ABSTRACT
Objective:To explore the value of using the mean gamma index (GI) in targer area receiving 50% prescribed dose as reference in VMAT planned dose verification through model-based dose calculation and measurement-based dose reconstruction.Methods:Based on Compass dose verificantion system, the VMAT plans for 70 patients were validated using two method. The mean GI and passing rate in target area receiving 50% of prescribed dose area for each validation plan were obtained to evaluate its application value in dose validation. First, plan information obtained by TPS calculation was input into the Compass system for performing independent dose calculation based on the accelerator data model, and obtain a three-dimensional dose based on the independent model calculation. The planned fluence measured for each patient′s treatment plan on the accelerator was reconstructed through the Compass system to obtain a three-dimensional dose based on measurement reconstruction. The three-dimensional dose obtained by the two method were compared with the three-dimensional dose calculated by TPS.Results:Combined with the gamma criteria of 3%/3 mm in the error setting condition of GI analysis, the mean GI in the area receiving 50% of prescribed dose was evaluated. GI≤0.4 was classified as PASS, 0.4 < GI ≤ 0.6 as being clinically acceptable, and GI > 0.6 as FAIL. The VMAT planned dose verification for 70 patients showed that the model-based independent calculation was in a better agreement with the TPS calculation. The GI values were all < 0.6: GI≤0.4 for 67 patients and 0.4 <GI≤ 0.6 for the other 3 patients, with gamma passing rate larger than 92%. The in- vivo measurement-based reconstructed 3D dose are slightly lesser than the model-based planed result ; theGI values were all < 0.6: GI ≤0.4 for 35 patients and 0.4 < GI ≤ 0.6 for other 35 patients, with gamma pass rate larger than 88%, of which gamma passing rate > 90% for 88 patients and < 90% for other 2, all meeting the requirements of clinical dose verification. The model-based independent dose verification is better than the measurement-based reconstructed dose verificantion, and the difference is statistically significant ( t=15.20, 10.71, P < 0.05). Conclusions:The mean GI in target area receiving 50% of prescribed dose can be used as a reference to judge the operatability of clinical plan in clinical dose verification. The mean GI value, in combination with the comprehensive result of gamma passing rate, is more convincing to evaluate dose verification. A combination of model-based dose verification, despite time-saving and labor-saving, and the measurement-based dose verification could become a reliable dose verification method for clinical application.
ABSTRACT
Dose verification, as part of the radiotherapy QA chain, is a significant method to ensure the patients' safety and efficacy of treatment. The increasing application of precision radiotherapy techniques in clinic has advanced the need of three-dimensional ( 3D) dose verification. Gel dosimeters, prevailing for its intrinsic 3D high-resolution measurement and good tissue equivalence, can serve as effective supplement to the clinical radiotherapy dosimetric system. This paper reviews the method ology, dose response mechanism, characterizations of the state-of-the-art gel dosimeters. Gel dosimeters, outstanding for 3D dose measurement, have a great potential to explore both for clinical application and academic research.
ABSTRACT
Objective To analyze the impact of electronic portal imagingdevice (EPID) position error on three-dimensional dose verification of volumetric modulated arc therapy (VMAT).Metbods Five Suremark SL-20 lead points were fixed on Elekta tray,and EPID images were collected in 0-360° rotation,one image per 5°.The position error relative to the accelerator was analyzed via Matlab.Then the images position error was corrected according to the analysis,and the 3D dose was reconstructed with the corrected images.The dose distributions of double arcs,clockwise arc(arc 1),and counterclockwise arc (arc 2) of 16 nasopharyngeal carcinoma patients' VMAT plan were evaluated by γ analysis,and the results of before and after position error correction were compared.Results Compared to 0° gantry angle,the error of source to the image distance (SID) was maximum (1.20 cm) when the gantry angle was 180°.On account of the SID change,the maximum error along the up-down (y) direction in the iso-center planar was 2.28 mm and the left-right (x) direction error was within ± O.5 mm.The 3D γ analyses of 16 nasopharyngeal carcinoma in VMAT plans were obviously increased after the position error along y was corrected.The double arcs,arc1 and arc 2 were increased by (4.12 ±1.67) ％ (t =-9.86,P＜ 0.05),(3.47±1.64) ％ (t=-8.46,P＜ 0.05) and (5.08±1.30) ％ (t=-15.63,P＜ 0.05) in 5％/3 mm standard,respectively.However,in 3％/3 mm standard,γ value of the double arcs,arc 1 and arc2 were increased by (7.63 ±2.24) ％ (t =-13.63,P＜ 0.05),(6.03 ±2.07) ％ (t =-11.66,P＜ 0.05),(9.17 ±2.23) ％ (t =-16.41,P＜ 0.05),respectively.Since the EPID position error along x was corrected after y,the 3D γ analysis of reconstruction dose indicated that the average of the 5％/3 mm and 3％/3 mm γ value were increased by 0.23％ and 0.24％,respectively.Conclusions EPID motion error along the gantry to table direction of the accelerator can't be ignored.When reconstruct dose based on EPID,a modification should be made for rebuilding more accurate patients' 3D dose distribution.