Gold Fiducial Markers Research Articles

SU-E-J-118: Quantifying Intrafractional Prostate Rotation From Cone-Beam Computed Tomography with Radiopaque Markers

Purpose: Rotation greater than one degree can severely compromise the accuracy of radiotherapy treatments. Prostate intrafractional rotation is not negligible because peristaltic motion has a time scale that is shorter than the delivery time of a single fraction. The objectives of this study were (a) to investigate a clinically feasible method to calculate tumor rotation with radiopaque markers; (b) to evaluate the accuracy of the method and (c) to quantify prostate intrafractional rotation in intensity‐modulated arc radiotherapy (IMAT). Methods: 2D kilovoltage (kV) cone‐beam computed tomography (CBCT) images were acquired for prostate cancer patients with three implanted gold fiducial markers as surrogates for the tumor position during fractionated IMAT. Post‐treatment, 3D marker trajectories were determined from the 2D projection images by maximum likelihood estimation of a 3D probability density function. Rotations about each axis (roll (right/left), pitch (superior/inferior) and yaw (anterior/posterior)) were then calculated based on the iterative closest point algorithm (ICP) method, with the origin set at the centroid of the three markers. Results: Intrafractional rotation was studied for10 patients, 268 fractions. Average roll, pitch and yaw were calculated as 0.85±1.16, 0.41±0.51 and 0.37±0.4 degrees, respectively. Roll was the predominant rotation among the three axes which is consistent with literature. Rotations showed a similar pattern among different fractions for the same patient. The pattern acquired at the first few fractions could be used as a prediction for the rest of the fractions. Conclusion: This study demonstrated a novel method to quantify intrafractional tumor rotation from 2D kV projections with radiopaque markers. Intrafractional rotation is small but significant for certain cases. Real‐time radiopaque marker‐based adaptive radiotherapy with rotation correction could make a positive impact on reducing treatment margins. This project is supported by an NHMRC Australia Fellowship and NHMRC project grant 1034060.

Stereotactic hypofracionated radiotherapy for prostate cancer patients.

e16097 Background: 3D conformal radiotherapy (78Gy in 2Gy per fraction) is the standard treatment for prostate cancer patients. It is a very long treatment, not comfortable both for patients and radiotherapy departments. Stereotactic radiotherapy has recently been recognized as an alternative technique. Prostate cancer has probably a low α/β ratio (1.5Gy) and 33.5Gy in 5 fractions has the predicted efficacy of 78Gy in 39 fractions. The objective of the study was to investigate the effectiveness and safety of stereotactic radiotherapy for localized prostate cancer. Methods: This is a prospective one-center clinical study. The treatment was done with IMRT. PTV received 33.5Gy in 5 fractions twice a week. Three gold fiducial markers were placed in the prostate under transrectal ultrasound guidance for image-guided positioning and motion tracking. Treatment planning CT and MRI scans were performed. IMRT plans were done. Before each treatment MV Cone Beam CT and portal imagines for markers were performed. Results: In this analysis 32 patients were included (age 65-83 years, median 73 years) with localized low- and intermediate-risk (according to NCCN) adenocarcinoma of prostate T2-T3N0M0. Combined Gleason scores were 5 to 7, the prostate-specific antigen (PSA) was 4.7-20.0ng/ml (median 10.2ng/ml). The median prostate volume calculated during treatment planning (on CT and MR) was 40.5cc (15.8cc to 113.6cc). Acute genitourinary toxicity Grade 1–2 was observed in about 50% patients and only Grade 1 acute gastrointestinal toxicity in 25% patients. No Grade 3 or higher toxicity was reported. Only 3 patients reported rectal bleeding after radiotherapy (Grade 1 late toxicity). PSA level were systematically decreased to 1.08ng/ml 3 months, 0.91ng/ml 6 months and 0.56ng/ml 9 months after treatment end. Conclusions: Stereotactic hypofractionated radiotherapy for prostate cancer patients is an effective and safe treatment in short term analysis.

Prostate and Patient Intrafraction Motion: Impact on Treatment Time-Dependent Planning Margins for Patients With Endorectal Balloon

To investigate intrafraction prostate and patient motion during different radiation therapy treatments as a function of treatment time; included were prostate patients with an endorectal balloon (ERB). Margins accounting for setup uncertainties and intrafraction motion were determined. The study included 17 patients undergoing prostate cancer radiation therapy. All patients received 3 fiducial gold markers implanted in the prostate and were then immobilized in the supine position with a knee support and treated with an ERB. Twelve patients with intermediate risk for pelvic lymph node metastases received intensity modulated radiation therapy (IMRT), and 5 patients at low risk received a 4-field box treatment. After setup based on skin marks, patients were imaged with a stereoscopic imaging system. If the marker displacement exceeded a 3-mm tolerance relative to planning computed tomography, patients were shifted and verification images were taken. All patients underwent additional imaging after treatment; IMRT patients also received additional imaging at halftime of treatment. Prostate and bone drifts were evaluated as a function of treatment time for more than 600 fractions, and margins were extracted. Patient motion evaluated by bone match was strongly patient dependent but in general was smallest in the superior-inferior (SI) direction. Prostate drifts were less patient dependent, showing an increase with treatment time in the SI and anterior-posterior (AP) directions. In the lateral (LAT) direction, the prostate stayed rather stable. Mean treatment times were 5.5 minutes for 4-field box, 10 minutes for 5-field boost IMRT, and 15 minutes or more for 9-field boost and 9-field pelvic IMRT treatments. Margins resulted in 2.2 mm, 3.9 mm, and 4.3 mm for 4-field box; 3.7 mm, 2.6 mm, and 3.6 mm for 5-field boost IMRT; 2.3 mm, 3.9 mm, and 6.2 mm for 9-field boost IMRT; and 4.2 mm, 5.1 mm, and 6.6 mm for 9-field pelvic IMRT in the LAT, SI, and AP directions, respectively. Intrafraction prostate and patient displacement increased with treatment time, showing different behaviors for the single directions of movement. Repositioning of the patients during long treatments or shorter treatment times will be necessary to further reduce the treatment margin.

Use of Implanted Gold Fiducial Markers With MV-CBCT Image Guided IMRT for Pancreatic Tumors

Use of Implanted Gold Fiducial Markers With MV-CBCT Image Guided IMRT for Pancreatic Tumors

Patterns of intrafractional motion and uncertainties of treatment setup reference systems in accelerated partial breast irradiation for right- and left-sided breast cancer

PurposeThis study investigated the patterns of intrafractional motion and accuracy of treatment setup strategies in 3-dimensional conformal radiation therapy of accelerated partial breast irradiation (APBI) for right- and left-sided breast cancers. Methods and MaterialsSixteen right-sided and 17 left-sided breast cancer patients were enrolled in an institutional APBI trial in which gold fiducial markers were strategically sutured to the surgical cavity walls. Daily pre- and postradiation therapy kV imaging were performed and were matched to digitally reconstructed radiographs based on bony anatomy and fiducial markers, respectively, to determine the intrafractional motion. The positioning differences of the laser-tattoo and the bony anatomy-based setups with respect to the marker-based setup (benchmark) were determined to evaluate their accuracy. ResultsStatistical differences were found between the right- and left-sided APBI treatments in vector directions of intrafractional motion and treatment setup errors in the reference systems, but less in their overall magnitudes. The directional difference was more pronounced in the lateral direction. It was found that the intrafractional motion and setup reference systems tended to deviate in the right direction for the right-sided breast treatments and in the left direction for the left-sided breast treatments. ConclusionsIt appears that the fiducial markers placed in the seroma cavity exhibit side dependent directional intrafractional motion, although additional data may be needed to further validate the conclusion. The bony anatomy-based treatment setup improves the accuracy over laser-tattoo. But it is inadequate to rely on bony anatomy to assess intrafractional target motion in both magnitude and direction.

Tracking the dynamic seroma cavity using fiducial markers in patients treated with accelerated partial breast irradiation using 3D conformal radiotherapy

The purpose of the present study was to perform an analysis of the changes in the dynamic seroma cavity based on fiducial markers in early stage breast cancer patients treated with accelerated partial breast irradiation (APBI) using three-dimensional conformal external beam radiotherapy (3D-CRT). A prospective, single arm trial was designed to investigate the utility of gold fiducial markers in image guided APBI using 3D-CRT. At the time of lumpectomy, four to six suture-type gold fiducial markers were sutured to the walls of the cavity. Patients were treated with a fractionation scheme consisting of 15 fractions with a fractional dose of 333 cGy. Treatment design and planning followed NSABP∕RTOG B-39 guidelines. During radiation treatment, daily kV imaging was performed and the markers were localized and tracked. The change in distance between fiducial markers was analyzed based on the planning CT and daily kV images. Thirty-four patients were simulated at an average of 28 days after surgery, and started the treatment on an average of 39 days after surgery. The average intermarker distance (AiMD) between fiducial markers was strongly correlated to seroma volume. The average reduction in AiMD was 19.1% (range 0.0%-41.4%) and 10.8% (range 0.0%-35.6%) for all the patients between simulation and completion of radiotherapy, and between simulation and beginning of radiotherapy, respectively. The change of AiMD fits an exponential function with a half-life of seroma shrinkage. The average half-life for seroma shrinkage was 15 days. After accounting for the reduction which started to occur after surgery through CT simulation and treatment, radiation was found to have minimal impact on the distance change over the treatment course. Using the marker distance change as a surrogate for seroma volume, it appears that the seroma cavity experiences an exponential reduction in size. The change in seroma size has implications in the size of the CTV, PTV, and percent of normal breast tissue irradiated when using 3D-CRT.

The Value of Repeat Cone Beam CT Scans in Image Guided Radiation Therapy for Prostate Cancer

The Value of Repeat Cone Beam CT Scans in Image Guided Radiation Therapy for Prostate Cancer

Fiducial Markers (FM) in the Prostate Bed for Postprostatectomy External Beam Radiation Therapy: Improved Accuracy as Compared to the Use of Surgical Clips

With the use of intensity modulated radiation therapy (IMRT) and dose escalation in post prostatectomy external beam radiation therapy (EBRT), daily target reproducibility has become more important to ensure adequate planning target volume (PTV) coverage while reducing the dose to the organs at risk (OAR). The aim of this study is to assess the reliability of daily matching with gold fiducial markers (FM) implanted in the prostatic bed as compared to the usual matching with surgical clips. Three to four gold FM were implanted transrectally in the prostate bed of 10 patients for either adjuvant or salvage EBRT. The stability of the FM' position (FM fixity) was analyzed for nine treatments from the first to the last week during a course of EBRT. We then performed for each patient a match with the surgical clips and compared both matches, taking the FM as the gold standard. Couch shifts required to align FM and surgical clips on digitally reconstructed radiograph (DRR) were analyzed using an in-house program. All the matches were performed by two experienced radiation therapy technologists. The variation in matching for clips and FM between the two technologists was then compared. FM fixity was very good. The mean FM migration during a course of EBRT was 1.1mm (SD ±0.8mm). The mean difference when comparing FM and surgical clips on the same kV image was clinically significant (mean 2.6 mm ± 1.5mm). Matches with surgical clips, compared to FM as a gold standard, showed a mismatch ≥ 2mm in 64%. The mismatch >2mm was on average 3.5 mm ± 1.2 mm. There was less inter-observer variability for FM (0.9 mm ± 0.7 mm) as compared to surgical clips (1.8 mm ± 1.3 mm). There was less variability between technologists for matching with FM (>2mm in 4%) vs. surgical clips (>2mm in 36%). FM in the prostate bed remains stable throughout an entire course of EBRT. FM are superior to surgical clips in showing less inter-observer variability in matching and less variation in position than surgical clips . Because of the superior precision, FM could ultimately help in reducing treatment margins for dose-escalation.

Treatment Dose Verification in Liver SBRT Using a Deformable Respiratory Motion Model and Treatment Beam's-Eye-View Fluoroscopy With Tumor Tracking

• A patient with hepatocellular carcinoma was treated with SBRT 40Gy/4fx. Gold fiducial markers were implanted to mark the tumor location. • 4D-CT was used for ITV definition/planning and modeling of deformable motion between respiratory phases. • In the TPS, beams were applied to each phase’s 3DCT. Each resulting dose distribution was spatially transformed via the motion model onto a common anatomical space (planning CT). • MVF with fiducial tracking yielded tumor motion during radiation delivery. Motion analysis determined the duty cycle for each respiratory phase. A duty cycle weighted average of the deformed dose distributions yielded the composite delivered dose distribution. • Breathing variability in SBRT causes deviation of delivered dose distribution from the planned dose. • The goal was to calculate the actual dose delivered to a moving tumor using measurements of implanted fiducial marker position obtained using beam’s-eye-view MV fluoroscopy (MVF).

Comparison of Cone Beam CT and kV/kV Imaging Using Fiducial Marker for PTV margin Definition in IGRT of Localized Prostate Cancer

Purpose/Objective(s): Image-guided radiation therapy (IGRT) using fiducial markers (FM) or cone beam CT (CBCT) is the preferred method in delivering RT for prostate cancer (PC) enabling precise targeting and dose escalation. These techniques should enable reduced CTV to PTV margins resulting in lower toxicity. We applied the Van Herk formula to calculate the margin for PTV of prostate using CBCT or FM. Materials/Methods: Twenty patients with low, intermediate or high risk PC received IGRT (VMAT) to a dose of 74Gy/37f, 60Gy/20f or 57Gy/ 19f. Three gold FM were implanted prior to treatment. Patients were planned and treated with rectal balloon (RB). CBCT was performed on day 1 to 3, then once every week. KV-image pairs (kV/kV) were used on all other days. For 10 patients CBCT was performed after each fraction to assess intra-fractional motion. Contouring of the prostate (P) for low risk and of the prostate/seminal vesicles (PSV) for intermediate/high risk PC was performed independently by 3 clinicians and mean differences in vertical, lateral and longitudinal directions between the intersections and the unions of the three contours were assessed. The Van Herk formula M Z 2.5S + 0.7s was used for PTV margin calculation, where S represents the systematic and s the random error. We calculated the systematic errors for intra-fractional motion, contouring as well as setup using CBCT, kV/kV and FM referring to prostate position in the CBCT and random errors for intra-fractional motion and patient setup. Results: Calculated PTV margins were 6.1 mm vertically, 7.1 mm longitudinally and 4.4 mm laterally for contouring and intra-fractional but not setup errors. Systematic error for contouring was more influential than intra-fractional motion (0.9-1.4mm) and more pronounced in intermediate/high risk (2.5-3.2mm) than in low risk PC (1.3-2.1mm). Including setup errors, enlargement was least for CBCT and kV/kVimaging using FM: +0.4-0.6mm (P) / +2.0-2.3mm (PSV) vertically, +0.1-0.7mm (P) / +2.5-2.9mm (PSV) longitudinally and +0.3mm (P) / +3.0-3.1mm (PSV) laterally. Higher increases were noticed for bone matching: +1.8-2.0mm (P) / +2.2-2.4mm (PSV) vertically, +1.4-1.5mm (P) / +3.6mm (PSV) longitudinally and +0.4-0.7mm (P) / +3.0-3.3mm (PSV) laterally. Largest increases were found for rectal balloon matching: +1.3-1.6mm (P) / +2.8-3.0mm (PSV) vertically, +0.9-4.5mm (P) / +3.1-6.2mm (PSV) longitudinally and +0.5-2.0mm (P) / +3.1-4.1mm (PSV) laterally. Conclusion: Calculated PTV margins are comparable using either CBCT or kV/kV-imaging FM matching. The systematic error for contouring was more influential than intra-fractional motion. Margins for intermediate/ high risk PC are notably larger than those for low risk PC due to seminal vesicles. Author Disclosure: C. Oehler: None. S. Lang: None. P. Dimmerling: None. C. Bolesch: None. A. Tini: None. C. Glanzmann: None. U.M. Luetolf: None. Y. Najafi: None. G. Studer: None. D.R. Zwahlen: None.

Intrafractional Tracking Accuracy in Surrogate Signal-based Dynamic Tumor Tracking Irradiation With 4DRT

We have developed an innovative four-dimensional (4D) image-guided radiation therapy system, and clinically applied its dynamic tumor-tracking (DTT) irradiation to patients with lung cancer since September 2011. 4DRT swings its gimbaled 6 MV x-ray head along the pan and tilt direction to track a moving tumor. The purpose of this study was to verify the intra-fractional tracking accuracy in DTT irradiation with 4DRT. Four lung cancer patients who had undergone the DTT irradiation were included in this study. The peak-to-peak amplitudes of lung tumor motions were in range of 0.6 to 5.5 mm in left-right (LR), of 10.2 to 26.0 mm in cranio-caudal (CC), and of 2.0 to 5.4 mm in anterior-posterior (AP) direction. Patient-specific tracking margins of 5 to 8 mm were anisotoripically added to compensate the uncertainties of DTT irradiation. 4DRT dynamically tracks a moving tumor by predicting its future position with a correlation model (4D-model) between external surrogate signals of the abdominal wall and tumor positions indicated by implanted fiducial gold markers. During beam delivery, tumor and fiducial markers' position were monitored by two orthogonal kV x-ray imaging subsystem every 1 s. Meanwhile, the predicted and tracked positions of the tumor were recorded on the log files every 5 ms. Intra-fractional tracking accuracy was verified by the detected tumor position on the fluoroscopy images and the corresponding predicted and tracked tumor positions calculated from the synchronized log files. The tracked tumor position was derived from the intersection of the tracking orientation of the gimbaled head with the tracked tumor plane. The tracked tumor plane was the perpendicular plane to the gantry angle of each port at the depth of the tracked tumor. The difference between the predicted and detected tumor positions was defined as the prediction error (EP), and the difference between the tracked and predicted tumor positions as mechanical error (EM), and the difference between the tracked and detected tumor positions as the total tracking system error (ET). Averaged 99th percentiles [range] of the EM, EP, and ET were (LR, CC, AP) = (0.2 mm [0.1-0.4 mm], 0.7 mm [0.5-1.2 mm], 0.2 mm [0.1-0.3 mm]), (1.3 mm [0.7-2.7 mm], 3.9 mm [2.9-5.5 mm], 1.6 mm [0.9-2.0 mm]), and (1.6 mm [1.1-2.8 mm], 3.4 mm [2.5-4.9 mm], 1.3 mm [1.0-1.5 mm]), respectively. The intrafractional tracking errors were fully ensured by the tracking margin. We demonstrated that DTT irradiation with 4DRT can substantially reduce the impacts of respiratory motion with high accuracy. The prediction error mainly caused the total tracking system error while the mechanical error was negligible.

Evaluation of Gold Fiducial Marker Migration After Implantation in Prostate Image Guided Radiation Therapy (IGRT)

Evaluation of Gold Fiducial Marker Migration After Implantation in Prostate Image Guided Radiation Therapy (IGRT)

Mediastinal lymph node position variability in non-small cell lung cancer patients treated with radical irradiation

Background and purposeDetailed knowledge on mediastinal lymph nodes position variability is lacking. In this study we quantified the variability over the irradiation course in non-small cell lung cancer (NSCLC) patients. MethodsA 0.35×5mm gold fiducial marker was inserted in mediastinal lymph nodes of 14 stage III NSCLC patients. A respiration correlated 4D-planning-CT (pCT) and daily 4D-Cone Beam (CB)CT-scans were acquired. To calculate the systematic and random baseline variations, and respiratory motion variability of the lymph nodes, all CBCT scans were registered to both the bony anatomy and marker in the pCT. Patient population statistics of the peak-to-peak amplitude and time averaged mean position relative to the bony anatomy were calculated. ResultsThe average peak-to-peak amplitude was 0.21cm, 0.52cm and 0.20cm in the Left–Right, Cranial–Caudal and Anterior–Posterior direction respectively, while the amplitude variability was ⩽0.1cm in each direction. Inter-fraction lymph node baseline variation was 0.21/0.2cm, 0.34/0.23cm, and 0.17/0.15cm systematic/random. PTV margins for these variations were 0.92cm, 1.24cm, 0.82cm for an online bone match and could be reduced to 0.77cm, 0.82cm and 0.86cm for an online carina match. ConclusionsSubstantial and anisotropic, systematic and random mediastinal lymph node baseline variations were found in NSCLC patients indicating that non-uniform margins could be beneficial.

Endoscopic ultrasound-guided gold fiducial marker placement for intraoperative identification of insulinoma

Insulinomas are the most common functioning pancreatic neuroendocrine neoplasm, comprising 30%–45% of these tumors. Although laparoscopic resection is safe, minimally invasive, and is associated with shorter length of hospital stay, tumor localization at surgery can be challenging. We describe endoscopic ultrasound (EUS)-guided fiducial placement as a new technique for intraoperative localization of pancreatic insulinoma. A 36-year-old woman with episodes of confusion that resolved with intake of glucose and whose laboratory tests were suggestive of insulinoma was referred for laparoscopic distal pancreatectomy. Linear-array EUS (GF-UCT 240; Olympus Corp., Center Valley, Pennsylvania, USA) confirmed a tail mass (Fig.1). After retracting the stylet of the fine-needle aspiration (FNA) needle (Expect 19-gauge Flex needle; Boston Scientific Corp., Natick, Massachusetts, USA) by 2cm, one gold fiducial (3 × 0.8mm; Best Medical International, Springfield, Virginia, USA) was back-loaded into the lumen of the needle and sealed with bone wax. At EUS, the fiducials were deployed within the tumor by advancing the stylet forward (Fig.2). Overall, two fiducials were deployed (Fig.3), and a preoperative computed tomography (CT) scan confirmed their position. At laparoscopy, the fiducials were identified using cross-table fluoroscopy (Fig.4), and distal pancreatectomy with splenectomy was performed. A frozen section confirmed negative tumor margins, and the explant specimen revealed the tumor with fiducials in place (Fig.5). Final pathological analysis revealed T1 N0 grade II pancreatic neuroendocrine neoplasm (Fig.6). Fig.1 Linear-array endoscopic ultrasound (EUS) image of the isoechoic mass lesion in the tail of pancreas, superior to the left kidney. Fig.2 Fluoroscopic image showing the linear-array echoendoscope with the 19-gauge fine-needle aspiration (FNA) needle and the first fiducial. Fig.3 Endoscopic ultrasound (EUS) image showing the markers within the lesion casting an acoustic shadow (arrow). Fig.4 Intraoperative fluoroscopic image with the stapler gun in place. The two fiducial markers can be easily visualized. Fig.5 Postoperative specimen of the pancreas, showing the fiducial marker within the lesion. Fig.6 Postoperative histological specimen showing uniform sheets of polyhedral cells with hyperchromatic nuclei and abundant eosinophilic cytoplasm, consistent with a neuroendocrine neoplasm (hematoxylin and eosin stain, ×40). Preoperative injection of India ink [1] or indocyanine green [2] under EUS guidance was developed to facilitate quick tumor localization and to decrease operative time and blood transfusion requirements [3]. However, the disadvantages of dyes include peritonitis, infection, allergic reactions, and reabsorption within tissue planes [4]. In this report, we have described the use of fiducials as an alternative technique for this indication. The fiducials are inexpensive ($80 for five fiducials), easy to deploy, can be identified readily using fluoroscopy or intraoperative ultrasound, and (unlike dyes) do not extravasate into surrounding tissue. Endoscopy_UCTN_Code_TTT_1AS_2AD

Clinical outcomes of image guided radiation therapy (IGRT) with gold fiducial vaginal cuff markers for high-risk endometrial cancer

Objective. To report two year clinical outcomes of image guided radiation therapy (IGRT) to the vaginal cuff and pelvic lymph nodes in a series of high-risk endometrial cancer patients. Methods. Twenty-six consecutive high-risk endometrial cancer patients requiring adjuvant radiation to the vaginal cuff and regional lymph nodes were treated with vaginal cuff fiducial-based IGRT. Seventeen (65%) received sequential chemotherapy, most commonly with a sandwich technique. Brachytherapy followed external radiation in 11 patients to a median dose of 18 Gy in 3 fractions. The median external beam dose delivered was 47.5 Gy in 25 fractions. Results. All 656 fractions were successfully imaged and treated. The median overall translational shift required for correction was 9.1 mm (standard deviation, 5.2 mm) relative to clinical set-up with skin tattoos. Shifts of 1 cm, 1.5 cm, and 2 cm or greater were performed in 43%, 14%, and 4% of patients, respectively. Acute grade 2 gastrointestinal (GI) toxicity occurred in eight patients (30%) and grade 3 toxicity occurred in one. At two years, there have been no local or regional failures and actuarial overall survival is 95%. Conclusion. Daily image guidance for high-risk endometrial cancer results in a low incidence of acute GI/genitourinary (GU) toxicity with uncompromised tumor control at two years. Vaginal cuff translations can be substantial and may possibly result in underdosing if not properly considered.

Determination of optimal fiducial marker across image‐guided radiation therapy (IGRT) modalities: visibility and artifact analysis of gold, carbon, and polymer fiducial markers

The purpose of this study was to evaluate the visibility and artifact created by gold, carbon, and polymer fiducial markers in a simple phantom across computed tomography (CT), kilovoltage (kV), and megavoltage (MV) linear accelerator imaging and MV tomotherapy imaging. Three types of fiducial markers (gold, carbon, and polymer) were investigated for their visibility and artifacts in images acquired with various modalities and with different imaging parameters (kV, mAs, slice thickness). The imaging modalities include kV CT, 2D linac‐based kilovoltage and megavoltage X‐ray imaging systems, kV cone‐beam CT, and normal and fine tomotherapy imaging. The images were acquired on a phantom constructed using Superflab bolus in which markers of each type were inserted into the center layer. The visibility and artifacts produced by each marker were assessed qualitatively and quantitatively. All tested markers could be identified clearly on the acquired CT and linac‐based kV images; gold markers demonstrated the highest contrast. On the CT images, gold markers produced a significant artifact, while no artifacts were observed with polymer markers. Only gold markers were visible when using linac‐based MV and tomotherapy imaging. For linac‐based kV images, the contrast increased with kV and mAs values for all the markers, with the gold being the most pronounced. On CT images, the contrast increased with kV for the gold markers, while decreasing for the polymer and carbon marker. With the bolus phantom used, we found that when kV imaging‐based treatment verification equipment is available, polymer and carbon markers may be the preferred choice for target localization and patient treatment positioning verification due to less image artifacts. If MV imaging will be the sole modality for positioning verification, it may be necessary to use gold markers despite the artifacts they create on the simulation CT images.PACS number: 87

SU-E-J-138: Fast 2-D Fiducial Marker Detection on Sequential MV Projections in Arc Therapy

To automatically detect intrafraction motion during arc radiotherapy for prostate cancer patients by tracking fiducial markers in two-dimensional MV images acquired using the treatment beam, in order to adjust radiation dose accordingly. Four fiducial gold markers are implanted in a patient's prostate. Patients are irradiated using a Varian Linac 2100 C/D with RapidArc upgrade (Varian Medical Systems, Palo Alto, CA). MV images (1024 × 768 pixels, 0.392 × 0.392 mm2 pixel size) acquired during a 360 degree gantry rotation at a one second interval (5 degrees) are preprocessed by subtracting a smoothed version of the image to retain only high image frequencies. Edge detection is then applied, followed by a one pixel wide dilation and erosion to transform the edges into contiguous regions. Next, our method searches the centers of visible markers (i.e. not covered by the MLC), constrained by marker estimates from the planning CT. This is done by finding all contiguous regions and maximizing a marker-region distance criterion for every visible marker. A two-dimensional estimate correction over consecutive projections is also implemented to improve marker estimates during gantry rotation. We applied our method on four treatment fractions of the same patient. As such, a total of 191 projections with manually indicated marker ends as ground truth were used as validation. Markers were indicated twice on all images, to include observer errors. Results show a mean detection error of less than 0.5 mm in the projection image (standard deviation 0.6 mm), with an execution time of less than one second per image in matlab. Undetected markers and false positives mostly occurred at moving leaf boundaries, where marker visibility was determined by the observer. Preliminary findings demonstrate that this method can be used to detect intrafraction motion during arc radiotherapy by only using projected MV images. Research sponsored by Varian Medical Systems, Palo Alto, CA.

SU-E-J-66: Evaluation of Proton Induced X-Ray Fluorescence from Gold Fiducial Markers for In-Vivo Determination of Proton Range and Energy.

To evaluate a method for in-vivo determination of proton range and post-Bragg peak straggling by detection of proton induced x-ray fluorescence of markers placed at known locations. Therapeutic beams from the UF Proton Therapy Institute were used to excite proton-induced x-ray fluorescence emission (PIXE) from cylindrical pure gold fiducial markers. The markers were embedded in a homogeneous water phantom and PIXE was measured using NaI photodetectors with energy dispersive spectral analysis. The geometry of the phantom and marker placement was chosen to model parallel-opposed beam treatment of prostate cancer by proton therapy. The fluorescence yield from these markers was further modeled using the GEANT4 Monte-Carlo package with low-energy corrections. Gold K and L shell fluorescence yield as determined by the GEANT4 simulations was verified quantitatively by comparison to measured Au yield at energies from 1 MeV to 68 MeV, and to semiempirical model calculations covering the energy range from 1 MeV to 15 MeV. The Au K-shell fluorescence cross section is significantly smaller thanthat of the L-shell, but the higher yield of the L-shell fluorescence isoffset by the larger absorption as the x-ray exits the phantom. The overallrelative detection efficiency of K and L shell fluorescence depends on thedetails of the shape of the phantom and location of the marker. Acharacteristic shape of fluorescence yield as it depends on proton range isfound, which can be used to extract an in-vivo PDD profile of a spread-outBragg peak (SOBP). A combination of a specific protocol for delivering a SOBP, geometriclocation of fiducial markers from CT, and simultaneous detection of protoninduced x-ray fluorescence, can determine the depth range of a primary protonbeam in-vivo. The fluorescence yield as measured at the Proton Therapy Institute is easily distinguished from background radiation.

SU-E-J-174: ITV Variations as a Function of CT Geometry and Scan Time : A Simulation Study Using Patient Data.

The internal target volume (ITV) accounts for uncertainties in tumor position and shape and is defined from images acquired with 4DCT. In this work, the locations of gold fiducial markers implanted in lung tumors of seven patients are used to represent tumor motion and investigate the role of the CT scanner geometry and scan duration on ITV definition. All of the simulations are geometric simulations performed in MATLAB (The Mathworks, Nattick,MA) and variations in image reconstruction are not considered. The CT scanner geometries simulated include a 4-slice CT scanner (with an axial field of view of 10 mm) and an 'ideal' CT scanner geometry that covers all potential tumor positions in the same bed position (axial field of view of 100 mm). The scan durations were varied from 6 seconds per bed position to 30 seconds per bed position. For all of the simulations, the tumor was modeled as a sphere of diameter 25 mm. The ITVs were compared by calculating the mean ITV for each patient and scan parameters. For all of the patients, the mean ITV increased as the scan duration increased. The maximum increase in mean ITV was 25%. This work highlights the potential benefit of modified CT protocols to determine the ITV. One example of a modified protocol would be to increase the scan duration (to decrease ITV uncertainties) while decreasing the mA of the scanner (to avoid giving extra radiation dose). The largest factor influencing the ITV is the scan duration (and not the scanner geometry).

Impact of Concurrent Androgen Deprivation on Fiducial Marker Migration in External-beam Radiation Therapy for Prostate Cancer

To determine the extent of gold fiducial marker (FM) migration in patients treated for prostate cancer with concurrent androgen deprivation and external-beam radiation therapy (EBRT). Three or 4 gold FMs were implanted in 37 patients with prostate adenocarcinoma receiving androgen deprivation therapy (ADT) in conjunction with 70-78 Gy. Androgen deprivation therapy was started a median of 3.9 months before EBRT (range, 0.3-12.5 months). To establish the extent of FM migration, the distance between each FM was calculated for 5-8 treatments once per week throughout the EBRT course. For each treatment, the distance between FMs was compared with the distance from the digitally reconstructed radiographs generated from the planning CT. A total of 281 treatments were analyzed. The average daily migration was 0.8 ± 0.3 mm, with distances ranging from 0.2 mm-2.6 mm. Two of the 281 assessed treatments (0.7%) showed migrations >2 mm. No correlation between FM migration and patient weight or time delay between ADT and start of EBRT was found. There was no correlation between the extent of FM migration and prostate volume. This is the largest report of implanted FM migration in patients receiving concomitant ADT. Only 0.7% of the 281 treatments studied had significant marker migrations (>2 mm) throughout the course of EBRT. Consequently, the use of implanted FMs in these patients enables accurate monitoring of prostate gland position during treatment.