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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.

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.

Purpose: Aim of the study is to evaluate mechanical and radiological accuracy of multi-fraction regimen and validate Gamma knife based fractionation using newly developed patient simulating multipurpose phantom. Methods: A patient simulating phantom was designed to verify fractionated treatments with extend system (ES) of Gamma Knife however it could be used to validate other radiotherapy procedures as well. The phantom has options to insert various density material plugs and mini CT/MR distortion phantoms to analyze the quality of stereotactic imaging. An additional thorax part designed to predict surface doses at various organ sites. The phantom was positioned using vacuum head cushion and patient control unit for imaging and treatment. The repositioning check tool (RCT) was used to predict phantom positioning under ES assembly. The phantom with special inserts for film in axial, coronal and sagittal plane were scanned with X-Ray CT and the acquired images were transferred to treatment planning system (LGP 10.1). The focal precession test was performed with 4mm collimator and an experimental plan of four 16mm collimator shots was prepared for treatment verification of multi-fraction regimen. The prescription dose of 5Gy per fraction was delivered in four fractions. Each fraction was analyzed using EBT3 films scanned with EPSON 10000XL Scanner. Results: The measurement of 38 RCT points showed an overall positional accuracy of 0.28mm. The mean deviation of 0.28% and 0.31 % were calculated as CT and MR image distortion respectively. The radiological focus accuracy test showed its deviation from mechanical center point of 0.22mm. The profile measurement showed close agreement between TPS planned and film measured dose. At tolerance criteria of 1%/1mm gamma index analysis showed a pass rate of > 95%. Conclusion: Our results show that the newly developed multipurpose patient simulating phantom is highly suitable for the verification of fractionated stereotactic radiosurgery using ES of Gamma knife. The study is a part of intramural research project of Research Section, All India Institute of Medical Sciences New Delhi India (A 247).

Purpose: To evaluate forward and inverse planning methods for acoustic neuroma cases treated in Gamma Knife Perfexion. Methods: Five patients with acoustic neuroma tumour abutting brainstem were planned twice in LGP TPS (Version 10.1) using TMR10 algorithm. First plan was entirely based on forward planning (FP) in which each shot was chosen manually. Second plan was generated using inverse planning (IP) for which planning parameters like coverage, selectivity, gradient index (GI) and beam-on time threshold were set. Number of shots in IP was automatically selected by objective function using iterative process. In both planning methods MRI MPRAGE sequence images were used for tumour localization and planning. A planning dose of 12Gy at 50% isodose level was chosen. Results and Discussion: Number of shots used in FP was greater than IP and beam-on time in FP was in average 1.4 times more than IP. One advantage of FP was that the brainstem volume subjected to 6Gy dose (25% isodose) was less in FP than IP. Our results showed use of more number of shots as in FP results in GI less than or equal to 2.55 which is close to its lower limit. Dose homogeneity index (DHI) analysis of FP and IP showed average values of 0.59 and 0.67 respectively. General trend in GK for planning in acoustic neuroma cases is to use small collimator shots to avoid dose to adjacent critical structures. More number of shots and prolonged treatment time causes inconvenience to the patients. Similarly overuse of automatic shot shaping as in IP results in increased scatter dose. A compromise is required in shot selection for these cases. Conclusion: IP method could be used in acoustic neuroma cases to decrease treatment time provided the source sector openings near brainstem are shielded or adjusted appropriately to reduce brainstem dose.

Purpose: To evaluate a calibration method using the depth-dose data of an electron beam for MRI-based polymer gel dosimetry. Methods: MAGAT was manufactured in-house to fill two 400mL-cylindrical phantoms and nine 22mL-glass vials. Phantom-A was irradiated along the cylinder axis with a 9MeV electron beam of 6 cm x 6 cm field size (FS). Phantom-B was irradiated with a 6MV photon beam of 3 cm x 3 cm FS by a 360-degree arc technique. Eight vials were irradiated in a water-bath to various doses with a 20 cm x 20 cm FS 6MV photon beam. All irradiated phantoms and one un-irradiated vial were scanned with a 3T MRI scanner to obtain the spin-spin relaxation rate (R2) distributions. By comparing the measured R2-to-depth data with the known depth-dose data for Phantom-A, R2-to-dose calibration data were obtained (e-beam method). Another calibration data were obtained from the 9 vials data (9-vial method). We tested two regression equations, i.e., third-order polynomial and tangent functions, and two dose normalization methods, i.e., one-point and two-point methods. Then, these two calibration methods were used to obtain the 3D dose distribution of Phantom-B and evaluated by comparing the measured data with the dose distribution from a treatment planning system. The comparison was made with gamma passing rate (2%/2mm criteria). Results: We did not observe a clear advantage of the e-beam method over the 9-vial method for the 3D dose comparison with the test case. Nevertheless, we found that the e-beam method required a smaller dose scaling for the dose comparison. Furthermore, the tangent function showed better data fitting than the polynomial function with smaller uncertainty of the estimated coefficients. Conclusions: Considering the overall superior performance, we recommend the e-beam method with the tangent function as the regression equation and one-point dose normalization for the MRI-based polymer gel dosimetry.

Purpose: Gamma Knife perfexion (GKPFX) with extend system is being used for multi-fraction or fractionated stereotactic radiosurgery (fGKSRS). Patient surveillance Unit (PSU) is used to attain a non invasive immobilization for fractionation. Calibrated digital probe is used to measure encoded mechanical positions on repositioning check tool (RCT). The objective of this study is to check consistency and statistical position variation during multi-fraction stereotactic radiosurgery using GKPFX extend system. Methods: 31 fractions through 7 patients being delivered to check the positional variation during patient setup. Variation is calculated after sampling the collected data at same point measurement for all the patients. Average value of three stage measurements or the reference values are compiled with the sample data using SPSS (statistical software). Measurement of encoded points has been taken and various statistical parameters are observed. Reproducibility is checked, comparing the reference to measured values through encoded points before treatment. Intra-fraction variation is measured on compiling pre and post treatment RCT measurements while inter-fractionation variation compiled along with fractions before treatment. Results: Pre-treatment radial setup difference auto calculated by the system varies from 0.5mm to 0.8 mm with mean variation of 0.7 mm. Positional variation was observed for post-treatment and was found that it varies from 0.9 mm - 1.0 mm. Paired sample test results the paired difference mean from the reference (for pre and post measurements) was 0.26 mm (0.1775 mm- 0.3475 mm) with only 25% of error significance. The distribution of the random component of observer uncertainty was 0.06mm. Conclusion: Post treatment positional variation is 20%-30% of pre treatment measurements, i.e. about treatment measurements, i.e. about 0.2 mm. Explaining complete procedure to patient, limiting head rotation/lift and preventing other movements, may maximize positional accuracy during fractionations. Contending positional accuracy, supports the extend system for fractionated stereotactic radiosurgery among other multisession radiosurgery techniques.

Purpose: To perform dosimetric evaluation of convolution algorithm in Gamma Knife (Perfexion Model) using solid acrylic anthropomorphic phantom. Methods: An in-house developed acrylic phantom with ion chamber insert was used for this purpose. The middle insert was designed to fit ion chamber from top(head) as well as from bottom(neck) of the phantom, henceforth measurement done at two different positions simultaneously. Leksell frame fixed to phantom simulated patient treatment. Prior to dosimetric study, hounsfield units and electron density of acrylic material were incorporated into the calibration curve in the TPS for convolution algorithm calculation. A CT scan of phantom with ion chamber (PTW Freiberg, 0.125cc) was obtained with following scanning parameters: Tube voltage-110kV, Slice thickness-1mm and FOV-240mm. Three separate single shot plans were generated in LGP TPS (Version 10.1.) with collimators 16mm, 8mm and 4mm respectively for both ion chamber positions. Both TMR10 and Convolution algorithm based planning (CABP) were used for dose calculation. A dose of 6Gy at 100% isodose was prescribed at centre of ion chamber visible in the CT scan. The phantom with ion chamber was positioned in the treatment couch for dose delivery. Results: The ion chamber measured dose was 5.98Gy for 16mm collimator shot plan with less than 1% deviation for convolution algorithm whereas with TMR10 measured dose was 5.6Gy. For 8mm and 4mm collimator plan merely a dose of 3.86Gy and 2.18Gy respectively were delivered at TPS calculated time for CABP. Conclusion: CABP is expected to perform accurate prediction of time for dose delivery for all collimators, but significant variation in measured dose was observed for 8mm and 4mm collimator which may be due collimator size effect. Effect of metal artifacts caused by pins and frame on the CT scan also may have role in misinterpreting CABP. The study carried out requires further investigation.

Purpose: To validate the three‐dimensional (3D) dose distributions calculated by Leksell Gamma Plan (LGP) for Gamma Knife stereotactic radiosurgery. (GKSRS) cases with multiple matrices using MRI‐based polymergeldosimetry.Method and Materials: BANG3 polymergel contained in a head size glass bottle simulated the entire treatment process of GKSRS. All phantoms were fixed in the Leksell stereotactic frame. For the first study one phantom was used for calibration and the second was irradiated with five 18‐mm shots in five dose matrices. For the second study the calibration was performed with 8‐mm shots and a treatment plan was generated with three 18‐mm shots and one 8‐mm shot. The 3D‐dose distribution recorded in the gel dosimeter was read using a 3T MRI scanner (Siemens Magnetom Trio). Scanning parameters were CPMG pulse sequence with 32 equidistant echoes, TR = 7 s, echo step = 13.6 ms, pixel size = 1mm × 1mm. Interleaved acquisition mode was used to obtain 15 to 45 2mm‐thick slices. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), we converted R2 images to dose images. The dose comparison was accomplished using an in‐house MATLAB‐based program. Results:Calibrations indicate the dose response of the current polymergel was non‐linear for doses higher than 10 Gy. The artifacts mainly caused by the metallic pins used to immobilize the phantom distorted the measured dose distributions. Away from the pin positions, however, we could compare the LGP dose distributions with measured dose distributions. Preliminary analyses indicated some spatial position discrepancies in addition to the dose differences. Conclusion: Through this study we could perform a true comparison between 3D dose distributions measured with a polymergel dosimeter and LGP calculations for plans using multiple matrices/targets.

Purpose:Doses to rectum, bladder and Point‐A in patients who received intracavitary HDR brachytherapy treatment were calculated by both orthogonal radiographsreconstruction method and by image based reconstruction method and were compared. Method and Materials: Twenty‐five patients of carcinoma cervix who received intracavitary brachytherapy treatment by micro‐Selectron HDR unit were included in this study. For these patients, in Plato treatment planning system, reconstruction of sources, applicator, organs at risk and Point‐A were done using conventional orthogonal radiographs and by using 1 mm thick CT scanimages. Bladder was identified on radiographs through a Foley's catheter inserted into it filled with 7 cc of radio‐opaque fluid and the rectum through a rectal marker. Doses were calculated at 5 points each on bladder and rectum, for the treatment dose of 7 Gy to Point‐A (right & left). On CTimages, these organs were delineated with the radiologists help and from dose‐volume‐histograms,doses to 1, 2 and 5 cc of these organs were calculated besides dose to Point‐A (right & left). Results: The critical organdoses estimated by the CT method were consistently higher when compared to those by the radiographs method. When compared to the radiographicdose maximum for bladder, doses to 1, 2 and 5 cc of bladder were higher by 89%, 69.7% and 41.6% on an average respectively. For rectum, the corresponding values were 36.6%, 21.7% and 0.4%. However dose to Point‐A did not differ by more than 3% between the two methods. Conclusion: The conventional orthogonal radiography based reconstruction method consistently underestimates the doses to critical organs while the doses to reference points are comparable in these two methods. Long‐term follow‐up in such patients can identify the threshold volumes for these critical organs beyond which complication rates are bound to rise.

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.

Purpose: To evaluate the penumbral characteristics as function of collimation systems, conventional detector, depth, energy and MLC travel orientations using diode detectors.Methods and Materials: The experiment was carried out in a Clinac 2300C/D linear accelerator for 6 and 15 MV. For penumbral width determination, cross‐beam profiles were measured using Blue phantom radiation field analyzer, 0.13cm3 ionchamber and diode detectors, CU‐500E dual‐channel electrometer and OmniPro beam‐data acquisition system. The penumbra was measured for both Jaw and MLC‐defined field sizes of 4×4 to 30×30cm2 depths at dmax, 5, 10, 15 and 20cm and also for cross‐plane and in‐plane MLC travel orientations at 100cm SSD. The penumbra width was calculated as the distance between 80%–20% intensity in the profiles. Results: The MLC penumbra was in the range of 3.6mm to 4mm for 6MV and 5.4mm to 7.1mm for 15MV at dmax depth for field sizes of 4×4 to 30×30cm2 as measured by diode detector. The mean differences between Jaw and MLC‐defined field's penumbra were 0.8mm ± 0.2, and 1mm ± 0.2 for 6MV and 1.2mm ±0.2 and 1.8mm ±0.5 for 15MV at dmax and 10cm depths. The maximum difference of 1.9mm ±0.3 for 6MV and 2.2mm ±0.2 for 15MV between diode and 0.13cm3detectors were found. Use of diode detectors, maximum differences of 3.4mm ±3 at 20cm depths between energies and the differences of 3.3mm ± 2.2 for 6 MV and 1.2mm ± 0.3 for 15MV between cross‐plane and in‐plane MLC penumbra were found. Conclusions: The rounded‐end MLC penumbra was larger as compared to Jaw penumbra. The penumbra measured by diode was increasing with energy upto a certain depths and field sizes and then it increases more with low‐energy at larger depths and field sizes as compared to high‐energy. The adequate accounting of penumbra is important in intensity modulated beam dose calculation.

Purpose: To evaluate the volume effect of detector's in multiple points dose in small and large field IMRT verification and to compare the planar dose distribution between fixed‐fields coronal‐plane and multifield axial‐plane verification methods. Method and Materials: Patient‐specific dosimetric verification was conducted for 20 patients of nasopharynx's small‐field, SIB‐IMRT large‐field and prostate tumors. Eclipse‐Helios system, Clinac‐2300C/D linear accelerator, Med‐Tec IMRT phantom and ionchambers of 0.6, 0.13 and 0.015cm3 and MOSFET detectors were used. For each patient, 15 IMRT plans were generated. The multiple points doses were measured at multifield and fixed‐fields on‐axis, 4‐offsets and inhomogeneous points. The agreements between calculated and measured doses were found for small and large field IMRT verification. For relative dosimetry, EDR2 films, Virtual Water phantom for fixed‐fields coronal‐plane and IMRT Body phantom for multifield axial‐plane verifications RITsystem and Vidar scanner were used. The agreements between calculated and measured dose distributions between two methods were compared using various gamma criteria. Results: The agreements between measured and calculated doses using 0.6 and 0.13cm3 chambers at multifield and fixed‐fields on‐axis points were within 4.7% ±2.3. The dose differences of 6% ±1.7, 19.7% ±5.5, and 12.1% ±3.2 at offset1 to 3 points were found. The 0.015cm3 chamber and MOSFET detectors were showed the dose differences of upto 8.4% in large field verification. The mean differences in percentage of pixels passing the gamma criteria of 3mm/5% were 98.2% ±0.9 and 90% ±5.4 for fixed‐fields coronal‐plane and multifield axial‐plane verifications. Conclusions: This study is useful in evaluating the detector's response at high and low dose, outside the fields and inhomogeneous points dose verification. In offset points, 0.6cm3 is more accurate than 0.13cm3 chamber in both small and large field verification. The fixed‐fields coronal‐plane verification is more accurate, however, the multifield axial‐plane is clinically realistic method of verification and to be adopted.