J Turcotte

COMPARISON OF TARGET REGISTRATION ERRORS FOR MULTIPLE IMAGE-GUIDED TECHNIQUES IN ACCELERATED PARTIAL BREAST IRRADIATION

PURPOSE External beam accelerated partial breast irradiation requires accurate localization of the target volume for each treatment fraction. Using the concept of target registration error (TRE), the performance of several methods of target localization was compared. METHODS AND MATERIALS Twelve patients who underwent external beam accelerated partial breast irradiation were included in this study. TRE was quantified for four methods of image guidance: standard laser-based setup, kilovoltage imaging of the chest wall, kilovoltage imaging of surgically implanted clips, and three-dimensional surface imaging of the breast. The use of a reference surface created from a free-breathing computed tomography scan and a reference surface directly captured with three-dimensional video imaging were compared. The effects of respiratory motion were also considered, and gating was used for 8 of 12 patients. RESULTS The median value of the TRE for the laser, chest wall, and clip alignment was 7.1 mm (n=94), 5.4 mm (n=81), and 2.4 mm (n=93), respectively. The median TRE for gated surface imaging based on the first fraction reference surface was 3.2 mm (n=49), and the TRE for gated surface imaging using the computed tomography-based reference surface was 4.9 mm (n=56). The TRE for nongated surface imaging using the first fraction reference surface was 6.2 mm (n=25). CONCLUSIONS The TRE of surface imaging using a reference surface directly captured with three-dimensional video and the TRE for clip-based setup were within 1 mm. Gated capture is important for surface imaging to reduce the effects of respiratory motion in accelerated partial breast irradiation.

Boron neutron capture therapy: cellular targeting of high linear energy transfer radiation.

Boron neutron capture therapy (BNCT) is based on the preferential targeting of tumor cells with10 B and subsequent activation with thermal neutrons to produce a highly localized radiation. In theory, it is possible to selectively irradiate a tumor and the associated infiltrating tumor cells with large single doses of high-LET radiation while sparing the adjacent normal tissues. The mixture of high- and low-LET dose components created in tissue during neutron irradiation complicates the radiobiology of BNCT. Much of the complexity has been unravelled through a combination of preclinical experimentation and clinical dose escalation experience. Over 350 patients have been treated in a number of different facilities worldwide. The accumulated clinical experience has demonstrated that BNCT can be delivered safely but is still defining the limits of normal brain tolerance. Several independent BNCT clinical protocols have demonstrated that BNCT can produce median survivals in patients with glioblastoma that appear to be equivalent to conventional photon therapy. This review describes the individual components and methodologies required for effect BNCT: the boron delivery agents; the analytical techniques; the neutron beams; the dosimetry and radiation biology measurements; and how these components have been integrated into a series of clinical studies. The single greatest weakness of BNCT at the present time is non-uniform delivery of boron into all tumor cells. Future improvements in BNCT effectiveness will come from improved boron delivery agents, improved boron administration protocols, or through combination of BNCT with other modalites.

Effects of organ motion on IMRT treatments with segments of few monitor units

Interplay between organ (breathing) motion and leaf motion has been shown in the literature to have a small dosimetric impact for clinical conditions (over a 30 fraction treatment). However, previous studies did not consider the case of treatment beams made up of many few-monitor-unit (MU) segments, where the segment delivery time (1-2 s) is of the order of the breathing period (3-5 s). In this study we assess if breathing compromises the radiotherapy treatment with IMRT segments of low number of MUs. We assess (i) how delivered dose varies, from patient to patient, with the number of MU per segment, (ii) if this delivered dose is identical to the average dose calculated without motion over the path of the motion, and (iii) the impact of the daily variation of the delivered dose as a function of MU per segment. The organ motion was studied along two orthogonal directions, representing the left-right and cranial-caudal directions of organ movement for a patient setup in the supine position. Breathing motion was modeled as sin(x), sin4(x), and sin6(x), based on functions used in the literature to represent organ motion. Measurements were performed with an ionization chamber and films. For a systematic study of motion effects, a MATLAB simulation was written to model organ movement and dose delivery. In the case of a single beam made up of one single segment, the dose delivered to point in a moving target over 30 fractions can vary up to 20% and 10% for segments of 10 MU and 20 MU, respectively. This dose error occurs because the tumor spends most of the time near the edges of the radiation beam. In the case of a single beam made of multiple segments with low MU, we observed 2.4%, 3.3%, and 4.3% differences, respectively, for sin(x), sin4(x), and sin6(x) motion, between delivered dose and motion-averaged dose for points in the penumbra region of the beam and over 30 fractions. In approximately 5-10% of the cases, differences between the motion-averaged dose and the delivered 30-fraction dose could reach 6%, 8% and 10-12%, respectively for sin(x), sin4(x), and sin6(x) motion. To analyze a clinical IMRT beam, two patient plans were randomly selected. For one of the patients, the beams showed a likelihood of up to 25.6% that the delivered dose would deviate from the motion-averaged dose by more than 1%. For the second patient, there was a likelihood of up to 62.8% of delivering a dose that differs by more than 1% from the motion-averaged dose and a likelihood of up to approximately 30% for a 2% dose error. For the entire five-beam IMRT plan, statistical averaging over the beams reduces the overall dose error between the delivered dose and the motion-averaged dose. For both patients there was a likelihood of up to 7.0% and 33.9% that the dose error was greater than 1%, respectively. For one of the patients, there was a 12.6% likelihood of a 2% dose error. Daily intrafraction variation of the delivered dose of more than 10% is non-negligible and can potentially lead to biological effects. We observed [for sin(x), sin4(x), and sin6(x)] that below 10-15 MU leads to large daily variations of the order of 15-35%. Therefore, for small MU segments, non-negligible biological effects can be incurred. We conclude that for most clinical cases the effects may be small because of the use of many beams, it is desirable to avoid low-MU segments when treating moving targets. In addition, dose averaging may not work well for hypo-fractionation, where fewer fractions are used. For hypo-fractionation, PDF modeling of the tumor motion in IMRT optimization may not be adequate.

A Voluntary Breath-Hold Treatment Technique for the Left Breast With Unfavorable Cardiac Anatomy Using Surface Imaging

PURPOSE Breath-hold (BH) treatments can be used to reduce cardiac dose for patients with left-sided breast cancer and unfavorable cardiac anatomy. A surface imaging technique was developed for accurate patient setup and reproducible real-time BH positioning. METHODS AND MATERIALS Three-dimensional surface images were obtained for 20 patients. Surface imaging was used to correct the daily setup for each patient. Initial setup data were recorded for 443 fractions and were analyzed to assess random and systematic errors. Real time monitoring was used to verify surface placement during BH. The radiation beam was not turned on if the BH position difference was greater than 5 mm. Real-time surface data were analyzed for 2398 BHs and 363 treatment fractions. The mean and maximum differences were calculated. The percentage of BHs greater than tolerance was calculated. RESULTS The mean shifts for initial patient setup were 2.0 mm, 1.2 mm, and 0.3 mm in the vertical, longitudinal, and lateral directions, respectively. The mean 3-dimensional vector shift was 7.8 mm. Random and systematic errors were less than 4 mm. Real-time surface monitoring data indicated that 22% of the BHs were outside the 5-mm tolerance (range, 7%-41%), and there was a correlation with breast volume. The mean difference between the treated and reference BH positions was 2 mm in each direction. For out-of-tolerance BHs, the average difference in the BH position was 6.3 mm, and the average maximum difference was 8.8 mm. CONCLUSIONS Daily real-time surface imaging ensures accurate and reproducible positioning for BH treatment of left-sided breast cancer patients with unfavorable cardiac anatomy.

Evaluation and commissioning of a surface based system for respiratory sensing in 4D CT

The purpose of this study is to assess the temporal and reconstruction accuracy of a surface imaging system, the GateCT under ideal conditions, and compare the device with a commonly used respiratory surrogate: the Varian RPM. A clinical CT scanner, run in cine mode, was used with two optical devices, GateCT and RPM, to detect respiratory motion. A radiation detector, GM‐10, triggers the X‐ray on/off to GateCT system, while the RPM is directly synchronized with the CT scanner through an electronic connection. Two phantoms were imaged: the first phantom translated on a rigid plate along the anterior–posterior (AP) direction, and was used to assess the temporal synchronization of each optical system with the CT scanner. The second phantom, consisting of five spheres translating 3 cm peak‐to‐peak in the superior–inferior direction, was used to assess the quality of rebinned images created by GateCT and RPM. Calibration assessment showed a nearly perfect synchronization with the scanner for both the RPM and GateCT systems, thus demonstrating the good performance of the radiation detector. Results for the volume rebinning test showed discrepancies in volumes for the 3D reconstruction (compared to ground truth) of up to 36% for GateCT and up to 40% for RPM. No statistical difference was proven between the two systems in volume sorting. Errors are mainly due to phase detection inaccuracies and to the large motion of the phantom. This feasibility study assessed the consistency of two optical systems in synchronizing the respiratory signal with the image acquisition. A new patient protocol based on both RPM and GateCT will be soon started. PACS number: 87

Analysis of setup uncertainties for extremity sarcoma patients using surface imaging.

PURPOSE Proper positioning of patients with extremity sarcoma tumors can be challenging. A surface imaging technique was utilized to quantify the setup uncertainties for sarcoma patients and to assess whether surface imaging could improve the accuracy of patient positioning. METHODS AND MATERIALS Pretreatment and posttreatment 3-dimensional (3D) surface images were obtained for 16 patients and 236 treatments. Offline surface registration was performed to quantify interfraction and intrafraction setup errors, and the required planning target volume (PTV) margins were calculated. Setup differences were also assessed using root mean square (RMS) error analysis. RESULTS For intrafraction variation, the mean 3D vector shift was 2.1 mm, and the systematic and random errors were 1.3 mm or less. When using a reference surface from the first fraction, the mean interfraction setup variation (3D vector shift) was 7.6 mm. Systematic and random errors were 3-4 mm in each direction. When using a computed tomographic based reference surface, the mean 3D vector shift was 9.5 mm. Systematic and random errors ranged from 3.1 to 7.9 mm. The required PTV margins were 1.0 cm, 1.2 cm, and 1.3 cm in the anterior-posterior, superior-inferior, and lateral directions, respectively. The mean (standard deviation) RMS errors for the uncorrected position were 4.7 mm (1.9 mm) and were reduced to 2.2 mm (0.8 mm) and 1.7 mm (0.8 mm), for 4 degree of freedom (DOF) and 6 DOF surface alignment, respectively. CONCLUSIONS Intrafraction motion is small. Interfraction motion can exceed typical PTV margins and daily imaging should be utilized to reduce setup variations. Surface imaging may reduce setup errors and is a feasible technique for daily image guidance.

SU-F-T-227: A Comprehensive Patient Specific, Structure Specific, Pre-Treatment 3D QA Protocol for IMRT, SBRT and VMAT - Clinical Experience

PURPOSE To present a 3D QA method and clinical results for 550 patients. METHODS Five hundred and fifty patient treatment deliveries (400 IMRT, 75 SBRT and 75 VMAT) from various treatment sites, planned on Raystation treatment planning system (TPS), were measured on three beam-matched Elekta linear accelerators using IBAs COMPASS system. The difference between TPS computed and delivered dose was evaluated in 3D by applying three statistical parameters to each structure of interest: absolute average dose difference (AADD, 6% allowed difference), absolute dose difference greater than 6% (ADD6, 4% structure volume allowed to fail) and 3D gamma test (3%/3mm DTA, 4% structure volume allowed to fail). If the allowed value was not met for a given structure, manual review was performed. The review consisted of overlaying dose difference or gamma results with the patient CT, scrolling through the slices. For QA to pass, areas of high dose difference or gamma must be small and not on consecutive slices. For AADD to manually pass QA, the average dose difference in cGy must be less than 50cGy. The QA protocol also includes DVH analysis based on QUANTEC and TG-101 recommended dose constraints. RESULTS Figures 1-3 show the results for the three parameters per treatment modality. Manual review was performed on 67 deliveries (27 IMRT, 22 SBRT and 18 VMAT), for which all passed QA. Results show that statistical parameter AADD may be overly sensitive for structures receiving low dose, especially for the SBRT deliveries (Fig.1). The TPS computed and measured DVH values were in excellent agreement and with minimum difference. CONCLUSION Applying DVH analysis and different statistical parameters to any structure of interest, as part of the 3D QA protocol, provides a comprehensive treatment plan evaluation. Author G. Gueorguiev discloses receiving travel and research funding from IBA for unrelated to this project work. Author B. Crawford discloses receiving travel funding from IBA for unrelated to this project work.

Clinical implementation and error sensitivity of a 3D quality assurance protocol for prostate and thoracic IMRT

This work aims at three goals: first, to define a set of statistical parameters and plan structures for a 3D pretreatment thoracic and prostate intensity‐modulated radiation therapy (IMRT) quality assurance (QA) protocol; secondly, to test if the 3D QA protocol is able to detect certain clinical errors; and third, to compare the 3D QA method with QA performed with single ion chamber and 2D gamma test in detecting those errors. The 3D QA protocol measurements were performed on 13 prostate and 25 thoracic IMRT patients using IBAs COMPASS system. For each treatment planning structure included in the protocol, the following statistical parameters were evaluated: average absolute dose difference (AADD), percent structure volume with absolute dose difference greater than 6% (ADD6), and 3D gamma test. To test the 3D QA protocol error sensitivity, two prostate and two thoracic step‐and‐shoot IMRT patients were investigated. Errors introduced to each of the treatment plans included energy switched from 6 MV to 10 MV, multileaf collimator (MLC) leaf errors, linac jaws errors, monitor unit (MU) errors, MLC and gantry angle errors, and detector shift errors. QA was performed on each plan using a single ion chamber and 2D array of ion chambers for 2D and 3D QA. Based on the measurements performed, we established a uniform set of tolerance levels to determine if QA passes for each IMRT treatment plan structure: maximum allowed AADD is 6%; maximum 4% of any structure volume can be with ADD6 greater than 6%, and maximum 4% of any structure volume may fail 3D gamma test with test parameters 3%/3 mm DTA. Out of the three QA methods tested the single ion chamber performed the worst by detecting 4 out of 18 introduced errors, 2D QA detected 11 out of 18 errors, and 3D QA detected 14 out of 18 errors. PACS number: 87.56.Fc

SU-E-T-152: Error Sensitivity and Superiority of a Protocol for 3D IMRT Quality Assurance

PURPOSE To test if the parameters included in our 3D QA protocol with current tolerance levels are able to detect certain errors and show the superiority of 3D QA method over single ion chamber measurements and 2D gamma test by detecting most of the introduced errors. The 3D QA protocol parameters are: TPS and measured average dose difference, 3D gamma test with 3mmDTA/3% test parameters, and structure volume for which the TPS predicted and measured absolute dose difference is greater than 6%. METHODS Two prostate and two thoracic step-and-shoot IMRT patients were investigated. The following errors were introduced to each original treatment plan: energy switched from 6MV to 10MV, linac jaws retracted to 15cmx15cm, 1,2,3 central MLC leaf pairs retracted behind the jaws, single central MLC leaf put in or out of the treatment field, Monitor Units (MU) increased and decreased by 1 and 3%, collimator off by 5 and 15 degrees, detector shifted by 5mm to the left and right, gantry treatment angle off by 5 and 15 degrees. QA was performed on each plan using single ion chamber, 2D ion chamber array for 2D gamma analysis and using IBAs COMPASS system for 3D QA. RESULTS Out of the three tested QA methods single ion chamber performs the worst not detecting subtle errors. 3D QA proves to be the superior out of the three methods detecting all of introduced errors, except 10MV and 1% MU change, and MLC rotated (those errors were not detected by any QA methods tested). CONCLUSION As the way radiation is delivered evolves, so must the QA. We believe a diverse set of 3D statistical parameters applied both to OAR and target plan structures provides the highest level of QA.

SU‐E‐T‐189: A Protocol for 3D IMRT Quality Assurance of Prostate Radiotherapy

PURPOSE Define a set of statistical parameters and plan structures that will characterize the pass/fail criteria for a 3D prostate pre-treatment IMRT quality assurance protocol. METHODS For this study, 3D IMRT QA measurements were performed on thirteen IMRT prostate patients. All plans were for an initial treatment course to the prostate and the seminal vesicles with prescription dose of 45Gy, and planned with step-and-shoot IMRT. All patients plans passed traditional QA methods, including point measurement with ion chamber and 2D gamma analysis at 3mm/3%. A total of 75 3D IMRT QA measurements were performed on all patients, on different days and on different beam matched linear accelerators. IBA COMPASS system was used for 3D QA measurements. For each patient, nine structures of interest were selected: femoral heads, two planning target volumes (PTV1, PTV2), prostate, seminal vesicles, anterior and posterior rectal wall and bladder. For each structure the following statistical parameters were evaluated: average dose, 3D gamma test, volume at 4% and 6% difference, and volume that receives between 5%-105% of the prescription dose. Two QA protocols are proposed, a conservative with pass/fail QA thresholds the maximum of these 75 passing measurements; standard protocol with pass/fail QA threshold value at the maximum of the 75 measurements +50% increase. RESULTS Out of the 153 statistical parameters investigated, 65 were excluded from the QA protocol. The reason is that their values were constant for all 75 measurements. Assessment of the validity of the conservative protocol and standard protocol are ongoing. CONCLUSION 3D IMRT QA is a powerful and versatile tool for pre-treatment prostate IMRT QA, with ability to perform variety of statistical tests and overlay dose and anatomy. However, because 3D QA provides more information than single ion chamber measurement and/or 2D gamma analysis, a protocol is needed to establish pass/fail thresholds.