Isodose Curves Research Articles

Treatment of absolute painful glaucoma with dynamic arcs using novalis shaped beam radiosurgery

Purpose: We assessed the effect of shaped beam conformal stereotactic radiosurgery (SRS) in 1 patient with chronic painful glaucoma in one eye refractory to medical treatment. Methods and Materials: Left eye ciliary body was targeted at 18 Gy (90% isodose curve) with a dedicated linear accelerator (Novalis, BrainLAB, Germany) SRS. Interval follow-up was performed weekly for the first month, and every 2 months until 1 year was completed with clinical examinations and intraocular pressure (IOP) measurements. Results: Ocular pain resolved at 6 weeks after SRS treatment. IOP decreased and normalized at 1 year. Conclusions: We present a case in which SRS appears to be an effective treatment of chronic refractory painful glaucoma. Further Phase I studies are needed to know the best parameters for radiation dose, tolerance of organs at risk, and pathophysiologic effects.

227: Computer Optimized Non-Coplanar Beam Set-ups for SBRT of Liver Tumors

In the Erasmus MC, patients with primary or metastatic liver tumors with radiotherapy as the treatment of choice are treated with hypofractionation in a stereotactic body frame. Prior to each of the three to five fractions a contrast enhanced CT-scan is acquired for image guidance. For plan generation, beam set-ups are currently selected by experienced dosimetrists in a trial and error procedure. Generally, most beams are in the axial plane, but also non-coplanar directions are selected. The 65% isodose curve that closely surrounds the PTV is used for dose prescription.

Dosimetric evaluation of the staff working in a PET/CT department

The dosimetric literature data concerning the medical personnel working in positron emission tomography/computed tomography (PET/CT) departments are limited. Therefore, we measured the radiation dose of the staff working in the first PET/CT department in Greece at the Diagnostic and Therapeutic Center of Athens HYGEIA—Harvard Medical International. As, for the time being, only 2-deoxy-2-[ 18F]fluoro-d-glucose (FDG) PET studies are performed, radiation dose measurements concern those derived from dispensing of the radiopharmaceutical as well as from the patients undergoing FDG-PET imaging. Our aim is to develop more effective protective measures against radionuclide exposure. To estimate the effective dose from external exposure, all seven members of the staff (two nurses, two medical physicists, two technologists, one secretary) had TLD badges worn at the upper pocket of their overall, TLD rings on the right hand and digital dosimeters at their upper side pocket. In addition, isodose curves were measured with thermoluminescence detectors for distances of 20, 50, 70 and 100 cm away from patients who had been injected with 18F-FDG. Dose values of the PET/CT staff were measured with digital detectors, TLD badges and TLD rings over the first 8 months for a total of 160 working days of the department's operation, consisting of a workload of about 10–15 patients/week who received 250–420 MBq of 18F-FDG each. Whole - body collective doses and hand doses for the staff were the following: Nurse #1 received 1.6 mSv as a whole body dose and 2,1 as a hand dose, Nurse #2 received 1.9 and 2.4 mSv respectively. For medical physicist #1 the dose values were 1.45 mSv whole body and 1.7 mSv hand dose, for medical physicist #2 1.67 mSv wholebody dose and 1.55 mSv hand dose and for technologists #1 & #2 the whole body doses were 0.7 and 0.64 mSv respectively. Lastly, the secretary received 0.1 mSv whole body dose. These preliminary data have shown that the dose levels of our PET/CT staff are within acceptable limits.

An Investigation of Operator Exposure in Interventional Radiology

A study was conducted to investigate how operator exposure in interventional radiology is affected by various common fluoroscopic imaging conditions. Stray radiation levels surrounding the imaging chain of a C-arm angiographic system were measured with an anthropomorphic abdomen phantom under different imaging conditions, and isodose curves were constructed. Operator exposure was shown to increase with patient dose-area product as the imaging field of view (FOV) is changed, with the highest scatter levels occurring with an intermediate-sized FOV. Use of copper spectral beam filtration was found to result in decreased operator exposure, whereas use of wedge-shaped equalization filters was found to increase exposure. The effect of increasing patient abdomen thickness was simulated by surrounding the phantom with plastic bolus material. Increasing the thickness by 5 cm resulted in a doubling of exposure at the operator's waist. Exposure to the operator's upper body was significantly reduced when the FOV was positioned on the far side of the patient. Operator exposure can be maintained at an acceptable level by taking these variables into consideration and incorporating the suggested dose reduction techniques into routine practice to the greatest extent possible.

Assessment of absorbed dose to thyroid, parotid and ovaries in patients undergoing Gamma Knife radiosurgery

Stereotactic radiosurgery was originally introduced by Lars Leksell in 1951. This treatment refers to the noninvasive destruction of an intracranial target localized stereotactically. The purpose of this study was to identify the dose delivered to the parotid, ovaries, testis and thyroid glands during the Gamma Knife radiosurgery procedure. A three-dimensional, anthropomorphic phantom was developed using natural human bone, paraffin and sodium chloride as the equivalent tissue. The phantom consisted of a thorax, head and neck and hip. In the natural places of the thyroid, parotid (bilateral sides) and ovaries (midline), some cavities were made to place TLDs. Three TLDs were inserted in a batch with 1 cm space between the TLDs and each batch was inserted into a single cavity. The final depth of TLDs was 3 cm from the surface for parotid and thyroid and was 15 cm for the ovaries. Similar batches were placed superficially on the phantom. The phantom was gamma irradiated using a Leksell model C Gamma Knife unit. Subsequently, the same batches were placed superficially over the thyroid, parotid, testis and ovaries in 30 patients (15 men and 15 women) who were undergoing radiosurgery treatment for brain tumours. The mean dosage for treating these patients was 14.48 ± 3.06 Gy (10.5–24 Gy) to a mean tumour volume of 12.30 ± 9.66 cc (0.27–42.4 cc) in the 50% isodose curve. There was no significant difference between the superficial and deep batches in the phantom studies (P-value < 0.05). The mean delivered doses to the parotid, thyroid, ovaries and testis in human subjects were 21.6 ± 15.1 cGy, 9.15 ± 3.89 cGy, 0.47 ± 0.3 cGy and 0.53 ± 0.31 cGy, respectively. The data can be used in making decisions for special clinical situations such as treating pregnant patients or young patients with benign lesions who need radiosurgery for eradication of brain tumours.

TU‐E‐ValB‐04: Impact On Nodal Dose Distribution From Daily Fiducial Tracking for IMRT Prostate and Pelvic Node Treatments

Purpose: In prostate only treatments, fiducial seeds can be implanted in the prostate and used for accurate daily targeting of the prostate. For high‐risk disease, the regional nodes are simultaneously treated with the prostate using intensity‐modulated radiotherapy thereby sparing the rectum, bladder, bowel, and femoral heads. However, the implanted fiducials do not indicate the position of the nodes as the nodes are anchored to the patient's bony anatomy. Method and Materials: We measured the displacement of the fiducials with respect to the bony anatomy for each treatment fraction as a surrogate for the displacement of the nodes with respect to the prostate from their original planning position. The displacements, taken from the clinical cases were measured to the nearest 0.01 cm, in LR, AP, and SI directions and were used to perform a forward calculation of the IMRT plan that the patient received during treatment. The dose coverage to the nodes, femoral heads and bowel were evaluated using isodose curves and dose volume histograms. Results: Our results indicate for an average patient, the maximum shifts of the seeds with respect to the original DRR position was 0.78 cm inferior, 0.28 cm left, and 0.46 cm posterior. A weighted average of the same directions would be: 0.46 cm, 0.05 cm and 0.13 cm. Forward planning using the weighted average shifts, shows changes in the nodal maximum dose of −0.6% and mean dose −0.2%. The maximum bowel dose changed by −0.5% and the mean dose by −1%. The dose to the left and right femoral heads increased by +2% and +1.2%, respectively. Conclusion: The results indicate that for an average patient the difference in the dose planned to the nodal targets and the organs‐at‐risk are not clinically significant to those received during treatment using fiducial tracking of the prostate.

SU-FF-T-12: A Critical Review of the Performance of Varian's New Anisotropic Analytical Algorithm (AAA) Utilized in Photon Treatment Planning

Purpose: To critically review the performance of the Anisotropic Analytical Algorithm with respect to accuracy across a complete range of 3D and IMRT treatment fields and its ability to account for tissue heterogeneities, hard and dynamic wedges, multileaf collimated beams and dynamic leaf motions. Method and Materials: AAA generated monitor units are compared with predicted monitor units generated by measurements at central axis in a water phantom for open fields, hard and dynamic wedges and multileaf collimated fields. AAA predicted doses are compared against measured doses in a anthropomorphic chest phantom in a complex arrangement of beams for both 6 and 18 MV photons, multileaf collimated beams and dynamic wedges. AAA predicted isodose curves are compared by digital subtraction against film recorded isodose curves. Results: For 6 and 18 MV photons open field beams had an average error of 0.01 and 0.00% and a maximum error of 1.17 and 1.24%; hard wedged fields had an average error of 0.12 and 0.53% and a maximum error of 1.16 and 2.32%; dynamic wedges had an average error of 1.28 and 1.87 and a maximum error of 3.3 and 4.8% respectively. These measurements had a measurement error of ±1%. In the chest phantom measurements the dose was evaluated for 1) Isocenter dose, 2) Off‐axis dose, and 3) Out‐of‐field dose. The results showed for 6 and 18 MV the average error was 1) 2.3%, 0.4%, 2) 0.7%, 4.4% and 3) 0.2%, 0.3% respectively. The digital subtraction analysis showed similar results and is displayed and analyzed. Conclusion: The AAA algorithm is a robust and accurate calculation engine. It shows consistent accuracy throughout the useful range of field sizes and depths. It demonstrates an improved accuracy with hard wedge beam hardening and tissue heterogeneities and successfully manages electron contamination above 5 cm in depth.

TH‐C‐230A‐09: Implementation of MINERVA/PEREGRINE as An ATC Review Tool

Purpose: To test the feasibility of a system of software (MINERVA/PEREGRINE) developed by the Idaho National Laboratory (INL) and the Lawrence Livermore National Laboratory (LLNL) for supporting quality assurance (QA) review of cooperative‐group clinical trials treatment planning data within the Advanced Technology QA Consortium (ATC). Method and Materials: MINERVA is an open architecture, open source code system, designed to accommodate any computation engine through a plugin structure. MINERVA supports two types of data storage ‐ relational databases and XML files. Patient data is stored in a relational database. XML‐based import/export tools have been developed to transfer patient information between QA Centers and reviewers. Tools have been implemented as plugins to allow addition of more advanced tools. The research version of the LLNL PEREGRINE Monte Carlo code has been relocated to UC Davis Medical Center. The basis has been created for an integration of PEREGRINE with ITC, and it has been integrated with MINERVA as a calculation engine. Results: MINERVA supports submission of digital treatment planning data using RTOG format. Ability to import DICOM‐RT objects that satisfies the ATC DICOM conformance statement is needed. MINERVA provides display of DVHs and axial patient images with overlaid organ‐at‐risk/target‐volume contours, as well as user‐defined isodose curves. Users can edit contours, recalculate DVHs for these user‐defined structures, and display point doses. Test cases for several body sites have been calculated using PEREGRINE to demonstrate feasibility. We believe that the use of Monte Carlo simulation will become a key tool for credentialing and QA review of clinical trials treatment planning and verification data in the near future. Conclusion: The MINERVA/PEREGRINE software system appears to be well suited to meet the needs regarding QA of data submitted for future ATC‐supported clinical trials. Conflict of Interest: This work was supported by NIH U24 Grant CA81647.

HI-ART tomotherapy megavoltage CT-based cumulative dose volume histograms and isodose curve plans for low-dose-rate brachytherapy of cervical cancer

Purpose: Evaluation of HiArt tomotherapy megavoltage CT imaging to determine cumulative dose volume histograms (DVH) and cumulative isodose cure plans for intracavitary low-dose-rate brachytherapy of cervical carcinoma. Megavoltage scanning reduces artifact from metal applicator sets and allows for accurate contouring of anatomic structures. Doses from the cumulative DVHs were compared with the ICRU 50 bladder and rectal points and Point A doses.

Empirical formula for the prediction of off axis ratios and isodose curves for a treatment planning system.

A mathematical model has been developed for prediction of off axis ratio (OAR), using Wood - Saxon term used to represent nuclear potential. This method has been satisfactorily applied for predicting OAR in case of 60Co γ-rays and high energy X-rays. Investigations are considered upto a depth of 25 cm in the case of 4MV LINAC for which measurements were carried out in our laboratory using indigenously developed Radiation Field Analyzer. For 60Co γ-rays as well as 6 and 18MV LINAC beams we could get off-axis profiles only upto 20 cm. The shift δ between measured and predicted OAR is within ±2 mm except for 20 cm depth near the falling edge of the penumbra, where it is 2.80 mm. Software has been developed in Visual Basic 6 on Windows platform to plot Isodose curves, which is based on the mathematical modeling of OAR and central axis percentage depth dose.

Application of mri for improved local control in complex radiotherapy of cervical cancer

Background: The aim of this study was to analyze the use of magnetic resonance imaging (MRI) as a modern medical imaging technique in radiotherapy with special emphasis on the integration of MRI and a novel technique in brachytherapy to optimize treatment outcome in cervical cancer. Methods: In addition to the CT based shrinking volume conformal teletherapy in 31 patients with locally advanced cervical cancer, MRI examination with a special adjustable applicator at the treatment site was performed for the brachytherapy planning. To avoid excessive doses to the healthy structures during complex cervical radiotherapy isodose curves were calculated upon the information of the MR image and dose distribution was evaluated. Results: The consecutive application of CT and MRI limited the possibility for overdosage of the critical organs and undertreatment of the advanced tumor spread in all cases. The overall response rate for the complex treatment was 74.2% with complete regression in 25.8% of the cases. Based on the exact information of the three dimensional digital data radiation doses could be optimized without increasing the possibility of acute complications rate. Conclusion: The introduction of 3D treatment planning for teletherapy pelvic and boost irradiation of cervical carcinoma as well as for the brachytherapy part of the complex treatment is to be recommended. .

Quality control of blood irradiation: determination T cells radiosensitivity to cobalt‐60 gamma rays

To identify the most appropriate dose for the prevention of transfusion-associated graft-versus-host disease, the radiosensitivity of T cells has been determined in blood bags irradiated with X-rays produced by a linear accelerator and gamma rays derived from the cesium-137 source of a specific irradiator. In this study, the influence of doses ranging from 500 to 2500 cGy was investigated on T cells isolated from red blood cell (RBC) units preserved with ADSOL and irradiated with a cobalt teletherapy unit. A thermal device consisting of acrylic and foam was constructed to store the blood bags during irradiation. Blood temperature was monitored with an automated data acquisition system. Dose distribution in the blood bags was analyzed based on isodose curves obtained with a polystyrene phantom constructed for this purpose. The influence of cobalt-60 gamma radiation on T cells was determined by limiting-dilution analysis, which measures clonable T cells. T-cell content of the mononuclear cell population plated was assessed by flow cytometry with a monoclonal antibody specific for CD3. Blood temperature ranged from 2 to 4.5 degrees C during irradiation. Dosimetry performed on the phantom showed a homogenous dose distribution when the phantom was irradiated with a parallel-opposite field. A radiation dose of 1500 cGy led to the inactivation of T cells by 4 log, but T-cell growth was observed in all experiments. At 2500 cGy, no T-cell growth was detected in any of the experiments and a greater than 5 log reduction in functional T cells was noted. The results showed that a dose of 2500 cGy completely inactivates T cells in RBC units irradiated with cobalt-60 source.

Computerized radiotherapy planning: retrospective analysis and current methods

To present the development of treatment planning methods at the National Institute of Oncology (NIO) from 1969 till 2005. The methods and devices of treatment planning is described chronologically. First we did the treatment planning with in-house made devices: body contour drawing instrument, simplified anatomical cross sections, treatment planning table for the cross section projection, archives of isodose curves adjusted to body contours, etc. It was a significant improvement when the graphical addition of isodose curves was followed by computerized dose calculation. In 1978 the work of the Computerized National Treatment Planning Network was started. The Network was organized by IAEA, Ministry of Health Hungary and NIO. The modern treatment planning started at NIO in 1981. From this year, the treatment planning was based on CT, using the CT apparatus of the Medical Postgraduate University. In 1991 a Siemens MEVAPLAN treatment planning system was installed at NIO. The CT data were transferred to the system via floppy disk. The 3D treatment planning program (Pinnacle software of ADAC) started in 2000. The CT, the treatment planning system and newer linear accelerators are connected through the computerized radiotherapy network. Patient positioning, fixing and control devices (mask, EPID etc.) increased the efficacy of the treatment. In-house made devices help this aim too: mirrors at the linear accelerators, special skin marks for CT, block verification unit, multileaf collimator for x-ray treatment simulator. In this year the intensity-modulated radiotherapy (IMRT) will be started at NIO. The treatment planning at NIO developed to high degree during the investigated time, and it had a considerable effect on the efficacy of radiotherapy.

1377 POSTER Movement of the cervix in after-loading brachytherapy: implications for designing external beam radiotherapy boost fields

This report presents the clinical applications of an automated treatment-planning program of high-dose-rate intracavitary brachytherapy (HDR-ICBT) for advanced uterine cervical cancer infiltrating the parametrium and the lower vagina.We adopted HDR-ICBT under optimized dose distribution for 22 cervical cancer patients with tumor infiltration of the lower half of the vagina. All patients had squamous cell carcinoma with International Federation of Gynecology and Obstetrics clinical stages IIB–IVA. After whole pelvic external beam irradiation with a median dose of 30.6 Gy, a conventional ICBT was applied as “pear-shaped” isodose curve. Then 3–4 more sessions per week of this new method of ICBT were performed. With a simple determination of the treatment volume, the cervix-parametrium, and the lower vagina were covered automatically and simultaneously by this program, that was designated as “utero-vaginal brachytherapy”. The mean follow-up period was 87.4 months (range, 51.8–147.9 months).Isodose curve for this program was “galaxy-shaped”. Five-year local-progression-free survival and overall survival rates were 90.7% and 81.8%, respectively. Among those patients with late complications higher than Grade 2 Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer morbidity score, only one (4.5%) developed severe proctitis.Because of the favorable treatment outcomes, this treatment-planning program with a simplified target-volume based dosimetry was proposed for cervical cancer with lower vaginal infiltration.

Effect of intravenous contrast on treatment planning system dose calculations in the lung?

Intravenous contrast-enhanced computed tomography is utilised in radiotherapy lung treatment planning to improve the delineation of the tumour volume and nodal areas. In the resultant CT images, the electron density is increased within the vascular structures of the lung and the overall density in the lung volume may also be increased. As yet, it is unclear whether the change in density affects the accuracy of dose calculations based on this CT data. Two investigations were undertaken. Firstly, contrast-enhancement was simulated using an anthropomorphic phantom. In the second investigation, bulk density corrections were performed in an existing patient dataset. In both investigations, treatment plans were generated using both pre- and post-contrast datasets. The numbers of monitor units calculated in each of the plans were compared, as were the resulting isodose curves, dose volume histograms and physical mean lung doses. The numbers of monitor units calculated from the contrast- and non contrast-enhanced datasets agreed within 2%. The isodose curves and dose volume histograms showed very minor differences in size and shape. With the introduction of contrast agent, the physical mean lung doses calculated remained below the limit recommended for an acceptable plan. These results indicate that the introduction of contrast agent has a minimal dosimetric impact upon lung cancer treatment plans.

Dose measurements for characterization of a semi-industrial cobalt-60 gamma-irradiation facility

Cobalt-60 irradiation facility has been put into operation at the National Centre of Nuclear Sciences and Technology, Sidi-thabet, Tunisia. Its technical specifications were controlled by dosimetry commissioning experiments and compared to the data specified by the plant manufacturer. Installation qualification has been carried out to measure absorbed dose distribution in the irradiation cell and products. Two dosimeter systems were used for measurements: Red and Amber Perspex and Cellulose Triacetate (CTA). The regions of minimum and maximum absorbed dose within a homogeneous dummy product (sawdust) with a bulk density of 114 kg / m 3 and the dose uniformity ratio were determined. The isodose curves and the three-dimensional views were built using an automatic geostatistical gridding method, the kriging method.

OEDIPE: A Personalized Dosimetric Tool Associating Voxel-Based Models with MCNPX

A new tool, named OEDIPE (a French acronym that stands for "Tool for Personalized Internal Dose Assessment") was developed to carry out personalized internal dosimetry calculations for nuclear medicine (for both therapeutic and diagnostic procedures) and for radiation safety (in the case of internal contamination). It was developed under the PV-Wave visual data analysis system by the Institute of Radioprotection and Nuclear Safety (IRSN) in collaboration with the French Institute of Health and Medical Research (INSERM). This software creates anthropomorphic voxel-based phantoms from computed tomography (CT) and magnetic resonance imaging (MRI) patient images through the use of a friendly graphical user interface (GUI). Several tools have been built-in to allow for image segmentation. Source data, including VOI localization and cumulated activities, are assessed by single photon emission computed tomography (SPECT) images, and the source may be specified in any number of organs either as a point source or a homogeneously distributed source. It is also possible to choose the dosimetric parameters required for the study (mean organ dose or a dose distribution). Phantom, source, and dosimetric parameters are automatically written into a file. That file is then processed by the Monte Carlo code MCNPX (LANL) to perform the actual dose calculation. OEDIPE can compute either the absorbed dose in each organ (in a few minutes), or the absorbed dose in each voxel of the phantom (i.e. the spatial dose distribution at a tissue level) in a few hours or more. OEDIPE automatically reads the MCNPX output file and processes results to give a list of absorbed doses in each organ or a plot of isodose curves superimposed onto the phantom. Because of the long calculation times required to compute an absorbed dose within an entire whole-body phantom at a spatial resolution of a few millimeters, modifications were made to reduce computational times to reasonable values. To illustrate this tool, results of a dosimetric study of technetium-99m labeling of a bone-scanning agent are presented. OEDIPE is a tool that can be used for patient-specific dosimetry--for example, in targeted radiotherapy--by taking into account the individual patient anatomy, including tumors.

SU-FF-T-291: Monte Carlo Calculation of Rectal Dose When Using An Endorectal Balloon During Prostate Radiation Therapy

Purpose: Air‐filled intrarectal balloons can be used to localize and immobilize the prostate for radiation therapy. This project investigates how well the Eclipse treatment planning system (Varian Inc.) includes the effect of this potentially significant heterogeneity on doses to the rectum. Method and Materials: The BEAMnrc/DOSXYZnrc Monte Carlo(MC) codes were used to simulate a 4‐field conformal therapy treatment for patients who have been treated for prostate cancer under an IRB‐approved protocol which includes a 27Gy cone‐down using a rectal balloon. The rectal doses calculated using MC were compared with those from Eclipse using isodose curves, dose‐volume histograms, and wall‐volume histograms. Results: The MC results showed that, for a 27Gy prescription to the 95% isodose line, Eclipse over‐estimates the volume of the rectum receiving more than 26 Gy by 2–10cc and the volume of the rectum receiving between 12–15 Gy by 10–20cc. Conclusion: The differences in the rectal dose calculated by Eclipse and MC are consistent and can be predicted. They can be explained by the scattering behavior of the PA and lateral fields inside the balloon: For each field, the lack of electronic equilibrium reduces the high dose region while the increased electron range widens the low dose region beyond the penumbra. The combined effect of the four fields divides the DVH into two major regions, corresponding to the AP‐PA and lateral fields, and four subregions, corresponding to the scattering behavior of electrons in air.

SU-FF-T-401: Development of Specific Treatment Plan Based On Physical Lattice Structure for Stereotactic Radiosurgery

Purpose: The stereotactic radiosurgery (SRS) describes a method of delivering a high dose of radiation to a small target volume in the brain, generally in a single fraction, while the dose delivered to the surrounding normal tissue should be minimized. To perform automatic plan of the SRS, a new method of multi-isocenter/shot linear accelerator (linac) and gamma knife (GK) radiosurgery treatment plan was developed, based on a physical lattice structure in target. Method and Materials: The optimal radiosurgical plan had constructed by many beam parameters in a linear accelerator or gamma knife-based radiation therapy. In this work, one isocenter/shot was modeled as a sphere, which is equal to the circular collimator/helmet hole size because the dimension of the 50% isodose level in the dose profile is similar to its size. In computer-aided system, it accomplished first an automatic arrangement of multi-isocenter/shot considering two parameters such as positions and collimator/helmet sizes for each isocenter/shot. Simultaneously, an irregularly shaped target was approximated by cubic structures through computation of voxel units. The treatment planning method by the technique was evaluated as a dose distribution by dose volume histograms (DVHs), dose conformity, and dose homogeneity to targets. Results: For irregularly shaped targets, the new method performed the optimal multi-isocenter packing, and it only took a few seconds in computer-aided system. The targets were included in a more than 50% isodose curve. The dose conformity was ordinarily acceptable levels and the dose homogeneity was always less than 2.0, satisfying for various targets referred to Radiation Therapy Oncology Group (RTOG) SRS criteria. Conclusion: This approach using new method could be an efficient radiosurgical plan used two beam parameters both the irregularly shaped targets and different modality techniques such as linac and GK for SRS.

TH-C-T-6C-07: Is Cone Beam CT Suitable for Dose Verification?

Purpose: To evaluate the influence of cone beam CT(CBCT) on dose calculation accuracy and check if CBCT is suitable for the daily dose verification of patient treatment.Method and Materials: A CT‐calibration phantom was first used to calibrate both conventional CT(GE Discovery‐ST) and CBCT (Varian Trilogy OBI system). CT and CBCTimages of the calibration phantom, an anthropomorphic phantom and a lung patient were then acquired for this study. The HU profiles along two orthogonal lines were acquired and compared for both CT and CBCT.CT and CBCTimages were then imported to a treatment planning system (Varian Eclipse). Treatment plans were generated based on both CT and CBCT and the resultant dose distributions were compared based on isodose curves, dose profiles and DVHs. The various factors causing the dosimetric inaccuracy in CBCT‐based planning were analyzed. Results: Even though the same calibration was used for both CT and CBCT‐based planning, the difference in the HUs between the CT and CBCT was found to be clinically significant. The discrepancy was most pronounced in the regions close to the phantom surface, with the maximum difference reaching 400 HUs. For the same planning condition (geometric setups, energy, MU, and dose calculation algorithm), ∼3% dose difference was found in the two planning schemes for the phantom. For the lung case, the maximum dose discrepancy was found to be ∼7% due to the motion artifacts in CBCT.Conclusion: Although CBCT provides useful volumetric anatomy information for patient positioning verification, when used for dose calculation, it could introduce clinically unacceptable dosimetric errors. The quality of current CBCT should be improved in order for it to be used routinely for dosimetric verification calculation.