of 1 cm thick with or without different cavities was constructed or. AAA was used to calculate the dose distribution on the central axis and the x-axis of different depth of the water model with different cavities respectively. The accelerator model, the same water phantom and the water film were constructed and the dose distributions of the same positions were calculated with Geant4. Based on the Geant4 calculation result, the calculation precision of AAA with different cavity were compared;Results For cavities with area of 2 cm × 2 cm, if the thickness is smaller than 0.5 cm, the AAA calculation error is about 2%. with the cavity thickness increase, the AAA would overestimate the dose in the shallow area under the cavity. With the cavity area increase, the area where AAA overestimate the shallow dose gradually moved out until near the edge of the radiation field, and the calculation error on the central area gradually reduced until there is basically no error. Conclusions The shallow dose would be increased according to the cavity size when planning with AAA; If there are cavities with large volume, it is better to reposition.
ABSTRACT
Objective To study the accuracy of collapsed cone convolution ( CCC) and anisotropic analytical algorithm ( AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo ( MC ) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry “blue phantom” 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water?equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system.
ABSTRACT
Objective To compare the dosimetric differences between pencil beam convolution (PBC) and anisotropic analytical algorithm (AAA) in Eclipse treatment planning system for intensitymodulated radiotherapy (IMRT) planning of lung cancer patients and dosimetric verification.Methods 10 IMRT plans of lung cancer patients were calculated using the PBC and AAA and the differences of dosimetric parameter were analyzed according to dose-volume histogram of planning target volume (PTV),lung and spinal cord.The verification measurements were performed on an inhomogeneous thorax phantom using a pinpoint ionization chamber.The agreement between calculated and measured doses was determined.The paired t test was used to compare the results.Results Compared with PBC,the AAA predicted higher maximum PTV dose (t =-4.03,P =0.010),lower minimum PTV dose (t =5.09,P =0.040),and a reduction of the volume of PTV covered by the prescribed dose.The AAA also predicted slightly increases than the PBC algorithm in the mean dose to the lung and the V20 as well as the maximum dose to the spinal cord,and the differences were statistically significant (t =-3.99,-2.79,-5.46,P =0.010,0.038,0.003).In the verification measurements,the agreement between the AAA and measurement was within 2％and superior to the PBC algorithm on isocenter (t =-3.82,P =0.012).Conclusions For IMRT treatment planning of lung cancer,the PBC algorithm overestimates the dose to the PTV and underestimates the dose to the lung and the spinal cord,so the AAA for treating planning in which the tissue inhomogeneous such as lung is present is recommended.
ABSTRACT
The purpose of this study is to evaluate the variation of radiation dose distribution for liver tumor located in liver dome and for the interest organs(normal liver, kidney, stomach) with the pencil beam convolution (PBC) algorithm versus anisotropic Analyticalal algorithm (AAA) of the Varian Eclipse treatment planning system, The target volumes from 20 liver cancer patients were used to create treatment plans. Treatment plans for 10 patients were performed in Stereotactic Body Radiation Therapy (SBRT) plan and others were performed in 3 Dimensional Conformal Radiation Therapy (3DCRT) plan. dose calculation was recalculated by AAA algorithm after dose calculation was performed by PBC algorithm for 20 patients. Plans were optimized to 100% of the PTV by the Prescription Isodose in Dose Calculation with the PBC algorithm. Plans were recalculated with the AAA, retaining identical beam arrangements, monitor units, field weighting and collimator condition. In this study, Total PTV was to be statistically significant (SRS: p=0.018, 3DCRT: p=0.006) between PBC and AAA algorithm. and in the case of PTV, ITV in liver dome, plans for 3DCRT were to be statistically significant respectively (p=0.013, p=0.024). normal liver and kidney were to be statistically significant (p=0.009, p=0.037). For the predictive index of dose variation, CVF ratio was to be statistically significant for PTV in the liver dome versus PTV (SRS r=0.684, 3DCRT r=0.732, p<0.01) and CVF ratio for Tumor size was to be statistically significant (SRS r=-0.193, p=0.017, 3DCRT r=0.237, p=0.023).