Feasibility of electromagnetic transponder use to monitor inter- and intrafractional motion in locally advanced pancreatic cancer patients

A Prospective Study on Deep Inspiration Breath Hold Thoracic Radiation Therapy Guided by Bronchoscopically Implanted Electromagnetic Transponders

BACKGROUND

Electromagnetic transponders bronchoscopically implanted near the tumor can be used to monitor deep inspiration breath hold (DIBH) for thoracic radiation therapy (RT). The feasibility and safety of this approach require further study.

METHODS

We enrolled patients with primary lung cancer or lung metastases. Three transponders were implanted near the tumor, followed by simulation with DIBH, free breathing, and 4D-CT as backup. The initial gating window for treatment was ±5 mm; in a second cohort, the window was incrementally reduced to determine the smallest feasible gating window. The primary endpoint was feasibility, defined as completion of RT using transponder-guided DIBH. Patients were followed for assessment of transponder- and RT-related toxicity.

RESULTS

We enrolled 48 patients (35 with primary lung cancer and 13 with lung metastases). The median distance of transponders to tumor was 1.6 cm (IQR 0.6-2.8 cm). RT delivery ranged from 3 to 35 fractions. Transponder-guided DIBH was feasible in all but two patients (96% feasible), where it failed because the distance between the transponders and the antenna was >19 cm. Among the remaining 46 patients, 6 were treated prone to keep the transponders within 19 cm of the antenna, and 40 were treated supine. The smallest feasible gating window was identified as ±3 mm. Thirty-nine (85%) patients completed one year of follow-up. Toxicities at least possibly related to transponders or the implantation procedure were grade 2 in six patients (six incidences, cough and hemoptysis), grade 3 in three patients (five incidences, cough, dyspnea, pneumonia, and supraventricular tachycardia), and grade 4 pneumonia in one patient (occurring a few days after implantation but recovered fully and completed RT). Toxicities at least possibly related to RT were grade 2 in 18 patients (41 incidences, most commonly cough, fatigue, and pneumonitis) and grade 3 in four patients (seven incidences, most commonly pneumonia), and no patients had grade 4 or higher toxicity.

CONCLUSIONS

Bronchoscopically implanted electromagnetic transponder-guided DIBH lung RT is feasible and safe, allowing for precise tumor targeting and reduced normal tissue exposure. Transponder-antenna distance was the most common challenge due to a limited antenna range, which could sometimes be circumvented by prone positioning.

The first internal electromagnetic motion monitoring implementation for stereotactic liver radiotherapy in China: procedures and preliminary results

BACKGROUND

Respiratory motion may compromise the dose delivery accuracy in liver stereotactic body radiation therapy (SBRT). Motion management can improve treatment delivery. However, external surrogate signal may be unstable and inaccurate. This study reports the first case of liver SBRT based on internal electromagnetic motion monitoring (Calypso, Varian Medical Systems, USA) in China.

MATERIALS AND METHODS

The patient with a primary liver cancer was treated with respiratory-gated SBRT guided by three implanted electromagnetic transponders. The treatment was carried out in breath-hold end-exhale with beam-on when the centroid of the three transponders drifted within 5 mm (left-right (LR), anterior-posterior (AP) and cranio-caudal (CC) directions) from the planned position. The motion monitoring treatments were delivered in breath-hold end-exhale mode with the energy of 6 MV in FFF mode with 1200 monitor units (MU) per minute. For each fraction, QA results, intertransponder distances, geometric checks as well as tumor motion logs were explicitly recorded.

RESULTS

Comparing with the plan data, distance variances between each two transponders were - 0.56 ± 0.32 mm, 0.17 ± 0.33 mm and - 0.82 ± 0.68 mm. Geometric residual, the pitch, roll and yaw angles were 0.48 ± 0.21 mm (threshold 2.0 mm), 2.17° ± 1.85° (threshold 10°), - 2.42° ± 1.51° (threshold 10°) and 1.67° ± 1.07° (threshold 10°), respectively. The delivery time of the five fields were 13.8 s, 13.1 s, 11.2 s, 11.6 s, and 11.6 s with the average value of 12.3 ± 1.1 s. Treatment duration of each fraction ranged from 6.2 to 21.4 min, with the average value of 11.3 ± 5.0 min.

CONCLUSIONS

The first case of liver SBRT patient of China based on internal electromagnetic motion monitoring was performed. The system had a high tracking accuracy, and it did not delay the treatment time. In addition, the patient did not show any severe side effects except for grade I myelotoxicity. The internal electromagnetic motion monitoring system provides a real-time and direct way to track liver tumor targets.

Clinical experience with lung-specific electromagnetic transponders for real-time tumor tracking in lung stereotactic body radiotherapy

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7 patients were implanted with lung-specific electromagnetic transponders (EMT).

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We report no complications from implantation and no migration of the EMT.

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7 non-small cell lung cancer patients underwent SBRT using EMT real-time tracking.

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SBRT was delivered in free-breathing (FB) or in deep inspiration breath-hold (DIBH).

Background and purposes

Motion management is crucial for optimal stereotactic body radiotherapy (SBRT) of moving targets. We aimed to describe our clinical experience with real-time tracking of lung-specific electromagnetic transponders (EMTs) for SBRT of early stage non-small cell lung cancer in free-breathing (FB) or deep inspiration breath-hold (DIBH).

Material and methods

Seven patients were implanted with EMTs. Simulation for SBRT was performed in FB and in DIBH. We prescribed 60 Gy in 3, 5 or 8 fractions to the tumor and delivered SBRT with volumetric modulated arcs and a 6 MV flattening filter free photon beam. Patients’ setup at the linac was performed using EMT positions and cone-beam CT (CBCT) verification. Four patients were treated in DIBH because of a dosimetric benefit. We analysed patient alignment and treatment delivery parameters using DIBH or FB and EMT real-time tracking.

Results

There were no complications from the EMT implantation. Visual inspection of CBCT before and/or after SBRT revealed good alignment of structures and EMTs. The median setup time was 9.8 min (range: 4.6–34.1 min) and the median session time was 14.7 min (range: 7.3–36.5 min). EMT positions in lungs remained stable during overall treatment and allowed real-time tracking both in FB and in DIBH SBRT. The treatment beam was gated when EMT centroid position exceeded tolerance thresholds ensuring correct delivery of radiation to the tumor.

Conclusion

Using EMTs for real-time tracking of tumor motion during lung SBRT proved to be safe, accurate and easy to integrate clinically for treatments in FB or DIBH.

Accuracy assessment of target tracking using two 5-degrees-of-freedom wireless transponders

Purpose

Surgical navigation systems are generally only applied for targets in rigid areas. For non-rigid areas, real-time tumor tracking can be included to compensate for anatomical changes. The only clinically cleared system using a wireless electromagnetic tracking technique is the Calypso^® System (Varian Medical Systems Inc., USA), designed for radiotherapy. It is limited to tracking maximally three wireless 5-degrees-of-freedom (DOF) transponders, all used for tumor tracking. For surgical navigation, a surgical tool has to be tracked as well. In this study, we evaluated whether accurate 6DOF tumor tracking is possible using only two 5DOF transponders, leaving one transponder to track a tool.

Methods

Two methods were defined to derive 6DOF information out of two 5DOF transponders. The first method uses the vector information of both transponders (TTV), and the second method combines the vector information of one transponder with the distance vector between the transponders (OTV). The accuracy of tracking a rotating object was assessed for each method mimicking clinically relevant and worst-case configurations. Accuracy was compared to using all three transponders to derive 6DOF (Default method). An optical tracking system was used as a reference for accuracy.

Results

The TTV method performed best and was as accurate as the Default method for almost all transponder configurations (median errors < 0.5°, 95% confidence interval < 3°). Only when the angle between the transponders was less than 2°, the TTV method was inaccurate and the OTV method may be preferred. The accuracy of both methods was independent of the angle of rotation, and only the OTV method was sensitive to the plane of rotation.

Conclusion

These results indicate that accurate 6DOF tumor tracking is possible using only two 5DOF transponders. This encourages further development of a wireless EM surgical navigation approach using a readily available clinical system.

Real-time intrafraction motion monitoring in external beam radiotherapy

Radiotherapy (RT) aims to deliver a spatially conformal dose of radiation to tumours while maximizing the dose sparing to healthy tissues. However, the internal patient anatomy is constantly moving due to respiratory, cardiac, gastrointestinal and urinary activity. The long term goal of the RT community to 'see what we treat, as we treat' and to act on this information instantaneously has resulted in rapid technological innovation. Specialized treatment machines, such as robotic or gimbal-steered linear accelerators (linac) with in-room imaging suites, have been developed specifically for real-time treatment adaptation. Additional equipment, such as stereoscopic kilovoltage (kV) imaging, ultrasound transducers and electromagnetic transponders, has been developed for intrafraction motion monitoring on conventional linacs. Magnetic resonance imaging (MRI) has been integrated with cobalt treatment units and more recently with linacs. In addition to hardware innovation, software development has played a substantial role in the development of motion monitoring methods based on respiratory motion surrogates and planar kV or Megavoltage (MV) imaging that is available on standard equipped linacs. In this paper, we review and compare the different intrafraction motion monitoring methods proposed in the literature and demonstrated in real-time on clinical data as well as their possible future developments. We then discuss general considerations on validation and quality assurance for clinical implementation. Besides photon RT, particle therapy is increasingly used to treat moving targets. However, transferring motion monitoring technologies from linacs to particle beam lines presents substantial challenges. Lessons learned from the implementation of real-time intrafraction monitoring for photon RT will be used as a basis to discuss the implementation of these methods for particle RT.

Successful treatment of platinum refractory ovarian clear cell carcinoma with secondary cytoreductive surgery and implantable transponder placement to facilitate targeted volumetric arc radiation therapy

We describe a case of the first successful treatment of platinum refractory clear cell ovarian cancer with secondary cytoreductive surgery and placement of Calypso transponders to facilitate post-operative volumetric arc radiation therapy. In the setting of both primary and recurrent disease, patients with clear cell ovarian cancer are less responsive to standard chemotherapy and those treated with radiation therapy may have improved outcomes compared to the use of other treatment modalities. Volumetric arc radiation therapy with implantable transponders is feasible, and allows for the targeted treatment of sites of metastatic disease while limiting toxicity to surrounding structures and can be considered for patients with recurrent ovarian cancer and oligometastatic disease.

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Post-operative VMAT is feasible for patients with recurrent ovarian cancer.

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VMAT minimizes toxicity and facilitates radiation therapy delivery.

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Implantable transponders are a novel approach for targeted radiation therapy.

Evaluation of Dynamic Tumor-tracking Intensity-modulated Radiotherapy for Locally Advanced Pancreatic Cancer

Intensity-modulated radiotherapy (IMRT) is now regarded as an important treatment option for patients with locally advanced pancreatic cancer (LAPC). To reduce the underlying tumor motions and dosimetric errors during IMRT as well as the burden of respiratory management for patients, we started to apply a new treatment platform of the dynamic tumor dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT) using the gimbaled linac, which can swing IMRT toward the real-time tumor position under patients' voluntary breathing. Between June 2013 and March 2015, ten patients were treated, and the tumor-tracking accuracy and the practical benefits were evaluated. The mean PTV size in DTT-IMRT was 18% smaller than a conventional ITV-based PTV. The root-mean-squared errors between the predicted and the detected tumor positions were 1.3, 1.2, and 1.5 mm in left-right, anterior-posterior, and cranio-caudal directions, respectively. The mean in-room time was 24.5 min. This high-accuracy of tumor-tracking with reasonable treatment time are promising and beneficial to patients with LAPC.

Feasibility study of ultrasound imaging for stereotactic body radiation therapy with active breathing coordinator in pancreatic cancer

PURPOSE

Stereotactic body radiation therapy (SBRT) allows for high radiation doses to be delivered to the pancreatic tumors with limited toxicity. Nevertheless, the respiratory motion of the pancreas introduces major uncertainty during SBRT. Ultrasound imaging is a non-ionizing, non-invasive, and real-time technique for intrafraction monitoring. A configuration is not available to place the ultrasound probe during pancreas SBRT for monitoring.

METHODS AND MATERIALS

An arm-bridge system was designed and built. A CT scan of the bridge-held ultrasound probe was acquired and fused to ten previously treated pancreatic SBRT patient CTs as virtual simulation CTs. Both step-and-shoot intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) planning were performed on virtual simulation CT. The accuracy of our tracking algorithm was evaluated by programmed motion phantom with simulated breath-hold 3D movement. An IRB-approved volunteer study was also performed to evaluate feasibility of system setup. Three healthy subjects underwent the same patient setup required for pancreas SBRT with active breath control (ABC). 4D ultrasound images were acquired for monitoring. Ten breath-hold cycles were monitored for both phantom and volunteers. For the phantom study, the target motion tracked by ultrasound was compared with motion tracked by the infrared camera. For the volunteer study, the reproducibility of ABC breath-hold was assessed.

RESULTS

The volunteer study results showed that the arm-bridge system allows placement of an ultrasound probe. The ultrasound monitoring showed less than 2 mm reproducibility of ABC breath-hold in healthy volunteers. The phantom monitoring accuracy is 0.14 ± 0.08 mm, 0.04 ± 0.1 mm, and 0.25 ± 0.09 mm in three directions. On dosimetry part, 100% of virtual simulation plans passed protocol criteria.

CONCLUSIONS

Our ultrasound system can be potentially used for real-time monitoring during pancreas SBRT without compromising planning quality. The phantom study showed high monitoring accuracy of the system, and the volunteer study showed feasibility of the clinical workflow.

Technical Note: Validation and implementation of a wireless transponder tracking system for gated stereotactic ablative radiotherapy of the liver

PURPOSE

Tracking soft-tissue targets has recently been cleared as a new application of Calypso, an electromagnetic wireless transponder tracking system, allowing for gated treatment of the liver based on the motion of the target volume itself. The purpose of this study is to describe the details of validating the Calypso system for wireless transponder tracking of the liver and to present the clinical workflow for using it to deliver gated stereotactic ablative radiotherapy (SABR).

METHODS

A commercial 3D diode array motion system was used to evaluate the dynamic tracking accuracy of Calypso when tracking continuous large amplitude motion. It was then used to perform end-to-end tests to evaluate the dosimetric accuracy of gated beam delivery for liver SABR. In addition, gating limits were investigated to determine how large the gating window can be while still maintaining dosimetric accuracy. The gating latency of the Calypso system was also measured using a customized motion phantom.

RESULTS

The average absolute difference between the measured and expected positional offset was 0.3 mm. The 2%/2 mm gamma pass rates for the gated treatment delivery were greater than 97%. When increasing the gating limits beyond the known extent of planned motion, the gamma pass rates decreased as expected. The 2%/2 mm gamma pass rate for a 1, 2, and 3 mm increase in gating limits was measured to be 97.8%, 82.9%, and 61.4%, respectively. The average gating latency was measured to be 63.8 ms for beam-hold and 195.8 ms for beam-on. Four liver patients with 17 total fractions have been successfully treated at our institution.

CONCLUSIONS

Wireless transponder tracking was validated as a dosimetrically accurate way to provide gated SABR of the liver. The dynamic tracking accuracy of the Calypso system met manufacturer's specification, even for continuous large amplitude motion that can be encountered when tracking liver tumors close to the diaphragm. The measured beam-hold gating latency was appropriate for targets that will traverse the gating limit each respiratory cycle causing the beam to be interrupted constantly throughout treatment delivery.

Neoadjuvant radiotherapeutic strategies in pancreatic cancer

This review summarizes the current status of neoadjuvant radiation approaches in the treatment of pancreatic cancer, including a description of modern radiation techniques, and an overview on the literature regarding neoadjuvant radio- or radiochemotherapeutic strategies both for resectable and irresectable pancreatic cancer. Neoadjuvant chemoradiation for locally-advanced, primarily non- or borderline resectable pancreas cancer results in secondary resectability in a substantial proportion of patients with consecutively markedly improved overall prognosis and should be considered as possible alternative in pretreatment multidisciplinary evaluations. In resectable pancreatic cancer, outstanding results in terms of response, local control and overall survival have been observed with neoadjuvant radio- or radiochemotherapy in several phase I/II trials, which justify further evaluation of this strategy. Further investigation of neoadjuvant chemoradiation strategies should be performed preferentially in randomized trials in order to improve comparability of the current results with other treatment modalities. This should include the evaluation of optimal sequencing with newer and more potent systemic induction therapy approaches. Advances in patient selection based on new molecular markers might be of crucial interest in this context. Finally modern external beam radiation techniques (intensity-modulated radiation therapy, image-guided radiation therapy and stereotactic body radiation therapy), new radiation qualities (protons, heavy ions) or combinations with alternative boosting techniques widen the therapeutic window and contribute to the reduction of toxicity.

Endobiliary Stent Position Changes during External-beam Radiotherapy

PURPOSE

Endobiliary stents can be used as surrogates for pancreatic localization when using cone-beam computed tomography (CBCT) during external-beam radiotherapy (EBRT). This work reports on interfraction stent position changes during EBRT for locally advanced pancreatic cancer (LAPC).

MATERIALS AND METHODS

Six patients with endobiliary stents who underwent EBRT for LAPC were assessed. Measurements from the most superior aspect of the stent (sup stent) and the most inferior aspect of the stent (inf stent) to the most inferior, posterior aspect of the L1 vertebra central spinous process were determined from daily treatment CBCTs and compared with those determined from the planning computed tomography (CT) scan. Changes in stent-L1 measurements were interpreted as changes in relative stent position.

RESULTS

Three patients showed mean interfraction stent position changes of ≥1 cm when treatment measurements were compared with planning measurements. The sup stent for patient A moved to the right (2.66 ± 2.77 cm) and inferiorly (3.0 ± 3.12 cm), and the inf stent moved to the right (1.92 ± 2.02 cm) inferiorly (3.23 ± 3.34 cm) and posteriorly (1.41 ± 1.43 cm). The inf stent for patient B moved superiorly (2.23 ± 0.49 cm) and posteriorly (1.72 ± 0.59 cm). The sup and inf stent for patient F moved inferiorly (0.98 ± 0.35 cm and 1.21 ± 0.38 cm, respectively). The remaining three patients C, D, and E showed interfraction position changes of <1 cm.

CONCLUSION

Endobiliary stent migration and deformation were observed in a small subset of patients. Further investigation is required before confirming their use as surrogates for LAPC target localization during image-guided EBRT.

Increased dose near the skin due to electromagnetic surface beacon transponder

The purpose of this study was to evaluate the increased dose near the skin from an electromagnetic surface beacon transponder, which is used for localization and tracking organ motion. The bolus effect due to the copper coil surface beacon was evaluated with radiographic film measurements and Monte Carlo simulations. Various beam incidence angles were evaluated for both 6 MV and 18 MV experimentally. We performed simulations using a general‐purpose Monte Carlo code MCNPX (Monte Carlo N‐Particle) to supplement the experimental data. We modeled the surface beacon geometry using the actual mass of the glass vial and copper coil placed in its L‐shaped polyethylene terephthalate tubing casing. Film dosimetry measured factors of 2.2 and 3.0 enhancement in the surface dose for normally incident 6 MV and 18 MV beams, respectively. Although surface dose further increased with incidence angle, the relative contribution from the bolus effect was reduced at the oblique incidence. The enhancement factors were 1.5 and 1.8 for 6 MV and 18 MV, respectively, at an incidence angle of 60°. Monte Carlo simulation confirmed the experimental results and indicated that the epidermal skin dose can reach approximately 50% of the dose at dmax at normal incidence. The overall effect could be acceptable considering the skin dose enhancement is confined to a small area (∼1 cm2), and can be further reduced by using an opposite beam technique. Further clinical studies are justified in order to study the dosimetric benefit versus possible cosmetic effects of the surface beacon. One such clinical situation would be intact breast radiation therapy, especially large‐breasted women.

PACS number: 87.53

Interfraction positional variation in pancreatic tumors using daily breath-hold cone-beam computed tomography with visual feedback

We assessed interfraction positional variation in pancreatic tumors using daily breath‐hold cone‐beam computed tomography at end‐exhalation (EE) with visual feedback (BH‐CBCT). Eleven consecutive patients with pancreatic cancer who underwent BH intensity‐modulated radiation therapy with visual feedback were enrolled. All participating patients stopped oral intake, with the exception of drugs and water, for >3 hr before treatment planning and daily treatment. Each patient was fixed in the supine position on an individualized vacuum pillow. An isotropic margin of 5 mm was added to the clinical target volume to create the planning target volume (PTV). The prescription dose was 42 to 51 Gy in 15 fractions. After correcting initial setup errors based on bony anatomy, the first BH‐CBCT scans were performed before beam delivery in every fraction. BH‐CBCT acquisition was obtained in three or four times breath holds by interrupting the acquisition two or three times, depending on the patient's BH ability. The image acquisition time for a 360° gantry rotation was approximately 90 s, including the interruption time due to BH. The initial setup errors were corrected based on bony structure, and the residual errors in the target position were then recorded. The magnitude of the interruptions variation in target position was assessed for 165 fractions. The systematic and random errors were 1.2 and 1.8 mm, 1.1 and 1.8 mm, and 1.7 and 2.9 mm in the left–right (LR), anterior–posterior (AP), and superior–inferior (SI) directions, respectively. Absolute interfraction variations of >5 mm were observed in 18 fractions (11.0%) from seven patients because of EE‐BH failure. In conclusion, target matching is required to correct interfraction variation even with visual feedback, especially to ensure safe delivery of escalated doses to patients with pancreatic cancer.

PACS number: 87.57.Q‐, 87.57.‐s, 87.55.Qr

Calypso® 4D Localization System: a review

Purpose

Calypso® 4D Localization System is a system based on electromagnetic transponders detection enabling precise 3D localisation and continuous tracking of tumour target. This review intended to provide information in order to (1) show how Calypso® 4D Localization System works, (2) to present advantages and disadvantages of this system, (3) to gather information from several clinical studies and, finally, (4) to refer Calypso® System as a tool in dynamic multileaf collimator studies for target motion compensation.

Methods

A structured search was carried out on B-On platform. The key words used in this research were ‘Calypso’, ‘Transponder’, ‘Electromagnetic Localization’, ‘Electromagnetic Tracking’, ‘Target Localization’, ‘Intrafraction Motion’ and ‘DMLC’.

Review

Treatment the implanted transponders are excited by an electromagnetic field and resonate back. These frequencies are detected and Calypso® software calculates the position of the transponders. If the movement detected is larger than the limits previously defined, irradiation can be stopped. The system has been proven to be submillimetre accurate.

Discussion

Calypso® System has been presented as an accurate tool in prostate radiotherapy treatments. The application of this system to other clinical sites is being developed.

Conclusion

The Calypso® System allows real-time localisation and monitoring of the target, without additional ionising radiation administration. It has been a very useful tool in prostate cancer treatment.

Determination of acquisition frequency for intrafractional motion of pancreas in CyberKnife radiotherapy

PURPOSE

To report the characteristics of pancreas motion as tracked using implanted fiducials during radiotherapy treatments with CyberKnife.

METHODS AND MATERIALS

Twenty-nine patients with pancreas cancer treated using CyberKnife system were retrospectively selected for this study. During the treatment, the deviation is examined every 3-4 nodes (~45 s interval) and compensated by the robot. The pancreas displacement calculated from X-ray images acquired within the time interval between two consecutive couch motions constitute a data set.

RESULTS

A total of 498 data sets and 4302 time stamps of X-ray images were analyzed in this study. The average duration for each data set is 634 s. The location of the pancreas becomes more dispersed as the time elapses. The acquisition frequency depends on the prespecified movement distance threshold of pancreas. If the threshold between two consecutive images is 1 mm, the acquisition frequency should be less than 30 s, while if the threshold is 2 mm, the acquisition frequency can be around 1 min.

CONCLUSIONS

The pancreas target moves significantly and unpredictably during treatment. Effective means of compensating the intrafractional movement is critical to ensure adequate dose coverage of the tumor target.

Interfractional position variation of pancreatic tumors quantified using intratumoral fiducial markers and daily cone beam computed tomography

PURPOSE

The aim of this study was to quantify interfractional pancreatic position variation using fiducial markers visible on daily cone beam computed tomography (CBCT) scans. In addition, we analyzed possible migration of the markers to investigate their suitability for tumor localization.

METHODS AND MATERIALS

For 13 pancreatic cancer patients with implanted Visicoil markers, CBCT scans were obtained before 17 to 25 fractions (300 CBCTs in total). Image registration with the reference CT was used to determine the displacement of the 2 to 3 markers relative to bony anatomy and to each other. We analyzed the distance between marker pairs as a function of time to identify marker registration error (SD of linear fit residuals) and possible marker migration. For each patient, we determined the mean displacement of markers relative to the reference CT (systematic position error) and the spread in displacements (random position error). From this, we calculated the group systematic error, Σ, and group random error, σ.

RESULTS

Marker pair distances showed slight trends with time (range, -0.14 to 0.14 mm/day), possibly due to tissue deformation, but no shifts that would indicate marker migration. The mean SD of the fit residuals was 0.8 mm. We found large interfractional position variations, with for 116 of 300 (39%) fractions a 3-dimensional vector displacement of >10 mm. The spread in displacement varied significantly (P<.01) between patients, from a vector range of 9.1 mm to one of 24.6 mm. For the patient group, Σ was 3.8, 6.6, and 3.5 mm; and σ was 3.6, 4.7 and 2.5 mm, in left-right, superior-inferior, and anterior-posterior directions, respectively.

CONCLUSIONS

We found large systematic displacements of the fiducial markers relative to bony anatomy, in addition to wide distributions of displacement. These results for interfractional position variation confirm the potential benefit of using fiducial markers rather than bony anatomy for daily online position verification for pancreatic cancer patients.

Effects on the photon beam from an electromagnetic array used for patient localization and tumor tracking

One of the main components in a Calypso 4D localization and tracking system is an electromagnetic array placed above patients that is used for target monitoring during radiation treatment. The beam attenuation and beam spoiling properties of the Calypso electromagnetic array at various beam angles were investigated. Measurements were performed on a Varian Clinac iX linear accelerator with 6 MV and 15 MV photon beams. The narrow beam attenuation properties were measured under a field size of 1 cm × 1 cm, with a photon diode placed in a cylindrical graphite buildup cap. The broad beam attenuation properties were measured under a field size of 10 cm × 10 cm, with a 0.6 cc cylindrical Farmer chamber placed in a polystyrene buildup cap. Beam spoiling properties of the array were studied by measuring depth-dose change from the array under a field size of 10 cm × 10 cm in a water-equivalent plastic phantom with an embedded Markus parallel plate chamber. Change in depth doses were measured with the array placed at distances of 2, 5, and 10 cm from the phantom surface. Narrow beam attenuation and broad beam attenuation from the array were found to be less than 2%-3% for both 6 MV and 15 MV beams at angles less than 40°, and were more pronounced at more oblique angles. Spoiling effects are appreciable at beam buildup region, but are insignificant at depths beyond dmax. Dose measurements in a QA phantom using patient IMRT and VMAT treatment plans were shown to have less than 2.5% dose difference with the Calypso array. The results indicate that the dose difference due to the placement of Calypso array is clinically insignificant.

Abdominal and pancreatic motion correlation using 4D CT, 4D transponders, and a gating belt

The correlation between the pancreatic and external abdominal motion due to respiration was investigated on two patients. These studies utilized four dimensional computer tomography (4D CT), a four dimensional (4D) electromagnetic transponder system, and a gating belt system. One 4D CT study was performed during simulation to quantify the pancreatic motion using computer tomography images at eight breathing phases. The motion under free breathing and breath‐hold were analyzed for the 4D electromagnetic transponder system and the gating belt system during treatment. A linear curve was fitted for all data sets and correlation factors were evaluated between the 4D electromagnetic transponder system and the gating belt system data. The 4D CT study demonstrated a modest correlation between the external marker and the pancreatic motion with R‐square values larger than 0.8 for the inferior–superior (inf‐sup). Then, the relative pressure from the belt gating system correlated well with the 4D electromagnetic transponder system's motion in the anterior–posterior (ant‐post) and the inf–post directions. These directions have a correlation value of −0.93 and 0.76, while the lateral only had a 0.03 correlation coefficient. Based on our limited study, external surrogates can be used as predictors of the pancreatic motion in the inf–sup and the ant–post directions. Although there is a low correlation on the lateral direction, its motion is significantly shorter. In conclusion, an appropriate treatment delivery can be used for pancreatic cancer when an internal tracking system, such as the 4D electromagnetic transponder system, is unavailable.

PACS number: 87.55.kh

Dose perturbations by electromagnetic transponders in the proton environment

Surgically implanted electromagnetic transponders have been used in external beam radiotherapy for target localization and position monitoring in real time. The effect of transponders on proton therapy dose distributions has not been reported. A Monte Carlo implementation of the transponder geometry is validated against film measurements in a proton SOBP and subsequently used to generate dose distributions for transponders at different positions and orientations in the proton SOBP. The maximum dose deficit is extracted in each case. Dose shadows of up to 60% occur for transponders positioned very near the end of range of the Bragg peak. However, if transponders are positioned further than 5 mm from the end of range, and are not oriented parallel to the beam direction, then the dose deficit can be kept below 10%.