Kodak XV Film Research Articles

The use of normoxic polymer gel for measuring dose distributions of 1, 4 and 30mm cones

Abstract This study demonstrates the use of normoxic polymer gel for measuring dose distributions of small fields that lack lateral electronic equilibrium. Two different types of normoxic polymer gel, MAGAT and PAGAT, are studied in a larger field (10 cm×10 cm) and 1, 4 and 30 mm cones to obtain cone factors, dose profiles and percentage depth doses. These results were then compared to KODAK XV film measurements and BEAMnrc Monte Carlo simulations. The results show that the sensitivity of PAGAT gel is 0.090±0.074 s −1 Gy −1 , which may not be suitable for small-field dosimetry with a 0.3 mm resolution scanned using a 3 T MR imager in a dose range lower than 2.5 Gy. There are good agreements between cone factors estimated using KODAK XV film and MAGAT gel. In a dose profile comparison, good dose agreement among MAGAT gel, XV film and MC simulation can be seen in the central area for a 30 mm cone. In penumbra, the distance to agreement is at most 1.2 mm (4 pixel), and less than 0.3 mm (1 pixel) for 4 and 1 mm cones. In a percentage depth dose comparison, there were good agreements between MAGAT and MC up to a depth of 8 cm. Possible factors for gel uncertainty such as MRI magnetic field inhomogeneity and temperature were also investigated.

SU-E-T-147: Film Dosimetry Verification for TSE Using An Epson Scanner.

To commission and verify an Epson scanner for film dosimetry for total skin electron beam therapy (TSEB). Use data from an IBA PPC40 parallel-plate ion chamber and Sun Nuclear QED skin diode detectors as standard; we have made comparisons to the film measurement using Kodak XV films. Hurter-Driffield (HD) curve are established for 6 MeV total skin electron beams at a source-to-surface distance (SSD) of 5 m. Also HD curves are built for 6 MeV at a 100 cm SSD. Dose profiles for a series of oblique incident large electron fields are measured using the film for approximately 80 cGy dose delivered at the peak. The film is then scanned using two scanners, an Epson expression 10000 XL and a Vidar VXR-16 Dosimetry Pro. The optimal scanning conditions (e.g., dot per pixel size, internal color correction scheme) are chosen for the Epson scanner. Matlab is then used to analyze the optical density (OD) of the scanned films. A transmission densitometer made by Tobias Associates transmission is used to analyze the films to give a classical standard. The analysis of the Epson scanner is presented in two forms: one with and one without the HD correction from the established HD curve. The error analysis gives an uncertainty of 5% without the HD correction. An improved result of approximately 3% is found when an HD correction is applied to the analysis. A simple Epson scanner satisfies the commissioning standards for TSEB when an HD curve correction is applied.

A procedure to determine the planar integral spot dose values of proton pencil beam spots

Planar integral spot dose (PISD) of proton pencil beam spots (PPBSs) is a required input parameter for beam modeling in some treatment planning systems used in proton therapy clinics. The measurement of PISD by using commercially available large area ionization chambers, like the PTW Bragg peak chamber (BPC), can have large uncertainties due to the size limitation of these chambers. This paper reports the results of our study of a novel method to determine PISD values from the measured lateral dose profiles and peak dose of the PPBS. The PISDs of 72.5, 89.6, 146.9, 181.1, and 221.8 MeV energy PPBSs were determined by area integration of their planar dose distributions at different depths in water. The lateral relative dose profiles of the PPBSs at selected depths were measured by using small volume ion chambers and were investigated for their angular anisotropies using Kodak XV films. The peak spot dose along the beam's central axis (D(0)) was determined by placing a small volume ion chamber at the center of a broad field created by the superposition of spots at different locations. This method allows eliminating positioning uncertainties and the detector size effect that could occur when measuring it in single PPBS. The PISD was then calculated by integrating the measured lateral relative dose profiles for two different upper limits of integration and then multiplying it with corresponding D(0). The first limit of integration was set to radius of the BPC, namely 4.08 cm, giving PISD(RBPC). The second limit was set to a value of the radial distance where the profile dose falls below 0.1% of the peak giving the PISD(full). The calculated values of PISD(RBPC) obtained from area integration method were compared with the BPC measured values. Long tail dose correction factors (LTDCFs) were determined from the ratio of PISD(full)∕PISD(RBPC) at different depths for PPBSs of different energies. The spot profiles were found to have angular anisotropy. This anisotropy in PPBS dose distribution could be accounted in a reasonable approximate manner by taking the average of PISD values obtained using the in-line and cross-line profiles. The PISD(RBPC) values fall within 3.5% of those measured by BPC. Due to inherent dosimetry challenges associated with PPBS dosimetry, which can lead to large experimental uncertainties, such an agreement is considered to be satisfactory for validation purposes. The PISD(full) values show differences ranging from 1 to 11% from BPC measured values, which are mainly due to the size limitation of the BPC to account for the dose in the long tail regions of the spots extending beyond its 4.08 cm radius. The dose in long tail regions occur both for high energy beams such as 221.8 MeV PPBS due to the contributions of nuclear interactions products in the medium, and for low energy PPBS because of their larger spot sizes. The calculated LTDCF values agree within 1% with those determined by the Monte Carlo (MC) simulations. The area integration method to compute the PISD from PPBS lateral dose profiles is found to be useful both to determine the correction factors for the values measured by the BPC and to validate the results from MC simulations.

Experimental measurements and Monte Carlo simulations of dose perturbation around a nonradioactive brachytherapy seed due to 6- and 18-MV photons

PurposeRadioactive seeds used in permanent prostate brachytherapy are composed of high-Z metals and may exceed 100 in a patient. If supplemental external beam treatment is administered afterward, the seeds may cause substantial dose perturbation, which is being investigated in this article. Methods and MaterialsFilm measurements using 6-MV beam were primarily carried out using Kodak XV2 film layered above and below a nonradioactive iodine-125 (125I) seed. Monte Carlo simulations were carried out using DOSXYZnrc. Other experimental comparisons looked at changing beam energy, depth, and field size, including two opposing fields’ pair. Effect of multiple seeds spatially spaced 0.5cm vertically was also studied. ResultsFor a single 125I seed, on XV film, there is a localized dose enhancement of 6.3% upstream and −10.9% downstream. With two opposing fields, a cold spot around the seed of ∼3% was noticed. Increasing beam energy and field size decreased the magnitude of this effect, whereas the effect was found to increase with the increasing Z of material. DOSXYZnrc and EBT-2 film verified maximum dose enhancement of +15% upstream and −20% downstream of the 125I seed surface. ConclusionsIn general, the dose perturbation because of the seeds was spatially limited to ∼2mm upstream and ∼5mm downstream to the incident beam. Similar to other heterogeneities, the seeds perturbation depends on incident beam energy, field size, and its Z. With multiple seeds spatially apart and multiple radiation fields routinely used in external beam radiotherapy, the cumulative effect may not result in clinically significant dose perturbation.

SU-E-T-144: Study of the Effect of the Optical Density to Dose Conversion Calibration Curves on the Measurement of Transverse Lateral Dose Profiles of Proton Beams Using Kodak XV Films

Purpose: To investigate the effect of the optical density (OD) to dose conversion calibration curves on the measurement of transverse lateral dose profiles of proton beams using Kodak XV films. Methods: A number of XV films were exposed to different doses in a 10 cm × 10 cm field of passively scattered range modulated beams of different incident energies. The films were placed at the center of spread out Bragg peak of the beam inside plastic water phantoms. The films were scanned by a Vidar Scanner using Scanditronix/ Wellhofer OmniPro‐Accept6.4 software. Different OD‐dose calibration curves were generated and compared. Transverse lateral dose profiles were obtained from an irradiated film using different OD‐dose calibration curves and were compared to evaluate the differences. The lateral dose profiles of proton pencil beam spots (PPBS) were also obtained using the XV films and one of these OD‐dose calibration curves and were compared with ion chamber measurement. Results: OD‐dose calibration curves obtained by giving same doses, but with beams of different incident energies were found to have differences in all dose regions. But, the relative lateral dose profiles (LDP) were not affected by the differences in the calibration curves. Thus, the incident energy of the range modulated beam is not an important consideration for obtaining the film OD‐dose calibration curves for relative LDP measurements. The XV film measured relative LDP of the PPBS agreed well with that from ion chamber measurement in the high dose region, but was seen to be noisier in the long tail region. Conclusions: OD‐dose calibration curves created with given doses from any range modulated passively scattered beam, irrespective of the energy of the incident beam, are found to be suitable for measuring the relative LDP of broad fields of passively scattered proton beams and PPBS with XV films.

SU‐E‐T‐467: The Trick in MLC Calibration to Correct Underdose Region in IMRT Delivery

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we figure out the cause of the low dose regions inside the treatment field which only occur in verification images, but not in the treatment plan. Methods: The treatment planning system is ADAC Pinnacle version 9.0 with SmartArc. The IMRT plan is transferred to IMPAC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two or three dimensional dose distribution, we take the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy LINAC. Results: Low dose regions are discovered in only Synergy LINAC and always in gun‐to‐target (G‐T) direction. By comparing the fluence maps between treatment plan and verification film segment by segment, we find these low dose regions result from the jaw position, i.e. the MLC directional backup jaw is too close to cause these unplanned underdoses in the treatment volume. We recheck the light‐field and the radiation field again; there is coincidence between the light‐field and the radiation field and just on the correct position. According to the statute, each radiation field edge is to be aligned with the displayed diaphragm within ±1 mm and each light‐field edge is to be aligned with the radiation field edge within ±1 mm. We recalibrate the MLC directional backup jaws and MLCs position to 0.5 mm larger than display and recheck the fluence maps, the low dose regions disappear eventually Conclusions: When doing the radiation field size calibration, one should calibrate the MLC moving directional radiation field size 0.5 mm larger than display each side on ELEKTA machine to prevent the unanticipated low dose regions inside the tumor in IMRT delivery.

Beam characteristics of megavoltage beams at low monitor unit settings

Beam characteristics of a linear accelerator are of great importance for intensity-modulated radiation therapy (IMRT) to ensure precise and accurate dose delivery to patients. In step-and-shoot IMRT, each beam is delivered through a series of small, segmented fields at low monitor unit (MU) settings. In this study, the beam characteristics of both static (ST) and segmental intensity-modulated (IM) beams were investigated at various dose rates for 6 and 18 MV at low MU settings. Dose linearity was investigated for both the ST and the IM beams. For the ST beams, standard 10 × 10 cm 2 fields were irradiated with MU values ranging from 1 to 100. For the IM beams, 10 × 10 cm 2 and 15 × 15 cm 2 fields were used as subfields. The normalized dose (ND)/MU was obtained. Beam flatness and symmetry for 2 and 10 MU was measured by in-plane (G–T) and cross-plane (R–L) profiles using Kodak XV films. The largest dose/MU discrepancies were observed for 1 MU. For the ST beams, the beam output decreased up to 4.5% for 1 MU at the high dose rates of 6 and 18 MV. Dose variations were less than 1% for doses above 5 MU. No significant variation was observed in the beam profiles of the ST and the IM groups. Beam flatness and symmetry were close to 3% and 2% for 6 and 18 MV, respectively. Our results showed that dose linearity and delivery errors were close to 1% for doses above 5 MU, which is considered acceptable for both 6- and 18-MV ST and IM therapy.

Analysis of variation in calibration curves for Kodak XV radiographic film using model‐based parameters

Film calibration is time‐consuming work when dose accuracy is essential while working in a range of photon scatter environments. This study uses the single‐target single‐hit model of film response to fit the calibration curves as a function of calibration method, processor condition, field size and depth. Kodak XV film was irradiated perpendicular to the beam axis in a solid water phantom. Standard calibration films (one dose point per film) were irradiated at 90 cm source‐to‐surface distance (SSD) for various doses (16–128 cGy), depths (0.2, 0.5, 1.5, 5, 10 cm) and field sizes (5×5,10×10 and 20×20cm2). The 8‐field calibration method (eight dose points per film) was used as a reference for each experiment, taken at 95 cm SSD and 5 cm depth. The delivered doses were measured using an Attix parallel plate chamber for improved accuracy of dose estimation in the buildup region. Three fitting methods with one to three dose points per calibration curve were investigated for the field sizes of 5×5,10×10 and 20×20cm2. The inter‐day variation of model parameters (background, saturation and slope) were 1.8%, 5.7%, and 7.7% (1 σ) using the 8‐field method. The saturation parameter ratio of standard to 8‐field curves was 1.083±0.005. The slope parameter ratio of standard to 8‐field curves ranged from 0.99 to 1.05, depending on field size and depth. The slope parameter ratio decreases with increasing depth below 0.5 cm for the three field sizes. It increases with increasing depths above 0.5 cm. A calibration curve with one to three dose points fitted with the model is possible with 2% accuracy in film dosimetry for various irradiation conditions. The proposed fitting methods may reduce workload while providing energy dependence correction in radiographic film dosimetry. This study is limited to radiographic XV film with a Lumisys scanner.PACS number: 87.55.Qr

SU‐GG‐T‐198: A Pitfall in IMRT Verification: The Jaw Position

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we try to figure out the cause of the high dose regions outside the treatment field which only shown in measured dose, but not in the treatment plan. Method and Materials: Our treatment planning system is ADAC Pinnacle version 8.0m. The IMRT plan is transferred to IMP AC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two dimensional dose distribution, we took the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy. Results: High dose regions are obviously in only one Precise LINAC and always perpendicular to MLC leakages. By comparing the fluence maps between treatment plan and verification film segment by segment, we find it occurs on the MLC 0.8 cm gap which is designed to prevent one side MLCs to collide with the opposite MLCs. The gap should be blocked by the primary jaw and MLC directional backup jaw. However, because of the machine limitation, the primary jaws are not allowed to cross the central line, the gap is simply blocked by the MLC directional backup jaw only (3cm tungsten) occasionally and slight transmission doses are generated (about 15%). If the backup jaw goes to the wrong position and does not block the MLC gap, a high dose region occurs during dose delivery, but not shown in plan. Conclusion: When the backup jaw position is not compatible with treatment plan, the unanticipated high dose region emerges not only outside of the field but also inside the field. Even if one checks the jaw position by any open field, this situation sometimes occurs. The best way is to recalibrate the jaw mechanical parameters and recheck the incorrect segment.

SU‐GG‐T‐313: A Procedure for Standardizing MLC Quality Assurance for Elekta Linacs

Purpose: As specified in TG142, MLC position accuracy needs to be tested on weekly/monthly basis, with 1mm tolerance. This study focuses on developing techniques, hardware and software tools for implementation of MLC QA tests for Elekta Linacs. Material and Methods: This process was tested with an Elekta Synergy S, Beam Modulator™, which has 40 leaf pairs of 4mm width (maximum 16cm×21cm field size). Based on the machine characteristics, two picket‐fence IMRT plans were designed: one has 5 2cm×16cm strips separated by 2cm gap; the other has the same setup with individual leafs intentionally displaced by ±1mm, ±1.2mm, etc. Both plans used 6MV x‐rays and 50MU on each strip. We overcame the limitation of Xio planning system in generating picket‐fence IMRT plan by modifying leaf positions from a DICOM RT plan file. In‐house software was executed to validate the files before imported into Record and Verify system (Mosaiq) for delivery. Radiographic images were acquired using Kodak XV films. The borders of a 16cm×21cm light field were first traced on the film. These reference lines helped reduce the orientation errors during image registration. Two sets of films were exposed with full buildup. After development, each film was digitized with 0.06mm resolution using a high‐resolution scanner. The images were then imported into Matlab. In‐house code was used to detect leafs exceeding the 1mm threshold. Results: The plans were delivered smoothly. Leaf positions in the first image were used as baselines, instead of using reference leaf positions from the same exposure. This reduced systematic errors. After image registration, leafs displacing from the baseline by 16 pixels (1mm) or more were detected. Conclusion: This efficient procedure provides a sufficiently accurate test for MLC positioning reproducibility. It is a simple and straightforward procedure that can be used for routine MLC position checks.

Dose discrepancies in the buildup region and their impact on dose calculations for IMRT fields

Dose accuracy in the buildup region for radiotherapy treatment planning suffers from challenges in both measurement and calculation. This study investigates the dosimetry in the buildup region at normal and oblique incidences for open and IMRT fields and assesses the quality of the treatment planning calculations. This study was divided into three parts. First, percent depth doses and profiles (for 5 x 5, 10 x 10, 20 x 20, and 30 x 30 cm2 field sizes at 0 degrees, 45 degrees, and 70 degrees incidences) were measured in the buildup region in Solid Water using an Attix parallel plate chamber and Kodak XV film, respectively. Second, the parameters in the empirical contamination (EC) term of the convolution/ superposition (CVSP) calculation algorithm were fitted based on open field measurements. Finally, seven segmental head-and-neck IMRT fields were measured on a flat phantom geometry and compared to calculations using gamma and dose-gradient compensation (C) indices to evaluate the impact of residual discrepancies and to assess the adequacy of the contamination term for IMRT fields. Local deviations between measurements and calculations for open fields were within 1% and 4% in the buildup region for normal and oblique incidences, respectively. The C index with 5%/1 mm criteria for IMRT fields ranged from 89% to 99% and from 96% to 98% at 2 mm and 10 cm depths, respectively. The quality of agreement in the buildup region for open and IMRT fields is comparable to that in nonbuildup regions. The added EC term in CVSP was determined to be adequate for both open and IMRT fields. Due to the dependence of calculation accuracy on (1) EC modeling, (2) internal convolution and density grid sizes, (3) implementation details in the algorithm, and (4) the accuracy of measurements used for treatment planning system commissioning, the authors recommend an evaluation of the accuracy of near-surface dose calculations as a part of treatment planning commissioning.

The evaluation of 6 and 18 MeV electron beams for small animal irradiation

A small animal irradiator is critical for providing optimal radiation dose distributions for pre-clinical animal studies. This paper focuses on the evaluation of using 6 or 18 MeV electron beams as small animal irradiators. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Two major approaches including simulation and measurement were used to evaluate the feasibility of applying electron beams in animal irradiation. These simulations and measurements were taken in three different fields (a 6 cm × 6 cm square field, and 4 mm and 30 mm diameter circular fields) and with two different energies (6 MeV and 18 MeV). A PTW Semiflex chamber in a PTW-MP3 water tank, a PTW Markus chamber type 23343, a PTW diamond detector type 60003 and KODAK XV films were used to measure PDDs, lateral beam profiles and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results show good agreement for comparisons of percentage depth doses (⩽2.5% for 6 MeV e; ⩽1.8% for 18 MeV e) and profiles (FWHM ⩽ 0.5 mm) between simulations and measurements on the 6 cm field. Greater deviation can be observed in the 4 mm field, which is mainly caused by the partial volume effects of the detectors. The FWHM of the profiles for the 18 MeV electron beam is 32.6 mm in the 30 mm field, and 4.7 mm in the 4 mm field at d90. It will take 1–13 min to complete one irradiation of 5–10 Gy. In addition, two different digital phantoms were also constructed, including a homogeneous cylindrical water phantom and a CT-based heterogeneous mouse phantom, and were implemented into Monte Carlo to simulate dose distribution with different electron irradiations.

SU-FF-J-166: The Development of a Precise Electron Irradiator for Small Animal Studies

Purpose: This study focuses on the development, validation, and evaluation of a precise MV-electron irradiator. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Method and Materials: Two major approaches including simulation and measurement were used to evaluate the feasibility of an electron animal irradiator. These simulations/measurements were taken in three different fields (a 6 cm square field, and 30 and 4 mm diameter circular fields) and with two different energies (6 and 18 MeV). A PTW Semiflex chamber i n a PTW-MP3 water tank, a PTW Markus chamber type 23343, a PTW diamond detector type 60003, and KODAK XV films were used to measure PDDs, lateral beam profiles, and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results: Results show good agreement for the comparisons of PDD (⩽ 2.5% for 6 MeV e; ⩽ 1.8% for 18 MeV e) and profiles (FWHM ⩽ 0.5 mm) between simulations and measurements on the 6 cm field. Greater deviation can be observed in the 4 mm field due to the partial volume effects of detectors. The output factor for the 18 MeV electron beam is 0.970 in the 30 mm field, and 0.610 in the 4 mm field; the FWHM of profiles for the 18 MeV electron beam is 32.6 mm in the 30 mm field, and 4.7 mm in the 4 mm field at the d90. Two different digital phantoms were also constructed, including a homogeneous cylindrical water phantom and a CT-based heterogeneous mouse phantom, and were implemented into Monte Carlo to simulate the dose distribution with different electron irradiations. Conclusion: These results ensure that our proposed electron irradiator is feasible for precise small animal irradiation.

SU‐GG‐T‐259: Dose Discrepancies in the Buildup Region and Their Impact On IMRT Field Dosimetry

Purpose: The accuracy of the buildup dose for radiotherapy treatment planning suffers from challenges in both measurement and calculation. This study was designed to improve the parameter fit in the contamination term of the convolution/superposition calculation algorithm based on more accurate measurements and to evaluate the impact of residual discrepancies on IMRT fields for normal and oblique incidences. Method and Materials: The percent depth and profile doses were measured in the buildup region in Solid Water using an Attix parallel plate chamber with over‐response correction factors and using Kodak XV film with dynamic calibration curves. The doses were measured at superficial depths ranging from 0.2 to 1.5 cm for 5×5, and 10×10 cm2 fields and 0°, 45°, and 70° incidences. Contamination term parameter fit preference was given to normal incidence. Two segmental head‐and‐neck IMRT fields were recalculated with the new fit parameters as a function of incident angle. Comparison indices (γ and dose‐gradient compensation) were used to quantify the agreement between measurements and calculations for the IMRT fields. Results: After parameter refit, the agreement between measurements and calculations were improved. Differences were still present for some dose profiles. The profile measurements showed larger field widths and sharper penumbras. The local deviations of percent depth dose (PDD) comparisons were within 2% and 5% for normal and oblique incidences, respectively. For IMRT fields, the incident angle did not greatly influence the agreement between calculation and measurement. Comparison indices were lower for shallower depths. Conclusion: Refitting the parameters in the contamination term using more accurate measured buildup doses improved the accuracy of calculations in the buildup region. Contamination term modifications may be required to better account for the influence of incident angle.Supported by NIHP01CA59827.

SU-GG-T-187: Predicting Calibration Curves for Kodak XV Film Using Model-Based Parameters

Purpose: Film calibration is time‐consuming work necessary to achieve good accuracy for film dosimetry. This study analyzed the calibration curves varying with the depth, field size and delivery day using model‐based parameters in order to predict calibration curves for future use. Method and Materials: The Kodak XV film was placed perpendicular to the beam axis in Solid Water phantom (30×30 or 40×40 cm2). Standard calibration films (one dose point per film) were irradiated at 90 cm SSD with various doses (0–128 cGy) at several depths (0.2, 0.5, 1.5, 5, 10 cm) for 5×5, 10×10, and 20×20 cm2 fields. Standard calibration responses were compared to an 8‐field calibration response (eight doses per film), irradiated at 5 cm depth and 95 cm SSD with doses from 16 to 128 cGy. All films were developed using a Kodak X‐OMAT 3000RA Processor and digitized with a Lumiscan75. All curves were fitted with single‐target‐single‐hit model (y=y 0 +a(1−e −bD )) . The parameters were compared for different delivered days, calibration methods, field sizes and depths. The method to predict the calibration curve was verified with previous data for 20×20 cm2 fields. Results: The daily variation of y0, a, and b parameters were 2.2%, 2.9%, and 11.4% using the 8‐field method. The “a” ratio of standard to 8‐field curves was 1.083. The “b” ratio ranged from 0.91 to 0.97 depending on the field size and depth. The “b” ratio decreases with increasing depth below 0.5 cm for the three field sizes. This ratio increases with increasing depths above 0.5 cm except for 5×5 cm2 field. The local differences between expected and measured calibration curves were within 5%. Conclusion: Predicting the calibration curve using one calibration film is possible by using a model‐based parameter relationship. This method reduces film processing and batch errors without re‐acquiring complete calibration curves.

SU-GG-T-81: Issues in Image-Guided Radiotherapy (IGRT) and Re-Planning with Megavoltage Cone Beam CT (MVCT)

Purpose: In this study, the MVCT dose and its distributions are determined with ion chamber and film measurements in phantom. The effect of the imagingdose for a number of disease sites is investigated. The application of MVCT in re‐planning is also addressed. Method and Materials: A solid water phantom is CT scanned and the arc rotation of MVCT is simulated in the Pinnacle planning system. A 0.6cc ion chamber is used to measure the MVCT dose at the center of the phantom. Dose distributions are determined with Kodak XV films and compared with calculation. To investigate the effect of imagingdose, DVHs for the target and critical organs are compared with and without correcting for the MVCT dose for four diseases sites (lung, H&N, mediastinum and prostate). To assess the potential application of MVCT in re‐planning, dose distributions on the planning CT of a Rando phantom are compared with those calculated on the MVCT image set. Image fusion is also performed between the CT study and its MVCT image set. Results: MVCT dose varies linearly with MU. The imagingdose cannot be completely corrected for in practice. For example in an H&N case, the dose/fraction may be adjusted to account for the MVCT dose. 3–5% additional doses to the left eye and the left parotid are still resulted. In IGRT and re‐planning applications, distributions calculated on both the planning CT and the MVCT agree to within 1%. However, erroneous contours occur on the MVCT image set because of the difference in slice thickness between the two studies. Conclusion: While the imagingdose can be determined easily for MVCT, it cannot be corrected completely in treatment planning. For re‐planning application, the MVCT slice thickness should be the same as the planning CT.Conflict of Interest: Research supported partially by Siemens.

SU-GG-T-215: Routine Linear Accelerator Electron Beam QA Using EPIDs

Purpose: To investigate the use of Electronic Portal Imaging Devices (EPIDs) for the routine quality assurance (QA) of linear accelerator (Linac) electron beams. Method and Materials: Routine electron beam QA is typically performed using radiographic film. The use of film for beam QA is time consuming and prone to many sources of error. EPIDs have become standard equipment on modern linacs, and their suitability for photon beam QA has been previously established. We have investigated the use of EPIDs for routing electron beam QA, including radiation field size, penumbra, flatness and symmetry. A Varian 2100iX with aSi imager was used. Service mode was used, as the linac does not support imaging with electron beams in clinical mode. Dark and flood field calibration of the EPID was obtained using the electron beams. EPID pixel response linearity with dose was verified for energies 6, 12, and 20 MeV with a 10×10 cm cone by exposing the imager to various MU. For radiation field size and beam flatness/symmetry QA, images were acquired using the 6×6 cm and 20×20 cm cones at energies of 6, 12 and 20 MeV. For comparison, Kodak XV films at a depth of 2 cm were exposed using the same configurations. The SDD for both film and EPID measurements was 105 cm. All films and images were analyzed using MATLAB code. Results: The EPID pixel response is linear with delivered dose. The radiation field size as measured by the EPID agreed with film to within 1.6 mm. Penumbra agreed to within 1.5 mm. It was not possible to obtain accurate flatness and symmetry results, but the EPID can be used for flatness and symmetry constancy. Conclusion: EPIDs can be used for the routine QA of electron beams. Radiation field size results are similar to those obtained with film.

Liquid embolisation material reduces the delivered radiation dose: a physical experiment

To test a new hypothesis that the glue/contrast admixture used for embolisation reduces the dose delivered to AVMs using an experimental model. A model was created using a block of "solid water" (6 x 5 x 2 cm) with twelve wells of different depths. Different concentrations of the glue admixture (Enbucrilate + Lipiodol) were used. The model was irradiated using a 5MV beam with a clinical LINAC system and the dose was checked upstream and downstream. Dose was measured using Kodak XV film, a Vidar 16 bit film scanner and software for therapeutic film dosimetry measurements (RIT software). The radiation dose varied with the distance beyond the glue solid water interface. For distances of 0, 2 and 5 mm to the film, the mean reduction was 13.65% (SD = 2.94), 6.87% (SD = 1.95) and 1.75% (SD = 1.14), respectively. There was also correlation with the Lipiodol concentration in the mixture. The maximum reductions for 80, 50 and 20% Lipiodol concentrations were 16.1% (SD = 1.32), 14.85% (SD = 0.98) and 10% (SD = 1.21), respectively. There was no correlation between the glue depth and the dose delivered. The hypothesis that the glue mixture used for embolisation reduces the radiation dose delivered was experimentally confirmed with this study.

SU-FF-T-281: Leakage Radiation of Elekta Synergy-S LINAC Machine

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy-S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy-S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x-ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulator treatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy-S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.

TU-FF-A2-05: Dose Verification of 2-, 4-, 6-Mm Cones for Stereotactic Radiosurgery Radiotherapy

Purpose: Small field dosimetry is critical in radiosurgery. Many researchers have reported the dose verification results for cones larger than 5 mm. Cones smaller than 5 mm, though are not common used in Stereotactic Radiosurgery, have the potential in benign tumor radiosurgery and small animal irradiation. This study demonstrates dose verification results for 2-, 4-, and 6-mm cones using several different detectors and determines their maximum spatial resolution for small fields without lateral electronic equilibrium. Method and Materials: Several different small-volume detectors including a small volume ion chamber (PTW 23323), a pin-point chamber (PTW31014), a PTW diamond detector (faced parallel or perpendicular), KODAK XV-film (scanned in 0.1 mm resolution), and BEAMnrc06 Monte Carlo (MC) simulation were used to study percentage depth doses, profiles, and cone factors for 2-, 4-, and 6-mm cones. Results: All detectors agree well with MC simulation for larger cones (30 mm or 14 mm). Notable dose deviation between small volume and pin point ion chambers can be observed for 6 mm cone. The dose measured using diamond detector faced perpendicular does not agree well with that measured using film, MC simulation, and diamond detector faced parallel. Finally, neither detectors nor simulation agree with measurement for 2-mm cone. In fact, this purported 2-mm cone is assumed to be 3.1 mm since the PDDs, profile, and cone factor measured using film and diamond detector faced parallel agree well with MC simulation of a 3.1-mm cone. Conclusion: Films and the diamond detector faced parallel are more suitable for small field dosimetry. Noting that the ECUT should be reduced to 521 keV for simulating cones smaller than 1 cm, the dose deviation of PDDs and profiles between MC and measurement (film, diamond) are less than 1.5%.