Oblique Gantry Research Articles

TU‐D‐201‐05: Validation of Treatment Planning Dose Calculations: Experience Working with MPPG 5.a

Purpose:Newly published medical physics practice guideline (MPPG 5.a.) has set the minimum requirements for commissioning and QA of treatment planning dose calculations. We present our experience in the validation of a commercial treatment planning system based on MPPG 5.a.Methods:In addition to tests traditionally performed to commission a model‐based dose calculation algorithm, extensive tests were carried out at short and extended SSDs, various depths, oblique gantry angles and off‐axis conditions to verify the robustness and limitations of a dose calculation algorithm. A comparison between measured and calculated dose was performed based on validation tests and evaluation criteria recommended by MPPG 5.a. An ion chamber was used for the measurement of dose at points of interest, and diodes were used for photon IMRT/VMAT validations. Dose profiles were measured with a three‐dimensional scanning system and calculated in the TPS using a virtual water phantom.Results:Calculated and measured absolute dose profiles were compared at each specified SSD and depth for open fields. The disagreement is easily identifiable with the difference curve. Subtle discrepancy has revealed the limitation of the measurement, e.g., a spike at the high dose region and an asymmetrical penumbra observed on the tests with an oblique MLC beam. The excellent results we had (> 98% pass rate on 3%/3mm gamma index) on the end‐to‐end tests for both IMRT and VMAT are attributed to the quality beam data and the good understanding of the modeling. The limitation of the model and the uncertainty of measurement were considered when comparing the results.Conclusion:The extensive tests recommended by the MPPG encourage us to understand the accuracy and limitations of a dose algorithm as well as the uncertainty of measurement. Our experience has shown how the suggested tests can be performed effectively to validate dose calculation models.

A motorized solid-state phantom for patient-specific dose verification in ion beam radiotherapy

For regular quality assurance and patient-specific dosimetric verification under non-horizontal gantry angles in ion beam radiotherapy, we developed and commissioned a motorized solid state phantom. The phantom is set up under the selected gantry angle and moves an array of 24 ionization chambers to the measurement position by means of three eccentrically-mounted cylinders. Hence, the phantom allows 3D dosimetry at oblique gantry angles. To achieve the high standards in dosimetry, the mechanical and dosimetric accuracy of the phantom was investigated and corrections for residual uncertainties were derived. Furthermore, the exact geometry as well as a coordinate transformation from cylindrical into Cartesian coordinates was determined. The developed phantom proved to be suitable for quality assurance and 3D-dose verifications for proton- and carbon ion treatment plans at oblique gantry angles. Comparing dose measurements with the new phantom under oblique gantry angles with those in a water phantom and horizontal beams, the dose deviations averaged over the 24 ionization chambers were within 1.5%. Integrating the phantom into the HIT treatment plan verification environment, allows the use of established workflow for verification measurements. Application of the phantom increases the safety of patient plan application at gantry beam lines.

SU‐E‐T‐724: Testing the Limitations of a Photon Dose Algorithm in Commissioning

Purpose:This work is to test the limitations of a photon dose algorithm during the commissioning of a treatment planning system.Methods:Commissioning tests for a model‐based dose calculation algorithm typically involve the verification of PDD, output and dose profiles, but often those tests are performed at one SSD, one depth and one nominal gantry position. Those tests are useful to validate the modeling of radiation beams, but they may not be sufficient to detect the limitations of an algorithm. Additional tests were performed at several SSDs, various depths, oblique gantry angles and off‐axis conditions to demonstrate the accuracy and limitations of the dose calculation algorithm. Dose profiles were measured by in‐water scanning with a three‐dimensional scanning system and calculated in the TPS using a virtual water phantom.Results:Dose profiles from both the measurement and the calculation were plotted for comparison. Also plotted were the difference in absolute dose between the measurement and the calculation. For the off‐axis MLC field tests, larger discrepancies were observed at the open portion and the far edge of the field, which became greater with short SSD and deep depth. In addition, the shape of the profile appeared different for the open portion of the field. For the oblique MLC field tests, the largest difference was seen around the high‐dose shoulder of the profile. Differences in some point doses were observed to be greater than 10% of the maximum field dose.Conclusion:We have learned the limitations of the superposition dose algorithm in the XiO planning system through some specific tests, which are related to the parallel kernel approximation and the simplified energy spectra employed by the algorithm to calculate field‐size dependent fluence.

TU‐CD‐304‐04: Scanning Field Total Body Irradiation Using Dynamic Arc with Variable Dose Rate and Gantry Speed

Purpose:Enable a scanning field total body irradiation (TBI) technique, using dynamic arcs, which is biologically equivalent to a moving couch TBI.Methods:Patient is treated slightly above the floor and the treatment field scans across the patient by a moving gantry. MLC positions change during gantry motion to keep same field opening at the level of the treatment plane (170 cm). This is done to mimic the same geometry as the moving couch TBI technique which has been used in our institution for over 10 years. The dose rate and the gantry speed are determined considering a constant speed of the moving field, variations in SSD and slanted depths resulting from oblique gantry angles. An Eclipse (Varian) planning system is commissioned to accommodate the extended SSD. The dosimetric foundations of the technique have been thoroughly investigated using phantom measurements.Results:Dose uniformity better than 2% across 180 cm length at 10cm depth is achieved by moving the gantry from −55 to +55 deg. Treatment range can be extended by increasing gantry range. No device such as a gravity‐oriented compensator is needed to achieve a uniform dose. It is feasible to modify the dose distribution by adjusting the dose rate at each gantry angle to compensate for body thickness differences. Total treatment time for 2 Gy AP/PA fields is 40–50 minutes excluding patient set up time, at the machine dose rate of 100 MU/min.Conclusion:This novel yet transportable moving field technique enables TBI treatment in a small treatment room with less program development preparation than other techniques. Treatment length can be extended per need, and. MLC‐based thickness compensation and partial lung blocking are also possible.

A preliminary study: The impact of manufacturing conditions on the dosimetric characteristics of N-vinylpyrrolidone based polymer gels

s / Physica Medica 30 (2014) e45ee74 e71 were also investigated with the Delta4PT phantom by performing the test in oblique gantry angles. Results: Results obtained through the two methods showed very good agreement as far as significant dispositions are concerned. For slight inaccuracies Delta4PT provided accurate numerical results. As far as gravity effects are concerned a very slight diversification was observed for an oblique gantry setup. Discussion: EBT2 films provide a relatively fast, qualitative visual inspection of the significant leaf dispositions. When slight inaccuracies need to be revealed or precise numerical results for each leaf position are needed, Delta4PT provides the desired accuracy. In treatment modalities where a higher accuracy is required in the delivered dose distribution, such as in IMRT, a 3D dosimetric phantom is recommended for the MLC positional inspection.

SU-E-J-44: Dual Energy Subtraction Imaging to Improve Tumor Visibility at Oblique Angles.

To characterize the contrast improvement of simulated tumors in an anthropomorphic phantom using Dual Energy (DE) subtraction with a clinical on-board imager (OBI) at oblique angles. An Alderson lung/chest anthropomorphic phantom with simulated tumors in the thoracic cavity was imaged using a sequential DE imaging methodology. High (120kVp) and low (60kVp) planar images were obtained in pairs every 100 in a full (3600) rotation using the OBI (Varian Medical Systems, Palo Alto, CA). Optimal mAs settings for DE component images were determined byvarying the x-ray exposure time, while maintaining a constant tube current. DE images were created to best suppress the bone overlaying the simulated tumors. Tumor visibility in DE images was quantified using the Contrast-to-Noise Ratio (CNR). The ratio of the CNR from the DE image relative to a single image (standard protocol) was evaluated as a function of gantry angle. CNR was improved with DE imaging by an average ratio of 1.66 over all gantry angles. The greatest improvement occurred at gantry angles where the tumor was obstructed by the ribs alone. More modest improvements were observed where the tumor overlapped other soft tissue structures (such as the heart) or the dense spine, on a given projection. This study illustrates the feasibility of performing DE imaging at oblique gantry angles using a clinical on-board imaging system. Incorporating DE imaging into clinical practice may allow for verification of tumor position at oblique gantry angles, and may facilitate the development of markerless motion tracking techniques. Supported by a grant from Varian Medical Systems.

SU-E-T-49: Verification of the Monte Carlo Model in the BrainLAB IPlan System for Clinical Applications

Purpose: To verify the Monte Carlo (MC) model in the BrainLab iPlan treatment planning system (TPS) in homogeneous and inhomogeneous media for clinical applications and to evaluate the BL Imaging Couch Top (BICT) in the TPS. Methods: One 30×30×12 cm3 solid water phantom and one 30×30×18 cm3 inhomogeneous phantom made of solid water and cork board were CT scanned and the images were transferred to the iPlan TPS. Single field plans (from 2×2 to 15×15 cm2) were calculated on the two phantoms for AP, PA and oblique gantry angles. An IMRT and a dynamic arc plans were also calculated. Each plan was calculated using both the Pencil Beam Convolution (PBC) and the MC models and with insertion of the BICT. All plans were delivered at the Novalis TX and the isocenter doses were measured using the A‐14 ion chamber.Results: For the homogeneous phantom, planned doses from the PBC and the MC models agree within 2% and all the planned doses agree with the measured doseswithin 2%. For the inhomogeneous phantom, the planned doses from the PBC and the MC models differ from 1.6% to 4.8% depending on the field sizes. The measurements agree with the MC plans within 1.2%. The differences between the measurements and the PBC plans vary from −1.3% to −4.0%. With the BICT included in the plans, the measured doses for the single PA fields differ with the MC planned doses from 0.5% to 3.7%. However, for the six‐beam IMRT and the six‐dynamic arc plans, the measured total doses agree with the planned doses with in 2%. Conclusions: This work verified that the MC model in the BrainLab iPlan TPS is ready for clinical applications in both homogeneous and inhomogeneous media. The couch model in the TPS was evaluated and is acceptable.

Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy

The use of solid carbon fiber table materials in radiotherapy has become more common with the implementation of image‐guided radiotherapy (IGRT), since the solid materials give less imaging artifacts than the so‐called tennis racket couchtops. The downside of the solid carbon fiber couch inserts is that they increase the beam attenuation, resulting in increased surface doses and inaccuracies in determine the dose in the patient. The purpose of this study was to evaluate the interaction of 6 and 15 MV photons with eight different couch inserts. The presented results enable direct comparison of the attenuation properties of the studied couchtops. With a direct posterior beam the maximum attenuations reach 3.6% and 2.4% with 6 and 15 M V, respectively. The measured maximum attenuation by a couchtop with an oblique gantry angle was 10.8% and 7.4% at 6 and 15 MV energies, respectively. The skin‐sparing effect was decreased substantially with every couchtop. The highest increases in surface doses were recorded to be four‐ and threefold, as compared to the direct posterior open field surface doses of 6 and 15 MV, respectively. In conclusion, the carbon fiber tabletops decrease the skin‐sparing effect of megavoltage photon energies. The increased beam attenuation and skin doses should be taken into account in the process of treatment planning.PACS number: 07.90.+c

Patient position verification with oblique radiation beams

Purpose In this study we investigated whether the position of head and neck cancer patients during radiotherapy could be determined from portal images of oblique radiation beams. Currently applied additional anterior posterior (AP) and lateral verification beams could then be abandoned. Method The patient position was determined from portal images of the oblique radiation beams and compared with that determined from AP and lateral verification beams. Seven hundred and fifty-one portal images of 18 different patients were analyzed. Results The set-up errors of patients that were treated with oblique gantry angles could be determined with the same accuracy from the oblique beams as from the AP and lateral verification beams in the ventrodorsal and craniocaudal direction. An additional AP beam was necessary to obtain the same accuracy in the lateral direction, because the used beam directions were relatively close to lateral. The position verification of patients treated with both oblique gantry angles and isocentric table rotations was more accurate if AP and lateral verification beams were used. Conclusions For patients treated with an irradiation technique with oblique gantry angles (and no isocentric table rotations) position verification can be performed by using these oblique radiation beams.

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.

The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert

The use of rigid carbon fiber couch inserts in radiotherapy treatment couches is a well-established method of reducing patient set-up errors associated with couch sag. Several published studies have described such inserts as radiotranslucent with negligible attenuation of the radiation field. Most of these studies were conducted with the radiation field normally incident on the couch and there appears to be no evidence in the literature of the effect of the gantry angle on the extent of beam attenuation by the carbon fiber insert alone during external beam radiotherapy. In this study we examined the magnitude of this effect over a range of posterior oblique gantry angles using a cylindrical solid water phantom containing an ionization chamber placed isocentrically. It was found that a 6 MV photon beam, field size 10 x 5 cm, was attenuated significantly as the gantry angle approached the plane of the couch, from 2% at normal incidence and reaching 9% attenuation at angle of incidence 70 degrees. This could have serious implications regarding dose to the treatment volume for treatments requiring posterior oblique angles of incidence with a possible correction factor necessary in monitor unit calculations.

Monte Carlo simulation of portal dosimetry on a rectilinear voxel geometry: a variable gantry angle solution

A software solution has been developed to carry out Monte Carlo simulations of portal dosimetry using the BEAMnrc/DOSXYZnrc code at oblique gantry angles. The solution is based on an integrated phantom, whereby the effect of incident beam obliquity was included using geometric transformations. Geometric transformations are accurate within ±1 mm and ±1° with respect to exact values calculated using trigonometry. An application in portal image prediction of an inhomogeneous phantom demonstrated good agreement with measured data, where the root-mean-square of the difference was under 2% within the field. Thus, we achieved a dose model framework capable of handling arbitrary gantry angles, voxel-by-voxel phantom description and realistic particle transport throughout the geometry.

Oblique gantry — an alternative solution for a beam delivery system for heavy-ion cancer therapy

Beams of protons and heavy ions have favorable physical and biological properties for their application in radiotherapy. Their advantages can be best exploited if the beam can be applied to the patient from any direction by a rotating gantry. The construction of a gantry is quite demanding for heavy ions due to the high magnetic rigidity of the therapy beams. In order to reduce the gantry size and weight, a novel gantry concept with an oblique 60° output beam is proposed. This concept allows a very compact gantry design even for the isocentric layout and normalconducting magnets. The overall gantry radius is 2.8 m. The gantry is equipped with a two-directional magnetic scanning system and an achromatic beam transport system. The scanning system is located upstream to the last gantry dipole and combines a parallel scanning mode in one direction with a low-angle scanning mode in the other direction in order to reduce the vertical gap of the last dipole. The beam transport system is designed with a high degree of ion-optical flexibility which is used to form a narrow pencil-like beam with adjustable spot-size in the gantry isocentre. The design principles and ion-optical properties of the gantry beam transport and scanning systems are discussed.

Oblique gantry — An alternative for heavy-ion cancer therapy

To design a gantry-type beam delivery system for heavy-ion therapy beams. In order to allow for installation of the gantry in hospital environment, small overall gantry size and low weight are required. The isocentric configuration is to be preferred. A new gantry concept with oblique output beam is introduced. Transformations between the treatment angle, angles of gantry and patient table rotations are derived. The isocentric gantry with the overall radius of 2.8 m is feasible using the 60 degrees output beam. The oblique output beam imposes some restrictions on gantry capabilities, which has to be carefully evaluated by radiation oncology community.

Optimization of the Oblique Angles in the Treatment of Prostate Cancer During Six-Field Conformal Radiotherapy

Historically, four perpendicular treatment fields or bilateral arcs have been used in the treatment of prostate cancer. As new techniques challenge the four-field box technique for their superiority of tumor coverage and adjacent critical structure sparing, oblique beam angles (in addition to right and left laterals) have been introduced as an alternative to anterior (AP) and posterior (PA) beams. Among the most popular of these alternative approaches is a six-field technique. Traditionally 45° angles have been used with this technique. In this study, opposed coplanar oblique beams angled 20, 25, 30, 35, 40, and 45° off the lateral beam position, were compared for their ability to minimize adjacent critical structure doses, while maintaining maximum clinical target volume (CTV) coverage. This analysis compared rectum, bladder, and femoral head dose volume histograms (DVH) for each of these varying oblique gantry angles. As the angle of the posterior oblique beams became more horizontal, it is more difficult to encompass the apex of the prostate in the 95% isodose value. On inferior CT slices near the apex of the prostate, the density of the pelvic bones in the path of the posterior oblique fields causes the beam to be slightly attenuated, thereby underdosing the CTV. The oblique angles most affected by this bone heterogeneity are beams angled from 20 to 30° off the lateral beam position. As the angles approach the vertical direction, rectal and bladder doses increase, while femoral head doses decrease. Oblique gantry angles approaching the horizontal direction result in a decrease in rectal and bladder doses, while femoral head doses increase. Of the oblique angles studied, 35° off the lateral position provides lower rectum and bladder doses than 30, 40, and 45°; lower femoral head doses than 20, 25, and 30°, and the maximum CTV coverage on all CT slices studied.