الفهرس 
Title : Studying the Accuracy of Treatment Planning Calculations
AbstractOnly 14 pages are availabe for public view
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Abstract
The dosimetric effect due to the presence of air cavity is one of the major concerns for lung cases. When tumors are near air cavities or low density material such as lung are treated with photon beam energy, the surface layers located beyond the low density/tissue interface are under-dosed due to a loss of both lateral and longitudinal electronic equilibrium. Although the Anisotropic Analytical Algorithm (AAA) is more widely used in clinical practice, Acuros XB (AXB) has been found to be more accurately calculate the dose distribution in heterogeneous media. Prediction of clinical outcomes should be based on dose calculation modeling to the patient as accurately as possible. To evaluate that one algorithm is more accurate than the other require knowledge about the actual absorbed dose in case of extreme heterogeneity. The aim of the present study was to investigate the dosimetric performance of Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). Also, to evaluate their accuracy in clinical plans of Intensity Modulated Radiation Therapy (IMRT) technique for patients with locally advanced Non-Small Cell Lung Cancer (NSCLC). A heterogeneous phantom to investigate the dose prediction accuracy of AXB and AAA beyond low-density medium by comparing the calculated results with the measurements was used. Also, a special phantom analogous to a lung cancer patient was designed for this study to view the dose distribution inside and around a tumor inside lung tissue using both the Radiochromic EBT-2 films and Thermoluminescent dosimeters (TLDs). Isodose comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The algorithms were evaluated for clinical practice as the study included 32 patients with locally advanced NSCLC were subjected to
radiation therapy with IMRT technique. The dose regimen was 60 Gy over 30 fractions. The effects on Planning Target Volume (PTV) and Organ At Risk (OAR) were evaluated using AXB and AAA inpatient’s plans to compare the accuracy of the calculation. Clinically acceptable treatment plans with AAA were re-calculated using AXB algorithm with the two modes of calculation Dose to water (Dw) and Dose to medium (Dm) at the same beam arrangements and multileaf collimator leaf settings as with AAA. The heterogonous phantom study showed that the results of AXB had better agreement with the measurements than AAA and the average dose difference for all points was about 0.5%. Conversely, AAA showed a lower agreement with measured values and the average difference was up to 5.9%. The results of TLDs and films in lung phantom showed that the dose agreement of AXB was within 3% of doses measured with TLD for all measurement points and that gamma analysis of the film measurements passed the ±3%/3 mm criteria with 97.5% passing rate, while, AAA tends to overestimate the dose over both TLDs and films and had a lower agreement. For the clinical cases, the mean differences observed between the AAA, AXB_Dm, and AXB_Dw were statistically significant (p < 0.05), for most of the parameters evaluated. The maximum relative difference was between AXB_Dm and AAA for PTV dose (D98%).Also the percentage dose differences of plans calculated by AAA, AXB_Dw and AAA, AXB_Dm revealed AAA overestimated the dose than AXB. Regarding Organs At Risk (OAR), results showed significant difference between both AAA and AXB_Dw, AAA and AXB_Dm for lungs-PTV as AAA showed higher mean dose, higher V5, V10 and V20 when compared to both AXB modes with p values 0.0001, 0.0001, 0.0001 and 0.0001, respectively. Heart evaluation showed significantly higher mean dose in AAA algorithm with p value 0.0001 in comparison with both AXB modes, however, there was a significant higher difference between AXB_Dw
and AAA in terms of V40 and a non-significant difference for V45 with P values 0.02 and 0.22, respectively. Statistical significance of the maximum dose differences between the AXB_Dw, AXB_Dm and AAA plans was observed for the spinal cord (P = 0.0001), but not for esophagus (p = 0.04 and 0.26). Moreover, mean dose of esophagus was also non significantly in AAA algorithm compared with AXB_Dw (p = 0.17), while for AAA with AXB_Dm there was a significant difference (p = 0.0001). Acuros XB algorithm has accurate dose prediction in heterogeneous medium in comparison to AAA. Differences in dose distribution were observed when plans re-calculated with AXB indicating AAA apparently overestimates dose, particularly the PTV dose. AXB principally predicts lower dose in case of tissue heterogeneity compared to AAA. The AXB algorithm should be used in preference to AAA for cases in which PTVs are involved with tissues of highly different densities, such as lung.