A convolution method of calculating dose for 15‐MV x rays

A pretreatment quality assurance program for volumetric techniques should include redundant calculations and measurement‐based verifications. The patient‐specific quality assurance process must be based in clinically relevant metrics. The aim of this study was to show the commission, clinical implementation, and comparison of two systems that allow performing a 3D redundant dose calculation. In addition, one of them is capable of reconstructing the dose on patient anatomy from measurements taken with a 2D ion chamber array. Both systems were compared in terms of reference calibration data (absolute dose, output factors, percentage depth‐dose curves, and profiles). Results were in good agreement for absolute dose values (discrepancies were below 0.5%) and output factors (mean differences were below 1%). Maximum mean discrepancies were located between 10 and 20 cm of depth for PDDs (‐2.7%) and in the penumbra region for profiles (mean DTA of 1.5 mm). Validation of the systems was performed by comparing point‐dose measurements with values obtained by the two systems for static, dynamic fields from AAPM TG‐119 report, and 12 real VMAT plans for different anatomical sites (differences better than 1.2%). Comparisons between measurements taken with a 2D ion chamber array and results obtained by both systems for real VMAT plans were also performed (mean global gamma passing rates better than 87.0% and 97.9% for the 2%/2 mm and 3%/3 mm criteria). Clinical implementation of the systems was evaluated by comparing dose‐volume parameters for all TG‐119 tests and real VMAT plans with TPS values (mean differences were below 1%). In addition, comparisons between dose distributions calculated by TPS and those extracted by the two systems for real VMAT plans were also performed (mean global gamma passing rates better than 86.0% and 93.0% for the 2%/2 mm and 3%/3 mm criteria). The clinical use of both systems was successfully evaluated.PACS numbers: 87.56.Fc, 87.56.‐v, 87.55.dk, 87.55.Qr, 87.55.‐x, 07.57.Kp, 85.25.Pb

Section: Methodsmentioning

confidence: 99%

“…Therefore, usual TPS commissioning measurements are required: relative distributions (profiles and PDDs), OFs, and absolute dose values. The model connects with a collapsed cone 29 , 31 convolution/superposition algorithm that calculates the dose on the patient CT.…”

Section: Methodsmentioning

confidence: 99%

Dosimetric validation and clinical implementation of two 3D dose verification systems for quality assurance in volumetric‐modulated arc therapy techniques

Clemente-Gutiérrez

Perez-Vara

2015

“…The photon dose‐calculation algorithm evaluated in this study is the convolution/superposition algorithm that was introduced by Mackie et al 9 and extended by Papanikolaou et al 10 to polyenergetic spectra. The implementation of the dose‐calculation algorithm in the particular commercial treatment planning system (Pinnacle 3 ; ADAC Laboratories, Milpitas, CA) has been described previously 2 …”

Section: Methodsmentioning

confidence: 99%

Verification of the accuracy of a photon dose‐calculation algorithm

Gifford

Followill

Liu

et al. 2002

An extensive set of measured data was developed for the purpose of verifying the accuracy of a photon dose‐calculation algorithm. Dose distributions from a linear accelerator were measured using an ion chamber in a water phantom and thermoluminescent dosimeters in a heterogeneous anthropomorphic phantom. Test cases included square fields, rectangular fields, fields having different source‐to‐surface distances, wedged fields, irregular fields, obliquely incident fields, asymmetrically collimated fields with wedges, multileaf collimator‐shaped fields, and two heterogeneous density cases. The data set was used to validate the photon dose‐calculation algorithm in a commercial radiation treatment planning system. The treatment planning system calculated photon doses to within the American College of Medical Physics (AAPM) Task Group 53 (TG‐53) criteria for 99% of points in the buildup region, 90% of points in the inner region, 88% of points in the outer region, and 93% of points in the penumbra. For the heterogeneous phantoms, calculations agreed with actual measurements to within ±3%. The monitor unit tests revealed that the 18‐MV open square fields, oblique incidence, oblique incidence with wedge, and mantle field test cases did not meet the TG‐53 criteria but were within ±2.5% of measurements. It was concluded that (i) the photon dose calculation algorithm used by the treatment planning system did not meet the TG‐53 criteria 100% of the time; (ii) some of the TG‐53 criteria may need to be modified, and (iii) the generally stated goal of accuracy in dose delivery of within 5% cannot be met in all situations using this beam model in the treatment planning system.PACS number(s): 87.53.–j, 87.66.–a

“…7.4f) radiation therapy planning system was used to model the EPID response. The convolution–superposition algorithm used in Pinnacle 3 is based on the work of Mackie et al (28) , (29) and Papanikolaou et al (30) For incident energy fluence, the algorithm calculates total energy released per unit mass (TERMA) in the medium, which is subsequently convolved with the energy deposition kernel to compute dose in the medium (5) . Using ImageJ, in‐plane and cross‐plane profiles and OFs were extracted from EPID images of the field geometries set out in Table 1.…”

Section: Methodsmentioning

confidence: 99%

An empirical model of electronic portal imager response implemented within a commercial treatment planning system for verification of intensity‐modulated radiation therapy fields

Khan¹,

Ostapiak²,

Szabo

2008

Quality assurance (QA) of an intensity‐modulated radiation therapy (IMRT) plan is more complex than that of a conventional plan. To improve the efficiency of QA, electronic portal imaging devices (EPIDs) can be used. The major objective of the present work was to use a commercial treatment planning system to model EPID response for the purpose of pre‐treatment IMRT dose verification.Images were acquired with an amorphous silicon flat panel portal imager (aS500: Varian Medical Systems, Palo Alto, CA) directly irradiated with a 6‐MV photon beam from a Clinac 21EX linear accelerator (Varian Medical Systems). Portal images were acquired for a variety of rectangular fields, from which profiles and relative output factors were extracted. A dedicated machine model was created using the physics tools of the Pinnacle3 (Philips Medical Systems, Madison, WI) treatment planning system to model the data. Starting with the known photon spectrum and assuming an effective depth of 7 cm, machine model parameters were adjusted to best fit measured profile and output factors. The machine parameters of a second model, which assumed a 0.8 MeV monoenergetic photon spectrum and an effective depth in water of 3 cm, were also optimized. The second EPID machine model was used to calculate planar dose maps of simple geometric IMRT fields as well as a 9‐field IMRT plan developed for clinical trials credentialing purposes.The choice of energy and depth for an EPID machine model influenced the best achievable fit of the optimized machine model to the measured data. When both energy and depth were reduced by a significant amount, a better overall fit was achieved. In either case, the secondary source size and strength could be adjusted to give reasonable agreement with measured data. The gamma evaluation method was used to compare planar dose maps calculated using the second EPID machine model with the EPID images of small IMRT fields. In each case, more than 95% of points fell within 3% of the maximum dose or 3 mm distance to agreement. These results are slightly poorer than those obtained using an ion chamber array, which confirms agreement to within 2% of the maximum dose or 2 mm distance to agreement for all points within these fields.PACS numbers: 87.55Qr, 87.56.Fc