Setup Correction Research Articles

Using an onboard kilovoltage imager to measure setup deviation in intensity‐modulated radiation therapy for head‐and‐neck patients

The purpose of the present study was to use a kilovoltage imaging device to measure interfractional and intrafractional setup deviations in patients with head‐and‐neck or brain cancers receiving intensity‐modulated radiotherapy (IMRT) treatment.Before and after IMRT treatment, approximately 3 times weekly, 7 patients were imaged using the Varian On‐Board Imager (OBI: Varian Medical Systems, Palo Alto, CA), a kilovoltage imaging device permanently mounted on the gantry of a Varian 21EX LINAC (Varian Medical Systems). Because of commissioning of the remote couch correction of the OBI during the study, online setup corrections were performed on 2 patients. For the other 5 patients, weekly corrections were made based on a sliding average of the measured data. From these data, we determined the interfractional setup deviation (defined as the shift from the original setup position suggested by the daily image), the residual error associated with the weekly correction protocol, and the intrafractional setup deviation, defined as the difference between the post‐treatment and pretreatment images. We also used our own image registration software to determine interfractional and intrafractional rotational deviations from the images based on the template‐matching method. In addition, we evaluated the influence of inter‐observer variation on our results, and whether the use of various registration techniques introduced differences. Finally, translational data were compared with rotational data to search for correlations.Translational setup errors from all data were 0.0±0.2 cm, −0.1±0.3 cm, and −0.2±0.3 cm in the right–left (RL), anterior–posterior (AP), and superior–inferior (SI) directions respectively. Residual error for the 5 patients with a weekly correction protocol was −0.1±0.2 cm (RL), 0.0±0.3 cm (AP), and 0.0±0.2 cm (SI). Intrafractional translation errors were small, amounting to 0.0±0.1 cm, −0.1±0.2 cm, and 0.0±0.1 cm in the RL, AP, and SI directions respectively. In the sagittal and coronal views respectively, interfractional rotational errors were −1.1±1.7 degrees and −0.5±0.9 degrees, and intrafractional rotational errors were 0.3±0.6 degrees and 0.2±0.5 degrees. No significant correlation was seen between translational and rotational data.The OBI image data were used to study setup error in the head‐and‐neck patients. Nonzero systematic errors were seen in the interfractional translational and rotational data, but not in the intrafractional data, indicating that the mask is better at maintaining head position than at reproducing it.PACS numbers:87.53.Kn, 87.53.Oq

Accuracy of a commercial optical 3D surface imaging system for realignment of patients for radiotherapy of the thorax

Accurate and reproducible patient setup is a prerequisite to fractionated radiotherapy. To evaluate the applicability and technical performance of a commercial 3D surface imaging system for repositioning of breast cancer patients, measurements were performed in a rigid anthropomorphic phantom as well as in healthy volunteers. The camera system records a respiration-gated surface model of the imaged object, which may be registered to a previously recorded reference model. A transformation is provided, which may be applied to the treatment couch to correct the setup of the patient. The system showed a high stability and detected pre-defined shifts of phantoms and healthy volunteers with an accuracy of 0.40 ± 0.26 mm and 1.02 ± 0.51 mm, respectively (spatial deviation between pre-defined shift and suggested correction). The accuracy of the suggested rotational correction around the vertical axis was always better than 0.3° in phantom measurements and 0.8° in volunteers, respectively. Comparison of the suggested setup correction with that detected by a second and independently operated marker-based optical system provided consistent results. The results demonstrate that the camera system provides highly accurate setup corrections in a phantom and healthy volunteers. The most efficient use of the system for improving the setup accuracy in breast cancer patients has to be investigated in routine patient treatments.

TU‐EE‐A3‐05: Assessment of Treatment Site‐Specific Setup Accuracy and Reproducibility in Patient Positioning Using Daily MVCT Imaging

Purpose: To assess treatment site‐specific corrections in patient positioning from TomoTherapy Hi‐ART® megavoltage images. Method and Materials: Initial positioning corrections, determined by fusion of daily pre‐treatment MVCT to planning CT images, were analyzed for different anatomical treatment sites.Over 600,1400, and 1200 fractions of head and neck (H&N) and brain, lung, and prostate tomotherapy treatments, respectively, were assessed. Translational and rotational, per‐fraction setup corrections were retrospectively compiled from individual patient archive files. Setup corrections were compared amongst anatomical treatment sites. Setup reproducibility was assessed by analyzing standard deviations and histograms of setup corrections. Setup accuracy was also assessed by analyzing 3D vector lengths and magnitudes of corrections. Results: Large variations in setup corrections were seen in all three disease sites. H&N treatments had a significantly smaller (p<0.001) vector length standard deviation of 2.54 mm compared with 6.99 mm and 6.46 mm for lung and prostate treatments, respectively, but had no significantly different standard deviation in roll rotations. The frequency of translations of vector lengths ⩾ 10 mm were 48.9% and 48.1% of all lung and prostate treatments, respectively, whereas corrections of that magnitude occurred in only 1.56% of H&N treatments. Frequencies of roll rotations were more comparable among the three disease sites. Conclusions: Analysis of patient positioning corrections indicates large variations between patients and treatments, suggesting a role for imaging every patient per‐fraction to ensure the most accurate reproducibility. H&N treatments had a higher setup reproducibility and accuracy in the lateral, longitudinal, and vertical directions. Even so, they were not significantly different rotationally than prostate and lung treatments, suggesting a role for daily imaging in all three treatment sites. In quantifying site‐specific positioning reproducibility and accuracy, this work may be useful for assessing new treatment planning margins for image‐guided procedures or for developing adaptive radiotherapy techniques.

SU‐FF‐J‐09: An Analysis of Cone‐Beam CT for Determining Setup Errors and Designing Treatment Margins for Non‐Small Cell Lung Cancer Patients

Purpose: KV cone‐beam CT imaging (CBCT) was used to analyze setup errors when treating patients with non‐small cell lung cancer (NSCLC). For patients where an off‐line correction strategy was adopted, CBCT was also used to determine the patient specific planning target volume (PTV). Method and Materials: Eighteen patients with NSCLC were treated with CBCT image‐guided radiotherapy using an Elekta Synergy linac. 4D‐CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free‐breathing and were immobilized with a wing‐board. The internal target volume (ITV) was determined using the maximum‐intensity‐projection (MiP) CT data to account for organ motion. A 5‐mm PTV margin was used to account for setup error. On‐line setup correction was based on the daily CBCT imaging prior to treatment. The systematic and random errors were analyzed retrospectively. The results were then fit to our CBCT off‐line correction strategy where a setup correction was made at the fifth treatment fraction to correct for the systematic error. The patient specific PTV margin was also analyzed. Results: The systematic and random errors were 5.7 ± 3.2 mm, 4.2 ± 2.6 mm and 4.8 ± 3.1 mm for anterior‐posterior (AP), medial‐lateral (ML) and superior‐inferior (SI) directions respectively. The 3D displacement vectors were 4.1 mm ± 3.1 mm (systematic ± random). If the patients were treated with the off‐line correction strategy, a patient specific PTV margin ranging from 7.0 to 17.4 mm would be needed. Conclusions: The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on‐line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. A larger patient specific PTV margin, ranging from 7.0 to 17.4 mm, would be needed if an off‐line correction is used.

TH-E-M100F-02: CTV-To-PTV Margins for Prostate Irradiations, a Quantitative Assessment Using Novel Voxel Tracking Techniques

Purpose: To quantify adequate anisotropic CTV-to-PTV margins for three different setup strategies used during prostate irradiations: (1) no setup corrections, (2) on-line corrections based on bony anatomy and (3) on-line corrections based on gold markers. Method and Materials: Three radiation oncologists independently delineated the CTV, the bladder and the rectum in CT images of 30 prostate patients with implanted gold markers using a 3D model-based segmentation technique. IMRT plans with zero CTV-to-PTV margins were generated for each of the 3 times 30 contoured image datasets. Eight repeat scans were acquired to allow simulation of the delivered dose distributions in changing geometry. Different registration approaches were taken to mimic the different setup strategies. A surface-model based deformable image registration system was used to warp the delivered dose distributions back to the dose in the planning CT. Based on the geometric extent of underdosed areas in this patient population, a set of anisotropic margins was derived. A second simulation was carried out to assess the target coverage of the derived margins. Results: Without setup correction, margins of 10 mm for the corpus of the prostate and 15 mm for the seminal vesicles were required. These margins could be reduced to 7 mm and 12 mm respectively with on-line correction using bony anatomy registration methods and to 3 mm and 7 mm using on-line repositioning based on gold markers. A larger margin at the apex was required to account for the significant observer variability and steep dose gradients at this location (11 mm — skin marker registration, 9 mm — bony anatomy registration, 7 mm — gold marker registration). Conclusion: Novel voxel tracking techniques enable us to calculate accumulated dose distributions and design accurate 3-D CTV-to-PTV margins for prostate irradiations.

MO‐E‐M100J‐04: Planning Target Volume Determination

Treatment planning should be based on a high fidelity representation of the patient's anatomy, as it will be at time of treatment. The imaging setup (couch, immobilization, alignment) should therefore be representative for the treatment situation. When basing a treatment plan on a single scan (which is current clinical practice), the optimal imaging strategy is therefore to create an anatomical model that is as close as possible to the “average” anatomy. For random organ motion, plan optimization can therefore be achieved by averaging the geometry of multiple scans (i.e., adaptive radiotherapy). For periodic motion it has been proposed to use a scan with the organs as close as possible to their time‐weighted average position — by selecting a single scan out of 4DCT. Using these methods, the systematic errors due to treatment preparation are minimized and the dose distributions will be very close to a full 4D planning (using all scans out of a 4DCT scan). Using image guidance some other error sources can be eliminated. However, even with image guidance there are residual errors: such as uncertainty in GTV and CTV delineation, in image registration, and in setup correction and uncertainty due to intra‐fraction motion. Currently, simple margin recipes are used to estimate the correct CTV‐PTV margin such that the net effect of all these uncertainties does not compromise the goal of the treatment: eradicate the tumor while sparing normal tissues. One should be aware, however, that the most simple margin recipes are based on many assumptions: such as Gaussian distributions, penumbra width in water, systematic error SD > random error SD, and plans with a more or less uniform dose distribution. In case of image guidance for lung tumors all of these assumptions break down, and simple recipes typically will overestimate the required margin. Based on a more detailed analysis, it appears that the margin that can be used for hypofractionated lung radiotherapy can be very small (1 cm or less), even for large respiratory amplitudes (2 cm).Educational Objectives:1. Understand the need for a representative planning CT, i.e., minimize systematic errors.2. Understand the difference between systematic and random errors, and the effect that respiratory motion has on the delivery of radiotherapy.3. Understand the need for margins, even in case of image‐guided. radiotherapy.4. Understand the derivation of margins for image‐guided radiotherapy of lung cancer.

SU-FF-J-108: Dose Variation of Critical Structures in Daily Treatment of Nasopharyngeal Cancer Patients Using Helical Tomotherapy Megavoltage CT System

Purpose: To study daily spinal cord and parotid gland dose variation for nasopharyngeal cancer patients undergoing helical tomotherapy treatments, as a result of patient weight loss during the course of treatments.Method and Materials: Five nasopharygeal cancer patients who received helical tomotherapy treatments were selected. At the end of the treatments, patients lost an average weight of 6.3 kg, or 8.4% relative to the weight at the beginning of the treatments. Daily megavoltage CT (MVCT) scans were performed prior to each treatment fraction, and MVCT images were registered with planning CTimages for patient setup corrections. Contours for spinal cord and parotid glands were drawn on MVCT images throughout the treatment course. Based on daily MVCT images, actual dose delivered to the critical structures were calculated using the Planned Adaptive application in the helical tomotherapy treatment planning system. The maximum dose to the spinal cord and the median doses to the parotid glands were analyzed. Results: The maximum dose to the spinal cord, after setup corrections, did not show significant variation as a result of patient weight loss. However, the median doses to the parotid glands showed significant increase due to facial tissue shrinkage during treatments. The average median dose to the parotid glands for the five patients were 83 cGy for the first treatment fraction, and it was 143 cGy per fraction for the last fraction. The average parotid gland volume decreased from 20.5 cc to 13.2 cc during treatments.Conclusion: Daily patient setup corrections based on MVCT image registrations prevented significant dose variation to the spinal cord due to setup uncertainty. During the course of radiotherapy, as a result of body weight loss, the parotid glands typically receive significantly higher dose compared to the dose dictated in the treatment plan.

SU‐FF‐J‐63: Comparison of Daily Setup Corrections Using On‐Board Cone‐Beam Computed Tomography, and Two Planar Kilovoltage Imaging Systems

Purpose: The purpose of this study is to compare bony landmark‐based setup corrections derived from a linac gantry‐mounted kilovoltage (kV) imaging system that can either be used in orthogonal radiography (OBI) or kV circular orbit computed tomography (CBCT) imaging mode. A third commercial image‐guided radiotherapy system, consisting of orthogonal room‐mounted planar x‐ray imaging system, Exac Trac (ET), was also tested. In addition to daily setup corrections, imaging doses were compared. Methods and Materials: Both patient and phantom data were used to perform this study. Five patients were selected and couch shifts ware determined using both the OBI and ET systems. Using a pelvic phantom, intentionally introduced shifts of 0, 5, and 20 mm in the anterior/posterior (AP/PA), superior/inferior (S/I), and lateral directions, respectively, were applied. The estimated couch shifts were compared using all three Image Guided Radiotheraphy (IGRT) systems. Two cranial cases, one liver case, one lung case, and one prostate case were selected and the estimated couch shifts using the OBI and the ET systems were determined while the patient is in place. Couch shifts are not applied until all imaging modalities are performed. Results: Comparison of simulated shifts using all three imaging modalities showed a standard deviation of setup uncertainties is within 1 mm in all directions. Patients' data showed that the biggest difference is in the order 4 mm in the superior/inferior direction for liver patient and 2 mm in the superior/inferior direction for cranial patient. Conclusions: This study showed that while using 3D/3D image fusion is an excellent choice for online daily patient setup, it is possible to determine couch shift to high degree of accuracy using either 2D/2D or 2D/3D image fusion. For lung and liver patient, tracking of organ motion must be used to any imaging modality.

Set-up verification on a belly-board device using electronic portal imaging

AbstractPurpose: Quantification of set-up errors is necessary to assess the accuracy of patient positioning and define set-up margins. In this article, we describe the analysis of two different set-up verification and correction procedures in pelvic irradiation for rectal cancer patients treated on a belly-board device.Methods: First, we conducted a retrospective study in ten patients. Skin marks were used for set-up and the position was verified and corrected at the start of treatment by portal imaging. Second, we analysed the implementation of a more rigorous verification and correction procedure in ten patients. The same set-up procedure was used, but verification was performed during the first three sessions and on a weekly basis thereafter. In both studies, systematic and random errors were linked with possible patient-related, treatment-unit-related and time-related factors.Results: The pooled data showed a significant reduction in systematic and random error in favour of the second verification procedure (p < 0.05). This resulted in a reduction in the size of the safety margin of more than 3 mm in all directions. Time trends were significant in four patients in the first analysis and in three patients in the second analysis. In six patients in the first and seven patients in the second study, a significant correlation was found between the vertical couch movement and the antero-posterior set-up error. Analysis of patient-related factors demonstrated a relationship between the abdominal contour and rotational errors in both studies.Conclusion: The results of these set-up analyses show that patient positioning on a belly-board device by laser alignment to skin marks is accurate and reproducible. However, in some patients we recommend the implementation of a fixed vertical couch position. The systematic error should be identified and corrected during the first fractions of treatment. Thereafter, verification should be performed at regular intervals to correct for possible time trends. Positioning of obese patients was found to be more prone to set-up errors and requires online position verification.

SU-FF-J-103: Intra-Fraction Prostate Motion Evaluation with On-Line Fluoroscopic Imaging

Purpose: This study attempts to quantify intra-fraction organ motion of prostate patients undergoing external beam radiotherapy. Method and Materials: Lateral kV fluoroscopic images are captured on the first 3 days of treatment and then weekly thereafter for a total of 10 sequences per patient. The fiducial markers (FM) are located in the apex, posterior aspect and base of the prostate. Each fluoro sequence is taken after 3D setup correction. Images are acquired at 5.5 fps for a duration of 30 seconds at a technique of 120kVp, 64mA, 20ms per frame. A normalized cross-correlation provides 2D localization of the three FM's in each frame. Displacement metrics were computed using the magnitude of the Euclidean distance of each FM from its initial position. Four patients are included in the study for a total of 30 fluoro sequences. The 2D vector displacements for all patient fractions in the study was computed and compared. Results: The standard deviation of the displacement distribution is (0.3, 0.9, 0.7) [mm] for the fiducial marker at the apex, posterior aspect and base of the prostate respectively. The posterior FM which is immediately adjacent to the rectum showed the largest motion. The ‘institutional mean’ (i.e. are all patients, on average, shifting in a particular direction) was quite small, with a max of 0.2 [mm] for the base FM. Conclusion: On-line kV fluoro is a useful tool for characterization of intra-fraction prostate motion supporting “evidence-based” margin design.

WE-D-AUD-02: Retrospective IMRT Dose Reconstruction Based On Cone-Beam Computed Tomography (CBCT) and the MLC Positional Log-File Recorded During Treatment

Purpose: Most CBCTs performed nowadays are only used for setup correction. In reality, CBCT‐derived 3D patient model can also be used for dosereconstruction to reflect the delivered dose taking into account the inter‐fraction organ motion/deformation. The purpose of this work is to establish a methodology to reconstruct the delivered IMRTdose by using on‐treatment CBCTimages and the actual dynamic leaf positions recorded during the treatment.Method and Materials: A CT phantom with different inserts was scanned using planning CT (pCT) and CBCT, respectively, and the corresponding electron density calibration curves were established. Five head‐and‐neck (HN) patients were selected and, for each patient, three CBCT scans with the patient at his/her treatment positions were taken at different fractions during the course of radiation therapy. The target and organ contours were mapped from the pCT onto the co‐registered CBCT for DVH analysis. The actual leaf positions in IMRT delivery were recorded by the MLC workstation and the log‐files were retrieved to reconstruct the fluence maps for the corresponding treatment fraction. The delivered dose was then obtained using the reconstructed fluence maps and the CBCT data, and the results were compared to the original pCT‐based treatment plan. Results: A retrospective dosereconstruction procedure has been implemented and applied to 5 HN cases. For cases in which the tumor shrinkage is not obvious, the reconstructed and planned doses are consistent to within 3% in high dose region. The DVHs of the target and other organs do not have significant differences. Large dosimetric changes (5–8%) were observed, however, for a case with tumor shrinkage, indicating the need for re‐planning or adaptive radiation therapy in this situation. Conclusion:IMRTdosereconstruction provides a powerful way to examine the actual patient dose delivery at a particular fraction and represents an essential step toward adaptive radiotherapy.

Setup Variations in Radiotherapy of Esophageal Cancer: Evaluation by Daily Megavoltage Computed Tomographic Localization

To use pretreatment megavoltage computed tomography (MVCT) scans to evaluate setup variations in anterior-posterior (AP), lateral, and superior-inferior (SI) directions and rotational variations, including pitch, roll, and yaw, for esophageal cancer patients treated with helical tomotherapy. Ten patients with locally advanced esophageal cancer treated by combined chemoradiation using helical tomotherapy were selected. After patients were positioned using their skin tattoos/marks, MVCT scans were performed before every treatment and automatically registered to planning kilovoltage CT scans according to bony landmarks. Image registration data were used to adjust patient setups before treatment. A total of 250 MVCT scans were analyzed. Correlations between setup variations and body habitus, including height, weight, relative weight change, body surface area, and patient age, were evaluated. The standard deviations for systematic setup corrections in AP, lateral, and SI directions and pitch, roll, and yaw rotations were 1.5, 3.7, and 4.8 mm and 0.5 degrees, 1.2 degrees, and 0.8 degrees, respectively. The appropriate averages of random setup variations in AP, lateral, and SI directions and pitch, roll, and yaw rotations were 2.9, 5.2, and 4.4 mm, and 1.0 degrees, 1.2 degrees, and 1.1 degrees, respectively. Setup variations were stable throughout the entire course of radiotherapy in all three translational and three rotational displacements, with little change in magnitude. No significant correlations were found between setup variations and body habitus variables. Daily MVCT scans before each treatment can effectively detect setup errors and thereby reduce planning target volume (PTV) margins. This will reduce radiation dose to critical organs and may translate into lower treatment-related toxicities.

Image‐guided patient positioning: If one cannot correct for rotational offsets in external‐beam radiotherapy setup, how should rotational offsets be managed?

When setting up patients via image-guided positioning for external-beam radiotherapy, one can often determine the rotational corrections needed for optimal alignment, but once measured, they are not always applied. However, in rigid-body setup calculations the optimal translational component of the setup correction will be different depending on whether rotations are included or excluded from the correction procedure. Furthermore, if rotations go uncorrected then the optimal translation becomes dependent on the relative locations of the registration landmarks, the treatment site, and the rotational axes. If one is not going to make rotational adjustments to the patient position, then two guidelines should be followed: (1) the registration landmarks should closely demarcate the treatment site, and (2) rotational degrees of freedom should not be included in the calculation of setup adjustments.

Assessment of secondary patient motion induced by automated couch movement during on-line 6 dimensional repositioning in prostate cancer treatment

Background and purpose The purpose of this study is to assess retrospectively secondary patient motion induced by 6D patient setup correction. Materials and methods For 104 patients, treated with Novalis ®, 6D setup correction prior to treatment was performed by ExacTrac5.0/NovalisBody ® in combination with the Robotic Tilt Module™ mounted underneath the Exact Couch ® top. This 6D correction might induce additional setup errors due to patient reaction against the rotations. To evaluate induced secondary motion, the 6D setup correction is verified and evaluated with respect to the tolerance limits. Results The majority of measured secondary motions are found within the tolerance limits. Detected secondary motions are mostly found in longitudinal shifts and lateral rotations, and mainly found in only 1 dimension during the same verification. The verifications indicate that the patient population can be divided into a group that hardly moves and a group that moves throughout all 6D setup corrections. The patient’s behavior can be predicted by the evaluation of the first five fractions as none of the patients demonstrate a learning curve during the treatment. Conclusions 6D setup correction does not induce secondary motion for the majority of the patients and can therefore be applied for all treatment indications.

Target Coverage in Image-Guided Stereotactic Body Radiotherapy of Liver Tumors

To determine the effect of image-guided procedures (with computed tomography [CT] and electronic portal images before each treatment fraction) on target coverage in stereotactic body radiotherapy for liver patients using a stereotactic body frame (SBF) and abdominal compression. CT guidance was used to correct for day-to-day variations in the tumor's mean position in the SBF. By retrospectively evaluating 57 treatment sessions, tumor coverage, as obtained with the clinically applied CT-guided protocol, was compared with that of alternative procedures. The internal target volume-plus (ITV(+)) was introduced to explicitly include uncertainties in tumor delineations resulting from CT-imaging artifacts caused by residual respiratory motion. Tumor coverage was defined as the volume overlap of the ITV(+), derived from a tumor delineated in a treatment CT scan, and the planning target volume. Patient stability in the SBF, after acquisition of the treatment CT scan, was evaluated by measuring the displacement of the bony anatomy in the electronic portal images relative to CT. Application of our clinical protocol (with setup corrections following from manual measurements of the distances between the contours of the planning target volume and the daily clinical target volume in three orthogonal planes, multiple two-dimensional) increased the frequency of nearly full (> or = 99%) ITV(+) coverage to 77% compared with 63% without setup correction. An automated three-dimensional method further improved the frequency to 96%. Patient displacements in the SBF were generally small (< or = 2 mm, 1 standard deviation), but large craniocaudal displacements (maximal 7.2 mm) were occasionally observed. Daily, CT-assisted patient setup may substantially improve tumor coverage, especially with the automated three-dimensional procedure. In the present treatment design, patient stability in the SBF should be verified with portal imaging.

ENAL: An Extension of the NAL Setup Correction Protocol for Effective Use of Weekly Follow-up Measurements

The no action level (NAL) protocol reduces systematic displacements relative to the planning CT scan by using the mean displacement of the first few treatment fractions as a setup correction in all subsequent fractions. This approach may become nonoptimal in case of time trends or transitions in the systematic displacement of a patient. Here, the extended NAL (eNAL) protocol is introduced to cope with this problem. The initial setup correction of eNAL is the same as in NAL. However, in eNAL, additional weekly follow-up measurements are performed. The setup correction is updated after each follow-up measurement based on linear regression of the available measured displacements to track and correct systematic time-dependent changes. We investigated the performance of eNAL with Monte Carlo simulations for populations without systematic displacement changes over time, with large gradual changes (time trends), and with large sudden changes (transitions). Weekly follow-up measurements were simulated for 35 treatment fractions. We compared the outcome of eNAL with NAL and optimized shrinking action level (SAL) protocol with weekly measurements. Without time-dependent changes, eNAL, SAL, and NAL performed comparably, but SAL required the largest imaging workload. For time trends and transitions, eNAL performed superiorly to the other protocols and reduced systematic displacements to the same magnitude as in case of no time-dependent changes (SD approximately 1 mm). Extended NAL can reduce systematic displacements to a minor level irrespective of the precise nature of the systematic time-dependent changes that may occur in a population.

Investigating the Visibility of Three Types of Surgical Clips as Fiducial Markers in a Breast Phantom

To facilitate online setup correction in breast radiotherapy, we studied the visibility of three types of surgical clips of differing materials using three different image verification methods. Titanium, tantalum and stainless steel clips of identical size were embedded in various orientations in the breast attachments of a tissue-equivalent phantom. Clip visibility was assessed for isocentric megavoltage, isocentric kilovoltage, and floor-mounted kilovoltage imaging. Based upon this phantom study, titanium clips are recommended for isocentric kilovoltage verification imaging, and tantalum clips are recommended for isocentric megavoltage and floor-mounted kilovoltage verification imaging.

Ultra small-mass AMS 14C sample preparation and analyses at KCCAMS/UCI Facility

We have developed techniques for accurately and precisely measuring samples containing less than a few hundred micrograms of carbon, using a compact AMS system (NEC 0.5MV 1.5SDH-1). Detailed discussions of the sample preparation, measurement setup, data analysis and background corrections for a variety of standard samples ranging from 0.002 to 1mgC are reported. Multiple aliquots of small amounts of CO2 were reduced to graphite with H2 over pre-baked iron powder catalyst. A reduction reaction temperature of 450°C was adopted for graphite samples below 0.05mgC, rather than the usual 550°C used on samples of 0.1–1mgC. In our regular reactors (∼3.1cm3), this reduction in temperature improved the graphite yield from ∼60 to 90–100% for samples ranging from 0.006–0.02mgC. The combination of lower reaction temperature with a reduced reactor volume (∼1.6cm3) gave yields as high as 100% on graphite samples <0.006mgC. High performance measurements on ultra-small samples are possible also due to a modified NEC MC-SNIC ion-source that generates C− currents of 1μA per μg of carbon for samples in the 0.002 to 0.010mgC range, combined with on-line measurement of 12C and 13C (AMS δ13C) to correct machine-induced isotopic fractionation. Source efficiencies are in excess of 10%, which enables 4–5% of the radiocarbon atoms in 0.005–0.010mgC samples to be measured. Examination of the background samples revealed two components: (a) 0.2–1μg of modern carbon and (b) 0.1–0.5μg of dead carbon. The latter component can be ignored when measuring unknown samples paired to small standards of precisely identical size (matching size normalizing standard method). Otherwise, one must make corrections for both background components. Ultra-small samples from 0.002 to 0.01mgC can be measured with accuracy and precision of a few percent, based on scatter in results for multiple aliquots of a primary standard and deviations of secondary standards from their known values.

Initial experience in treating lung cancer with helical tomotherapy

Helical tomotherapy is a new form of image-guided radiation therapy that combines features of a linear accelerator and a helical computed tomography (CT) scanner. Megavoltage CT (MVCT) data allow the verification and correction of patient setup on the couch by comparison and image registration with the kilovoltage CT multi-slice images used for treatment planning. An 84-year-old male patient with Stage III bulky non-small cell lung cancer was treated on a Hi-ART II tomotherapy unit. Daily MVCT imaging was useful for setup corrections and signaled the need to adapt the delivery plan when the patient’s anatomy changed significantly.

Implementing of the offline setup correction protocol in pelvic radiotherapy: safety margins and number of images

Background. Patient positioning errors in pelvic radiotherapy at Department of Oncology and Radiotherapy in Osijek are explored in order to establish the offline setup correction protocol and determine the safety margins. Material and methods. Film portal imaging is used during the whole treatment in order to find patient positioning errors. Eleven patients are included in the study and 420 images are analyzed. Setup errors are found by measuring distances between the center of the field and bony landmarks. Systematic and random errors are analyzed. Results. Safety margins that should be employed at our department are 11 mm, 13 mm and 14 mm in mediolateral, craniocaudal and anteroposterior direction, respectively. Time trend is found only in an aged, obese patient with a hip problem. No action level offline setup protocol was employed by taking and averaging first four images in mediolateral and craniocaudal and 5 images in anteroposterior direction. Conclusions. Since time trend is found only in a patient who was hard to position because of his age, obesity and the hip problem, we decided that such patients are to be positioned without a bellyboard and in supine position. Time trends are not found in all of the other patients so we employed the offline setup error protocol by averaging setup errors from the first few consecutive images. Safety margins that will ensure 90 % probability of depositing at least 95 % of the prescribed dose in the target are calculated. Safety margins and number of images that should be taken showed that the most inaccurate positioning was in the anteroposterior direction.