3D Treatment Planning System Research Articles

Commissioning and Comprehensive Quality Assurance of commercial 3D Treatment Planning System using IAEA Technical Report Series — 430

The purpose of this work is to report the results of commissioning and to establish a quality assurance (QA) program for commercial 3D treatment planning system (TPS) based on IAEA Technical Report Series 430. Eclipse v 7.3.10, (Varian Medical Systems, Palo Alto, CA, U.S.A.) TPS was commissioned for a Clinac 6EX (Varian Medical Systems, Palo Alto, CA, USA) linear accelerator. CT images of a phantom with various known in-homogeneities were acquired. The images were transferred to TPS and tested for various parameters related to patient data acquisition, anatomical modeling, plan evaluation and dose calculation. Dosimetric parameters including open, asymmetric and wedged shaped fields, oblique incidence, buildup region behavior and SSD dependence were evaluated. Representative clinical cases were tested for MU calculation and point doses. The maximum variation between the measured and the known CT numbers was 20 +/- 11.7 HU (1 SD). The results of all non-dosimetric tests were found within tolerance, however expansion at the sharp corners was found distorted. The accuracy of the DVH calculations depends on the grid size. TPS calculations of all the dosimetric parameters were in good agreement with the measured values, however for asymmetric open and wedged fields, few points were found out of tolerance. Smaller grid size calculation showed better agreement of dose calculation in the build-up region. Independent tests for MU calculation showed a variation within +/-2% (relative to planning system), meanwhile variation of 3.0% was observed when the central axis was blocked. The test results were in agreement with the tolerance specified by IAEA TRS 430. A subset of the commissioning tests has been identified as a baseline data for an ongoing QA program.

Upright 3D Treatment Planning Using a Vertical CT

In this report, we describe a novel technique used to plan and administer external beam radiation therapy to a patient in the upright position. A patient required reirradiation for thymic carcinoma but was unable to tolerate the supine position due to bilateral phrenic nerve injury and paralysis of the diaphragm. Computed tomography (CT) images in the upright position were acquired at the Northern Illinois University Institute for Neutron Therapy at Fermilab. The CT data were imported into a standard 3-dimensional (3D) treatment planning system. Treatment was designed to deliver 24 Gy to the target volume while respecting normal tissue tolerances. A custom chair that locked into the treatment table indexing system was constructed for immobilization, and port films verified the reproducibility of setup. Radiation was administered using mixed photon and electron AP fields.

Photon spectral characteristics of dissimilar linear accelerators

This work measures and compares the energy spectra of four dosimetrically matched beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three‐dimensional (3D) treatment planning system. The spectra of beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one waveguide and one waveguide), one Varian with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a waveguide. All four accelerators had percent depth dose curves for the beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto‐modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.

A comprehensive clinical 3-dimensional dosimetric analysis of forward planned IMRT and conventional wedge planned techniques for intact breast radiotherapy

A forward planned intensity modulated technique was initiated for intact breasts radiotherapy (FPIMRT). Forty-three patients were selected to compare dose distributions achieved by FPIMRT to dose distributions produced by conventional wedge techniques (CW). For the simulation process, the treatment field margins were clinically defined by a physician, and a set of fiducial reference markers was placed on the patient. A computed tomography (CT) scan was then performed and the images were transferred to a 3-dimensional (3D) treatment planning system (TPS). The breast tissue was then contoured to allow for a quantitative dose volume analysis. The treatment plan was initially generated with conventional tangential beam arrangements and open fields. Multiple multileaf collimator (MLC) shaped segments were created for each tangential beam in an effort to produce dose homogeneity throughout the breast. 6-MV photon beams were used for treatment unless acceptable dose homogeneity could not be achieved due to large breast size. In this case, the beam energies of selected segments were modified to 15-MV. Once the FPIMRT plan was created, additional plans were generated using the same beam geometry and 2 tangential open fields with CW techniques and 15° wedges (15DW), 30° wedges (30DW), 45° wedges (45 DW), and 60° wedges (60DW). The dose distributions generated by the CW plans were then compared to the FPIMRT plan. This process was repeated for each patient, and the patient group was divided into 3 categories based on breast volume (small, medium, and large). Both point dose relationships, which compared global hot spot (GHS) magnitude and location and dose volume relationships, which compared breast volume coverage of the 105% and 110% isodose lines (IDL) relative to the prescribed dose (PD), were explored. For the patient group in our study, FPIMRT produced the smallest average GHS and the most evenly distributed location of GHS for all breast size categories when compared to all CW techniques. FPIMRT also produced the smallest average breast volume receiving greater than 105% of the PD (V a105) for the small- and medium-size breast patients and the smallest average breast volume receiving greater than 110% of the PD (V a110) for all breast size categories when compared to all CW techniques.

The treatment of a large acoustic tumor with fractionated stereotactic radiotherapy.

The treatment of acoustic neuromas (AN) usually involves surgical excision or stereotactic radiosurgery. However, for large AN (mean diameter > 3 cm), stereotactic radiosurgery is rarely used, leaving patients with limited noninvasive treatment options. Recently, the use of fractionated stereotactic radiotherapy (FSRT) has been effective in treating small to medium-sized AN. We present a patient with a large AN treated with FSRT. The patient was a 43-year-old man presenting with imbalance, tinnitus, vertigo, and right-sided hearing decline associated with vomiting and hydrocephalus. Magnetic resonance (MR) imaging revealed a large, 3.8-cm, right cerebellopontine-angle tumor compressing the fourth ventricle. Following right frontal ventriculoperitoneal shunt placement, the patient underwent FSRT for treatment of the tumor. Using the Radionics X-Knife 4.0 3D treatment planning system, a total of 54 Gy was delivered in 1.8-Gy daily fractions with the prescription isodose line of 90%. Treatments were delivered using a dedicated Varian 6/100 linear accelerator, and head immobilization was achieved with the Gill-Thomas-Cosman relocatable stereotactic frame. The patient was subsequently evaluated with serial contrast-enhanced MR imaging. Following FSRT, local control (defined as the absence of tumor progression) was achieved, and treatment was well tolerated. There was no hearing-related, trigeminal, or facial-nerve morbidity following FSRT at 63-month follow-up. Treating a patient with a large AN with FSRT resulted in local tumor control, with no trigeminal nerve, facial nerve, or hearing-related morbidity. These results support FSRT as a potential noninvasive treatment modality for AN some would consider too large for single-fraction stereotactic radiosurgery (SRS).

SU-FF-T-111: Commissioning and Comprehensive Quality Assurance of Commercial 3D Treatment Planning System Using IAEA Technical Report Series - 430

Purpose: To report the results of commissioning and to structure a quality assurance program for commercially available 3D treatment planning system (TPS) based on IAEA Technical Report Series 430. Method and Materials: Eclipse v–7.3.1 TPS was commissioned for 6MV photons from Clinac–6EX linear accelerator. CT images of a phantom with various known in-homogeneities were acquired. The images were transferred to TPS and tested for orientation, correlation of CT number and density, image geometry, anatomical modeling, dose volume histogram, plan output and transfer. Accuracy of external beam dose calculation was checked by comparing various dosimetric parameters computed from TPS with measured data. These parameters includes depth dose, beam profiles and MU for square, rectangle, asymmetric, MLC shaped fields, hard and enhanced dynamic wedges. Representative clinical test cases were tested for MU calculation and point doses. Results: Maximum variation of CT number observed was ±20 HU. The results of anatomical modeling tests were within the tolerance limits, however 3.7% reduction in the absolute volume was found for a cylindrical object of volume 350cc due to distortion of object expansion at the sharp corners and edges. For PDD curves, variation of 2mm was observed in the buildup region. Maximum variation of ⩽1mm was observed in penumbra region for the profiles of square, rectangle, asymmetric, wedged, and MLC fields, except in the case where central axis was blocked (2.5mm). All tests for MU calculation had variation within ±2%, meanwhile variation of 3.8% was observed when the central axis was blocked. Clinical test cases resulted in point doses within ±2%. Conclusion: The results of quality assurance tests are in agreement with the validation software of Eclipse, which improves the confidence levels of the physicists. Periodic QA protocol and a subset of the commissioning tests recommended by TRS-430 have been customized for an ongoing QA program.

Commissioning of motorized wedge for the first Equinox-80 telecobalt unit and implementation in the Eclipse 3D treatment planning system

A new model of the telecobalt unit (TCU), Theratron Equinox-80, (MDS Nordion, Canada) equipped with a single 60 degree motorized wedge (MW), four universal wedges (UW) for 15 degrees, 30 degrees, 45 degrees and 60 degrees have been evaluated. MW was commissioned in Eclipse (Varian, Palo Alto, USA) 3D treatment planning system (TPS). The profiles and central axis depth doses (CADD) were measured with Wellhofer blue water phantom for MW and the measured data was commissioned in Eclipse. These profiles and CADD for MW were compared with UW in a homogeneous phantom generated in Eclipse for various field sizes. The dose was also calculated in the same phantom at 10 cm depth. For the particular MW angle and the respective open and MW beam weights, the dose was measured for a field size of 10 cm x 10 cm in a MEDTEC water phantom at 10 cm depth with a 0.13 cc thimble ion chamber (Scanditronix Wellhofer, Uppsala, Sweden) and a NE electrometer (Nuclear Enterprises, UK). Measured dose with ion chamber was compared with the TPS calculated dose. MW angle verification was also done on the Equinox for four angles (15 degrees, 30 degrees, 45 degrees and 60 degrees). The variation in measured and calculated dose at 10 cm depth was within 2%. The measured and the calculated wedge angles were in good agreement within 2 degrees. The motorized wedges were successfully commissioned in Eclipse for four wedge angles.

SU-FF-T-346: Photon Spectral Characteristics of Dissimilar 6MV Linear Accelerators

Purpose: To determine and compare the energy spectra of four 6MV beams, generated from four physically different accelerators. Also, to demonstrate that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a 3D treatment planning system. Methods and Materials: The spectra of 6MV beams that are dosimetrically matched were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had different physical designs. The four accelerators were two Varian 2100C/Ds (one 6MV/18MV waveguide and one 6MV/10MV waveguide), one Varian 600C with a vertically mounted waveguide and no bending magnet, and one Siemens MD 67/40 with a 6MV/10MV waveguide. All four accelerators had Percent Depth Dose curves for 6MV that were matched. Spectral measurements were performed by narrow beam transmission measurements through pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. Results: Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. Comparisons of the measured spectra to the spectra found by the auto‐modeling algorithm in a commercial treatment planning system also showed excellent agreement. Plots of the measured spectra are shown. Conclusions: The measured spectra of different 6MV beams are similar, regardless of accelerator design. The measured spectra agree with those derived by the treatment planning system. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.

Three‐dimensional heart dose reconstruction to estimate normal tissue complication probability after breast irradiation using portal dosimetry

Irradiation of the heart is one of the major concerns during radiotherapy of breast cancer. Three-dimensional (3D) treatment planning would therefore be useful but cannot always be performed for left-sided breast treatments, because CT data may not be available. However, even if 3D dose calculations are available and an estimate of the normal tissue damage can be made, uncertainties in patient positioning may significantly influence the heart dose during treatment. Therefore, 3D reconstruction of the actual heart dose during breast cancer treatment using electronic imaging portal device (EPID) dosimetry has been investigated. A previously described method to reconstruct the dose in the patient from treatment portal images at the radiological midsurface was used in combination with a simple geometrical model of the irradiated heart volume to enable calculation of dose-volume histograms (DVHs), to independently verify this aspect of the treatment without using 3D data from a planning CT scan. To investigate the accuracy of our method, the DVHs obtained with full 3D treatment planning system (TPS) calculations and those obtained after resampling the TPS dose in the radiological midsurface were compared for fifteen breast cancer patients for whom CT data were available. In addition, EPID dosimetry as well as 3D dose calculations using our TPS, film dosimetry, and ionization chamber measurements were performed in an anthropomorphic phantom. It was found that the dose reconstructed using EPID dosimetry and the dose calculated with the TPS agreed within 1.5% in the lung/heart region. The dose-volume histograms obtained with EPID dosimetry were used to estimate the normal tissue complication probability (NTCP) for late excess cardiac mortality. Although the accuracy of these NTCP calculations might be limited due to the uncertainty in the NTCP model, in combination with our portal dosimetry approach it allows incorporation of the actual heart dose. For the anthropomorphic phantom, and for fifteen patients for whom CT data were available to test our method, the average difference between the NTCP values obtained with our method and those resulting from the dose distributions calculated with the TPS was 0.1% +/- 0.3% (1 SD). Most NTCP values were 1%-2% lower than those obtained using the method described by Hurkmans et al. [Radiother. Oncol. 62, 163-171 (2002)], using the maximum heart distance determined from a simulator image as a single pre-treatment parameter. A similar difference between the two methods was found for twelve patients using in vivo EPID dosimetry; the average NTCP value obtained with EPID dosimetry was 0.9%, whereas an average NTCP value of 2.2% was derived using the method of Hurkmans et al. The results obtained in this study show that EPID dosimetry is well suited for in vivo verification of the heart dose during breast cancer treatment, and can be used to estimate the NTCP for late excess cardiac mortality. To the best of our knowledge, this is the first study using portal dosimetry to calculate a DVH and NTCP of an organ at risk.

Comparison between measured and calculated dynamic wedge dose distributions using the anisotropic analytic algorithm and pencil‐beam convolution

We used the two available calculation algorithms of the Varian Eclipse 7.3 three‐dimensional (3D) treatment planning system (TPS), the anisotropic analytic algorithm (AAA) and pencil‐beam convolution (PBC), to compare measured and calculated two‐dimensional enhanced dynamic wedge (2D EDW) dose distributions, plus implementation of the dynamic wedge into the TPS. Measurements were carried out for a 6‐MV photon beam produced with a Clinac 2300C/D linear accelerator equipped with EDW, using ionization chambers for beam axis measurements and films for dose distributions. Using both algorithms, the calculations were performed by the TPS for symmetric square fields in a perpendicular configuration. Accuracy of the TPS was evaluated using a gamma index, allowing 3% dose variation and 3 mm distance to agreement (DTA) as the individual acceptance criteria. Beam axis wedge factors and percentage depth dose calculation were within 1% deviation between calculated and measured values. In the non‐wedged direction, profiles exhibit variations lower than 2% of dose or 2 mm DTA. In the wedge direction, both algorithms reproduced the measured profiles within the acceptance criteria up to 30 degrees EDW. With larger wedge angles, the difference increased to 3%. The gamma distribution showed that, for field sizes of 10×10 cm or larger, using an EDW of 45 or 60 degrees, the field corners and the high‐dose region of the distribution are not well modeled by PBC. For a 20×20 cm field, using a 60‐degree EDW and PBC for calculation, the percentage of pixels that do not reach the acceptance criteria is 28.5%; but, using the AAA for the same conditions, this percentage is only 0.48% of the total distribution. Therefore, PBC is not reliable for planning a treatment when using a 60‐degree EDW for large field sizes. In all the cases, AAA models wedged dose distributions more accurately than PBC did.PACS numbers: 87.53.Bn, 87.53.Dq, 87.53.Kn

Adaptation of telecobalt unit for stereotactic irradiation

We investigated the feasibility of using an isocentric telecobalt unit for advanced treatment techniques, such as stereotactic radiotherapy. To adapt the telecobalt unit (Th780 C) for stereotactic irradiation, collimator inserts of various sizes, collimator mount, and a couch mount suitable for the telecobalt unit were developed, and the characteristics of the narrow beams of Cobalt-60 ( 60Co) were studied. Comparative study was carried out between the stereotactic radiotherapy plans of 6 MV and 60Co beams using a 3-dimensional (3D) treatment planning system. The beam penumbra of 60Co beams was found to be larger than those of 6 MV beams. The dose-volume histograms (DVH) obtained from the 60Co beam plan were comparable to those obtained from the 6 MV plan. The DVH of nontarget tissue obtained from the plans of the 2 beams were found to be in good agreement to each other. The difference in equivalent fall-off distance (EFOD) for all 3 cases was found insignificant; hence, it can be concluded that the fall-off dose in the dose distribution of the 60Co stereotactic plan is as good as that of the 6 MV stereotactic plan. In all 3 cases for which the treatment plans were compared between 60Co and 6 MV beams, it was observed that the fall-off doses outside the target were similar; therefore, considering 60Co with 5-mm margin is a cost effective alternative for the linac-based stereotactic radiotherapy.

Characterizing and configuring motorized wedge for a new generation telecobalt machine in a treatment planning system.

A new generation telecobalt unit, Theratron Equinox-80, (MDS Nordion, Canada) has been evaluated. It is equipped with a single 60-degree motorized wedge (MW), four universal wedges (UW) for 15°, 30°, 45° and 60°. MW was configured in Eclipse (Varian, Palo Alto, USA) 3D treatment planning system (TPS). The profiles and central axis depth doses (CADD) were measured with radiation field analyzer blue water phantom for MW. These profiles and CADD for MW were compared with UW in a homogeneous phantom generated in Eclipse for various field sizes. The absolute dose was measured for a field size of 10 × 10 cm2 only in a MEDTEC water phantom at 10 cm depth with a 0.13 cc thimble ion chamber (Scanditronix Wellhofer, Uppsala, Sweden) and a NE electrometer (Nuclear Enterprises, UK). Measured dose with ion chamber was compared with the TPS predicted dose. MW angle was verified on the Equinox for four angles (15°, 30°, 45° and 60°). The variation in measured and calculated dose at 10 cm depth was within 2%. The measured and the calculated wedge angles were in well agreement within 2°. The motorized wedges were successfully configured in Eclipse for four wedge angles.

2580: Clinical Results in 46 Pediatric Patients Treated by Craniospinal Technique in Supine Position Using Asymetric Half Fields

2580: Clinical Results in 46 Pediatric Patients Treated by Craniospinal Technique in Supine Position Using Asymetric Half Fields

The prediction of transmitted dose distributions using a 3D treatment planning system

Patient dose verification is becoming increasingly important with the advent of new complex radiotherapy techniques such as conformal radiotherapy (CRT) and intensity-modulated radiotherapy (IMRT). An electronic portal imaging device (EPID) has potential application for in vivo dosimetry. In the current work, an EPID has been modelled using a treatment planning system (TPS) to predict transmitted dose maps. A thin slab of RW3 material used to initially represent the EPID. A homogeneous RW3 phantom and the thin RW3 slab placed at a clinical distance away from the phantom were scanned using a CT simulator. The resulting CT images were transferred via DICOM to the TPS and the density of the CT data corresponding to the thin RW3 slab was changed to 1 g/cm3. Transmitted dose maps (TDMs) in the modelled EPID were calculated by the TPS using the collapsed-cone (C-C) convolution superposition (C/S) algorithm. A 6 MV beam was used in the simulation to deliver 300 MU to the homogenous phantom using an isocentric and SSD (source-to-surface) technique. The phantom thickness was varied and the calculated TDMs in the modelled EPID were compared with corresponding measurements obtained from a calibrated scanning liquid-filled ionisation chamber (SLIC) EPID. The two TDMs were compared using the gamma evaluation technique of Low et al. The predicted and measured TDMs agree to within 2 % (averaged over all phantom thicknesses) on the central beam axis. More than 90 % of points in the dose maps (excluding field edges) produce a gamma index less than or equal to 1, for dose difference (averaged over all phantom thicknesses), and distance-to-agreement criteria of 4 %, 3.8 mm, respectively. In addition, the noise level on the central axis in the predicted dose maps is less than 0.1 %. We found that phantom thickness changes of approximately 1 mm, which correspond to dose changes on the central beam axis of less than 0.6 %, can be detected in the predicted transmitted dose distributions.

Commissioning and quality assurance of a commercial stereotactic treatment-planning system for extracranial IMRT

A 3D treatment-planning system (TPS) for stereotactic intensity-modulated radiotherapy (IMRT) using a micro-multileaf collimator has been made available by Radionics. In this work, we report our comprehensive quality assurance (QA) procedure for commissioning this TPS. First, the accuracy of stereotaxy established with a body frame was checked to ensure accurate determination of a target position within the planning system. Second, the CT-to-electron density conversion curve used in the TPS was fitted to our site-specific measurement data to ensure the accuracy of dose calculation and measurement verification in a QA phantom. Using the QA phantom, the radiological path lengths were verified against known geometrical depths to ensure the accuracy of the ray-tracing algorithm. We also checked inter- and intraleaf leakage/transmission for adequate jaw settings. Measurements for dose verification were performed in various head/neck and prostate IMRT treatment plans using the patient-specific optimized fluence maps. Both ion chamber and film were used for point dose and isodose distribution verifications. To ensure that adjacent organs at risk receive dose within the expectation, we used the Monte Carlo method to calculate dose distributions and dose-volume histograms (DVHs) for these organs at risk. The dosimetric accuracy satisfied the published acceptability criteria. The Monte Carlo calculations confirmed the measured dose distributions for target volumes. For organs located on the beam boundary or outside the beam, some differences in the DVHs were noticed. However, the plans calculated by both methods met our clinical criteria. We conclude that the accuracy of the XKnife™ RT2 treatment-planning system is adequate for the clinical implementation of stereotactic IMRT. PACS numbers: 87.53.Xd, 87.53.Ly, 87.53.Wz

A simple technique for craniospinal radiotherapy in the supine position

Purpose Craniospinal irradiation poses technical difficulties that may be addressed with the use of the newer technologies that have become available over the past decade. The use of CT simulation allows improved target localisation and beam geometry definition while significantly reducing the treatment simulation time. We have developed a CT-based technique for whole CNS irradiation in the supine position that uses fixed field parameters, asymmetric jaws for field matching and drastically reduces simulation and treatment times. Methods The patient is CT scanned and treated in the supine position. The clinical target volume and relevant critical structures are outlined on a planning CT scan. Half beam blocked lateral fields with a collimator rotation are used to match the beam divergence from the superior border of the spinal field at the C2 vertebral body. The shielding for the cranial fields is generated automatically, and the dose distribution is calculated using a 3D treatment planning system. Fixed field parameters are used for the planning and treatment. The position of the isocenter of the spine field is always a fixed longitudinal distance from the isocenter of the brain fields. If multiple posterior fields are required, the isocenter of the second spine field is always a fixed longitudinal distance from that of the first and the gap between the fields is determined using virtual simulation and feathered during treatment using the asymmetric jaws of the linear accelerator. All treatment portals are filmed daily during the first week of treatment, and after each junction change thereafter. Results and conclusion The supine position provides numerous advantages. Patients are more comfortable, the treatment position is more reproducible, and access to the airway is possible, if necessary, for patient sedation. The use of CT simulation decreases the simulation time, allows for increased planning accuracy, and enables the use of multimodality image registration, and 3D treatment planning. The use of asymmetric jaws allows for junction feathering without changing the patient setup or using a couch angle.

Technical note: evaluation of dosimetric performance in a commercial 3D treatment planning system

The aim of this work was to evaluate the dosimetric performance of a commercial treatment planning system (TPS) which employs a three-dimensional calculation algorithm (Nucletron Plato version 2.2.3), following the guidelines of the AAPM Task Group 23 (TG23). Seven test cases were used to test the TPS dosimetric performance in homogeneous water. These cases involved absolute dose measurements on central as well as off-axis points situated at various depths, using simple field arrangements, and comparison with corresponding TPS calculations. This comparison yielded differences within +/-2% at all points, for all test cases. To test the ability of the TPS to account for tissue inhomogeneities, corresponding comparisons were performed with the presence of a low-density material in the beam to resemble an air inhomogeneity. Absolute dose measurements and corresponding TPS calculations showed a mean deviation of the order of +/-3.5%, reaching a maximum of 11.5% for small field sizes (5 cm x 5 cm). In summary, observed deviations are well within the set tolerance levels while comparison with previous TPS versions showed that Plato version 2.2.3 is significantly improved, especially in dose calculations in the presence of low density inhomogeneities.

280 Predicting 2D transmitted dose maps using a 3D treatment planning system

280 Predicting 2D transmitted dose maps using a 3D treatment planning system

A Clinical Interactive Technique for MR-CT Image Registration for Target Delineation of Intracranial Tumors

Replacement of current CT-based, three-dimensional (3D) treatment planning systems by newer versions capable of automated multi-modality image registration may be economically prohibitive for most radiation oncology clinics. We present a low-cost technique for MR-CT image registration on a "first generation" CT-based, 3D treatment planning system for intracranial tumors. The technique begins with fabrication of a standard treatment mask. A second truncated mask, the "minimask," is then made, using the standard mask as a mold. Two orthogonal leveling vials glued onto the minimask detect angular deviations in pitch and roll. Preservation of yaw is verified by referencing a line marked according to the CT laser on the craniocaudal axis. The treatment mask immobilizes the patient's head for CT. The minimask reproduces this CT-based angular treatment position, which is then maintained by taping the appropriately positioned head to the MR head coil for MR scanning. All CT and MR images, in DICOM 3.0 format, are entered into the treatment planning system via a computer network. Interactive registration of MR to CT images is controlled by real-time visual feedback on the computer monitor. Translational misalignments at the target are eliminated or minimized by iterative use of qualitative visual inspection. In this study, rotational errors were measured in a retrospective series of 20 consecutive patients who had undergone CT-MR image registration using this technique. Anatomic structures defined the three CT orthogonal axes from which angular errors on MR image were measured. Translational errors at the target isocenter were within pixel size, as judged by visual inspection. Clinical setup using the minimask resulted in overall average angular deviation of 3 degrees +/-2 degrees (mean +/- SD) and translational deviation within the edges of the target volume of typically less than 2 mm. The accuracy of this registration technique for target delineation of intracranial tumors is compatible with practice guidelines. This method, then, provides a cost-effective means to register MR and CT images for target delineation of intracranial tumors.

Validation of physics improvements for IMRT with a commercial treatment-planning system

A new Pinnacle 3D treatment-planning system software release has recently become available (v7.4, Philips Radiation Oncology Systems, Milpitas, CA), which supports modeling of rounded multileaf collimator (MLC) leaf ends; it also includes a number of other software enhancements intended to improve the overall dose calculation accuracy. In this report, we provide a general discussion of the dose calculation algorithm and new beam-modeling parameters. The accuracy of a diode dosimeter was established for measurement of MLC-shaped beam profiles required by the new software version by comparison with film and ion chamber measurements in various regions of the field. The results suggest that a suitable diode or other small volume dosimeter with appropriate energy sensitivity should be used to obtain profiles for commissioning the planning system. Film should be used with caution, especially for larger field profile measurements. The dose calculation algorithm and modeling parameters chosen were validated through various test field measurements including a bar pattern, a strip pattern, and a clinical head and neck IMRT field. For the bar and strip patterns, the agreement between Pinnacle calculations and diode measurements was generally very good. These tests were helpful in establishing the new model parameter values, especially tongue-and-groove width, additional interleaf leakage, rounded leaf tip radius, and MLC transmission. For the clinical head and neck field, the comparison between Pinnacle and film measurements showed regions of approximately 2 cGy under- or overdose. However, the Pinnacle calculations agreed with diode measurements at all points to within 1 cGy or 1% of the maximum dose for the field (67 cGy). The greatest discrepancy between film and diode measurements for the clinical field (maximum of 2.8%) occurred in low-dose regions in the central part of the field. The disagreement may be due to the overresponse of film to scattered radiation in the low-dose regions, which have significant shielding by the MLCs. PACS numbers: 87.53.Bn, 87.53.Dq