S.H. Patel

Examining Margin Reduction and Its Impact on Dose Distribution for Prostate Cancer Patients Undergoing Daily Cone-Beam Computed Tomography

PURPOSE To examine the dosimetric impact of margin reduction and quantify residual error after three-dimensional (3D) image registration using daily cone-beam computed tomography (CBCT) for prostate cancer patients. METHODS AND MATERIALS One hundred forty CBCTs from 5 prostate cancer patients were examined. Two intensity-modulated radiotherapy plans were generated on CT simulation on the basis of two planning target volume (PTV) margins: 10 mm all around the prostate and seminal vesicles except 6 mm posteriorly (10/6) and 5 mm all around except 3 mm posteriorly (5/3). Daily CBCT using the Varian On-Board Imaging System was acquired. The 10/6 and 5/3 simulation plans were overlaid onto each CBCT, and each CBCT plan was calculated. To examine residual error, PlanCT/CBCT intensity-based 3D image registration was performed for prostate localization using center of mass and maximal border displacement. RESULTS Prostate coverage was within 2% between the 10/6 and 5/3 plans. Seminal vesicle coverage was reduced with the 5/3 plan compared with the 10/6 plan, with coverage difference within 7%. The 5/3 plan allowed 30-50% sparing of bladder and rectal high-dose regions. For residual error quantification, center of mass data show that 99%, 93%, and 96% of observations fall within 3 mm in the left-right, anterior-posterior, and superior-inferior directions, respectively. Maximal border displacement observations range from 79% to 99%, within 5 mm for all directions. CONCLUSION Cone-beam CT dosimetrically validated a 10/6 margin when soft-tissue localization is not used. Intensity-based 3D image registration has the potential to improve target localization and to provide guidelines for margin definition.

Clinical commissioning and use of the Novalis Tx linear accelerator for SRS and SBRT

The purpose of this study was to perform comprehensive measurements and testing of a Novalis Tx linear accelerator, and to develop technical guidelines for commissioning from the time of acceptance testing to the first clinical treatment. The Novalis Tx (NTX) linear accelerator is equipped with, among other features, a high‐definition MLC (HD120 MLC) with 2.5 mm central leaves, a 6D robotic couch, an optical guidance positioning system, as well as X‐ray‐based image guidance tools to provide high accuracy radiation delivery for stereotactic radiosurgery and stereotactic body radiation therapy procedures. We have performed extensive tests for each of the components, and analyzed the clinical data collected in our clinic. We present technical guidelines in this report focusing on methods for: (1) efficient and accurate beam data collection for commissioning treatment planning systems, including small field output measurements conducted using a wide range of detectors; (2) commissioning tests for the HD120 MLC; (3) data collection for the baseline characteristics of the on‐board imager (OBI) and ExacTrac X‐ray (ETX) image guidance systems in conjunction with the 6D robotic couch; and (4) end‐to‐end testing of the entire clinical process. Established from our clinical experience thus far, recommendations are provided for accurate and efficient use of the OBI and ETX localization systems for intra‐ and extracranial treatment sites. Four results are presented. (1) Basic beam data measurements: Our measurements confirmed the necessity of using small detectors for small fields. Total scatter factors varied significantly (30% to approximately 62%) for small field measurements among detectors. Unshielded stereotactic field diode (SFD) overestimated dose by ~ 2% for large field sizes. Ion chambers with active diameters of 6 mm suffered from significant volume averaging. The sharpest profile penumbra was observed for the SFD because of its small active diameter (0.6 mm). (2) MLC commissioning: Winston Lutz test, light/radiation field congruence, and Picket Fence tests were performed and were within criteria established by the relevant task group reports. The measured mean MLC transmission and dynamic leaf gap of 6 MV SRS beam were 1.17% and 0.36 mm, respectively. (3) Baseline characteristics of OBI and ETX: The isocenter localization errors in the left/right, posterior/anterior, and superior/inferior directions were, respectively, −0.2±0.2 mm, −0.8±0.2 mm, and −0.8±0.4 mm for ETX, and 0.5±0.7 mm, 0.6±0.5 mm, and 0.0±0.5 mm for OBI cone‐beam computed tomography. The registration angular discrepancy was 0.1±0.2°, and the maximum robotic couch error was 0.2°. (4) End‐to‐end tests: The measured isocenter dose differences from the planned values were 0.8% and 0.4%, measured respectively by an ion chamber and film. The gamma pass rate, measured by EBT2 film, was 95% (3% DD and 1 mm DTA). Through a systematic series of quantitative commissioning experiments and end‐to‐end tests and our initial clinical experience, described in this report, we demonstrate that the NTX is a robust system, with the image guidance and MLC requirements to treat a wide variety of sites — in particular for highly accurate delivery of SRS and SBRT‐based treatments. PACS numbers: 87.55.Qr, 87.53.Ly, 87.59.‐e

WE‐C‐BRC‐08: A Method to Evaluate Region‐Specific Pulmonary Function Using 4D CT Images for Lung Cancer Patients Undergoing Radiation Therapy

Purpose: Collateral radiation exposure to healthy lungtissue during radiation therapy can result in changes in structural and biomechanical properties of the lung. These changes may cause various clinical symptoms. The purpose of this study was to develop a functional imaging technique to assess the lungs region‐specific ventilation and pressure during or after radiation treatment. Method and Materials: With an in‐house developed finite element framework, a heterogeneous elastic model was developed for a lung patient and its Youngs moduli were derived from a set of 4D CTimages, acquired during radiation treatment. Each phase of the 4D dataset was registered, using deformable image registration (ITK demons algorithm) with the end‐inhale reference dataset. The resultant deformation matrix was used first to calculate the volumetric variation of each image voxel to generate a 3D ventilation image, and then to compute its corresponding transpulmonary pressure with the mechanical model. Results:Lung volumes on each phase of the acquired 4D dataset were compared with those derived from the deformed model, and were found to be within 1% of each other. The maximum ventilation occurs from phase 1 to phase 2, the earliest expiration phase. The average ventilation increased from 20.2% in phase 2 to 30.8% in phase 5 and their correspondent pressures increased from 1.57 Kpa to 2.25 Kpa. This result is generally consistent with published measurements. Conclusion: This study describes a theoretical approach to calculate the region‐specific ventilation and mechanical functions using deformable image registration. The method may be applied toward understanding how the mechanical properties of damaged lung differ from that of healthy lungtissue, and therefore it has potential applicability as a diagnostic indicator, as well as a tool for predicting radiation‐induced lung damages. Work is underway to correlate this approach with other traditional functional‐imaging modalities used to assess lung function.

Improving safety and efficiency during emergent central venous catheter placement with a needleless securing clamp

Objective To compare the needleless securing clamp to the traditional suture-secured clamp for central venous catheters. Methods Compare the holding strength of each type of clamps by measuring the amount of kinetic energy absorbed, ask 20 physicians to evaluate the clamp placement using sutures or staples, and summarise the clamps effectiveness and complications in 10 patients. Results Compared to sutured clamp, the needleless clamp was more secure. The needleless clamp was also significantly better with regard to ease of use, safety, perceived strength (p value <0.002), and insertion time was reduced by 63%. No adverse events or skin infections occurred while using the needleless clamps. Conclusions Without incurring complications or increasing risk to patients, the needleless clamp is secure and improves safety and efficiency for physicians.

TH‐E‐BRC‐11: Practical Methods for Improving Dose Non‐Uniformity in Monte Carlo‐ Based IMRT Planning of Lung Tumors Treated with Stereotactic Body Radiotherapy (SBRT)

Purpose: Current commercially available planning systems which utilize MC algorithm‐based final dose calculation in IMRT planning employ pencil‐beam algorithms in the optimization process. Consequently, dose coverage for SBRTlung plans can be quite non‐uniform, featuring cold‐ spots in the tumor periphery for “island” lesions within the lung, and, for other locations, hot‐spots within nearby normal organs (example: rib‐cage). This study evaluated practical approaches to reducing dose non‐uniformity within the target and surrounding normal organs in MC‐based IMRT planning. Methods: We evaluated two different IMRT‐based approaches. (A) Iterative planning where the MC calculation (with pencil‐beam‐based optimization) is initially performed. The resultant cold spot is then contoured and used as a simulatneous boost volume. The MC‐based dose is re‐computed and the prescription dose re‐normalized to 95% of the PTV. Ten SBRTlung cases with tumors seated near the lung‐wall/rib‐cage interface were planned. (B) Planning in which coplanar and non‐coplanar beam angles with limited path through lung tissue were selected. Both techniques were evaluated against the conventional coplanar‐beam approach: a single MC calculation and prescription dose normalization to 95% of the PTV. Results: Technique A: conformity index (CI) and PTV dose uniformity (U_PTV) improved in seven of ten plans. Average improvement (+/− standard error) was 10.8%+/−2.7%, and 22.4%+/−5.4%, respectively. Non‐significantly improved plans had PTVs near the skin, trachea and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in nine often plans (average 10.6%+/−4.3%), with only the skin‐adjacent PTV plan showing no improvement. Technique B: we demonstrated an improvement of 11.2% and 2.6% in CI and U_PTV, respectively, and a D1cc reduction of 7.8% to surrounding OARs. Conclusions: The proposed practical approaches improve dose conformity in MC‐based IMRT planning of lungtumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs. Supported in part by NIH/NCI Grant No. 106770.

SU‐FF‐T‐560: Evaluation of ExacTrac and CBCT Patient Positioning On the Novalis TX

Purpose: To evaluate the precision and accuracy of patient localization using a cone‐beam CT(CBCT) and an orthogonal x‐ray pair with infrared markers on the Novalis Tx treatment unit. Method and Materials: Twenty stereotactic radiosurgery/therapy patients with a total of twenty‐five lesions over ninety sessions (ranging from 1−5 fractions per patient) were localized daily using both ExacTrac (ETX, BrainLab) and on‐board CBCT (Varian) coupled to the Novalis Tx treatment unit. Each patient was first positioned using the ETX system accounting for variances in all six dimensions using a robotic couch top. Following these shifts, a CBCT was performed and further translations were made (x, y, z, table rotation) based on image fusion between the CBCT and simulation CT. A phantom study was also performed, mimicking the patient set‐up method to assess the reproducibility of each system and to determine any systematic differences between the ETX and CBCT localization approaches. Results: Patient positioning between ETX and CBCT was consistent in all four dimensions within 1.1mm and 0.1°. The average discrepancy between each system across all sessions was 1.1±1.2mm A/P, 1.0±1.2mm S/I, 0.1±1.4mm M/L, and 0.1°±0.5° couch rotation. Phantom testing showed that both systems were reproducible within 1.5mm and 0.5° in all dimensions. A systematic discrepancy of 0.3mm A/P, 1.2mm S/I, 0.8mm M/L, and 0.6° rotation was found between the two systems; however, this difference was deemed to be within the calibration tolerance of both systems. Conclusions: The ETX and on‐board CBCT systems were found to agree on tumor localization within 1.1mm (all dimensions) and 0.1° (couch rotation). Phantom studies showed the reproducibility of each system to be acceptable for stereotactic treatments. The Novalis Tx treatment unit incorporates both fiducial marker‐based, and volume‐based localization for reproducible and accurate SRS/SBRT patient treatments.

SU‐FF‐J‐55: Treatment Verification for Lung Cancer Patients Undergoing Fractionated Stereotactic Body Radiation Therapy (SBRT) Utilizing Cone Beam CT (CBCT)

Purpose: Recent increase in hypofractionated radiation therapy for early stage lungcancer necessitates accurate patient setup. The acquisition of CBCT prior to treatment provides a three dimensional setup and verification tool. We assessed the utility of CBCT for SBRT through examination of dosimetric parameters. Methods and Materials: Four patients with early stage non small cell lungcancer were used for the analysis. Each patient underwent a 4DCT simulation for internal target volume (ITV) determination. A five to seven field IMRT plan was generated for PTV = ITV + 3mm. Four fractions were delivered (12 Gy/fraction) and CBCT was acquired prior to each fraction. Online image registration between CBCT and simulation CT (simCT) was performed and the resulting shifts were applied to move the couch prior to treatment. In the offline evaluation, the GTV was contoured on each CBCT and compared to the ITV. To assess the ITV coverage, we overlaid the ITV onto CBCT (CBCT‐ITV) based on the shift data and performed dose calculations using the respective simCT plan. Dosimetric parameters including volumes receiving 95% (V95), 90% (V90), and minimum (min) dose were used for analysis.Results: The average set up shifts in the AP, SI, and LR directions were −0.16±0.33, −0.29±0.44, and 0.33±0.34 cm, respectively. CBCT volumes varied when compared to the respective ITV with a range of 0–50% decrease in volume. Due to this variability ITVs were used for the dosimetricanalysis. The average PTV/CBCT‐ITV coverages for V95, V90, and min were: 94%/94.9%, 99.3%/96.7%, 85.4%/88.3%, respectively. Conclusion: The CBCT volumes were not representative of the ITV as seen by the volume discrepancy. Our dosimetricanalysis showed good correlation between PTV and CBCT‐ITV coverage, supporting our current PTV margin (3mm). Our data affirms that CBCT provides further assurance in regards to target localization for hypofractionated SBRT of lungcancer.