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

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 × 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%/2mm, 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.

MRI-based polymer gel dosimetry for validating plans with multiple matrices in Gamma Knife stereotactic radiosurgery.

One of treatment planning techniques with Leksell GammaPlan (LGP) for Gamma Knife stereotactic radiosurgery (GKSRS) uses multiple matrices with multiple dose prescriptions. Computational complexity increases when shots are placed in multiple matrices with different grid sizes. Hence, the experimental validation of LGP calculated dose distributions is needed for those cases. For the current study, we used BANG3 polymer gel contained in a head-sized glass bottle to simulate the entire treatment process of GKSRS. A treatment plan with three 18 mm shots and one 8 mm shot in separate matrices was created with LGP. The prescribed maximum dose was 8 Gy to three shots and 16 Gy to one of the 18 mm shots. The 3D dose distribution recorded in the gel dosimeter was read using a Siemens 3T MRI scanner. The scanning parameters of a CPMG pulse sequence with 32 equidistant echoes were as follows: TR = 7 s, echo step = 13.6 ms, field-of-view = 256 mm × 256 mm, and pixel size = 1 mm × 1 mm. Interleaved acquisition mode was used to obtain 15 to 45 2-mm-thick slices. Using a calibration relationship between absorbed dose and the spin-spin relaxation rate (R2), we converted R2 images to dose images. MATLAB-based in-house programs were used for R2 estimation and dose comparison. Gamma-index analysis for the 3D data showed gamma values less than unity for 86% of the voxels. Through this study we accomplished the first application of polymer gel dosimetry for a true comparison between measured 3D dose distributions and LGP calculations for plans using multiple matrices for multiple targets.

Quantification of narrow band UVB radiation doses in phototherapy using diacetylene based film dosimeters.

Narrow band ultraviolet B (NB UVB) radiation doses are administered during phototherapy for various dermatological ailments. Precise quantification of these doses is vital because the absorbed irradiation can cause adverse photochemical reactions which can lead to potential phototherapeutic side effects. The paper presents development of diacetylene based dosimeter for the determination of therapeutic NB UVB doses during phototherapy. The amide terminated diacetylene analogues have been synthesized by tailoring them with different functional groups. The synthesized diacetylene monomers have been introduced in a polyvinyl alcohol binder solution to obtain a film dosimeter. The influence of different headgroups on the colorimetric response to UV radiation has been studied. Among all the synthesized diacetylene analogues, the naphthylamine substituted diacetylene exhibited excellent color transition from white to blue color at 100 mJ cm-2 NB UVB radiation dose. The developed amide films can be easily pasted on multiple sites of the patient's skin to monitor doses during phototherapy simultaneously at different anatomical regions. The digital image processing of the scanned images of the irradiated films facilitates rapid dose measurement which enables facile implementation of the developed film dosimeters and promising application in routine clinical dosimetry.

Three-dimensional dosimetry of TomoTherapy by MRI-based polymer gel technique.

Verification of the dose calculation model and the software used for treatment planning is an important step for accurate radiation delivery in radiation therapy. Using BANG3 polymer gel dosimeter with a 3 Tesla magnetic resonance imaging (MRI) scanner, we examined the accuracy of TomoTherapy treatment planning and radiation delivery. We evaluated one prostate treatment case and found the calculated three-dimensional (3D) dose distributions agree with the measured 3D dose distributions with an exception in the regions where the dose was much smaller (25% or less) than the maximum dose (2.5 Gy). The analysis using the gamma-index (3% dose difference and 3 mm distance-to-agreement) for a volume of 12 cm × 11 cm × 9 cm containing the planning target volume showed that the gamma values were smaller than unity for 53% of the voxels. Our measurement protocol and analysis tools can be easily applied to the evaluation of other newer complex radiation delivery techniques, such as intensity-modulated arc therapy, with a reasonably low financial investment.

Three-dimensional radiation dosimetry using polymer gel and solid radiochromic polymer: From basics to clinical applications.

Accurate dose measurement tools are needed to evaluate the radiation dose delivered to patients by using modern and sophisticated radiation therapy techniques. However, the adequate tools which enable us to directly measure the dose distributions in three-dimensional (3D) space are not commonly available. One such 3D dose measurement device is the polymer-based dosimeter, which changes the material property in response to radiation. These are available in the gel form as polymer gel dosimeter (PGD) and ferrous gel dosimeter (FGD) and in the solid form as solid plastic dosimeter (SPD). Those are made of a continuous uniform medium which polymerizes upon irradiation. Hence, the intrinsic spatial resolution of those dosimeters is very high, and it is only limited by the method by which one converts the dose information recorded by the medium to the absorbed dose. The current standard methods of the dose quantification are magnetic resonance imaging, optical computed tomography, and X-ray computed tomography. In particular, magnetic resonance imaging is well established as a method for obtaining clinically relevant dosimetric data by PGD and FGD. Despite the likely possibility of doing 3D dosimetry by PGD, FGD or SPD, the tools are still lacking wider usages for clinical applications. In this review article, we summarize the current status of PGD, FGD, and SPD and discuss the issue faced by these for wider acceptance in radiation oncology clinic and propose some directions for future development.

Polymer gel dosimetry for measuring the dose near thin high-Z materials irradiated with high energy photon beams.

PURPOSE: To investigate the feasibility of three-dimensional (3D) dose measurements near thin high-Z materials placed in a water-like medium by using a polymer gel dosimeter (PGD) when the medium was irradiated with high energy photon beams. METHODS: PGD is potentially a useful tool for this application because it can record the dose around a small object made of a high-Z material in a continuous 3D medium. In this study, the authors manufactured a methacrylic acid-based normoxic PGD, nMAG. Two 0.5 mm thick lead foils (1 × 1 cm) were placed in foil supports with 0.7 cm separation in a 1000 ml polystyrene container filled with nMAG. The authors used two foil configurations, i.e., orthogonal and parallel. In the orthogonal configuration, two foils were placed in the direction orthogonal to the beam axis. The parallel configuration had two foils arranged in parallel to the beam axis. The phantom was irradiated with an 18 MV photon beam of 5 × 5 cm field size. It was imaged with a three-Tesla (3 T) magnetic resonance imaging (MRI) scanned using the Car-Purcell-Meiboom-Gill pulse sequence. The spin-spin relaxation time (R2) to-dose calibration data were obtained by using small vials filled with nMAG and exposing to known doses. The DOSXYZnrc Monte Carlo (MC) code was used to get the expected dose distributions. More than 35 × 106 of histories were simulated so that the average error was less than 1%. An in-house matlab-based software was used to obtain the dose distributions from the measured R2 data as well as to compare the measurements and the MC predictions. The dose change due to the presence of the foils was studied by comparing the dose distributions with and without foils (or the reference). RESULTS: For the orthogonal configuration, the measured dose along the beam axis showed an increase in the upstream side of the first foil, between the foils, and on the downstream side of the second foil. The range of increased dose area was 1.1 cm in the upstream of the first foil. However, in the downstream of the second foil, it was 0.2 cm, beyond which the dose fell below the reference dose by 10%. The dose profile between the foils showed a well-like shape with the minimum dose still larger than the reference dose by 1.8%. The minimum dose point was closer to the first foil than to the second foil. For the parallel configuration, the dose between foils was the largest at the center. The increased dose area opposite to the gap between foils extended outward to 1 cm. The spatial dose distributions of PGD and MC showed the same geometrical patterns except for the points inside the foils for both orthogonal and parallel foil arrangements. CONCLUSIONS: The authors demonstrated that the nMAG PGD with MRI could be used to measure the 3D dosimetric structures at the mm-scale in the vicinity of the foil. The current study provided more accurate 3D spatial dose distribution than the previous studies. Furthermore, the measurements were validated by the MC simulation.

SU-E-T-398: Verification of Gamma Knife EXtend System Based Fractionated Treatment Planning Using EBT2 Film.

PURPOSE: To present EBT2 film verification of treatment planning with the eXtend System, a relocatable frame system for multiple-fraction or serial multiple-session radiosurgery. METHODS: A human head shaped phantom simulated the verification process for fractionated Gamma Knife (GK) treatment. Phantom preparation for eXtend Frame based treatment planning involved creating a dental impression, fitting the phantom to the frame system, acquiring a stereotactic computed tomography (CT) scan. A CT scan (Siemens, Emotion6) of the phantom was obtained with following parameters: Tube Voltage - 110 kV, Tube Current - 280 mA, pixel size - 0.5 mm × 0.5 mm and 1 mm slice thickness. A treatment plan with two 8 mm collimator shots and three sector blocking in each shot was made. Dose prescription of 4.0 Gy at 100% was delivered for the first fraction out of the two fractions planned. Gafchromic EBT2 film (ISP Wayne, NJ) was used as 2D verification dosimeter in this process. Films were cut and placed inside the film insert of the phantom for treatment dose delivery. Meanwhile a set of films from the same batch were exposed from 0 Gy to 12 Gy doses for calibration purpose. EPSON (Expression 10000XL) scanner was used for scanning the exposed films in transparency mode. Scanned films were analyzed with in-house made Matlab codes. RESULTS: Gamma index analysis of film measurement in comparison with TPS calculated dose resulted in high pass rates >90% for different tolerance criteria of 2%/2mm, 1%/1mm, and 0.5%/0.5mm. The isodose overlay and linear dose profiles of film measured and computed dose distribution on sagittal and coronal plane was in close agreement. CONCLUSIONS: Through this study we propose a treatment verification QA method for eXtend frame based fractionated Gamma Knife radiosurgery using EBT2 film. Acknowledgement: Authors acknowledge the help of Andre Micke, ISP for sharing his expertise on EBT2 film.

SU-E-T-268: Evaluation of Photoneutron Contamination in Elekta Synergy-S High-Energy Linear Accelerator and Indigenous Novel Solution: The AIIMS Experience.

PURPOSE: The photoneutron contamination problem was encountered due to laminated barrier wall and short maze. The purpose of this study was to report our experience in evaluating the photoneutron contamination during radiation safety survey and solution. METHODS: The photoneutron contamination measurement was carried out in Elekta Synergy-S high-energylinear accelerator for 15MV beam. A NE Neutron survey meter and for photon, Victoreen and RADOS survey meters were used. The laminated barrier wall composed of 37cm steel with 30cm concrete both side and short maze length of 5 meter. During safety survey, higher photoneutron levels for 15MV X-rays at treatment room door found. The effect of photoneutron contamination as function of neutron shielding materials of wood, polyethylene and boron and thickness, distance, locations and directions to the control console at distance upto 7 meter were investigated for 4 gantry angles at locations of treatment room entry doors namely door1(A), door2(B), console(C), conduit (D) and above-ceiling(G) for 15MV. RESULTS: The initial safety survey showed that neutron level of 47mR/h and photon leakage of 3.2mR/hr at the treatment entry room door1. The neutron values could bring down to the level of acceptance at the treatment entry door2, but the photon values are not acceptable. Therefore, 30cm concrete wall block was made at the location of door2 and another bend was taken. Finally, treatment entrance room door was made using 3cm polyethylene neutron shielding materials in order to achieve the both neutron contamination and photon leakage within the acceptable levels. CONCLUSIONS: The neutron sliding-door is operated manually in finger-push by technologist for day-to-day usage. This simple solution is cost effective and increases the patient throughput. This study underlines that one needs to take appropriate safety measures prior to facility design whenever the space constraints situations arises for high energy linear accelerator.

Interstitial brachytherapy guided intensity modulated radiation therapy (IBGIMRT) in cervical cancer: a dosimetric study.

PURPOSE: Interstitial brachytherapy (IBT) is used as an alternative to intracavitary radiotherapy in the management of cervical carcinoma. We have devised a new technique called interstitial brachytherapy guided intensity modulated radiotherapy (IBGIMRT) which can potentially reduce doses to organs at risk (OaRs). It utilizes IMRT planning on the target volume (TV) defined by implantation of IBT needles. This study compares the dosimetry of IBT and IBGIMRT. MATERIAL AND METHODS: CT scan images of 18 patients with cervical cancer, who have been already treated by HDR-BT, were used to generate two rival plans, IBT and IBGIMRT, for a prescription dose of 10 Gy. Following dosimetric factors were used for comparison: volume receiving 95% of prescription dose (V95), conformity index (COIN) and external volume index (EI) for target and for OaR, dose received by volume of 1 cm3 (D1cc), 2 cm3 (D2cc), 5 cm3 (D5cc) and also volume receiving 50% and 75% of prescription dose (V50 and V75). RESULTS: The two plans resulted in COIN difference of 49.8% (p < 0.0001) and EI difference of 36.4% (p < 0.0028) in favor of IBGIMRT. Mean D1cc, D2cc and D5cc values for bladder were 8.3 Gy, 7.6 Gy and 6.4 Gy; and 7.8 Gy, 7.3 Gy and 5.8 Gy with IBT and IBGIMRT, respectively (p > 0.05). Similar figures for rectum with IBT vs. IBGIMRT were 11.2 Gy vs. 7.02 Gy, 10.5 Gy vs. 6.4 Gy and 9.1 Gy vs. 4.8 Gy respectively (p < 0.01). CONCLUSIONS: Our novel technique, IBGIMRT, has shown its dosimetric superiority and therefore needs to be studied in clinical set up.

Impact of different breathing conditions on the dose to surrounding normal structures in tangential field breast radiotherapy.

Cardiac toxicity is an important concern in tangential field breast radiotherapy. In this study, the impact of three different breathing conditions on the dose to surrounding normal structures such as heart, ipsilateral lung, liver and contralateral breast has been assessed. Thirteen patients with early breast cancer who underwent conservative surgery (nine left-sided and four right-sided breast cancer patients) were selected in this study. Spiral CT scans were performed for all the three breathing conditions, viz., deep inspiration breath-hold (DIBH), normal breathing phase (NB) and deep expiration breath-hold (DEBH). Conventional tangential fields were placed on the 3D-CT dataset, and the parameters such as V30 (volume covered by dose >30 Gy) for heart, V20 (volume covered by dose >20 Gy) for ipsilateral lung and V(50) (volume receiving >50% of the prescription dose) for heart and liver were studied. The average reduction in cardiac dose due to DIBH was 64% (range: 26.5-100%) and 74% (range: 37-100%) as compared to NB and DEBH respectively. For right breast cancer, DIBH resulted in excellent liver sparing. Our results indicate that in patients with breast cancer, delivering radiation in deep inspiration breath-hold condition can considerably reduce the dose to the surrounding normal structures, particularly heart and liver.