Index of content:
Volume 39, Issue 7, July 2012
Radiation-induced lung injury (RILI) is the primary dose-limiting toxicity for radiation therapy of the lung, and although the effects of radiation dose on RILI development have been well characterized, the influence of chronic obstructive pulmonary disease (COPD) on the development of RILI and other outcomes is not well understood. The purpose of this small pilot study was to evaluate the relationship between hyperpolarized3He magnetic resonance imaging(MRI) measurements of COPD with RILI and 12-month survival in lungcancer patients undergoing radical radiotherapy and to evaluate the feasibility of pulmonary functional MRI as an image guidance/planning tool for radiation therapy.Methods:
Fifteen non-small cell and small cell lungcancer patients underwent pulmonary function tests, x-ray computed tomography(CT), and hyperpolarized3He MRI prior to radical radiation therapy (≥60 Gy). Conventional thoracic 1H and hyperpolarized 3He MRI were acquired to generate ventilation defect percent and the apparent diffusion coefficient for the ipsilateral and contralateral lungs independently. CT was acquired postradiation therapy and qualitatively evaluated for radiological evidence of RILI and 12-month survival was reported.Results:
Hyperpolarized3He MRI measurements of COPD classified 10/15 subjects with contralateral lung COPD (CLC), and five subjects without COPD [contralateral lung normal (CLN)]. Of the 10 subjects with CLC, only four had a previous clinical diagnosis of COPD. CTimages were acquired postradiation therapy for 13 subjects, and for eight (62%) of these there was qualitative evidence of RILI, including 5/9 CLC and 3/4 CLN subjects. The one-year survival was 2/10 for CLC and 3/5 for CLN subjects.Conclusions:
In this small pilot study, we report the use of3He MRI to stratify lungcancer patients based on MRI evidence of COPD and showed that comorbid COPD was present in the majority of lungcancer subjects stratified for radiation therapy.Lungcancer patients with imaging evidence of COPD did not have an increased incidence of RILI compared to patients without COPD. However, preliminary data presented here indicated that one-year survival in COPD subjects was lower than expected based on previously published survival rates, which may have implications for radiation therapy in lungcancer patients with comorbid COPD.
- FIFTY‐FOURTH ANNUAL MEETING OF THE CANADIAN ORGANIZATION OF MEDICAL PHYSICISTS AND THE CANADIAN COLLEGE OF PHYSICISTS IN MEDICINE
- J.R. Cunningham Young Investigator Symposium
Sci—Thur PM: YIS — 01: Inverse treatment planning for modulated electrons and mixed photon and electron radiotherapy39(2012); http://dx.doi.org/10.1118/1.4740098View Description Hide Description
Modulated electron radiotherapy (MERT) takes advantage of the low distal dose of electrons to reduce dose to healthy tissue. The dosimetric advantage of MERT is clear when compared against single‐field electron irradiation where MERT demonstrates superior target homogeneity and sparing; however the dosimetric advantage is unclear when comparing MERT with photon intensity‐modulated radiotherapy(IMRT) where MERT techniques struggle to match the IMRT target homogeneity but with less total energy delivered to healthy tissues. In an effort to improve dosimetric benefits of MERT, this study investigated an inverse planning technique for the creation of hybrid MERT‐IMRT mixed beam radiotherapy (MBRT) plans. The optimization process decouples the photon and electron beamlets for combined modality optimization. The input to the optimization algorithm was a series of patient‐specific 3D dose distributions for the corresponding electron and photon beamlets, while the output was a list of weights that satisfied the optimization constraints. A photonIMRT Eclipse (Varian, Palo Alto, CA) plan and a MERT plan were created for a patient‐specific sarcoma irradiation. The MERT plan was competitive in its ability to reduce dose to organs at risk and total‐body dose; however, the plan suffered from poorer target conformity compared with the IMRT plan. The MBRT plan was created by adding two photon fields, divided into beamlets, to the electron beamlets of the MERT plan for reoptimization. The MBRT plan improved MERT target coverage with only minimal cost to healthy tissuedose. The MBRT plan provided clear dosimetric advantages over the IMRT and MERT plan.
Sci—Thur PM: YIS — 02: A validated approach for clinical linacs to accurately determine the photon spectra and the incident electron energy39(2012); http://dx.doi.org/10.1118/1.4740099View Description Hide Description
In clinical photon beams, independent determination of the photon spectra and the incident electron energy is useful for beam (re)commissioning and for detector response modelling. In this study, an approach is developed for that purpose, and validated on a research linac whose photon spectra and electron beams are directly and independently known. In this approach, an optimized combination of transmission curves is measured using multiple attenuators and detectors to maximize energy differentiation. For validation, transmission measurements are made for 8 beams from 10–30 MV, with bremsstrahlung targets from Be to Pb. A protocol is established to account for many influence quantities including linac drifts (2%), polarity (6%), ion recombination (0.2%), leakage (0.3%), room scatter (0.8%), non‐ideal attenuation (1.5%), attenuator mass thickness (4%), and photonuclear effect (5.6%). The experimental accuracy on the smallest signals is 0.4%. EGSnrc is upgraded to model photonuclear attenuation (without tracking secondary particles), and then used to model the full experiment. For direct transmission comparisons, the agreement is 2%. This allows for an estimate of 0.5% on the upper limit of photon cross section uncertainties, which is much better than the current estimate of 1–2%. The unfolded spectra agree with the benchmark ones within 4.5%. The incident electron energy is accurate within 5%, with 95% confidence. The overall improvement over the commonly used methods is a factor of 3. This transmission study is the first to independently determine the incident electron energy, and to recognize the significant role of the photonuclear effect at higher energies.
39(2012); http://dx.doi.org/10.1118/1.4740100View Description Hide Description
Purpose: To evaluate the treatment plan qualities of 4D‐VMAT, gated‐VMAT and 3D‐VMAT in the treatment of non‐small cell lungcancer(NSCLC) in stereotactic body radiation therapy(SBRT).Methods: 4D‐VMAT is a motion compensation strategy that aims to exploit relative target and OAR motion to increase OAR sparing over 3D‐VMAT without the long treatment times associated with gated‐VMAT. The 4D‐VMAT algorithm incorporates the entire patient respiratory cycle and 4D‐CT in the optimization process. Resulting treatment plans synchronize the delivery of each MLC aperture to a specific phase of the target motion. Using software developed in Matlab™, SBRTtreatment plans for 4D‐VMAT, gated‐VMAT and 3D‐VMAT were generated on 3 patients with NSCLC.Tumour motion ranged from 1.4–3.4 cm. The fractionation scheme was 48Gy in 4 fractions with the GTV receiving 100% of the prescribed dose. For gated‐VMAT, the treatment window constrained residual tumour motion to 3 mm or less corresponding to duty cycles of 40–60%. In 3D‐VMAT, the ITV was generated by merging the GTV from all phases. A b‐spline transformation model was used to register the 4D‐CT images and DVHs were calculated from total dose accumulated on the max expiration phase. Results and Conclusion: For the majority of OARs, gated‐VMAT provided the greatest radiation sparing but significantly extended treatment times (25–35 gantry interruptions/arc). For 3D‐VMAT, only 2 patients had clinically acceptable plans that met all the strict dose limits. OAR sparing in 4D‐VMAT was comparable to gated‐VMAT but with significantly improved delivery efficiency.
Sci—Thur PM: YIS — 04: Forcing lateral electron disequilibrium to spare lung tissue: A novel technique for SBRT of small lung tumours39(2012); http://dx.doi.org/10.1118/1.4740101View Description Hide Description
Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x‐ray fields, improves local control of lungcancer. However, small MV x‐ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT:LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lungtissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantom's diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm3) shells surrounding a small water tumor (volume = 0.8 cm3). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm2(RDDF∼0.5) and 3×3cm2(RDDF∼1). Analyzing dose results, the 1×1cm2 arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm2 arc. Although, central tumordose was also reduced by 15% using the 1×1cm2 arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, it's possible to design a SBRT treatment plan that reduces lungdose while maintaining relatively high tumordose levels. Clinical application requires an accurate dose algorithm and may lower SBRTdose‐induced toxicity levels in patients.
Sci—Thur PM: YIS — 05: Tomographic dosimetry using scintillating fibers and its application to 2D and 3D dosimetry39(2012); http://dx.doi.org/10.1118/1.4740102View Description Hide Description
Purpose: To present the proof of concept and the experimental validation of tomographic dosimetry (tomodosimetry), where a tomographic acquisition of the incident deposited dose is performed using long scintillating fibers. Method: 2D tomodosimetry: 50 long scintillating fibers were aligned on a 20cm diameter disk inside a 30cm diameter masonite phantom. 3D tomodosimetry: 128 long scintillating fibers of various orientation were simulated on the surface of two cylindrical regions of radius 7.5 and 3.75cm inside a 20cm diameter, 20cm long cylindrical phantom. In both case, the dose projections were acquire each 5 degrees over a 180 degrees (2D) or 360 degrees (3D) rotation of the device, and the dose in each scintillating fiber plane was reconstructed using a total variation minimization reconstruction iterative algorithm at a resolution of 1×1mm2. The 3D dose was obtained by interpolating between in each cylindrical plane in the 3D prototype. Results: 3%/3mm gamma tests conducted in the isocentre plane for both configurations achieved a success rate of more than 99% of the dose pixels in the region over 50% of the maximum dose. Absolute dose differences in the high dose low gradient region of each scintillating fiber plane were on average below 1% for the 2D configuration and below 1.3% for the 3D configuration. Conclusions: This work illustrates the potential and capacity of scintillating fiber based 2D and 3D tomodosimeters. The presented methodology allows for millimeter resolution dosimetry in a whole 2D plane or 3D volumes in real‐time using only a limited number of detectors.
39(2012); http://dx.doi.org/10.1118/1.4740103View Description Hide Description
Compressed Sensing MRSI (CS‐MRSI) offers the ability to accelerate MRSI sequences while suffering minimal artifacts compared to conventional fast MRSI techniques. CS‐MRSI exploits the inherent sparsity of MRSIimages and incoherent artifacts of pseudo‐random sub‐Nyquist sampling of k‐space combined with non‐linear reconstruction to produces MRSIimages. CS‐MRSI can be used as an acceleration tool to decrease the scan time while maintaining acceptable spatial definition or to enable the acquisition of higher resolution scans while minimizing the associated time penalty. In this work we adopt the compressed sensing technique to accelerate a clinically relevant 2‐D point resolved spectroscopy sequence. However, the process of weighing the cost and benefit of applying such a fast imaging technique is complicated due to the unique non‐linear nature of the reconstruction process and has largely relied on qualitative assessments. Moreover, pseudo‐random sub‐Nyquist sampling of k‐space can have unwanted effects on the modulation transfer function. In this work we set out to quantify the loss in image quality associated with CS‐MRSI. We used simulations of a phantom based method to investigate the MTF behaviour of CS‐MRSI with regard to different k‐space sampling patterns. As expected, the k‐space sampling patterns tested were found to have a direct effect on the MTFs. Moreover, limiting the deviation of the resulting k‐space sampling pattern from the prescribed probability distribution function had a positive effect on the MTF overall. Not only was low‐resolution response improved, but we also noticed an improvement of ∼ 26% in resolution at 0.1 MTF.
Sci—Thur PM: YIS — 07: Monte Carlo simulations to obtain several parameters required for electron beam dosimetry39(2012); http://dx.doi.org/10.1118/1.4740104View Description Hide Description
When current dosimetry protocols were written, electron beam data were limited and had uncertainties that were unacceptable for reference dosimetry.. Protocols for high‐energy reference dosimetry are currently being updated leading to considerable interest in accurate electron beam data. To this end, Monte Carlo simulations using the EGSnrc user‐code egs_chamber are performed to extract relevant data for reference beam dosimetry. Calculations of the absorbed dose to water and the absorbed dose to the gas in realistic ion chamber models are performed as a function of depth in water for cobalt‐60 and high‐energy electron beams between 4 and 22 MeV. These calculations are used to extract several of the parameters required for electron beamdosimetry — the beam quality specifier, R50, beam quality conversion factors, kQ and kR50, the electron quality conversion factor, k′R50, the photon‐electron conversion factor, kecal, and ion chamber perturbation factors, PQ. The method used has the advantage that many important parameters can be extracted as a function of depth instead of determination at only the reference depth as has typically been done. Results obtained here are in good agreement with measured and other calculated results. The photon‐electron conversion factors obtained for a Farmer‐type NE2571 and plane‐parallel PTW Roos, IBA NACP‐02 and Exradin A11 chambers are 0.903, 0.896, 0.894 and 0.906, respectively. These typically differ by less than 0.7% from the contentious TG‐51 values but have much smaller systematic uncertainties. These results are valuable for reference dosimetry of high‐energy electron beams.
39(2012); http://dx.doi.org/10.1118/1.4740105View Description Hide Description
Current generation electronic portal imaging devices(EPID) contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When using these EPIDs for low‐Z target imaging, the conversion plate largely attenuates the large populations of diagnostic energy photons, thereby decreasing the benefits of low‐Z target imaging. In this work we measure directly the effect the variation in thickness of a copper conversion plate has on image quality in planar and cone beam computed tomographyimaging. Monte Carlo modeling was used to quantify changes to the diagnostic spectrum and detector response for low‐Z target beams generated with 2.35 and 7.00 MeV electrons incident on a carbon target. Planar contrast‐to‐noise ratio (CNR) measurements were made as a function of copper thickness. Cone beam computed tomography(CBCT)imageCNR measurements were made as a function of dose both with and without the copper plate present in the EPID. The presence of copper in the EPID decreased the diagnostic photon population by up to 20% and suppressed the peak detector response at 60 kV by a factor of 6.4. Planar CNR was increased by a factor ranging from 1.4 to 4.0 with no copper present compared to 1.0 mm thickness. Increases in CBCTimageCNR ranged from a factor of 1.3 to 2.1 with the copper plate removed. As a result of this we suggest that the copper conversion plate be removed from the EPID when used for low‐Z target planar or CBCTimaging.
Sci—Thur PM: YIS — 09: Development of a graphite probe calorimeter for absolute clinical dosimetry: Numerical design optimization, prototyping and experimental proof‐of‐concept39(2012); http://dx.doi.org/10.1118/1.4740106View Description Hide Description
In this work, the feasibility of absolute dose to water measurements using a small‐scale graphite probe calorimeter (GPC) in a clinical environment is established. A numerical design optimization study was conducted by simulating the heat transfer in the GPC resulting from irradiation using a finite element method software package. The choice of device shape, dimensions and materials was made to minimize the heat loss in the sensitive volume of the GPC. The resulting design, which incorporates a novel aerogel‐based thermal insulator, was built in‐house. Absorbed dose to water measurements were made under standard conditions in a 6 MV 1000 MU/min photon beam and subsequently compared against TG‐51 derived values. The average measured dose to water was 95.7 ±1.4 cGy/100 MU, as compared to an expected value of 96.6 cGy/100 MU. The Monte Carlo‐calculated graphite to water dose conversion factor was 1.099, while the derived heat loss correction factors varied between 1.005 and 1.013. The most significant sources of uncertainty were the repeatability (type A, 1.4%) and thermistor calibration (type B, 2.1%). The contribution of these factors to the overall uncertainty is expected to decrease significantly upon the implementation of active thermal stabilization provided by a temperature controller and direct electrical calibration, respectively. This work demonstrates the feasibility of using the GPC as a practical clinical absolute photon dosimeter and will serve as the basis for a miniaturized version applicable to small and composite fields.
Sci—Thur PM: YIS — 10: A new optically encoded single‐fiber plastic scintillation detector for multi‐point radiation dosimetry39(2012); http://dx.doi.org/10.1118/1.4740107View Description Hide Description
Purpose: To develop a new multi‐point plastic scintillation detector (mPSD) that allows for simultaneous dose measurements at multiple points and uses a single optical guide. Materials and Methods: Two different prototypes were built. A two‐point mPSD was built and light discrimination was based on the use of multiple color filters at the outputs of a network of optical fiber splitters. Light intensity was measured by an EMCCD camera. For the three‐point mPSD, the light discrimination setup was replaced by a low‐noise spectrometer. Depth‐dose and profiles measurements were obtained on a 6 MV photon beam with the mPSDs inside a water phantom. An ion chamber was also used for comparison purpose. Finally, the three‐point mPSD was tested under an Ir‐192 high‐dose‐rate (HDR) brachytherapydose delivery and compared to the treatment planning system. Results: A good agreement was found between the measured and expected dose for both mPSDs. The average relative differences to the ion chamber measurement for the two‐point mPSD were of (2.4 ± 1.6)% and (1.3 ± 0.8)%. For the three‐point mPSD, these differences were of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)%. The latter mPSD was shown very versatile, being able to measure dose from HDR brachytherapy with an average accuracy of (2.3±1.0)% per catheter. Conclusions: The practical feasibility of mPSDs using a single optical guide has been demonstrated under irradiation from a 6 MV photon beam and an Ir‐192 HDR brachytherapy source. Their application for pre‐treatment quality assurance and in vivo dosimetry will be various.
- Poster Session
Poster — Thur Eve — 01: Development of simple and fast EBT2 film calibration procedure using PDD table39(2012); http://dx.doi.org/10.1118/1.4740108View Description Hide Description
Standard calibration procedure for EBT films is laborious and time‐consuming. The objective of this work was to develop a simple and fast approach of EBT2 film calibration using PDD tables. EBT2 sheet is cut into 3 stripes of 5×25.5cm2. The strips were exposed to dose of 600, 200 and 70cGy at dmax each while placed horizontally in the middle of a 30×30×30cm3 solid water phantom. Varian 21EX 6MV 10×10cm2 beam was used with the gantry rotated to 90° and SSD of 100cm to the phantom surface. After at least 24 hours, the films were digitized with flatbed scanner (Epson10000XL), according to a modified ISP scanning protocol. All images were analysed using an in‐house Matlab code and ImageJ software. The net‐optical densities against depths in the solid phantom were calibrated using PDD tables measured with ionization chamber for same machine. For verification, another calibration curve was generated for the same film batch following the same calibration protocol. Seven pieces of films were exposed to known doses and these doses were reconstructed using two derived calibration curves. The proposed approach was 3.6 times faster than the standard considering the number of films used in each methods, 3 stripes compared to11 pieces. The mean relative dose difference calculated for these films using the PDD calibration and the standard methods was 1.0±1.2% and 0.5±2.2% with maximum relative differences of 3.0% and 4.7% respectively. Our results show that PDD calibration approach is much easier, faster and predicts dose more reproducibly and accurately than the standard approach.
39(2012); http://dx.doi.org/10.1118/1.4740109View Description Hide Description
Following a recent review of the Class II Nuclear Facilities and Prescribed Equipment Regulations and regulatory oversight of particle accelerators, the Canadian Nuclear Safety Commission (CNSC) has changed its policy concerning the regulation of particle accelerators. In November 2011, the CNSC began to exercise its regulatory authority with respect to all particle accelerators operating at a beam energy of 1 (one) MeV or greater. The CNSC already licences and inspects particle accelerators capable of operating at or above 10 MeV. The decision to now include low energy particle accelerators (i.e., those operating at or above 1 MeV) ensures adequate, uniform and consistent regulatory oversight for all Class II accelerators. The CNSC expects these facilities to comply with CNSC requirements by December 2013. Besides conventional linear accelerators of lower energy (6 MeV or below) typically found in cancer clinics, two types of equipment now fall under the CNSC's regulatory oversight as a result of the above change: roboticradiosurgery and tomotherapy equipment and facilities. A number of clinics in Canada already operates these types of equipment and facilities. The safety aspects of radiosurgery equipment differ slightly from those for conventional linear accelerators. This poster aims to present an approach taken by the CNSC to regulate roboticradiosurgery equipment and facilities. The presentation will explain how to meet regulatory requirements of the Class II Nuclear Facilities and Prescribed Equipment Regulations by licensees operating or planning to acquire these types of equipment and facilities.
39(2012); http://dx.doi.org/10.1118/1.4740110View Description Hide Description
The RADPOS in vivo dosimetry system combines an electromagnetic positioning sensor and either one or five MOSFET dosimeters. The feasibility of using the system for quality control has been explored for a range of radiotherapy treatment techniques including most recently transperineal interstitial permanent prostate brachytherapy and high dose rate (HDR) treatments. Dose and position information was collected by a RADPOS array detector inside a Foley catheter within patients' urethra during permanent seed implantation. Ten patients were studied, and average displacement during implantation was Δr = (1.4–5.1) mm, with movements up to 9.7 mm due to the removal of the transrectal ultrasound probe. Maximum integral dose in the prostatic urethra ranged from 110–195 Gy, and it was found that the dose can change up to 63 cGy (62.0%) depending on whether the rectal probe is in place. For HDR, a RADPOS detector was first calibrated with an Ir‐192 source. A treatment was then simulated using a total of 50 dwell positions in 5 catheters in an acrylic phantom. Dwell positions ranged from 1 to 10 cm away from the RADPOS detector and dose was measured for each source position. An average calibration coefficient of 0.74±0.11 cGy/mV was calculated for the detector and the average absolute difference between measured values and expected dose was 0.7±5.4 cGy (5±20%). The demonstrated accuracy of RADPOS dose measurements along with its ability to simultaneously measure displacement makes it a powerful tool for brachytherapy treatments, where high dose gradients can present unique in vivo dosimetry challenges.
Poster — Thur Eve — 04: Online review of beam output and profile constancy using statistical process control39(2012); http://dx.doi.org/10.1118/1.4740111View Description Hide Description
The objective of this work was to create a comprehensive online tool to evaluate and review the performance of quality assurance measurements that assess beam output and profile constancy as soon as they are acquired using statistical process control. As part of routine quality assurance: output, flatness and symmetry measurements are acquired daily and weekly with DQA3 and the Matrix and symmetry and flatness are acquired on a monthly basis with Profiler2. An individuals control chart and a moving range control chart was plotted for each set of data. Upper and lower control limits were calculated using measurements acquired during a several month period when the linear accelerators were operating optimally. The existing action levels, established according to TG142 and CAPCA guidelines were compared with the calculated statistical control limits. Tighter tolerance limits were recommended for output, symmetry and flatness Matrix measurements and DQA3 flatness measurements.
Poster — Thur Eve — 05: Safety systems and failure modes and effects analysis for a magnetic resonance image guided radiation therapy system39(2012); http://dx.doi.org/10.1118/1.4740112View Description Hide Description
Introduction: An online Magnetic Resonance guided Radiation Therapy (MRgRT) system is under development. The system is comprised of an MRI with the capability of travel between and into HDR brachytherapy and external beam radiation therapy vaults. The system will provide on‐line MRimages immediately prior to radiation therapy. The MRimages will be registered to a planning image and used for image guidance. With the intention of system safety we have performed a failure modes and effects analysis.Methods: A process tree of the facility function was developed. Using the process tree as well as an initial design of the facility as guidelines possible failure modes were identified, for each of these failure modes root causes were identified. For each possible failure the assignment of severity, detectability and occurrence scores was performed. Finally suggestions were developed to reduce the possibility of an event. Results/Discussion: The process tree consists of nine main inputs and each of these main inputs consisted of 5 – 10 sub inputs and tertiary inputs were also defined. The process tree ensures that the overall safety of the system has been considered. Several possible failure modes were identified and were relevant to the design, construction, commissioning and operating phases of the facility. The utility of the analysis can be seen in that it has spawned projects prior to installation and has lead to suggestions in the design of the facility.
Poster — Thur Eve — 06: Comparison of an open source genetic algorithm to the commercially used IPSA for generation of seed distributions in LDR prostate brachytherapy39(2012); http://dx.doi.org/10.1118/1.4740113View Description Hide Description
In early stage prostate cancer, low dose rate (LDR) prostate brachytherapy is a favorable treatment modality, where small radioactive seeds are permanently implanted throughout the prostate. Treatment centres currently rely on a commercial optimization algorithm, IPSA, to generate seed distributions for treatment plans. However, commercial software does not allow the user access to the source code, thus reducing the flexibility for treatment planning and impeding any implementation of new and, perhaps, improved clinical techniques. An open source genetic algorithm (GA) has been encoded in MATLAB to generate seed distributions for a simplified prostate and urethra model. To assess the quality of the seed distributions created by the GA, both the GA and IPSA were used to generate seed distributions for two clinically relevant scenarios and the quality of the GA distributions relative to IPSA distributions and clinically accepted standards for seed distributions was investigated. The first clinically relevant scenario involved generating seed distributions for three different prostate volumes (19.2 cc, 32.4 cc, and 54.7 cc). The second scenario involved generating distributions for three separate seed activities (0.397 mCi, 0.455 mCi, and 0.5 mCi). Both GA and IPSA met the clinically accepted criteria for the two scenarios, where distributions produced by the GA were comparable to IPSA in terms of full coverage of the prostate by the prescribed dose, and minimized dose to the urethra, which passed straight through the prostate. Further, the GA offered improved reduction of high dose regions (i.e hot spots) within the planned target volume.
39(2012); http://dx.doi.org/10.1118/1.4740115View Description Hide Description
The Accelerators and Class II Facilities Division (ACFD) of the Canadian Nuclear Safety Commission (CNSC), is responsible for the oversight of radiotherapy facilities containing Class II prescribed equipment in Canada. This poster will highlight a number of new initiatives that the CNSC has implemented recently that have an impact on radiotherapy facility licensees. The presentation will discuss the recent policy decision to regulate particle accelerators of above 1 MeV. Challenges and progress with respect to the implementation of the policy will be presented. Other initiatives which will be described include:
• The new ACFD webspace on the CNSC website, with direct links to relevant information on licensing, compliance and Class II prescribed equipment
• The improved structure of the Appendix of Licence Documents that is part of every Class II licence
• Updated licence application guides
• Changes to Annual Compliance reporting requirements and progress on the ACR‐Online initiative
• Changes to some regulatory expectations related to medical accelerator facilities
• Consolidation of Class II facility licences
The poster will also include other initiatives that may be of particular interest to COMP membership.
Poster — Thur Eve — 08: A1SL ion chamber charged particle disequilibrium corrections for lung dose measurements using Monte Carlo39(2012); http://dx.doi.org/10.1118/1.4740116View Description Hide Description
An in house inhomogeneous insert for use with ArcCHECK ™ was developed for dose calculation verification of Stereotactic Body Radiation Therapy(SBRT)lung plans. The inhomogeneous insert has various ion chamber inserts for different geometrical configurations (lung, soft tissue, bone, air). However, the insertion of an ion chamber in a low density medium perturbs the dose to that region by creating Charged Particle Disequilibrium (CPD), limiting the accuracy of ion chamber measurements. By simulating the ion chamber and phantom using Monte Carlo, a correction factor could be calculated and measured to verify the dose difference caused by CPD. BEAMnrc was used to generate a phase space input file for DOSXYZnrc with beam characteristics that matched clinical commissioning data. A model of the A1SL ion chamber geometry (shell, collector, stem, guard) was simulated in a simple water—lung—water slab phantom. Dose to the active area of the ion chamber was measured in several locations throughout the phantom. The active area of the ion chamber was replaced by the surrounding medium; i.e., water or lung within the phantom, and the dose to the same voxels was calculated. The dose was measured on a Linac and the results agreed within 3% and confirmed that the presence of the ion chamber in low density lung perturbs the dose measured in the field by over 31%.
Poster — Thur Eve — 09: Novel radiation safety challenges of a brachytherapy redevelopment at the Credit Valley Hospital39(2012); http://dx.doi.org/10.1118/1.4740117View Description Hide Description
Carlo Fidani Peel Regional Cancer Center at the Credit Valley Site of the Credit Valley Hospital and Trillium Health Center is currently undergoing a redevelopment to build a brachytherapy suite and associated areas with a projected start date of April 2013. The new brachytherapy suite will be located in the PRCC, and is a redevelopment of office area into clinical space. The workload for the full brachytherapy program is expected to be 20 patients per week, in cervix, prostate, skin, lung and other sites. There were challenges for shielding due services in the slab to the renal clinic located above the redevelopment area. The presence of over 30 voids in the slab and upper walls made the solution to add shielding to the underside of the current slab unsuitable. To overcome this, a second ceiling built below the slab to allow for an uninterrupted shielding for the brachytherapy suite. The additional ceiling allows for a crawl space between slab and shielding allowing for servicing if needed for the drains from the renal clinic. The appropriate finalized shielding design is 69mm of lead brick supported by steel plate and steel beams. Final shielding for the walls is 690mm of concrete that allows public access to all hallways around the facility. The final design for the new brachytherapy site at the PRCC all services for the room are located within the shielding and all services for areas outside the room are located outside the shielding.
39(2012); http://dx.doi.org/10.1118/1.4740118View Description Hide Description
The rapidly growing use of volumetric modulated arc therapy (VMAT) treatments in radiation therapy calls for a quantitative, automated, and reliable quality assurance (QA) procedure that can be used routinely in the clinical setting. In this work, we present a series VMAT QA procedures used to assess dynamic multi‐leaf collimator (MLC) positional accuracy, variable dose‐rate accuracy, and MLC leaf speed accuracy. The QA procedures were performed using amorphous silicon electronic portal imaging devices (EPID) to determine the long term stability of the measured parameters on two Varian linear accelerators. The measurements were repeated weekly on both linear accelerators for a period of three months and the EPID images were analyzed using custom Matlab software. The results of the picket fence tests indicate that MLC leaf positions can be identified to within 0.11 mm and 0.15 mm for static gantry delivery and VMAT delivery respectively. In addition, the dose‐rate, gantry speed and MLC leaf speed tests both show very good stability over the measurement period. The measurements thus far, suggest that a number of the dosimetry tests may be suitable for quarterly QA for Varian iX and Trilogy linacs. However, additional measurements are required to confirm the frequency with which each test is required for safe and reliable VMAT delivery at our centre.