Spine SBRT Research Articles

SU‐E‐J‐202: Is Pretreatment Imaging at Each Treatment Fraction Needed in Spine SBRT to Enable Margin Reduction?

Purpose:To study translational setup errors of Spine SBRT treatments using BodyFIX immobilization system and the potential for margin reduction using pretreatment imaging.Methods:148 online match results of 39 Spine SBRT patients treated at our institution were reviewed. All treatments performed using Varian, TrueBeam and MI™, BodyFIX immobilization device. Prescriptions (dose‐fractionation schedules) for the patients in the study are 8 Gy X 3 (n=24), 6 Gy X 4 (n=10) and 16Gy X 1 (n=5). For all patients, OBI was performed on verification day and before each treatment. Recorded shifts are from adopted couch positions after physician online review of KV image‐pair and CBCT. Data analyzed by computing Mean error (M), Systematic error (Σ) and Random error (σ) for three translation directional shifts and 3D Vector length. To estimate correlation between imaging sessions, Anova‐test and T‐Test with alpha of 0.05 performed over the data.Results:Mean error (M), Systematic error (Σ) and Random error (σ) of Vertical direction are ‐1.81mm, 4.05mm and 2.01mm respectively. Similarly, Errors in Longitudinal direction are 0.76mm (M), 4.21mm (Σ) and 1.34mm (σ). Errors in Lateral direction are – 0.16mm (M), 3.22mm (Σ) and 1.83 mm (σ). Mean, SD and CI95% of 3D Vector lengths are 7.93mm, 4.41mm and 0.7mm respectively. Anova‐test results show, P‐Values for longitudinal and lateral shifts are > 0.3 and P‐value of vertical shift is 0.054. This signifies mean of the each imaging session are not significantly different. T‐Test on treatment sessions data of vertical shifts with respect to verification day shows, mean of Vertical shifts on verification day are different from Vertical shifts on treatment sessions.Conclusion:Results shows immobilization with BodyFIX is reproducible in the mm range. To eliminate systematic error component and to enable margin reduction, pretreatment imaging prior to each treatment fraction is indispensable.

SU‐E‐T‐518: Dosiemtric Impact of Patient Positioning Uncertainty in SBRT Treatments

Purpose:To investigate the dosimetric impact of patient positioning uncertainties on conventionally fractionated IMRT, hypofractionated and single‐fraction SBRT in light of the recently published AAPM practice guideline on IGRT quality assurance (MPPG2a) .Methods:Treatment plans of three prostate IMRT, three lungs SBRT and three spine SBRT patients were selected for this study. For each patient, uncertainties in isocenter positioning were simulated by displacing the planned isocenter with variable amount in three cardinal directions. The resulting dose distribution was obtained by recalculating the original treatment plan for each displaced isocenter position with the same planned monitor units. Dosimetric impact was assessed by comparing the isocenter‐shifted plan with the original plan with respect to the image‐treatment isocenter coincidence tolerance recommended in MPPG2a (1 mm for SRS and 2 mm for SBRT) .Results:For prostate IMRT, a 2‐mm uncertainty in lateral and superior‐inferior direction had minimal dosimetric impact on either PTV or rectum. However, a 2‐mm deviation in the anterior‐posterior position could Result in up to 8% change in PTV minimum dose (PTVDmin) and up to 7% increase in rectum D25. For lung SBRT, uncertainties of ≤ 2mm had minimal impact on PTV mean dose and mean lung dose. However, up to 21% reduction in PTVDmin was observed with a 2‐mm uncertainty in superior‐inferior direction. For single‐fraction spine SBRT, a 1‐mm uncertainty could cause up to 10% change in PTVDmin and cord maximum dose. Formal statistical analysis for patient‐specific impact over a given course of treatment is being performed .Conclusion:In addition to SRS, single‐fraction SBRT should require isocenter alignment tolerance to be ≤ 1 mm. The 2‐mm positioning tolerance for SBRT may not be sufficient for certain patients. Our analysis can be integrated into the treatment planning system for prospective prediction of the susceptibility of individual plans to setup uncertainties.

SU‐E‐T‐508: End to End Testing of a Prototype Eclipse Module for Planning Modulated Arc Therapy On the Siemens Platform

Purpose:The latest clinical implementation of the Siemens Artiste linac allows for delivery of modulated arcs (mARC) using full‐field flattening filter free (FFF) photon beams. The maximum doserate of 2000 MU/min is well suited for high dose treatments such as SBRT. We tested and report on the performance of a prototype Eclipse TPS module supporting mARC capability on the Artiste platform.Method:our spine SBRT patients originally treated with 12/13 field static‐gantry IMRT (SGIMRT) were chosen for this study. These plans were designed to satisfy RTOG0631 guidelines with a prescription of 16Gy in a single fraction. The cases were re‐planned as mARC plans in the prototype Eclipse module using the 7MV FFF beam and required to satisfy RTOG0631 requirements. All plans were transferred from Eclipse, delivered on a Siemens Artiste linac and dose‐validated using the Delta4 system.Results:All treatment plans were straightforwardly developed, in timely fashion, without challenge or inefficiency using the prototype module. Due to the limited number of segments in a single arc, mARC plans required 2‐3 full arcs to yield plan quality comparable to SGIMRT plans containing over 250 total segments. The average (3%/3mm) gamma pass‐rate for all arcs was 98.5±1.1%, thus demonstrating both excellent dose prediction by the AAA dose algorithm and excellent delivery fidelity. Mean delivery times for the mARC plans(10.5±1.7min) were 50‐70% lower than the SGIMRT plans(26±2min), with both delivered at 2000 MU/min.Conclusion:A prototype Eclipse module capable of planning for Burst Mode modulated arc delivery on the Artiste platform has been tested and found to perform efficiently and accurately for treatment plan development and delivered‐dose prediction. Further investigation of more treatment sites is being carried out and data will be presented.

SU‐E‐T‐269: Quality Assurance of Spine Volumetric Modulated Arc Therapy with Flattening Filter Free Beams Using Gafchromic EBT3 Films

Purpose:We implemented the Gafchromic film‐based patient specific QA of volumetric modulated arc therapy (VMAT) with flattening‐filter free (FFF) beams for spine metastases and validated the accuracy of fast arc delivery.Methods:EBT3 films and a homemade cylindrical QA phantom were employed for dosimetric verification of VMATs. For 14 FFF VMAT plans (10 with 10‐MV FFF beams and 4 with 6‐MV FFF beams), the doses were recalculated on the phantom and delivered by a TrueBeam STx accelerator equipped with a high‐definition 120 leaf MLC. The EBT3 films were scanned using an Epson 10000XL scanner through the FilmQA Pro software. All the irradiated film images were converted to dose map using a calibration response curve. The resulting dose map of film measurement was compared with treatment plan and evaluated using gamma analysis with dose tolerance of 2% within 2 mm. In addition, the point‐dose measurement in the phantom using an ion chamber was evaluated as a reference in a ratio of measured and planned doses.Results:The gamma pass rates averaged over all FFF plans for composite‐field measurements were 96.0 ± 3.6% (88.9%–99.5%). When adopting a tolerance level of 3% – 3 mm, the gamma pass rates were improved with the ranges from 98% to 100%. In addition, dose profiles and dose distributions showed that spinal cord was protected by the rapid dose fall‐off and by delivering the treatment with high precision. In point‐dose measurements, the average differences between the measured and planned doses were 0.5% ± 1.0% of the prescription dose.Conclusion:We demonstrated that Gafchromic EBT3 film would be an effective patient‐specific QA tool, especially for VMAT of spine SBRT with treatment of small fields and highly gradient dose distributions. The results of film QA verified that the dosimetric accuracy of spine SBRT utilizing RapidArc with FFF beams in our institution is reliable.

SU‐E‐T‐316: The Design of a Risk Index Method for 3D Patient Specific QA

Purpose:To suggest a new guidance for the evaluation of 3D patient specific QA, a structure‐specific risk‐index (RI) method was designed and implemented.Methods:A new algorithm was designed to assign the score of Pass, Fail or Pass with Risk to all 3D voxels in each structure by improving a conventional Gamma Index (GI) algorithm, which implied the degree of the risk of under‐dose to the treatment target or over‐dose to the organ at risks (OAR). Structure‐specific distance to agreement (DTOA), dose difference and minimum checkable dose were applied to the GI algorithm, and additional parameters such as dose gradient factor and dose limit of structures were used to the RI method. Maximum passing rate (PR) and minimum PR were designed and calculated for each structure with the RI method. 3D doses were acquired from a spine SBRT plan by simulating the shift of beam iso‐center, and tested to show the feasibility of the suggested method.Results:When the iso‐center was shifted by 1 mm, 2 mm, and 3 mm, the PR of conventional GI method between shifted and non‐shifted 3D doses were 99.9%, 97.4%, and 89.7% for PTV, 99.8%, 84.8%, and 63.2% for spinal cord, and 100%, 99.5%, 91.7% for right lung. The minimum PRs from the RI method were 98.9%, 96.9%, and 89.5% for PTV, and 96.1%, 79.3%, 57.5% for spinal cord, and 92.5%, 92.0%, 84.4% for right lung, respectively. The maximum PRs from the RI method were equal or less than the PRs from the conventional GI evaluation.Conclusion:Designed 3D RI method showed more strict acceptance level than the conventional GI method, especially for OARs. The RI method is expected to give the degrees of risks in the delivered doses, as well as the degrees of agreements between calculated 3D doses and measured (or simulated) 3D doses.

SU‐E‐T‐302: Spine SBRT Planning for Patients with Spinal Hardware

Purpose:To investigate the effects of different imaging artifact correction strategies and susceptibility to setup errors for multiple spinal SBRT planning approaches in treating a patient with titanium hardware.Methods:Two‐step‐IMRT, IMRT and two arc VMAT plans were created for a C‐spine SBRT case with titanium hardware. The plans were normalized to the maximum point dose to the thecal sac. Density overrides for the planning CT were applied to mitigate imaging artifacts. All plans were recalculated on CTs with no density overrides and with the density of all structures set to 1g/cm^3. Setup errors up to 2mm toward the spinal cord were simulated.Results:The 2‐step IMRT plans achieved a mean dose to the cord of 6.5Gy compared to 8.0 and 8.7Gy for the IMRT and VMAT plans. The PTV coverage was comparable for both the 2‐step‐IMRT and the IMRT plan. The VMAT plan achieved a higher mean dose, but had hot spots >130%. If no density overrides were applied for the same MUs the increase in maximum dose to the cord was 0.3, 1.2 and 0.0% for 2‐step‐IMRT, IMRT and VMAT plans, respectively, compared to an increase of 2.3, 2.0 and 1.0% for unit density override.An isocenter shift of 2mm toward the cord resulted in an increase in maximum dose to the cord of 10.7, 8.0 and 5.0% for the 2‐step IMRT, IMRT and VMAT plan, respectively, with the absolute dose to the cord still being lowest for the 2‐step‐IMRT plan compared to the other plans without simulated setup errors.Conclusion:The 2‐step‐IMRT plan achieved the clinically most acceptable dose distribution. The plan quality was relatively insensitive to density overrides. The 2‐step‐IMRT plan was the most sensitive to setup errors due to the steep gradient but this was offset by the initial improvement in sparing of the cord.

TH‐A‐9A‐01: Active Optical Flow Model: Predicting Voxel‐Level Dose Prediction in Spine SBRT

Purpose:To predict voxel‐level dose distribution and enable effective evaluation of cord dose sparing in spine SBRT.Methods:We present an active optical flow model (AOFM) to statistically describe cord dose variations and train a predictive model to represent correlations between AOFM and PTV contours. Thirty clinically accepted spine SBRT plans are evenly divided into training and testing datasets. The development of predictive model consists of 1) collecting a sequence of dose maps including PTV and OAR (spinal cord) as well as a set of associated PTV contours adjacent to OAR from the training dataset, 2) classifying data into five groups based on PTV's locations relative to OAR, two “Top”s, “Left”, “Right”, and “Bottom”, 3) randomly selecting a dose map as the reference in each group and applying rigid registration and optical flow deformation to match all other maps to the reference, 4) building AOFM by importing optical flow vectors and dose values into the principal component analysis (PCA), 5) applying another PCA to features of PTV and OAR contours to generate an active shape model (ASM), and 6) computing a linear regression model of correlations between AOFM and ASM.When predicting dose distribution of a new case in the testing dataset, the PTV is first assigned to a group based on its contour characteristics. Contour features are then transformed into ASM's principal coordinates of the selected group. Finally, voxel‐level dose distribution is determined by mapping from the ASM space to the AOFM space using the predictive model.Results:The DVHs predicted by the AOFM‐based model and those in clinical plans are comparable in training and testing datasets. At 2% volume the dose difference between predicted and clinical plans is 4.2±4.4% and 3.3±3.5% in the training and testing datasets, respectively.Conclusion:The AOFM is effective in predicting voxel‐level dose distribution for spine SBRT.Partially supported by NIH/NCI under grant #R21CA161389 and a master research grant by Varian Medical System.

Effects of flattening filter‐free and volumetric‐modulated arc therapy delivery on treatment efficiency

Flattening filter‐free (FFF) beams are available on an increasing number of commercial linear accelerators. FFF beams have higher dose rates than flattened beams of equivalent energy which can lead to increased efficiency of treatment delivery, especially in conjunction with increased FFF beam energy and arc‐based delivery configurations. The purpose of this study is to quantify and assess the implications of improved treatment efficiency for several FFF delivery options on common types of linac applicable radiotherapy. Eleven characteristic cases representative of a variety of clinical treatment sites and prescription doses were selected from our patient population. Treatment plans were generated for a Varian TrueBeam linear accelerator. For each case, a reference plan was created using DMLC IMRT with 6 MV flat beams. From the same initial objectives, plans were generated using DMLC IMRT and volumetric‐modulated arc therapy (VMAT) with 6 MV FFF and 10 MV FFF beams (max. dose rates of 1400 and 2400 MU/min, respectively). The plans were delivered to a phantom; beam‐on time, total treatment delivery time, monitor units (MUs), and integral dose were recorded. For plans with low dose fractionations (1.8–2.0 & 3.85 Gy/fraction), mean beam‐on time difference between reference plan and most efficient FFF plan was 0.56 min (41.09% decrease); mean treatment delivery time difference between the reference plan and most efficient FFF plan was 1.54 min (range: 0.31–3.56 min), a relative improvement of 46.1% (range: 29.2%‐59.2%). For plans with high dose fractionations (16–20 Gy/fraction), mean beam‐on time difference was 6.79 min (74.9% decrease); mean treatment delivery time difference was 8.99 min (range: 5.40–13.05 min), a relative improvement of 71.1% (range: 53.4%‐82.4%). 10 MV FFF VMAT beams generated the most efficient plan, except in the spine SBRT case. The distribution of monitor unit counts did not vary by plan type. In cases where respiratory motion management would be applicable, 10 MV FFF DMLC IMRT reduced beam‐on time/field to less than 12 sec. FFF beams significantly reduced treatment delivery time. For radiosurgical doses, the efficiency improvement for FFF beams was clinically significant. For conventional fractionation, a large improvement in relative treatment delivery time was observed, but the absolute time savings were not likely to be of clinical value. In cases that benefit from respiratory motion management, beamon/field was reduced to a time for which most patients can comfortably maintain deep inspiratory breath hold.PACS numbers: 87.55.D‐, 87.55.de, 87.56.bd, 87.56.N‐

Postoperative Stereotactic Body Radiation Therapy (SBRT) for Patients With Spinal Metastasis: Predictive and Prognostic Factors Analysis

Spine SBRT is increasingly being applied to the post-operative patient, as an alternative to conventional palliative radiation therapy, with the aim to improve upon existing rates of local control. As an emerging indication, our aim was to identify clinical and dosimetric predictors of local control and survival.

Rotational Patient Positioning Corrections for Cervical Spine SBRT: Evaluation of 6DOF Couch Applicability

A retrospective analysis of registrations of individual cervical vertebrae between planning CT and pre-treatment CBCT was performed to estimate how many patients would be expected to require rotational corrections >3 degrees in magnitude. This information is expected to be predictive of the utility of patient tables capable of adjustment of patient roll and pitch angles over a limited range. A 3 degree rotation results in a translation of ∼2 mm for tissue 4 cm from the rotation axis, which could affect outcomes following spinal SBRT.

A Minimum Tumor to Spinal Cord Distance of 3-4 mm Is Needed for Optimal Planning of Spine SBRT

A Minimum Tumor to Spinal Cord Distance of 3-4 mm Is Needed for Optimal Planning of Spine SBRT

Cord and Thecal Sac Dosimetric Correlation with Toxicity for Spine SBRT

Cord and Thecal Sac Dosimetric Correlation with Toxicity for Spine SBRT

Spine SBRT Plans: Achieving Dose Coverage, Conformity, and Dose Fall-Off

Spine SBRT Plans: Achieving Dose Coverage, Conformity, and Dose Fall-Off

SU‐E‐T‐433: Evaluation of the ArcCheck Device for Patient‐Specific Quality Assurance in Step‐And‐Shoot IMRT for Single‐Fraction Spinal SBRT

Purpose: Current standard for patient specific spine SBRT IMRT QA in our institution is a combination of a point dose measurement in solid water under delivery of the beams at actual treatment angles, and 2D planar verification using an ion chamber (IC) array with all beams delivered at standard gantry angle of zero. The very small active area of ArcChecks diodes (∼0.6mm2) has the potential to improve measurement accuracy in high dose gradient regions present in spinal SBRT. Given high clinical workload we seek to improve ease of workflow and decrease time necessary for patient‐specific QA, by obtaining 2D verification and point dose measurement at actual treatment angles in one single measurement. Methods: The results of 10 patient specific QA measurements, as performed with single‐point and 2D IC array were compared to results obtained with ArcCheck. ArcCheck and 2D IC array pass rates are evaluated for 3%/3mm gamma criteria, single‐point measurement based on percent deviation from QA plan dose. The constancy of the device was assessed by re‐measuring the same treatment plan over the course of three weeks, and evaluating pass rate variationResults: The gamma pass rates are generally higher for ArcCheck (average 99.3%) than for Matrixx (average 93.1%). Average absolute deviation for the single‐point IC measurement was 3.4%. Maximum variation in pass rate between any two ArcCheck measurements within the three‐week evaluation period was 0.5%. Since ArcCheck allows for the use of an insert with an ion chamber in place, simultaneous redundant point measurement is possible with no time addedConclusion: Based on our results patient‐specific QA for spine SBRT treatments will be performed using ArcCheck with pass/fail criteria 98%. High sensitivity towards geometrical shifts necessitates careful initial alignment of the device.

SU-E-J-207: Dosimetric Effects of Diaphragm Motion On Spine SBRT Treatments

Purpose: To evaluate the dosimetric effects of diaphragm motion on spine SBRT treatments with IMRT and VMAT deliveries. Methods: Five 4D-CTs were acquired for lung and liver SBRT patients. The vertebral body closest to the diaphragm level was contoured as the CTV. The thecal sac was used as a surrogate for the spinal cord PRV. A 3 mm expansion was used for the PTV with the PTV cropped to the cord PRV. Plans were optimized for PTV maximal coverage at 1800 cGy and cord PRV Dmax was no more than 1200 cGy. Plans were created for 7 and 11 field IMRT and partial arc VMAT modalities, all on the average intensity CT (AvgCT) and recalculated on the 4D CT data set. D95, D1, and Davg were recorded for the different delivery methods and all 11 CT sets (AvgCT+10 phases). Dose differences were calculated for each phase versus the planning AvgCT and for the maximum difference amongst the phases to evaluate worst-case scenarios. Results: Regarding the PTV coverage, the IMRT-7 plans always exhibited the largest dosimetric differences amongst metrics with Davg having the largest range (Davg[max-min]: 63 cGy) versus the IMRT-11 (51 cGy) and VMAT (49 cGy) out of the prescription dose of 1800 cGy. All other dosimetric points had smaller differences for PTV metrics. The maximum difference always occurred between exhale and inhale phases. The cord PRV D1% had smaller differences than PTV metrics with the maximum dose difference of 44 cGy for the IMRT-7 plans, and IMRT-11 had a maximum of 17 cGy and VMAT 23 cGy. Conclusions: Clinically relevant spine SBRT plans display less than 4% maximum difference in PTV dose (Davg) and 3% variability in the cord PRV D1% metric between inhale and exhale phases for IMRT-7 plans in the worst case scenario.

MO‐F‐108‐09: Single Fraction Spine SBRT End to End Testing On TomoTherapy and Vero Treatment Platforms Using a Novel Anthropomorphic Phantom

Purpose: Single fraction spine SBRT involves very high doses delivered to targets adjacent to the spinal cord. Highly conformal planning and accurate delivery of such plans is imperative for successful treatment without catastrophic adverse affects. End to end testing is an important practice for evaluating the entire treatment process from simulation through treatment delivery. We performed end‐to‐end testing for a set of representative spine SBRT targets planned and delivered using different treatment planning systems (TPSs) and delivery systems. Methods: An anthropomorphic E2E SBRT phantom (IMT, Troy,NY) accepting ion chambers in the thoracic region and film in the lumbar region was simulated and treated to evaluate agreement between measured and calculated doses. Four representative targets were developed within each region (thoracic and lumbar) to represent different presentations of spine metastases and planned according to RTOG 0631 constraints. Plans were created using the TomoTherapy TPS for delivery using the Hi‐ART system (Accuray, Sunnyvale,CA) and the iPlan TPS for delivery using the Vero system (BrainLAB, Feldkirchen,Germany). Delivered doses were measured using 0.007cc ion chambers (Standard Imaging, Middleton,WI) in the thoracic region and EBT3 Gafchromic film (Ashland, Wayne,NJ) in the lumbar region. Results: Both treatment platforms met all dose constraints required by RTOG 0631. Ion chamber measurements in the 4 thoracic targets delivered on TomoTherapy all agreed within 2.2%, with an average difference of 0.8%. Similar measurements on the Vero system agreed within 3.3%. Film measurements for the 4 lumbar targets resulted in gamma index passing rates over 99.7% at 3%/3mm, 99.2% at 2%/2mm, and 90% at 1%/1mm for all plans on both platforms. Conclusion: In spite of the complexity of the representative targets and their proximity to the spinal cord, both treatment platforms were able to create plans meeting all RTOG dose constraints and produced exceptional agreement between calculated and measured doses.

Spine SBRT treatment: A case report

Spine SBRT treatment: A case report

SU‐E‐T‐196: Pretreatment Patient Clinical Objective IMRT Quality Assurance Using a 3D Diode Array

Purpose: To evaluate the accuracy and sensitivity of a patient clinical objective IMRT QA tool using a 3D diode array. Methods: A 3D QA device (Delta4, Scandidos Inc.) with 2 orthogonal diode array planes was used for IMRT QA. The diode spacing is 5mm in central 6cmx6cm region and 1cm in the peripheral 20cmx20cm region. 3D volumetric dose is interpolated from the measured planar dose. The Delta4 Anatomy software (Anatomy) calculates the entrance energy fluence for each beam, which is then applied to the patient anatomy to calculate dose to the patient using a pencil‐beam dose calculation algorithm. The accuracy of Anatomy was evaluated in both solid water phantom using open fields and in patient anatomy using a prostate IMRT plan. The dose calculated by Eclipse was used as the gold standard. The sensitivity of the Anatomy was also evaluated by introducing leaf positioning errors in the delivery of a spine SBRT plan. Results: Excellent agreement between Anatomy and Eclipse was achieved for the solid water phantom with open fields. Discrepancy was observed for the PTV DVH calculated in patient anatomy using the prostate IMRT plan. This is mainly due to sharp dose fall‐off outside PTV and phantom setup errors during QA. Good agreement between Anatomy and Eclipse was achieved for bladder and rectum DVH. For spine case with introduced 1cm single leaf positioning error, the traditional gamma analysis using Delta4 still showed 95% pass rate using 3%, 3mm criteria. However, Anatomy showed 40% increase of cord max dose due to this error, which was consistent with Eclipse. Conclusion: Patient clinical objective QA using Delta4 Anatomy can potentially provide valuable information about the clinical significance of the QA results. It can potentially catch clinically significant delivery errors that would otherwise be missed in the traditional QA process using gamma analysis. Funding from Scandidos Inc.

SU‐E‐T‐179: Evaluating Treatment Couch Dosimetric Effects in Spine SBRT Treatment Planning

Purpose: To evaluate the dosimetric effects of the treatment couch on skin dose and field attenuation for Spine SBRT. Methods: Measurements were acquired with an Elekta Synergy S in a 10.4 cm x 10.4 cm open 6 MV field with a Marcus parallel plate ionization chamber for varying thickness of water equivalent plastic. Measurements were repeated with and without the Hexapod carbon fiber treatment couch. A program was written to superimpose a CT scan of the treatment couch onto the patient planning CT. Dose calculations were performed with and without the treatment couch in a solid water phantom in Raystation Treatment planning software and compared to the measured values. All doses are in reference to Dmax of the open field without couch. Results: The measurements displayed a 69% higher dose (93% versus 24%) at the surface with the addition of the couch due to the electron contamination from the couch. Our beam model in Raystation underestimates the surface dose by 10% and agreement is achieved at 7 mm and within 4% at 4 mm depth. At depths greater than Dmax, the couch introduced an attenuation factor of 2%. Conclusion: A carbon fiber couch significantly affects the surface dose for beams that pass through the couch. The couch introduces electron contamination that increases the dose by 69% at the surface. High doses to the skin can cause skin erythema. Couch dose enhancement combined with high dose fractions and several beams passing through the couch for spine SBRT treatments should be accurately modeled to evaluate the actual dose delivered to the skin.

SU-E-T-368: Effect of Oblique Beam Incidence On 3D Dose Reconstruction for Small Field IMRT QA

Purpose: To compare dose reconstruction accuracy with different oblique beam angles when 2d detector arrays are used for small field IMRT QA. Methods: 2D detector arrays are convenient to use but their low resolution limits the number of sampling points inside small radiation fields. In this study, we demonstrate that with oblique beam incidence, more points can be measured and 3D dose volume can be reconstructed more accurately. QA plans were simulated and measured using four oblique angles: 90 degree (straight incidence), 60 degree, 30 degree, and 15 degree. 3D dose volume was reconstructed based on dose measurements through back‐projection and inverse attenuation correction. Total delivering dose was calculated after rotating individual beam doses into actual beam angles. Two IMRT QA devices were used in our study, including a PTW /Octavius phantom and a Delta4 QA device. A spine SBRT plan was chosen as the study case. Gamma passing rate, mean PTV dose, and maximum dose received 2cc of spinal cord were calculated to compare reconstructed dose to original planning dose. Effects of measurement error and detector resolution were also investigated. Results: Oblique beam incidence will improve the sampling frequency of dose measurments. When the detector spacing is 10mm, 30 degree is the optimal angle for the Delta4 phantom and 15 degree is the opitmal angle for the PTW/Octavius phantom. Compared with standard straight incidence setup, QA with the optimal angle has 10% higher gamma passing rate (with 2mm/2% criteria) in both simulation and actual measurement. However, oblique incidence didn't improve QA accuracy consistently when the detector spacing is 5mm. Conclusion: Oblique incidence can be used to improve IMRT QA accuracy for 2d‐array devices which don't have sufficient spatial resolution for small field measurement.