P Chow

Respiratory correlated cone-beam computed tomography on an isocentric C-arm.

A methodology for 3D image reconstruction from retrospectively gated cone-beam CT projection data has been developed. A mobile x-ray cone-beam device consisting of an isocentric C-arm equipped with a flat panel detector was used to image a moving phantom. Frames for reconstruction were retrospectively selected from complete datasets based on the known rotation of the C-arm and a signal from a respiratory monitor. Different sizes of gating windows were tested. A numerical criterion for blur on the reconstructed image was suggested. The criterion is based on minimization of an Ising energy function, similar to approaches used in image segmentation or restoration. It is shown that this criterion can be used for the determination of the optimal gating window size. Images reconstructed from the retrospectively gated projection sequences using the optimal gating window data showed a significant improvement compared to images reconstructed from the complete projection datasets.

Visual Acuity, Contrast Sensitivity and Color Vision Three Years After Iodine-125 Brachytherapy for Choroidal and Ciliary Body Melanoma

Purpose : To report visual acuity, contrast sensitivity and color vision prior to, 1 year after, 2 years after and 3 years after iodine-125 brachytherapy for choroidal and ciliary body melanoma (CCM). Design : Prospective interventional case series. Participants : Thirty-seven patients (37 eyes) with CCM. Methods : Patients had best-corrected Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity, Pelli-Robson contrast sensitivity and Hardy-Rand-Rittler color vision measurement; comprehensive ophthalmology examination; optical coherence tomography; and ultrasonography at baseline prior to, 1 year after, 2 years after and 3 years after I-125 brachytherapy. Main Outcome Measures : Visual acuity, contrast sensitivity and color vision prior to, 1 year after, 2 years after and 3 years after brachytherapy. Results : Nineteen (19) men and 18 women with mean age of 58 years (SD 13, range 30-78) prior to, 1 year after, 2 years after and 3 years after brachytherapy had mean best-corrected visual acuity of 77 letters (20/32), 65 letters (20/50), 56 letters (20/80) and 47 letters (20/125); contrast sensitivity of 30, 26, 22 and 19 letters; color vision of 26, 20, 17 and 14 test figures, respectively. Decrease in visual acuity, contrast sensitivity and color vision was statistically significant from baseline at 1 year, 2 years, and 3 years after brachytherapy. Decreased acuity at 3 years was associated with mid-choroid and macula melanoma location, ≥ 4.1 mm melanoma height, radiation maculopathy and radiation optic neuropathy. Conclusion : 1, 2 and 3 years after brachytherapy, eyes with CCM had significantly decreased visual acuity, contrast sensitivity and color vision.

SU-F-J-125: Effects of Couch Position Variability On Dosimetric Accuracy with An MRI-Guided Co-60 Radiation Therapy Machine

PURPOSE Magnetic resonance imaging (MRI) guidance in radiation therapy brings real-time imaging and adaptive planning into the treatment vault where it can account for interfraction and intrafraction movement of soft tissue. The only commercially-available MRI-guided radiation therapy device is a three-head 60Co and MRI system with an integrated treatment planning system (TPS). An up to 20% attenuation of the beam by the couch is well modeled in the TPS. However, variations in the patients day-to-day position introduce discrepancies in the actual couch position relative its location as modeled in the treatment plan. For this reason, our institution avoids plans with beams that pass through or near the couch edges. This study looks at plans without restriction on beam angles and investigates the effects of couch shift by simulating shifts of the couch relative to the patient, in order to determine whether couch edge avoidance restrictions can be lifted. METHODS A total of 27 plans from 23 patients were investigated. Couch shifts of 1 and 2 cm were introduced in combinations of lateral and vertical direction to simulate variations in patient positioning on the couch giving 16 shifted plans per reference plan. The shift values of 1 and 2 cm were based on shifts recorded in 320 treatment fractions. RESULTS Measured couch attenuation versus TPS modeled agreed within 2.1%. Planning Target Volume (PTV) D95 changed less than 1% for 1 and 2 cm couch shifts in only the x-direction and less than 3% for all directions. CONCLUSION The dosimetry of all plans with shifts up to ±2 cm was within reasonable clinical tolerances. Robustness of a plan to couch shifts can be tested in the TPS. Inclusion of beams traversing the couch edges should be considered if an improvement in plan quality or delivery time can be achieved.

SU‐C‐BRD‐04: Automatic Detection of Patient Identification and Patient Positioning Errors Using 3D Setup Images

PURPOSE To develop an automated system to detect patient identification and patient positioning errors using algorithmic comparison between megavoltage CT (MVCT) and kilovoltage CT (kVCT) planning images. METHODS MVCT images from 35 head and neck (H&N) patients and 19 pelvis patients were collected from a Tomotherapy machine, along with the corresponding planning kVCTs. MVCTs and kVCTs were manually aligned according to clinical protocols at our institution. Patient identification errors were simulated by aligning MVCTs and kVCTs from different patients. Positioning errors were simulated by misaligning MVCTs and kVCTs by 1cm to 5cm in the each of the six anatomical directions. For each image pair, a pixel-by-pixel cross-correlation metric was computed within the MVCT image body contour. To eliminate the effect of daily variations in bowel gas, the metric was limited to voxels with HU>-700. The kVCT voxel intensities were remapped to the MVCT scale using a third-order polynomial from a publicly available software package. RESULTS A threshold pixel-by-pixel cross-correlation value was found that distinguished between correct and incorrect patient setup with a high degree of accuracy. A stratified 10-fold cross-validation analysis yielded average misclassification probabilities of 0.0030 for H&N and 0.00 for pelvis. For misaligned image pairs, cross-validation analysis yielded average misclassification probabilities of 0.00 and 0.0013 for H&N shifts ≥20mm and ≥10mm across all six anatomical directions, respectively. Misclassification probabilities were 0.00, 0.011, and 0.10 for pelvic shifts ≥30mm, ≥20mm, and ≥10mm, respectively. Receiver operator characteristic analysis for misaligned patients yielded areas under the curve ranging from 0.99 to 1.0 for H&N and 0.86 to 1.0 for pelvis. CONCLUSION This proof-ofconcept study shows that pixel-by-pixel cross-correlation of MVCT setup images with their corresponding planning CT images can be used to detect wrong-patient errors as well as incorrect patient shifts in the pelvis and H&N regions.

SU‐E‐J‐242: Post‐Treatment Planning Tool for Estimating Dose Distribution Delivered to Spinal Radiosurgery Patients Based On Measured Intra‐Fraction Positional Data

PURPOSE To demonstrate feasibility and utility of a planning tool for post treatment reconstruction of delivered dose distribution based on measured patient motion. The spinal cord dose limits in clinical use are based on planned dosimetric data which can significantly differ from delivered dose. This tool can be utilized to correlate clinical outcomes to delivered dose METHODS AND MATERIALS: ur previous studies have shown that without intrafraction motion management, patients can move up to 3mm. The current institutional protocol requires intrafraction motion management by stereoscopic imaging prior to each treatment field. Patients are repositioned when 1mm tolerance is exceeded. Dosimetric effects of 62 spinal radiosurgery patients were investigated by simulating patient motion. For each patient plan, three additional plans were created, where the planning isocenter was shifted towards the cord by 1mm, 2mm and 3mm, for a total of 248 plans. D95% was evaluated for the target volume and D0.035cc and V10Gy were evaluated for the spinal cord. RESULTS D95% decreased up to 3%, 9% and 15% for 1mm, 2mm and 3mm shifts correspondingly. These shifts resulted in D0.035cc exceeding 12Gy in 26%, 55% and 76% of patients, respectively, while previous work revealed that patients with a 1mm shift exceeded the 12Gy dose Dmax constraint 51% of the time. These respective shifts also resulted in 53%, 65% and 74% exceeding the constraint limit of 10Gy dose to 10% of the spinal cord outlined 6 mm above and below the target volume. CONCLUSION We have shown that delivered dose distributions can significantly differ from planned dose distributions. Spinal cord dose limits in clinical use are based on planned data, assuming no patient motion. This tool can be utilized to estimate delivered dose distribution and correlate clinical outcomes to delivered dose data.

SU-F-BRD-11: Prediction of Dosimetric Endpoints From Patient Geometry Using Neural Nets

PURPOSE The previously-published overlap volume histogram (OVH) technique lends itself naturally to prediction of the dose received by a given volume of tissue (e.g. D90) in intensity-modulated radiotherapy (IMRT) treatment plans. Here we extend the OVH technique using artificial neural networks in order to predict the volume of tissue receiving a given dose (e.g. V90) in both prostate IMRT and conventional breast radiotherapy. METHODS Twenty-nine prostate treatment plans and forty-three breast treatment plans were analyzed. The spatial relationships between the prostate and rectum and between the breast and ipsilateral lung were characterized using OVHs. The OVH is a cumulative histogram representing the fractional volume of the risk organ overlapped by a series of isotropic expansions of the planning target volume (PTV). Seven cases were identified as outliers and replanned. OVH points were used as inputs to a one hidden layer feed forward artificial neural network with quality parameters of the corresponding plan, such as the rectum V50, as targets. A 3-fold cross-validation was used to estimate the prediction error. RESULTS The root mean square (RMS) error between the predicted rectum V50s and the planned values was 2.3, which was 35% of the standard deviation of V50 for the twenty-nine plans. The RMS error of prediction of V20 of the ipsilateral lung in breast cases was 3.9, which was 90% of the standard deviation of the V20 values in the breast plan database. CONCLUSION This study demonstrates that artificial neural nets can be used to extend the OVH technique to predict dosimetric endpoints taking the form of a volume receiving a given dose, rather than the minimum dose received by a given volume. Prediction of ipsilateral lung dose in breast radiotherapy using the OVH technique remains a work in progress.

SU‐GG‐T‐573: Total Scalp Irradiation: Comparison between Volumetric Modulated Arc Therapy, Helical Tomotherapy and Conventional Electron and Photon Field Combination

Purpose: Homogeneous irradiation of the scalp is challenging due to relatively complex and superficial treatment volume. Conventional treatment approach of combining matched electron and photon fields produces dose heterogeneity, especially at matched lines. Fixed gantry IMRT techniques have not been able to provide clinically acceptable plans but helical tomotherapy has been demonstrated to be an alternative option. We have implemented total scalp treatment with volumetric modulated arc therapy (VMAT) and compared the dosimetric characteristics to tomotherapy and conventional method. Materials and Methods: Two patients have been planned for total scalp irradiation using RapidArc VMAT, helical tomotherapy and conventional electron and photon field combination. The same anatomy and structure sets have been used for the planning purposes. After each operator achieved clinically acceptable results for the particular method, the dose distributions and dose volume histograms were compared. Results: VMAT and tomotherapy provide significantly more homogeneous dose distribution and target coverage. Maximum PTV dose with VMAT and tomotherapy were 20% and 30% less than the conventional method respectively. Although, the conventional method provided 2/3 less dose to brain at D50% and D75%, tomotherapy and VMAT gave lower dose to the brain in high‐dose regions. Brainstem was best spared by conventional method with 1/4 of the maximum dose delivered with tomotherapy and VMAT. Tomotherapy provided more homogeneous dose distribution than VMAT but the brain and brainstem sparing were comparable. Conclusions: VMAT for total scalp irradiation is a clinically acceptable and comparable to tomotherapy. VMAT provides significantly more homogeneous distribution than conventional method. VMAT and Tomotherapy give lower dose to the brain in high‐dose regions than conventional method. Moreover, VMAT and tomotherapy plans can be delivered more reproducibly and reliably than conventional treatments. Average beam on time for VMAT and tomotherapy were 2.5 mins and 7.7 mins respectively. Research sponsored by Varian Medical Systems.