B Miller

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

FMEA of MR-only Treatment Planning in the Pelvis

Purpose To evaluate the implementation of a magnetic resonance (MR)-only workflow (ie, implementing MR simulation as the primary planning modality) using failure mode and effects analysis (FMEA) in comparison with a conventional multimodality (MR simulation in conjunction with computed tomography simulation) workflow for pelvis external beam planning. Methods and Materials To perform the FMEA, a multidisciplinary 9-member team was assembled and developed process maps, identified potential failure modes (FMs), and assigned numerical values to the severity (S), frequency of occurrence (O), and detectability (D) of those FMs. Risk priority numbers (RPNs) were calculated via the product of S, O, and D as a metric for evaluating relative patient risk. An alternative 3-digit composite number (SOD) was computed to emphasize high-severity FMs. Fault tree analysis identified the causality chain leading to the highest-severity FM. Results Seven processes were identified, 3 of which were shared between workflows. Image fusion and target delineation subprocesses using the conventional workflow added 9 and 10 FMs, respectively, with 6 RPNs >100. By contrast, synthetic computed tomography generation introduced 3 major subprocesses and propagated 46 unique FMs, 15 with RPNs >100. For the conventional workflow, the largest RPN scores were introduced by image fusion (RPN range, 120-192). For the MR-only workflow, the highest RPN scores were from inaccuracies in target delineation resulting from misinterpretation of MR images (RPN = 240) and insufficient management of patient- and system-level distortions (RPN = 210 and 168, respectively). Underestimation (RPN = 140) or overestimation (RPN = 192) of bone volume produced higher RPN scores. The highest SODs for both workflows were related to changes in target location because of internal anatomy changes (conventional = 961, MR-only = 822). Conclusions FMEA identified areas for mitigating risk in MR-only pelvis RTP, and SODs identified high-severity process modes. Efforts to develop a quality management program to mitigate high FMs are underway.

SU-G-BRC-07: Evaluation of AAA Focal Spot Size for SRS Planning Using End-To-End Dosimetric Data

PURPOSE To use end-to-end dosimetric measurements with Gafchromic film to evaluate the effects of focal spot size parameter for small-field dose calculations using AAA for SRS lesions. METHODS A total of 13 plans, corresponding to 7 patients previously treated with cranial SRS, were analyzed in this study (target volume range:[0.67cc,13.9cc]). The plans included DCA delivery (4 plans total) and VMAT delivery (9 plans total). All plans were mapped to a solid water phantom (15 cm thickness; isocenter and film plane at 7.5 depth). Dose calculation was performed with AAA v.11 (1.0mm grid size); three focal spot size settings were tested: 0mm, 0.5mm, and 1.5mm. For each plan, three calculated doses (corresponding to each focal spot size setting) were compared to measured film dose using quantitative methods [Gamma Analysis(1%,1mm,10% threshold criteria)] and qualitative methods (visual dose profile comparison). Film calibration and analysis were performed using in-house calibration methods and software package. RESULTS Gamma(1%,1mm) analysis passing rate results [mean(st.dev){%}] were as follows. For DCA plans: 98.74(0.54)-[0mm Focal Spot Size]; 98.24(1.26)-[0.5mm Focal Spot Size]; 95.42(2.29)-[1.5mm Focal Spot Size]. For VMAT plans: 98.75(0.54)-[0mm Focal Spot Size]; 98.89(0.73)-[0.5mm Focal Spot Size]; 97.43(1.30)-[1.5mm Focal Spot Size]. The majority of failing points (Gamma value>1.0) were found to be within the high dose region for all Focal Spot Size calculation models. Visual inspection of the dose profile, showed that the 1.5mm Focal Spot size calculation exhibited blurring in the high dose region (defined as >85% of the peak dose), resulting in a more gradual shoulder of the dose profile relative to measurements. CONCLUSION The dose calculation accuracy of DCA and VMAT plans is paramount for SRS treatment planning. Our results indicate similar behavior of the AAA model with focal spot sizes of 0mm and 0.5mm, while 1.5mm focal spot size tends to result in blurring of the high dose region. Henry Ford Health System has research agreements with Varian and Philips.

WE‐E‐141‐09: FMEA and Fault Tree Ananlysis Applied to Vendor Customer Technical Bulletins

PURPOSE Over the past 3 years our institution has received 44 Customer Technical Bulletins (CTBs) from vendors that supply us with hardware of software used for patient care. We have ranked the failure modes presented to our institution in these CTBs using Failure Mode and Effects Analysis (FMEA) tools as described in the AAPM report TG-100. METHODS FMEA applied to the failure modes comes up with a Risk Probability Number based on the severity, probability of occurrence and probability it will go undetected. In addition to FMEA, the adverse effects on our patient population are examined by looking at the number of people affected. For example, CTBs applying to dose calculations would affect 100% of our patients while CTBs applying to IMRT calculations will affect approximately 50% or our patient population. RESULTS Of the 44 CTBs received from vendors we identified nineteen with Risk Priority Numbers (RPN) greater than 200 and four of those had RPN greater than 300. Of the four with RPN greater than 300, two dealt with incorrect dose calculations due to incorrect commissioning of the treatment planning system, one with the CT datasets getting flipped and the last with isocenter being transferred incorrectly. Each of these failure pathways would have resulted in a systematic error that would have affected a large population of patients. For the failure modes with RPN greater than 200 the average RPN was reduced from 275 to 70 after modifying policy, checklists and improving clinical flow. CONCLUSION Analysis of vendor CTBs and review by a departments QA committee is an essential part of any QA program. By applying FMEA and analyzing the fault trees discussed in these bulletins, we were able to reduce the RPN from an average of 275 to 70.

MO‐D‐105‐07: Results of Applying FMEA and Fault Tree Analysis to the Online Incident Reporting Database

PURPOSE As part of the continuous quality of care improvement, an internal online database for processing reported incidents was established. In 3 years, 710 incidents were reported by 5 clinics. We have ranked the fault trees of the reported incidents using the AAPM report TG-100 Failure Mode and Effects Analysis (FMEA) tools Methods: Risk Probability Number (RPN) generated as a Result of applying FMEA is based on the severity, probability of occurrence and probability of going undetected. The reports were sorted in two categories. Potentially affecting dose delivery, e.g. incorrect setup instructions; and deviations from an established workflow defined by policies and procedures (P&P), e.g., incorrect naming of the fields. In addition to FMEA, the impact of new, as well as periodic reviews of P&P by staff members is assessed Results: Of 710 reports 676 were analyzed, 374 were variation in the workflow not directly affecting quality of care, 302 were potentially affecting dose delivery. 19 of 302 had dosimetric impact; however, due to low occurrence only 4 instances, related to bolus placement, reached the RPN above 200. Review of current P&P reduced the RPN from 270 to 9. Periodic review, introduction of the new or revising the existing P&P had a dual effect: drop in dose-affecting incidents and increased reporting of process deviations Conclusion: Analysis of reported incidents and review by the departmental QA committee is an essential part of any QA program. By defining the fault trees and applying FMEA to the reported incidents, we were able to reduce the RPN from an average of 150 for dose related incidents to 9, and for process variations from 295 to 28 on average. Event-triggered revising of P&Ps and periodic review with staff of the existing P&P is an effective tool in incident reduction.

SU‐E‐J‐19: A Feasility Study: Tube Current Modulation as a Dose Savings Technique for Cone‐Beam CT‐Based IGRT

Purpose: In image‐guidedradiotherapy(IGRT), cone‐beam CT(CBCT) scan parameters do not take into account patient size information, which can result in unnecessary dose. So‐called “smart scans” in radiology attempt to customize exposure parameters to reduce dose without sacrificing image quality. This work presents the feasibility of tube current modulation as a dose reduction technique in CBCT‐based IGRT. Methods: A CATPHAN® phantom with water‐equivalent annulus was used to evaluate noise. A CIRS phantom was used to evaluate dose characteristics. Gies et. al.[Med Phys, 26:1999], tells us that noise for the central pixel is minimal for tube current modulated by the square root of the given angular attenuation. This formulation is used to calculate modulation factors for each 10 degree imaging arc. Modulated image dataset was generated by compositing projection data from scans performed with each modulated current setting. Noise characterization was performed by analyzing the standard deviation of the uniformity section of the CATPHAN. Imagingdose was measured using an ionization chamber located in the phantom. For the modulated scan, each ten degree segment was measured separately and then accumulated to get a composite dose reading.Results: Central axis dose reduction of 31% was attained using this modulation scheme. Visual inspection of a modulated CBCT slices shows appreciable image quality relative to the conventional CBCT. The noise characteristics of the modulated scan were about 20 HU worse than a conventional scan, likely due to the presence of scatter in the enlarged CATPHAN phantom because theoretical predictions are based on primary beam attenuation alone Conclusions: Initial results demonstrate that the use of a modulated scanning technique, developed by taking into account patient anatomic variation, may be a feasible method to reduce dose while preserving image quality in CBCT‐based IGRT.

MO‐FF‐A1‐05: On‐line CT Verification of Needle Applicator Positions in HDR Prostate Brachytherapy

Purpose: Needle applicators have shown displacement between fractions during HDR prostate brachytherapy, which will degrade plan quality. Typically, displacement is determined by analyzing catheters on a verification CT (vCT) or port films obtained before treatment relative to fiducials or bony landmarks. We present a procedure to verify applicator positions to achieve planned dose distributions by registering vCT directly to planning CTs (pCT). Method and Materials: Patients received a vCT prior to each fraction. This was imported to BrachyVision (Varian Medical Systems) and fused to the pCT by rigid‐body registration based on matching urethra and three prostate‐implanted gold fiducials. Then, applicators in the vCT were compared to applicators in the pCT in a reconstructed plane through each catheter and any difference larger than 3mm was manually adjusted by a radiation oncologist before treatment. To assess treatment quality, the prostate volume was copied from pCT to vCT and the planned dwell positions/time was applied to the vCT applicators. Results: Two consecutive patients were treated using our CT registration needle verification method. Both patients required applicator adjustments in the first two fractions. Patient one had an average 7mm cranial‐to‐caudal applicator displacement, corresponding to 23% and 12% fractional drop in V100prostate respectively; Patient two had displacement in both cranial‐to‐caudal and caudal‐to‐cranial directions up to 9mm, but due to minimum loading of affected applicators, no significant decrease in V100prostate was seen. Neither patient required applicator adjustment at the Srd/last fraction. Despite differences in applicator displacement, post‐adjustment, the achieved V100prostate at each fraction deviated from planned value by less than 4% for both patients. The distances between fiducials in vCTs were consistent with those in pCTs within 3mm, confirming that fiducials are appropriate landmarks for registration. Conclusion: We have successfully treated two patients using this method. This technique ensures the treatment quality closely matches plan quality.

SU‐GG‐T‐51: CT Verification of Tissue‐Balloon Conformance and Skin Distance for HDR Partial Breast Irradiation Using a Multi Catheter Balloon

Purpose: Intracavitary HDR partial breast irradiation applicators require a planning CT to evaluate appropriateness for treatment. Issues such as tissue‐balloon conformance, balloon symmetry and minimal skin distance are considered. Daily kV and ultrasound imaging are sufficient to verify balloon symmetry and treatment position but are not sufficient to evaluate tissue‐balloon conformance or skin distance Plan quality can be compromised if tissue‐balloon conformance or skin distance change during the course of treatment. To evaluate the changes in tissue‐balloon conformance and skin distance we acquired daily CT scans on 5 patients. Methods and Materials: Daily CT scans were acquired and evaluation plans were generated from the treatment plan. In compliance with the NSABP B‐39 /RTOG 0413 protocol, tissue‐balloon conformance and skin distance were evaluated on the daily CT scans. Tissue‐balloon conformance was determined by measuring the air/seroma volume (ASV). Results: Daily imaging facilitated the identification of non‐compliant skin distances and ASVs. Patient 2 needed to be replanned to reduce the skin dose because the skin distance decreased from 6 to 3 mm over the course of treatment. After re‐planning, the skin dose was reduced from 156.5% to 128.1%. Patients 3, 4, and 5 required suctioning air to achieve a DVH with adequate coverage for the PTV. Before suctioning, the minimum V90PTVeval for patients 3, 4, and 5 was 83.2%, 71.8%, and 78.7%, respectively. After suctioning, V90PTVeval for patients 3, 4, and 5 was 97.6%, 99.2%, and 96.8%, respectively. Conclusions: This study shows that both skin distance and ASV can change over the course of treatment. These changes can compromise the quality of treatment if the plan is not adapted to account for these changes. It is important to treat with an acceptable skin distance to limit skin toxicity. Treating with a compliant ASV insures dosimetric coverage of the PTVeval.

SU‐FF‐J‐91: Assessment of Prostate Position During External Beam Treatments for Patients Post Brachytherapy Implantation

Purpose: To localize the prostate for patients undergoing an external beam boost after receiving an I‐125 implant to within 3mm and confirm the position of the prostate post‐treatment using marker seeds implanted in the prostate as a surrogate for its position. Method and Materials: Twenty patients receiving an I‐125 prostate implant had three marker seeds implanted in the prostate during the procedure. After the implant the patients underwent a CT simulation for an external beam radiotherapy boost. The marker seeds were contoured and projected onto a DRR with a reference field. Prior to each treatment, radiation therapists imaged the patient with an EPID. The therapists then localized the seeds relative to reference field and determined the error in the alignment of the prostate. If the alignment error was greater than 3mm in any one direction the patient was shifted and re‐imaged to verify the shift. A portal image was acquired after the patients treatment to confirm the position of the prostate post‐treatment. Results: Averages and standard deviations for pre‐treatment prostate alignment error ranged from −0.10 ± 0.18 to 0.15 ± 0.14 cm, −0.13 ± 0.16 to 0.16 ± 0.16 cm, and −0.17 ± 0.24 to 0.14 ± 0.19 cm in the right/left, superior/inferior, and anterior/posterior direction, respectively. Averages and standard deviations for the difference between the pre‐treatment and post‐treatment prostate alignment error ranged from −0.13 ± 0.08 to 0.09 ± 0.16 cm, −0.12 ± 0.18 to 0.21 ± 0.26 cm, and −0.15 ± 0.17 to 0.09 ± 0.15 cm in the right/left, superior/inferior, and anterior/posterior direction, respectively. Conclusion: Pre‐treatment the prostate was localized to within 3mm using markers seeds implanted in the prostate and an EPID. Post‐treatment portal images confirmed the prostate position to be within 3mm of its pre‐treatment position.