Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon‐diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison

The purpose of this study was to validate the newly designed acrylic phantom for routine dosimetric purpose in radiotherapy. The phantom can be used to evaluate and compare the calculated dose and measured dose using film and gel dosimetric methods. In this study, a doughnut‐shaped planning target volume (8.54 cm3) and inner organ at risk (0.353 cm3) were delineated for an IMRT test plan using the X‐ray CT image of the phantom. The phantom consists of acrylic slabs which are integrated to form a human head with a hole in the middle where several dosimetric inserts can be positioned for measurement. An inverse planning with nine coplanar intensity‐modulated fields was created using Pinnacle TPS. For the film analysis, EBT2 film, flatbed scanner, in‐house developed MATLAB codes and ImageJ software were used. The 3D dose distribution recorded in the MAGAT gel dosimeter was read using a 1.5 T MRI scanner. Scanning parameters were CPMG pulse sequence with 8 equidistant echoes, TR=5600, echo step=22 ms, pixel size=0.5 times 0.5, slice thickness=2 mm. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), R2 images were converted to dose images. The dose comparison was accomplished using in‐house MATLAB‐based graphical user interface named “IMRT3DCMP”. For gel measurement dose grid from the TPS was extracted and compared with the measured dose grid of the gel. Gamma index analysis of film measurement for the tolerance criteria of 2%/2 mm, 1%/1 mm showed more than 90% voxels pass rate. Gamma index analysis of 3D gel measurement data showed more than 90% voxels pass rate for different tolerance criteria of 2%/2 mm and 1%/1 mm. Overall both 2D and 3D measurement were in close agreement with the Pinnacle TPS calculated dose. The phantom designed is cost‐effective and the results are promising, but further investigation is required to validate the phantom with other 3D conformal techniques for dosimetric purpose.PACS numbers: 87.53.Kn, 87.55.km, 87.56.N‐

Section: Introductionmentioning

confidence: 99%

Indigenously developed multipurpose acrylic head phantom for verification of IMRT using film and gel dosimetry

Natanasabapathi¹,

Subbiah²,

Rath

et al. 2013

“…For example, a standard 0.6 cc Farmer chamber is commonly used in the radiation beam calibration protocols (10) . A microsized PinPoint chamber can be used to determine relative and absolute dosimetry of small photon fields in the SRS and SBRT procedures (12) . A plane‐parallel chamber, on the other hand, has flat surfaces with thin foils or membranes, which can cause minimal perturbation for incident photons and electrons, and can be also used in radiation beam calibration (10) .…”

Section: Introductionmentioning

confidence: 99%

“…By collecting all the charges created in the gas while an electric field is applied, these ion chambers can quantify the radiation dose. However, there is a recombination process when positive ions collide with negative ions (12) to induce charge loss and collection inefficiency. According to Attix, (13) there are two types of recombination: initial and general recombination.…”

Section: Introductionmentioning

confidence: 99%

Ion recombination corrections of ionization chambers in flattening filter‐free photon radiation

Wang¹,

Easterling²,

Ting³

2012

The flattening filter free (FFF) X‐rays can provide much higher dose rate at the treatment target compared to the conventional flattened X‐rays. However, the substantial increase of dose rate for FFF beams may affect the ion recombination correction factor, which is required for accurate measurements using ionization chambers in clinical dosimetry. The purpose of this work is to investigate the ion recombination of three types of commonly used ion chambers (Farmer, PinPoint and plane‐parallel) in the FFF photon radiation. Both 6 MV and 10 MV flattened and FFF beams were fully commissioned on a Varian TrueBeam linear accelerator. The ion recombination correction factor, normalPion, was determined using the two‐voltage technique for a 0.6 cc Farmer chamber, a 0.015 cc PinPoint chamber, and a 0.02 cc parallel‐plate chamber at different source‐to‐axis distances (SAD) in a solid water phantom or water tank phantom at a depth of 10 cm in a 10×10 cm2 field. Good repeatability of measurements was demonstrated. Less than 1% difference in Pion between the flattened and FFF photons for all three ion chambers was observed. At a SAD of 100 cm and a depth of 10 cm for a 10×10 cm2 field, normalPion for the Farmer chamber was 1.004 and 1.008 for the 6 MV flattened and FFF beams, respectively. At the same setup using the Farmer chamber, normalPion was 1.002 and 1.015 for the 10 MV flattened and FFF beams, respectively. All normalPion results for the Farmer, PinPoint, or parallel plate chamber in the 6 MV and 10 MV flattened and FFF beams were within 2% from the unity (1 ≤ normalPion < 1.02). The normalPion ratio of the FFF to flattened beams was 0.99~1.01 for both 6 MV and 10 MV photons. The ion recombination effect of the Farmer, PinPoint, and plane‐parallel chamber in the FFF beams is not substantially different from that in the conventional flattened beams.PACS number: 87.56.bd

“…In the past, MC techniques were considered to consume great amounts of computing resources, too much to be of practical use for patient‐specific QA (18) . However in recent times, MC simulation of radiation transport in patient geometry with the high‐performance computing (parallel processing), or a multicore processor workstation, can bring the results for the GK plan verification within one hour.…”

Section: Introductionmentioning

confidence: 99%

MAGAT gel and EBT2 film‐based dosimetry for evaluating source plugging‐based treatment plan in Gamma Knife stereotactic radiosurgery

Natanasabapathi

Subbiah

Kale

et al. 2012

This work illustrates a procedure to assess the overall accuracy associated with Gamma Knife treatment planning using plugging. The main role of source plugging or blocking is to create dose falloff in the junction between a target and a critical structure. We report the use of MAGAT gel dosimeter for verification of an experimental treatment plan based on plugging. The polymer gel contained in a head‐sized glass container simulated all major aspects of the treatment process of Gamma Knife radiosurgery. The 3D dose distribution recorded in the gel dosimeter was read using a 1.5T MRI scanner. Scanning protocol was: CPMG pulse sequence with 8 equidistant echoes, TR=7 s, echo step=14 ms, pixel size=0.5 mm x 0.5 mm, and slice thickness of 2 mm. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), we converted R2 images to dose images. Volumetric dose comparison between treatment planning system (TPS) and gel measurement was accomplished using an in‐house MATLAB‐based program. The isodose overlay of the measured and computed dose distribution on axial planes was in close agreement. Gamma index analysis of 3D data showed more than 94% voxel pass rate for different tolerance criteria of 3%/2 mm, 3%/1 mm and 2%/2 mm. Film dosimetry with GAFCHROMIC EBT 2 film was also performed to compare the results with the calculated TPS dose. Gamma index analysis of film measurement for the same tolerance criteria used for gel measurement evaluation showed more than 95% voxel pass rate. Verification of gamma plan calculated dose on account of shield is not part of acceptance testing of Leksell Gamma Knife (LGK). Through this study we accomplished a volumetric comparison of dose distributions measured with a polymer gel dosimeter and Leksell GammaPlan (LGP) calculations for plans using plugging. We propose gel dosimeter as a quality assurance (QA) tool for verification of plug‐based planning.PACS number: 87.53.Ly, 87.55.‐x, 87.56.N‐