Gamma Index Pass Rates Research Articles

Comparing cone-beam CT intensity correction methods for dose recalculation in adaptive intensity-modulated photon and proton therapy for head and neck cancer

ABSTRACTBackground. Adaptive intensity-modulated photon and proton radiotherapy (IMRT and IMPT) of head and neck (H&N) cancer requires frequent three-dimensional (3D) dose calculation. We compared two approaches for dose recalculation on the basis of intensity-corrected cone-beam (CB) x-ray computed tomography (CT) images.Material and methods. For nine H&N tumor patients, virtual CTs (vCT) were generated by deformable image registration of the planning CT (pCT) to the CBCT. The second intensity correction approach used population-based lookup tables for scaling CBCT intensities to the pCT HU range (CBCTLUT). IMRT and IMPT plans were generated with a commercial treatment planning system. Dose recalculations on vCT and CBCTLUT were analyzed using a (3%, 3 mm) gamma-index analysis and comparison of normal tissue and tumor dose/volume parameters. A replanning CT (rpCT) acquired within three days of the CBCT served as reference. Single field uniform dose (SFUD) proton plans were created and recalculated on vCT and CBCTLUT for proton range comparison.Results. Dose/volume parameters showed minor differences between rpCT, vCT and CBCTLUT in IMRT, but clinically relevant deviations between CBCTLUT and rpCT in the spinal cord for IMPT. Gamma-index pass-rates were found increased for vCT with respect to CBCTLUT in IMPT (by up to 21 percentage points) and IMRT (by up to 9 percentage points) for most cases. The SFUD-based proton range assessment showed improved agreement of vCT and rpCT, with 88–99% of the depth dose profiles in beam's eye view agreeing within 3 mm. For CBCTLUT, only 80–94% of the profiles fulfilled this criterion.Conclusion. vCT and CBCTLUT are suitable options for dose recalculation in adaptive IMRT. In the scope of IMPT, the vCT approach is preferable.

Real-time patient transit dose verification of volumetric modulated arc therapy by a 2D ionization chamber array

Objective To study the real-time dose verification with 2D array ion chamber array in volumetric modulated arc therapy(VMAT)with a 2D array ion chamber array. Methods The 2D ion chamber array was fixed on the panel of electronic portal imaging device(EPID). Source-detector distance(SDD)was 140 cm. 8 mm RW3 solid water was added to the 2D array to improve the signal noise ratio. Patient plans for esophageal, prostate and liver cancers were selected to be delivered on the cylindrical Cheese phantom 5 times in order to validate the reproducibility of doses. Real-time patient transit dose measurements were performed at each fraction. Dose distributions were evaluated using gamma index criteria of 3 mm DTA and 3% dose difference referred to the first time result. Results The gamma index pass rate in the Cheese phantom were about 98% ; the gamma index pass rate for esophageal, prostate and liver cancer patient were about 92%, 92% and 94%, respectively. Gamma pass rate for all single fraction were more than 90%. Conclusions The 2D array is capable of monitoring the real time transit doses during VMAT delivery. It is helpful to improve the treatment accuracy. Key words: Volumetric modulated arc radiotherapy; 2D ion chamber array; Dosimetry

Fast dose algorithm for generation of dose coverage probability for robustness analysis of fractionated radiotherapy

A fast algorithm is constructed to facilitate dose calculation for a large number of randomly sampled treatment scenarios, each representing a possible realisation of a full treatment with geometric, fraction specific displacements for an arbitrary number of fractions. The algorithm is applied to construct a dose volume coverage probability map (DVCM) based on dose calculated for several hundred treatment scenarios to enable the probabilistic evaluation of a treatment plan.For each treatment scenario, the algorithm calculates the total dose by perturbing a pre-calculated dose, separately for the primary and scatter dose components, for the nominal conditions. The ratio of the scenario specific accumulated fluence, and the average fluence for an infinite number of fractions is used to perturb the pre-calculated dose. Irregularities in the accumulated fluence may cause numerical instabilities in the ratio, which is mitigated by regularisation through convolution with a dose pencil kernel.Compared to full dose calculations the algorithm demonstrates a speedup factor of ~1000. The comparisons to full calculations show a 99% gamma index (2%/2 mm) pass rate for a single highly modulated beam in a virtual water phantom subject to setup errors during five fractions. The gamma comparison shows a 100% pass rate in a moving tumour irradiated by a single beam in a lung-like virtual phantom. DVCM iso-probability lines computed with the fast algorithm, and with full dose calculation for each of the fractions, for a hypo-fractionated prostate case treated with rotational arc therapy treatment were almost indistinguishable.

SU‐E‐T‐127: Application of TG‐119 for Evaluation of Proton Spot Scanning Based Planning and Treatment Delivery

Purpose:The clinical test cases presented in AAPM TG‐119 are used to evaluate the accuracy of treatment planning and delivery through spot scanning proton beams.Methods:An IBA spot scanning delivery system has been commissioned to be used with the RayStation treatment planning system. Various test cases provided in TG‐119 were used for planning and delivery verification. The CT dataset and structures as provided by TG‐119 were imported into a mock patient. The plans were optimized using the multi field optimization (MFO) to achieve the desired goals. The planner was given the flexibility to achieve the given dose‐volume goals by creating appropriate objectives and constraints. Beams were delivered to a phantom and measurements were performed at multiple depths using the MatrixxPT detector array. The analyses were performed on beam by beam basis and quantified using the gamma index. A tolerance of 3%/3 mm in 2D was used for gamma index analysis along with dose threshold of 10%.Results:The clinical goals for targets and critical structures were met or improved for all cases except the C‐Shape target with difficult constraints. The minimum gamma index using the 3%/3mm as a criterion is 93.3% for one of the planes measured for C‐Shape target. Using 2%/2mm as a criterion, the minimum gamma index drops to 70%. Only Prostate target has all the planes above >90% pass using the 2%/2mm criterion.Conclusion:The overall accuracy of the treatment planning and delivery is deemed clinically acceptable. The test cases with highly modulated beams can have steep gradients in the dose profiles that can reduce the gamma index pass rate. Gamma analysis based on 3D data may be needed for routine use of 2%/2mm criterion. In addition, improvements in modelling of spot profiles in dose engine may be required for further improving the gamma index pass rate.

SU‐E‐P‐35: Real‐Time Patient Transit Dose Verification of Volumetric Modulated Arc Radiotherapy by a 2D Ionization Chamber Array

Purpose:To explore the real‐time dose verification method in volumetric modulated arc radiotherapy (VMAT) with a 2D array ion chamber array.Methods:The 2D ion chamber array was fixed on the panel of electronic portal imaging device (EPID). Source‐detector distance (SDD)was 140cm. 8mm RW3 solid water was added to the detector panel to achieve maximum readings.The patient plans for esophageal, prostate and liver cancers were selected to deliver on the cylindrical Cheese phantom 5 times in order to validate the reproducibility of doses. Real‐time patient transit dose measurements were performed at each fraction. Dose distributions wereevaluated using gamma index criteria of 3mm DTA and 3% dose difference referred to the firsttime Result.Results:The gamma index pass rate in the Cheese phantom were about 98%; The gamma index pass rate for esophageal, liver and prostate cancer patient were about 92%,94%, and 92%, respectively; Gamma pass rate for all single fraction were more than 90%.Conclusion:The 2D array is capable of monitoring the real time transit doses during VMAT delivery. It is helpful to improve the treatment accuracy.

Comparison of 2D and 3D Gamma calculations for an IMRT QA phantom

Gamma index pass rates were evaluated for an anthropomorphic phantom using both 2D and 3D calculations. The phantom was irradiated with the traditional dosimetry insert loaded with radiochromic film and TLD, and then with a 3D dosimetry insert. A comparison with the calculated dose distribution showed that both the PRESAGE® dosimeter and the film and TLD system agreed with the plan to within 5% using 2D gamma index criteria. The 3D gamma index showed a slightly higher pass rate than the 2D gamma index at 3%/3mm, and comparable pass rates using more generous constraints. The acceptable number of pixels passing the tighter constraints (3%/3mm) might be dependent upon the choice between a 2D calculation versus a 3D calculation.

FoCa: a modular treatment planning system for proton radiotherapy with research and educational purposes

FoCa is an in-house modular treatment planning system, developed entirely in MATLAB, which includes forward dose calculation of proton radiotherapy plans in both active and passive modalities as well as a generic optimization suite for inverse treatment planning.The software has a dual education and research purpose. From the educational point of view, it can be an invaluable teaching tool for educating medical physicists, showing the insights of a treatment planning system from a well-known and widely accessible software platform. From the research point of view, its current and potential uses range from the fast calculation of any physical, radiobiological or clinical quantity in a patient CT geometry, to the development of new treatment modalities not yet available in commercial treatment planning systems.The physical models in FoCa were compared with the commissioning data from our institution and show an excellent agreement in depth dose distributions and longitudinal and transversal fluence profiles for both passive scattering and active scanning modalities. 3D dose distributions in phantom and patient geometries were compared with a commercial treatment planning system, yielding a gamma-index pass rate of above 94% (using FoCa’s most accurate algorithm) for all cases considered.Finally, the inverse treatment planning suite was used to produce the first prototype of intensity-modulated, passive-scattered proton therapy, using 13 passive scattering proton fields and multi-leaf modulation to produce a concave dose distribution on a cylindrical solid water phantom without any field-specific compensator.

Multicentre validation of IMRT pre-treatment verification: Comparison of in-house and external audit

Background and purposeWe performed a multicentre intercomparison of IMRT optimisation and dose planning and IMRT pre-treatment verification methods and results. The aims were to check consistency between dose plans and to validate whether in-house pre-treatment verification results agreed with those of an external audit. Materials and methodsParticipating centres used two mock cases (prostate and head and neck) for the intercomparison and audit. Compliance to dosimetric goals and total number of MU per plan were collected. A simple quality index to compare the different plans was proposed. We compared gamma index pass rates using the centre’s equipment and methodology to those of an external audit. ResultsWhile for the prostate case, all centres fulfilled the dosimetric goals and plan quality was homogeneous, that was not the case for the head and neck case. The number of MU did not correlate with the plan quality index. Pre-treatment verifications results of the external audit did not agree with those of the in-house measurements for two centres: being within tolerance for in-house measurements and unacceptable for the audit or the other way round. ConclusionsAlthough all plans fulfilled dosimetric constraints, plan quality is highly dependent on the planner expertise. External audits are an excellent tool to detect errors in IMRT implementation and cannot be replaced by intercomparison using results obtained by centres.

SU-E-T-199: How Number of Control Points Influences the Dynamic IMRT Plan Quality and Deliverability

Purpose: To study the influence of number of control points on plan quality and deliverability. Methods: Five previously treated patients of carcinoma of rectum were selected. Planning target volume (PTV) and organs at risk (OARs) i.e. bladder and bowel were contoured. Dynamic IMRT plans (6MV, 7-fields, 45Gy/25 fractions and prescribed at 95% isodose) were created in Eclipse (Varian medical system, Palo Alto, CA) treatment planning system (TPS) for Varian CL2300C/D linear-accelerator. Base plan was calculated with 166 control points, variable mode (Eclipse Default). For generating other plans, all parameters were kept constant, only number of control points (Fixed mode) was varied as follows: 100, 166 and 200. Then, plan quality was analyzed in terms of maximum and mean dose received by the PTV and OARs. For plan deliverability, TPS calculated fluence was verified with I’matriXX (IBA Dosimetry, Germany) array and compared with TPS dose-plane using gamma index criteria of 3% dose difference and 3mm distance to agreement (DTA). Total number of monitor units (MU) required to deliver a plan was also noted. Results: The maximum variation for the PTV maximum with respect to eclipse default control point (166) was 0.28% (0.14Gy). Similarly, PTV mean varied only up to 0.22 %(0.11Gy). Bladder maximum and bladder mean varied up to 0.51% (0.24Gy) and 0.16% (0.06Gy). The variation for the bowel maximum and bowel mean was also only 0.39% (0.19Gy) and 0.33% (0.04Gy). Total MU was within 0.32 % (4MU). Average gamma pass rate using different control points for five patients are 98.75±0.33%, 99.37±0.09%, 99.29±0.12%, 98.14±0.13% and 99.25±0.14% respectively. Conclusion: Slight variation (<1%) in PTV and OARs maximum and mean doses was observed with varying number of control points. Monitor unit was also not varied much. Reducing number of control points did not showed any comprise in plan deliverability in terms of gamma index pass rate.

The impact of leaf width and plan complexity on DMLC tracking of prostate intensity modulated arc therapy

Intensity modulated arc therapy (IMAT) is commonly used to treat prostate cancer. The purpose of this study was to evaluate the impact of leaf width and plan complexity on dynamic multileaf collimator (DMLC) tracking for prostate motion management during IMAT treatments. Prostate IMAT plans were delivered with either a high-definition MLC (HDMLC) or a Millennium MLC (M-MLC) (0.25 and 0.50 cm central leaf width, respectively), with and without DMLC tracking, to a dosimetric phantom that reproduced four prostate motion traces. The plan complexity was varied by applying leaf position constraints during plan optimization. A subset of the M-MLC plans was converted for delivery with the HDMLC, isolating the effect of the different leaf widths. The gamma index was used for evaluation. Tracking errors caused by target localization, leaf fitting, and leaf adjustment were analyzed. The gamma pass rate was significantly improved with DMLC tracking compared to no tracking (p < 0.001). With DMLC tracking, the average gamma index pass rate was 98.6% (range 94.8%-100%) with the HDMLC and 98.1% (range 95.4%-99.7%) with the M-MLC, using 3%, 3 mm criteria and the planned dose as reference. The corresponding pass rates without tracking were 87.6% (range 76.2%-94.7%) and 91.1% (range 81.4%-97.6%), respectively. Decreased plan complexity improved the pass rate when static target measurements were used as reference, but not with the planned dose as reference. The main cause of tracking errors was leaf fitting errors, which were decreased by 42% by halving the leaf width. DMLC tracking successfully compensated for the prostate motion. The finer leaf width of the HDMLC improved the tracking accuracy compared to the M-MLC. The tracking improvement with limited plan complexity was small and not discernible when using the planned dose as reference.

SU‐E‐T‐198: Dosimetric Effect of Multi‐Leaf Collimator Transmission On Intensity‐Modulated Radiation Therapy

Purpose: To investigate the dosimetric effect of multi‐leaf collimator (MLC) transmission on Intensity‐modulated radiation therapy. Methods: MLC transmission through the leaves and rounded ends were measured with ion chamber for Varian Trilogy linear accelerator with the energy of 6 MV and 10 MV. The intraleaf and interleaf transmission were also measured with the electronic portal imaging device of aS1000. 10 cancer patients treated with intensity‐modulated radiation therapy were used to evaluate the MLC parameters modeling in the Eclipse treatment planning system. Results: The average transmissions of Millennium MLC were 1.6% and 1.8% for 6 MV and 10 MV X‐rays. The transmission increased with the field size and depth of measurement. The transmissions at off axis position were a little lower than those on the central axis. The intraleaf transmissions measured with aS1000 were 0.8%–1.2% and 1.1%–1.6%, the interleaf transmission were 1.3%–1.9% and 1.6%–2.5% for 6 MV and 10 MV X‐rays respectively. Modeling with the measured MLC parameters, the Eclipse treatment planning system could calculate the dose distribution accurately. The gamma index pass rate at 3%3 mm was above 95%, except for two patients with nasopharyngeal cancer with the pass rate of 93.6% and 94.5%. Conclusion: The transmissions through the MLC leaves and the leaf ends contribute to the dose throughout the target significantly for IMRT. MLC transmission varied with the field size, depth of measurement and off axis position. This work was supported within the grant project: National Natural Science Foundation of China (No. 81071237).

Three-dimensional radiochromic film dosimetry for volumetric modulated arc therapy using a spiral water phantom

We validated 3D radiochromic film dosimetry for volumetric modulated arc therapy (VMAT) using a newly developed spiral water phantom. The phantom consists of a main body and an insert box, each of which has an acrylic wall thickness of 3 mm and is filled with water. The insert box includes a spiral film box used for dose-distribution measurement, and a film holder for positioning a radiochromic film. The film holder has two parallel walls whose facing inner surfaces are equipped with spiral grooves in a mirrored configuration. The film is inserted into the spiral grooves by its side edges and runs along them to be positioned on a spiral plane. Dose calculation was performed by applying clinical VMAT plans to the spiral water phantom using a commercial Monte Carlo-based treatment-planning system, Monaco, whereas dose was measured by delivering the VMAT beams to the phantom. The calculated dose distributions were resampled on the spiral plane, and the dose distributions recorded on the film were scanned. Comparisons between the calculated and measured dose distributions yielded an average gamma-index pass rate of 87.0% (range, 91.2–84.6%) in nine prostate VMAT plans under 3 mm/3% criteria with a dose-calculation grid size of 2 mm. The pass rates were increased beyond 90% (average, 91.1%; range, 90.1–92.0%) when the dose-calculation grid size was decreased to 1 mm. We have confirmed that 3D radiochromic film dosimetry using the spiral water phantom is a simple and cost-effective approach to VMAT dose verification.

Dosimetric and geometric evaluation of a novel stereotactic radiotherapy device for breast cancer: The GammaPod™

A dedicated stereotactic gamma irradiation device, the GammaPod™ from Xcision Medical Systems, was developed specifically to treat small breast cancers. This study presents the first evaluation of dosimetric and geometric characteristics from the initial prototype installed at University of Maryland Radiation Oncology Department. The GammaPod™ stereotactic radiotherapy device is an assembly of a hemi-spherical source carrier containing 36 (60)Co sources, a tungsten collimator, a dynamically controlled patient support table, and the breast immobilization system which also functions as a stereotactic frame. The source carrier contains the sources in six columns spaced longitudinally at 60° intervals and it rotates together with the variable-size collimator to form 36 noncoplanar, concentric arcs focused at the isocenter. The patient support table enables motion in three dimensions to position the patient tumor at the focal point of the irradiation. The table moves continuously in three cardinal dimensions during treatment to provide dynamic shaping of the dose distribution. The breast is immobilized using a breast cup applying a small negative pressure, where the immobilization cup is embedded with fiducials also functioning as the stereotactic frame for the breast. Geometric and dosimetric evaluations of the system as well as a protocol for absorbed dose calibration are provided. Dosimetric verifications of dynamically delivered patient plans are performed for seven patients using radiochromic films in hypothetical preop, postop, and target-in-target treatment scenarios. Loaded with 36 (60)Co sources with cumulative activity of 4320 Ci, the prototype GammaPod™ unit delivers 5.31 Gy/min at the isocenter using the largest 2.5 cm diameter collimator. Due to the noncoplanar beam arrangement and dynamic dose shaping features, the GammaPod™ device is found to deliver uniform doses to targets with good conformity. The spatial accuracy of the device to locate the radiation isocenter is determined to be less than 1 mm. Single shot profiles with 2.5 cm collimator are measured with radiochromic film and found to be in good agreement with respect to the Monte Carlo based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all hypothetical treatment plans corresponding to three target scenarios for each of the seven patients demonstrated good agreement with gamma index pass rates of better than 97% (99.0% ± 0.7%). Dosimetric evaluation of the first GammaPod™ stereotactic breast radiotherapy unit was performed and the dosimetric and spatial accuracy of this novel technology is found to be feasible with respect to clinical radiotherapy standards. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy.

SU‐E‐T‐359: Comparison of Three Array Calibration Methods of MapCheck for Elekta Beam Modulator LINAC

Purpose: To compare three array calibration methods of MapCheck for an Elekta Beam Modulator LINAC. Methods: Both Elekta Synergy and Elekta Beam Modulator LINACs are used in our clinic. While an Elekta Synergy LINAC is a conventional Linac with the maximu field size 40 × 40 cm2, an Elekta Beam Modulator LINAC only has a maximum field size of 21 × 16 cm2. Because the MapCheck requires a field size of 26 × 26 cm2 for array calibration according to its user manual, it is a challenge to calibrate MapCheck array for Beam Modulator. We have tested three methods for this Purpose: 1) use the array calibration file from Elekta Synergy; 2) calibrate the MapCheck using Beam Modulator at 135cm SSD; 3) calibrate the MapCheck with both Beam Modulator and Elekta Synergy, then combine the calibration files. For the third method, the MapCheck was first calibrated with Beam Modulator at both collimator angles 0 and 90 degrees at 110cm SSD. Then the MapCheck was calibrated with Elekta Synergy using the standard method at 100cm SSD. The three array calibration files were combined such that it best describes the response of MapCheck detectors to Beam Modulator and all the detectors are covered. After calibrating the MapCheck detector array with the three methods, four prostate and four H&amp;N IMRT treatment plans were delivered. The measurements were compared with the calculations. The Gamma Index (Gamma Index &lt; 1) pass rates with a criterion of 3% and 3mm from all the three calibration methods were compared. Results: The pass rate of Gamma Index ranges from 90.4% to 100%. The null hypothesis that all three methods come with the same QA pass rates can not be rejected (p&gt;0.1). Conclusions: For the testing cases, no significant difference was found among the three array calibration methods in term of Gamma Index pass rate.

SU-E-T-399: Dosimetric and Geometric Evaluation of a Novel Stereotactic Radiotherapy Device for Breast Cancer: The GammaPod.

A dedicated stereotactic irradiation device, the GammaPodTM, was developed to treat early stage breast cancer. This study presents the first description of the dosimetric and geometric characteristics from the prototype unit. The GammaPod stereotactic radiotherapy device is an assembly of a hemi-spherical source carrier containing 36 Co-60 sources, a tungsten collimator, a dynamically controlled treatment table and a breast immobilization cup embedded with a stereotactic coordinate system. The source carrier and the variable-size collimator rotate synchronously to form 36 non-coplanar, concentric arcs focused at the isocenter. The treatment table enables motion in three dimensions facilitating continuous dose painting in comparison to a sphere packing approach. Geometric and dosimetric evaluations and a method for absorbed dose calibration are provided. Dosimetric verifications of the dynamically delivered plans are performed for eight patients in hypothetical pre-op, post-op and dose painting treatment scenarios. Loaded with a cumulative activity of 4320 Ci, the GammaPod unit delivers 5.31 Gy/min at the isocenter. Due to non-coplanar beam arrangement and dynamic dose shaping features, the GammaPod delivers uniform doses to the targets with excellent conformity. The spatial accuracy of the device is less than 1 mm. Single shot profiles with the 25 mm collimator are measured with radiochromic film and found to be in good agreement with respect to the MC based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all treatment plans corresponding to three target scenarios for each of the eight patients demonstrated Gamma index pass rates greater than 97%. The first description of the dosimetric and geometric evaluation of the GammaPod was performed. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy. C Yu and J Zhang have commercial affiliations with Xcision Medical Systems.

SU-E-T-50: Updating a Familiar Clinical Tool: Including Structures in Gamma Index Calculations

Purpose: In order to improve the correlation between gamma index pass rates and plan quality, a new method was developed to incorporate DVH structures into the gamma index analysis. Methods: Five treatment machines were created in the Varian Eclipse platform. The original machine was created using Varian golden beam data. Deviations, representing clinical errors, were introduced in the other machines by modifying MLC transmission factors and smoothing beam penumbras. Doses for 9 Head and Neck dynamic IMRT plans were calculated, creating 36 modified and 9 original plans. The max spinal cord dose and dose to 95% of the PTV (PTV-95%) were calculated. The modified plans were compared to the original using a gamma analysis commonly used in IMRT QA. A new method of gamma analysis was performed by using the cord and PTV structures to vary the passing criteria (%/mm). The structures were extracted using Matlab and projected from a “beams eye view” onto the QA dose planes. To emphasize the importance of the dose to the structures, the passing criteria were tightened in proportion to the thickness of the structure that the beam traveled through. Pearson correlation coefficients(r-values) comparing the PTV-95%/spine dose differences and the gamma pass rates were calculated. Results: Pearson coefficients for the max cord dose were −0.717/−0.720/−.750 for traditional 3/2/1 (%/mm) passing criteria, respectively. This improved to −0.831 when using the structure dependent passing criteria. Pearson coefficients for PTV-95% were −0.729/−0.703/−.711 for the 3/2/1 (%/mm) criteria and −0.812 for the structures dependent criteria. Conclusions: Using structural information to vary gamma analysis passing criteria can result in improved correlation between gamma index pass rates and clinically relevant dose objectives. This enhances the ability of the gamma index, making it a more powerful IMRT QA tool.

SU-E-T-576: A Comprehensive Approach for SBRT Irradiation on RPC Spine Phantom

Purpose: To evaluate the entire spine SBRT process and perform comprehensive quality assurance (QA) for RPC Spine Phantom irradiation. Methods: A RPC spine phantom was obtained for RTOG0631 protocol credentialing. A 7 beam, 6MV photon, step‐and‐shoot IMRT treatment plan was generated using Pinnacle3 treatment planning system (TPS). Total 6Gy in 1 fraction was prescribed to the target PTV. Stringent objectives for OARs were set during plan optimization. Treatment table attenuation factor was measured for all posterior beams penetrating through the table. A comprehensive quality assurance (QA) of the plan was performed: I'mrt phantom (Scanditronix Wellhoffer, Germany) ‐based ion chamber point‐ dose measurement and GAF chromic film axial plane dose measurement, and Mapcheck planar dose measurements. The treatment plan was loaded on the front part of the I'mrt phantom and the planar dose at planned film location was generated for 2D gamma comparison. Similarly, the treatment plan was loaded on the rear part of the I'mrt phantom and point dose at ion chamber location was recorded. A routine beam‐by‐beam IMRT planar dose QA was performed. Results: The treatment plan has met protocol requirement of PTV coverage and OAR constraints. Greater than 90% of the PTV received prescribed dose. Max point dose of cord, heart and esophagus were 3.23, 1.74 and 5.20Gy, respectively. 1000cc lung received 0.75Gy. Table attenuation factors were 0.043, 0.052 and 0.08. Ion chamber measurement was agreed with TPS PTV point dose within 0.1%. Axial film measurement had excellent agreement with plan, the gamma index pass rate was 98.6% with 3% and 3mm criteria. Conclusion: Spine SBRT requires accurate TPS commissioning, careful planning and good image guidance before final patient treatment. The comprehensive QA effort needed for spine SBRT was more than the regular fractionated IMRT cases but was warranted by its potential high gain and high risk nature.

WE‐E‐BRB‐05: The Effects of Low Dose Regions On 3D IMRT QA Delivery Using a MatriXX 2D Array and MULTICUBE

Purpose: IMRT quality assurance performed at treatment gantry angles using 2D arrays allows for the evaluation of rotational aspects of beam delivery. We investigate aspects of angular‐based IMRT QA that contribute to lower‐than‐average passing rates when compared to treatment planning calculations. Method and Materials: Planar dose profiles calculated on a CT of the IBA MatriXX 2D array device with the MULTICUBE phantom at iso‐center were exported from the Philips Pinnacle TPS. Beam angles of 0, 90, 180, and 270° were delivered using a Varian Clinac 2300iX. IBA OmniPro‐I'mRT software was used for analysis. Individual beams of 4×4 and 10×10 cm2 were compared to calculated profiles using gamma index criteria of 3 % and 3 mm. Additionally, composite profiles of 0 and 90° beams were assessed for each field size. Finally, IMRT‐delivered clinical beams were assessed. Results: The gamma index pass rates of individual 4×4 and 10×10 cm2 beams at 0° were greater than 98%. However, 90° beams were found to achieve below 90% pass rates, with the 4×4 being below 85%. The measured and calculated results compared worse in the exit dose region. Composite profiles of 0 and 90° beams agreed with calculated to within 98 % for both field sizes. Clinical IMRT beams showed good agreement in high‐dose regions, but passing rates in exit‐dose regions demonstrated reduced passing rates. Conclusion: An angular dependency is evident in the measurement of radiation fields when using the MatriXX device. Gamma index pass rates can show significant improvement through composite dose comparison. However, individual beam information can be lost, particularly the low‐dose beam exit contribution, when applying composite evaluation. This becomes important when considering the dose to organs‐at‐risk which lay in these low dose regions.