Varian Eclipse Treatment Planning System Research Articles

TH-C-17A-03: Dynamic Visualization and Dosimetry of IMRT and VMAT Treatment Plans by Video-Rate Imaging of Cherenkov Radiation in Pure Water

Purpose: A novel optical dosimetry technique for the QA and verification of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) radiotherapy plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: An intensified CCD camera (ICCD) was used to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank. The ICCD acquisition was gated to the Linac, operated for single pulse imaging, and binned to a resolution of 512×512 pixels. The resulting videos were analyzed temporally for regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR) and summed to obtain an overall light distribution, which was compared to the expected dose distribution from the TPS using a gammaindex analysis. Results: The chosen camera settings resulted in data at 23.5 frames per second. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.2% and 95.6% agreement between the light distribution and expected TPS dose distribution based upon a 3% / 3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans respectively. Conclusion: The results from this initial study demonstrate the first documented use of Cherenkov radiation for optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications. NIH R01CA109558 and R21EB017559.

SU-E-T-105: Development of 3D Dose Verification System for Volumetric Modulated Arc Therapy Using Improved Polyacrylamide-Based Gel Dosimeter

Purpose: The aim of this dosimetric study was to develop 3D dose verification system for volumetric modulated arc therapy (VMAT) using polyacrylamide-based gel (PAGAT) dosimeter improved the sensitivity by magnesium chloride (MgCl2). Methods: PAGAT gel containing MgCl2 as a sensitizer was prepared in this study. Methacrylic-acid-based gel (MAGAT) was also prepared to compare the dosimetric characteristics with PAGAT gel. The cylindrical glass vials (4 cm diameter, 12 cm length) filled with each polymer gel were irradiated with 6 MV photon beam using Novalis Tx linear accelerator (Varian/BrainLAB). The irradiated polymer gel dosimeters were scanned with Signa 1.5 T MRI system (GE), and dose calibration curves were obtained using T2 relaxation rate (R2 = 1/T2). Dose rate (100-600 MU min−1) and fractionation (1-8 fractions) were varied. In addition, a cubic acrylic phantom (10 × 10 × 10 cm3) filled with improved PAGAT gel inserted into the IMRT phantom (IBA) was irradiated with VMAT (RapidArc). C-shape structure was used for the VMAT planning by the Varian Eclipse treatment planning system (TPS). The dose comparison of TPS and measurements with the polymer gel dosimeter was accomplished by the gamma index analysis, overlaying the dose profiles for a set of data on selected planes using in-house developed software. Results: Dose rate and fractionation dependence of improved PAGAT gel were smaller than MAGAT gel. A high similarity was found by overlaying the dose profiles measured with improved PAGAT gel dosimeter and the TPS dose, and the mean pass rate of the gamma index analysis using 3%/3 mm criteria was achieved 90% on orthogonal planes for VMAT using improved PAGAT gel dosimeter. Conclusion: In-house developed 3D dose verification system using improved polyacrylamide-based gel dosimeter had a potential as an effective tool for VMAT QA.

SU-E-T-640: Dosimetric Comparison of Intensity-Modulated Arc Therapy and Intensity-Modulated Tadiotherapy of Cervical Carcinoma

Purpose: To compare the dosimetric differences between intensity-modulated arc therapy (ARC) and intensity-modulated tadiotherapy (IMRT) plans for cervical carcinoma. Methods: 15 cervical carcinoma cases were sdudied. All plans were prescribed 50 Gy in 25 fractions.Two treatment plans (ARC and IMRT) were generated using Varian Eclipse treatment planning system. Evaluate the dose parameters of targets, organs at risk (OAR), monitor units and treatment time, using dose-volume histogram (DVH). Results: The CI and HI of PTV was better in ARC plan than IMRT plan,but there were no statistical differences (P>0.05). There were statistical differences between V20,V25,V30,V35of health tissues. There were statistical differences between V15,V20,V25of femurs. There were statistical differences between V20,V25,V30of small bowel. The V35,V40,V45 of bladder and rectum increased in ARC plan but there were statistical differences. The number of MU resulted to be MUIMRT=1677±212, MUARC=894±213. The MU of ARC plan were 47% less on average than IMRT plan. The mean treatment time for ARC was decreased 40% compared with IMRT. Conclusion: The ARC was equal to IMRT on the target coverage and the dose of OAR. The ARC plan has fewer machine monitor units and lesss treatment time and significantly improves the treatment efficiency.

SU-E-T-80: Comparison of Fluence-Based RapidArc QAs Using EPID and MapCHECK 2

Purpose: To compare the Varian aS-1000 EPID imager to the isocentrically mounted MapCHECK 2 diode array for RapidArc QAs as a function of photon beam energy. Methods: A Varian TrueBeam STx with an aS-1000 digital imaging panel was used to acquire RapidArc QA images for 13 patient plans; each plan QA was performed at 6, 8, 10 and 15MV energies. The Portal Dose Image Prediction algorithm in the Varian Eclipse treatment planning system (TPS) was used to create the comparison image for the EPID acquisition. A Sun Nuclear MapCHECK 2 diode array on an isocentric mounting fixture with 5 cm water-equivalent buildup was also used for the RapidArc QAs. A composite dose plane was taken from the Eclipse TPS for comparison to the MapCHECK 2 measurements. A gamma test was implemented in the Sun Nuclear Patient software with 10% threshold and absolute comparison for both QA methods. The two-tailed paired t-test was employed to analyze the statistical significance between two methods at the 95% confidence level. Results: The average gamma passing rates were greater than 95% at 3%/3mm using both methods for all four energies. The average passing rates were within 2.5% and 1.1% of each other when analyzed at 2%/2mm and 3%/3mm conditions, respectively. The EPID passing rates were somewhat better than the MapCHECK 2 when analyzed at 1%/1mm condition; this difference decreased with increasing energy (9.1% at 6MV to 2.7% at 15MV). The differences were not statistically significant for all criteria and energies (p-value ã 0.05). Conclusion: EPID-based RapidArc QA results are comparable to MapCHECK 2 when using 3%/3mm criteria at all four energies. EPID-based QA shows potential for being the superior device under strict gamma criteria.

SU-E-T-618: Dosimetric Comparison of Manual and Beam Angle Optimization of Gantry Angles in IMRT for Cervical Cancer

Purpose: To evaluate the dosimetric characteristics of intensity-modulated radiotherapy (IMRT) treatment plan with beam angle optimization. Methods: Ten post-operation patients with cervical cancer were included in this analysis. Two IMRT plans using seven beams were designed in each patient. A standard coplanar equi-space beam angles were used in the first plan (plan 1), whereas the selection of beam angle was optimized by beam angle optimization algorithm in Varian Eclipse treatment planning system for the same number of beams in the second plan (plan 2). Two plans were designed for each patient with the same dose-volume constraints and prescription dose. All plans were normalized to the mean dose to PTV. The dose distribution in the target, the dose to the organs at risk and total MU were compared. Results: For conformity and homogeneity in PTV, no statistically differences were observed in the two plans. For the mean dose in bladder, plan 2 were significantly lower than plan 1(p<0.05). No statistically significant differences were observed between two plans for the mean doses in rectum, left and right femur heads. Compared with plan1, the average monitor units reduced 16% in plan 2. Conclusion: The IMRT plan based on beam angle optimization for cervical cancer could reduce the dose delivered to bladder and also reduce MU. Therefore there were some dosimetric advantages in the IMRT plan with beam angle optimization for cervical cancer.

Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation.

A novel technique for optical dosimetry of dynamic intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 (TG-119) C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank doped with the fluorophore quinine sulfate. The ICCD acquisition was gated to the Linac target trigger pulse to reduce background light artifacts, read out for a single radiation pulse, and binned to a resolution of 512 × 512 pixels. The resulting videos were analyzed temporally for various regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR), and summed to obtain an overall light intensity distribution, which was compared to the expected dose distribution from the TPS using a gamma-index analysis. The chosen camera settings resulted in 23.5 frames per second dosimetry videos. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.9% and 96.2% agreement between the experimentally captured Cherenkov light distribution and expected TPS dose distribution based upon a 3%/3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans, respectively. The results from this initial study demonstrate the first documented use of Cherenkov radiation for video-rate optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications.

A Study of the Anatomic Changes and Dosimetric Consequences in Adaptive CRT of Non-Small-Cell Lung Cancer Using Deformable CT and CBCT Image Registration

The aim of this study is to evaluate anatomic lung tumor changes and dosimetric consequences utilizing the deformable daily kilovolt (KV) cone-beam computer tomography (CBCT) image registration. Five patients diagnosed with NSCLC were treated with three-dimensional conformal radiotherapy (3D CRT) and 10 daily KV CBCT image sets were acquired for each patient. Each CBCT image and plan CT were imported into the deformable image registration (DIR) system. The plan CT image was deformed by the DIR system and a new contour on CBCT was obtained by using the auto-contouring function of the DIR. These contours were individually marked as CBCT f1, CBCT f2,..., and CBCT f10, and imported into a treatment planning system (TPS). The daily CBCT plan was individually generated with the same planning criteria based on new contours. These plans were individually marked as CBCTp1, CBCTp2,..., and CBCTp10, followed by generating a dose accumulation plan (DA plan) in original pCT image contour sets by adding all CBCT plans using Varian Eclipse TPS. The maximum, minimum and mean doses to the plan target volume (PTV) in the 5 DA plans were the same with the CT plans. However, the volume of radiation 5, 10, 20, 30, and 50 Gy of the total lungs in DA plans were less than those of the CT plans. The maximum dose of the spinal cord in the DA plans were average 27.96% less than the CT plans. The mean dose for the left, right, and total lungs in the DA plans were reduced by 13.80%, 23.65%, and 12.96%, respectively. The adaptive 3D CRT based on the deformable registration can reduce the dose to the lung and the spinal cord with the same PTV dose coverage. Moreover, it provides a method for further adaptive radiotherapy exploration.

A Comparison Study of Volumetric Modulated Arc Therapy Quality Assurances Using Portal Dosimetry and MapCHECK 2

A Varian Portal Dosimetry system was compared to an isocentrica lly mounted MapCHECK 2 diode array for volumetric modulated arc therapy (VMAT) QA. A Varian TrueBeam STx with an aS-1000 digital imaging panel was used to acquire VMAT QA images for 13 plans using four photon energies (6, 8, 10 and 15 MV). The EPID-based QA images were compared to the Portal Dose Image Prediction calculated in the Varian Eclipse treatment planning system (TPS). An isocentrically mounted Sun Nuclear MapCHECK 2 diode array with 5 cm water-equivalent buildup was also used for the VMAT QAs and the measurements were compared to a composite dose plane from the Eclipse TPS. A γtest was implemented in the Sun Nuclear Patient software with 10% threshold and absolute comparison at 1%/1 mm (dose difference/distance-to-agreement), 2%/2 mm, and 3%/3 mm criteria for both QA methods. The two-tailed paired Student’s t-test was employed to analyze the statistical significance at 95% confidence level. The average γpassing rates were greater than 95% at 3%/3 mm using both methods for all four energies. The differences in the average passing rates between the two methods were within 1.7% and 1.6% of each other when analyzed at 2%/2 mm and 3%/3 mm, respectively. The EPID passing rates were somewhat better than the MapCHECK 2 when analyzed at 1%/1 mm; the difference was lower for 8 MV and 10 MV. However, the differences were not statistically significant for all criteria and energies (p-values ＞0.05). The EPID-based QA showed large off-axis over-response and dependence of γpassing rate on energy, while the MapCHECK 2 was susceptible to the MLC tongue-and-groove effect. The two fluence-based QA techniques can be an alternative tool of VMAT QA to each other, if the limitations of each QA method (mechanical sag, detector response, and detector alignment) are carefully considered.

Monte Carlo calculation of dose distributions in oligometastatic patients planned for spine stereotactic ablative radiotherapy

Dosimetric consequences of plans optimized using the analytical anisotropic algorithm (AAA) implemented in the Varian Eclipse treatment planning system for spine stereotactic body radiotherapy were evaluated by re-calculating with BEAMnrc/DOSXYZnrc Monte Carlo. Six patients with spinal vertebral metastases were planned using volumetric modulated arc therapy. The planning goal was to cover at least 80% of the planning target volume with a prescribed dose of 35 Gy in five fractions. Tissue heterogeneity-corrected AAA dose distributions for the planning target volume and spinal canal planning organ-at-risk volume were compared against those obtained from Monte Carlo. The results showed that the AAA overestimated planning target volume coverage with the prescribed dose by up to 13.5% (mean 8.3% +/− 3.2%) when compared to Monte Carlo simulations. Maximum dose to spinal canal planning organ-at-risk volume calculated with Monte Carlo was consistently smaller than calculated with the treatment planning system and remained under spinal cord dose tolerance. Differences in dose distribution appear to be related to the dosimetric effects of accounting for body composition in Monte Carlo simulations. In contrast, the treatment planning system assumes that all tissues are water-equivalent in their composition and only differ in their electron density.

Errors of two CT simulation positioning methods in intensity-modulated radiotherapy: a comparative study

Objective To compare the errors of final isocenter marking method and reference point marking method for CT simulation positioning in intensity-modulated radiotherapy (IMRT).Methods From 2009 to 2012,327 patients with head and neck cancer for IMRT underwent CT simulation positioning using the Philips Brilliance CT Big Bore scanner and Philips Tumor LOC workstation and were divided into final isocenter marking group (n =208) and reference point marking group (n =119) according to positioning methods.Target volume delineation and treatment plan design were performed on the Varian Eclipse treatment planning system (TPS).Before treatment,kilovoltage cone-beam CT scans and registration were performed with the Varian EX on-board imager system to obtain beam position errors in the right-left (RL),superior-inferior (SI),and anterior-posterior (AP) directions,and then comparisons of errors between the two groups were made by independent-samples t test.Finally,the TPS was used to measure the changes in the doses to the organs at risk after moving isocenters in the RL,SI,and AP directions among 5 patients with nasopharyngeal carcinoma.Results The mean beam position errors in the three directions were less in the final isocenter marking group than in the reference point marking group (P =0.02,0.01,0.03).After moving isocenters in the three directions,the target dose was reduced and the dose to the normal tissue around the target tumor was increased significantly.The error in the AP direction had the maximum influence on the spinal cord and brainstem.Conclusions Final isocenter marking method leads to less beam position error than reference point marking method in CT simulation positioning.Small isocenter motion can cause large changes in the doses to the organs at risk. Key words: CT simulation positioning; Final isocenter marking method; Reference point marking method; Head and neck neoplasms/intensity-modulated radiotherapy

SU-E-T-327: Dosimetry and Implementation of High-Dose Rate MLC-Based GRID Therapy Using a 6MV Flattened Filter Free (FFF) Photon Beam

Purpose: To dosimetrically characterize high-dose rate multileaf collimator (MLC)-based, spatially-fractionated GRID therapy (MLC GRID) using a 6MV flattened filter free (FFF) photon beam delivered from a Varian TrueBeam linear accelerator. Methods: Planning for MLC GRID therapy was done by a Varian Eclipse treatment planning system. The dose plane at depth of 5cm was exported for a dose comparison between the calculated and measured data. MLC GRID with the projected square openings of 10 x 10 mm at the isocenter was studied using 6MV FFF photons at a dose rate of 1400MU/min. Four separate MLC-shaped beams of each with two columns of GRID openings were automated for delivery by a Varian TrueBeam linear accelerator. MLC GRID was characterized with depth dose, profile and output using an IBA scanning system. Results: 6MV FFF GRID patterns recorded using films and EPID were found to be consistent in both aperture sizes and alignment. A MapCheck was used to validate the 16–20 cm field of a 6MV FFF GRID with a pass rate of 97% at 3%/3mm criteria. The ratio of the openings to the blocked area was quantified to be 25% with a valley–to–peak ratio of about 16.5% from the beam profiles in the central aperture. GRID therapy is a technique to treat advanced bulky tumor in a single or a few fractions by delivering a dose of 12 to 20Gy to tumors that do not respond to conventional fractionation schemes. Four abutting GRID fields with 3000 MU to each field took only about 9 minutes using a 6MV FFF beam, but took more than 30 minutes for delivery at 400 MU/min. Conclusion: 6MV FFF MLC–based GRID therapy aligned accurately with IGRT showed advantages in the higher dose delivery by more than 2 times reducing treatment time over a conventional delivery.

SU‐E‐T‐403: Intensity Modulated Proton Therapy Plans with Multiple Fields for Prostate Cancer

Purpose: To investigate the possible improvements in the dose conformity in intensity modulated proton therapy (IMPT) plans with multiple fields as compared to widely used single field optimized (SFO) plans with two lateral fields for spot scanned proton therapy (SSPT) and to verify the accuracy of the delivered dose using the Radiological Physics Center Proton Prostate Phantom (RPCPPP). Methods: The CT images of RPCPPP were used to design multiple IMPT plans using both SFO and multi field optimization (MFO) methods in the Varian Eclipse treatment planning system (TPS). The standard SFO clinical plan used two lateral fields going through the femoral heads. Non‐standard plans were generated using three or more oblique fields based on the SFO and MFO options of the TPS. The angles were carefully chosen to avoid bladder and rectum as much as possible. The RPC phantom containing films and TLDs was irradiated using the standard clinical SFO plan and the four field MFO plan to compare the planned and delivered dose. Results: Based on the DVHs, MFO plan with multiple oblique fields is found to have superior target conformity with reduced dose to organs at risk (OAR) and normal tissue, especially in the high dose regions, as compared to SFO plans. The point doses measured by TLDs and 2‐D dose distribution measured by GafChromic films in the RPCPPP irradiation passed the commonly used RPC acceptance criteria, namely +/− 7% agreement in point does and more than 85% of the voxels meeting the gamma index criteria of 7% dose or 4 mm distance agreement. Conclusion: The MFO SSPT plans for prostatic targets with multiple fields lead to better target conformity with lower doses to OARs as compared to SFO plans. The results of irradiations of the RPCPPP show that these plans can be delivered with acceptable accuracy

SU-E-T-331: Dose Comparison of IVD2 Measurement and Eclipse Treatment Planning Calculation for Whole Brain Irradiation

Purpose: To compare the dmax doses measured by Sun Nuclear IVD2 system with the doses calculated by Varian Eclipse treatment planning system (TPS) in the whole brain irradiation (WBI) of a humanoid unsliced phantom using standard lateral fields. Methods: The IVD2 system was carefully calibrated for measuring the dmax dose (at 1.5cm) of Varian Trilogy 6 MV photon beams. This system was also experimentally validated for the dependencies of field size, SSD, dose rate, and beam incident angle. The head and neck part of the phantom was scanned by a Phillips CT at the slice thickness of 2mm. A WBI plan was created using 6MV photon beams and parallel opposed lateral fields with the multiple leave collimator to spare the orbital structures. The plan was normalized to the weight point and with >=98% of brain volume covered by >=98% of prescription dose (300cGy/fraction). The phantom setup and target location were verified by the kV imaging and CBCT; the diodes were carefully placed at various locations. The comparison was made between the measured dose and the planned dose for all interested anatomical sites. Results: Our results indicated that the measured doses agree with the planned dose within 7.9% for the sites in the treatment fields such as mid right/left parietal bones. For the sites outside or partially outside the fields, such as the low chin and right/left orbits, the difference between the measurement and the TPS prediction was varied between 8.3% and 33.8%. Conclusion: Though the human head is relatively small in size, the dose at dmax of WBI can be accurately predicted by TPS and measured by IVD2 system if the points of the interest are inside of the fields. A difference up to 33.8% could be expected if the points are located on the boarder of the fields.

SU-E-T-716: Comparison Between Two and Three Beam Intensity Modulated Proton Therapy (IMPT) Plans for Base of Skull Chordomas

#Purpose: To investigate the dependency of the quality of treatment plans on the number of beams using Intensity Modulated Proton Therapy (IMPT) with pencil beam scanning for base of skull chordomas. Methods: IMPT plans were generated with two (lateral) and three (lateral + Superior anterior oblique) beams on a Varian Eclipse treatment planning system for five patients treated previously at our institution. All patients were on our clinical protocol and this study was approved by our internal Institutional Review Board (IRB). IMPT plans were optimized aiming to achieve 95% of planned treatment volume (PTV) receiving 95% of the dose prescription (D95) and all normal tissue dose constraints. The organs-at-risk (OARs) reviewed were brainstem, spinal cord, optic chiasm, optic nerves, and cochlea. Results: Dose volume histograms (DVH) for each plan were calculated for dose evaluations. We compared the two beam plans and three beam plans for all patients with respect to target coverage and doses to normal tissue structures mentioned previously. The average percent difference (two field values subtracted from three field values) in mean dose and max dose, respectively, between two beam and three beam plans for all patients was as follows: −2.5%, −1.1% for brainstem; +7.9%, +4.0% for spinal cord, +14.7%, +15.6% for optic chiasm, +12.8%, +3.8% for optic nerves, and −15.8%, −6.7% for cochlea, with a +1.0% difference in coverage to D95 of Clinical Target Volume (CTV). Conclusion: Although the quality of the IMPT plans is patient-specific, the two beam plans tended to spare optical structures while the three beam plans tended to spare cochlea. In clinical practice, it is always preferable to reduce the treatment time, and thus number of beams in IMPT. Since target coverage was comparable for both beam arrangements, importance of specific OAR sparing for each patient will determine the number of beams chosen.

SU‐C‐108‐04: To Better Understand IMRT and Planned Dose Distributions: A Measure of Entropy

Purpose: Traditional IMRT QA procedures yield close approximations to the patients planned dose distribution. However, the relationship between QA data and the patients planned distribution is difficult to quantify. This study presents an approach to investigate the relationship between a planar QA distribution and the same doseplane within the patient using complexity analysis. Methods: Shannon entropy was the decided metric to quantify the complexity of three calculated dose distributions: 1) the IMRT QA distribution, 2) the in‐patient dose distributions with heterogeneity correction on, and 3) the in‐patient dose distribution without heterogeneity correction. Varian Eclipse treatment planning system was used to construct the plans and calculate dose. MapCheck 1175 diode array was used for the IMRT QA acquisition. Global entropy was calculated without threshold for each distribution, and local entropies were found for low dose thresholds ranging from 0% to 100% of both prescribed and maximum dose. All calculations were done using MATLAB. Results: The QA data proved to have the highest global entropy of the three dose distributions analyzed. Local entropies, calculated using subsets of all dose points, converged at a low dose threshold of &gt;∼70% of maximum dose. For low dose thresholds &lt;∼70%, the entropies of both patient distributions remained similar, while the entropy for the QA distribution diverged, increasing in value. Conclusion: The greater entropy values for the QA distribution at low dose thresholds &lt;∼70% suggests a more disordered dose distribution than the two planned dose distributions. This is thought to be due to the larger phantom size with respect to the patient, which tends to spread out the dose gradient outside the target. This Result is suggestive that an optimal threshold for meaningful gamma analysis could be &gt;70% of maximum dose, or where the QA and planned dose distributions entropies, or complexities, are accurately matched.

SU‐E‐T‐423: Effect of X‐Ray Source Spot Size Modeling in AcurosXB Dose Calculation Algorithm On VMAT Based SBRT Treatments

Purpose: Aim of this study is to evaluate AcurosXB(AXB) dose calculation algorithm dosimetrically with different X‐ray source spot size modeling and its clinical effect on Volumetric Modulated Arc Therapy(VMAT) based Stereotactic Body Radiotherapy(SBRT). Methods: 10 SBRT cases were purposely selected to analyze the effect of source spot size for smaller field size. All the cases were planned in Varian Eclipse(V10.0) treatment planning system(TPS) which uses AXB algorithm for VMAT dose computation. For this study source spot sizes varied from 0.5mm, 1.0mm, 1.5mm and 2.0mm in X as well as Y‐direction in the beam data configuration. SBRT plans were optimized using Progressive Resolution Optimizer III(PRO‐III) and dose was calculated by AXB with different source spot sizes. Plan quality index such as conformity index(CI) and gradient measure for PTV and for normal tissues volume of 90% and 25% prescribed dose were used for analysis. VMAT plans were delivered with Varian Clinac‐iX and 2D‐planner dose was measured using iba iMatrixx 2D array in Multicube phantom. Gamma (3mm/3%) analyses were performed between measured and predicted doses for different spot sizes. Results: Conformity index value for PTV were found to be 1.16±0.21, 1.14±0.20, 1.13±0.2 &amp; 1.08±0.17 and gradient measure were 1.45±0.26, 1.46±0.26, 1.47±0.25 &amp; 1.52±0.27 for spot size of 0.5mm, 1.0mm, 1.5mm &amp; 2.0mm respectively. Volume of normal tissue receiving 90% of prescribed dose for 2mm spot size was 2.7% more in compare to 0.5mm spot size. Gamma analysis(3mm/3%) were found to be 98.77±1.03%, 98.96±0.91%, 99.18±0.76% &amp; 99.43±0.60% for spot size 0.5mm, 1.0mm, 1.5mm &amp; 2.0mm respectively. Conclusion: The Result showed that spot size configuration of 1.5–2.0mm were matching very closely with our measurements. Based on the variation in the PTV coverage and dose to normal tissues it is highly recommended to fine tune spot size parameters for VMAT based SBRT.

SU‐E‐T‐247: Design and Implement of the Radiation Therapy Case Conference Multi‐Media System Based On Windows Remote Desktop Feature

Purpose: To explore a method of using a computer system for multi‐media cases conference in clinical radiation oncology, in order to replace the traditional use of film images and paper. Methods: In a conference room without any treatment planning system(TPS), on a dedicated networked computer, we use Remote Desktop feature of Windows XP to remote access an Varian Eclipse TPS and the radiation therapy information management system (RTIMS), which is an in‐house developed system based on the flow of radiation therapy. Then in this room, we can online read the information of CT images, target volumes, treatment plans, electronic medical records and etc, and project it on the screen. Results: There has been half a year since we successfully set up a radiation therapy case conference multi‐media system(RTCCMMS) in the department. With this method, we can share the TPS in another room, and do not need buy another TPS. It is convenient and effective that the oncologists and physicists conduct a collective case discussion. Conclusion: This method required equipments are simple, and it is a safe and reliable technology, greatly improving the clinical efficiency and quality of medical care. Project supported by the National Natural Science Foundation of China (Grant No.81101694).

Beam angle selection for intensity-modulated radiotherapy (IMRT) treatment of unresectable pancreatic cancer: are noncoplanar beam angles necessary?

External beam radiation therapy with concurrent chemotherapy (CRT) is widely used for the treatment of unresectable pancreatic cancer. Noncoplanar (NCP) 3D conformal radiotherapy (3DCRT) and coplanar (CP) IMRT have been reported to lower the radiation dose to organs at risk (OARs). The purpose of this article is to examine the utility of noncoplanar beam angles in IMRT for the management of pancreatic cancer. Sixteen patients who were treated with CRT for unresectable adenocarcinoma of the pancreatic head or neck were re-planned using CP and NCP beams in 3DCRT and IMRT with the Varian Eclipse treatment planning system. Compared to CP IMRT, NCP IMRT had similar target coverage with slightly increased maximum point dose, 5,799 versus 5,775cGy (p=0.008). NCP IMRT resulted in lower mean kidney dose, 787 versus 1,210cGy (p<0.0001) and higher mean liver dose, 1,208 versus 1,061cGy (p<0.0001). Also, NCP IMRT resulted in similar mean stomach dose, 1,257 versus 1,248cGy (p=0.86) but slightly higher mean small bowel dose, 981 versus 866cGy (p<0.0001). The NCP IMRT was able to significantly decrease bilateral kidney dose, but did not improve other dose-volume criteria. The use of NCP beam angles is preferred only in patients with risk factors for treatment-related kidney dysfunction.

A Practical Method to Evaluate and Verify Dose Calculation Algorithms in the Treatment Planning System of Radiation Therapy

Purpose: To introduce a practical method of using an Electron Density Phantom (EDP) to evaluate different dose calculation algorithms for photon beams in a treatment planning system (TPS) and to commission the Anisotropic Analytical Algorithm (AAA) with inhomogeneity correction in Varian Eclipse TPS. Methods and Materials: The same EDP with various tissue-equivalent plugs (water, lung exhale, lung inhale, liver, breast, muscle, adipose, dense bone, trabecular bone) used to calibrate the computed tomography (CT) simulator was adopted to evaluate different dose calculation algorithms in a TPS by measuring the actual dose delivered to the EDP. The treatment plans with a 6-Megavolt (MV) single field of 20 × 20, 10 × 10, and 4 × 4 cm2 field sizes were created based on the CT images of the EDP. A dose of 200 cGy was prescribed to the exhale-lung insert. Dose calculations were performed with AAA with inhomogeneity correction, Pencil Beam Convolution (PBC), and AAA without inhomogeneity correction. The plans were delivered and the actual doses were measured using radiation dosimetry devices MapCheck, EDR2-film, and ionization chamber respectively. Measured doses were compared with the calculated doses from the treatment plans. Results: The calculated dose using the AAA with inhomogeneity correction was most consistent with the measured dose. The dose discrepancy for all types of tissues covered by beam fields is at the level of 2%. The effect of AAA inhomogeneity correction for lung tissues is over 14%. Conclusions: The use of EDP and Map Check to evaluate and commission the dose calculation algorithms in a TPS is practical. In Varian Eclipse TPS, the AAA with inhomogeneity correction should be used for treatment planning especially when lung tissues are involved in a small radiation field.

Projection imaging of photon beams by the Čerenkov effect

A novel technique for beam profiling of megavoltage photon beams was investigated for the first time by capturing images of the induced Čerenkov emission in water, as a potential surrogate for the imparted dose in irradiated media. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire 2D projection images of Čerenkov emission from a 4 × 4 cm(2) 6 MV linear accelerator (LINAC) x-ray photon beam operating at a dose rate of 400 MU∕min incident on a water tank with transparent walls. The ICCD acquisition was gated to the LINAC sync pulse to reduce background light artifacts, and the measurement quality was investigated by evaluating the signal to noise ratio and measurement repeatability as a function of delivered dose. Monte Carlo simulations were used to derive a calibration factor for differences between the optical images and deposited dose arising from the anisotropic angular dependence of Čerenkov emission. Finally, Čerenkov-based beam profiles were compared to a percent depth dose (PDD) and lateral dose profile at a depth of d(max) from a reference dose distribution generated from the clinical Varian ECLIPSE treatment planning system (TPS). The signal to noise ratio was found to be 20 at a delivered dose of 66.6 cGy, and proportional to the square root of the delivered dose as expected from Poisson photon counting statistics. A 2.1% mean standard deviation and 5.6% maximum variation in successive measurements were observed, and the Monte Carlo derived calibration factor resulted in Čerenkov emission images which were directly correlated to deposited dose, with some spatial issues. The dose difference between the TPS and PDD predicted by Čerenkov measurements was within 20% in the buildup region with a distance to agreement (DTA) of 1.5-2 mm and ±3% at depths beyond d(max). In the lateral profile, the dose difference at the beam penumbra was within ±13% with a DTA of 0-2 mm, ±5% in the central beam region, and 2%-3% in the beam umbra. The results from this initial study demonstrate the first documented use of Čerenkov emission imaging to profile x-ray photon LINAC beams in water. The proposed modality has several potential advantages over alternative methods, and upon future refinement may prove to be a robust and novel dosimetry method.