An evaluation of the use of DirectSPR images for proton planning in the RayStation treatment planning software.

An important source of uncertainty in proton therapy treatment planning is the assignment of stopping-power ratio (SPR) from CT data. A commercial product is now available that creates an SPR map directly from dual-energy CT (DECT). This paper investigates the use of this new product in proton treatment planning and compares the results to the current method of assigning SPR based on a single-energy CT (SECT). Two tissue surrogate phantoms were CT scanned using both techniques. The SPRs derived from single-energy CT and by DirectSPR™ were compared to measured values. SECT-based values agreed with measurements within 4% except for low density lung and high density bone, which differed by 13% and 8%, respectively. DirectSPR™ values were within 2% of measured values for all tissues studied. Both methods were also applied to scanned containers of three types of animal tissue, and the expected range of protons of two different energies was calculated in the treatment planning system and compared to the range measured using a multi-layer ion chamber. The average difference between range measurements and calculations based on SPR maps from dual- and single-energy CT, respectively, was 0.1 mm (0.07%) versus 2.2 mm (1.5%). Finally, a phantom was created using a layer of various tissue surrogate plugs on top of a 2D ion chamber array. Dose measurements on this array were compared to predictions using both single- and dual-energy CTs and SPR maps. While standard gamma pass rates for predictions based on DECT-derived SPR maps were slightly higher than those based on single-energy CT, the differences were generally modest for this measurement setup. This study showed that SPR maps created by the commercial product from dual-energy CT can successfully be used in RayStation to generate proton dose distributions and that these predictions agree well with measurements.

Seismic reinforcement of a linac installation.

The local building requirements to secure medical equipment in seismically active areas in the United States are based on recommendations of the American Society of Civil Engineers. In our institution we have recently acquired new linear accelerators, one of which had to be installed in an existing vault and one in a new vault. Since we are in a seismic active area, changes in the local code required us to start placing the new linacs seismically stable. Here, we describe the necessary steps taken to ensure a seismically sound installation of our linacs. For the linac installation to be seismically stable, the linac base frame has to be seismically fixed into the vault floor. The installation of a new linac into an existing vault requires verification of a structurally sound base frame. Knowledge of the previously applied fixation of such is needed and exploratory removal of grouted floor helped in the verification. Understanding the additional load requirements for the locality allows to account for the existing fixation and can potentially reduce the work needed to achieve seismic fixation requirements. For a prospective seismic installation the new linac base frame can be directly installed with the necessary strength. In addition the actual workflow is straight forward and vendor recommendations can be used. In both cases the vendor provided seismic calculations serve as baseline from which a facility should be work from. It is the facilities task to verify the correct installation of a linac in their specific location. An understanding of the seismic landscape can facilitate an appropriate installation at minimal additional cost.

FMEA occurrence values for four failure modes occurring using look-up tables for dose calculations.

PURPOSE: For a number of different treatment types [such as Total Body Irradiation (TBI), etc.] most institutions utilize tables from commissioned databooks to perform the dose calculations. Each time one manually looks up data from a large table and then copies the numbers for a manual calculation, there is potential for errors. While a second check effectively mitigates the potential error from such calculations, information regarding the frequency and nature of such mistakes is important to develop protocols and workflows that avoid related errors. METHODS: Five years' worth of TBI calculations were reviewed. Each calculation was re-performed and evaluated against the original calculation and original second check. Any discrepancies were noted and those discrepancies were checked to see if the number was the result of misreading from the look-up table, a typo, copying/skipping partially redundant steps, or rounding/avoiding interpolation. The number of calculations that contained these various types of discrepancies was tallied and percentages representing the frequency of said discrepancies were derived. RESULTS: All of the discrepancies only resulted in a monitor unit (MU) calculation difference of <1.7%. Typos, looking up wrong values from tables, rounding/avoiding interpolation, and skipping steps occurred in 10.4% ( ± 3.1%), 6.3% ( ± 2.5%), 53.1% ( ± 5.1%), and 4.2% ( ± 2.0%) of MU calculations, respectively. CONCLUSIONS: While all of the discrepancies only resulted in a monitor unit (MU) calculation difference of <1.7%, this review shows how frequently various discrepancies can occur. Typos and rounding/avoiding interpolation are the steps most likely to potentially cause a miscalculation of MU. To avoid direct human interaction on such a large repetitive scale, creating forms that calculate MU automatically from initial measurement data would reduce the incidences that numbers are written/transcribed and eliminate the need to look up data in a table, thus reducing the chance for error.

Using failure mode and effects analysis (FMEA) to generate an initial plan check checklist for improved safety in radiation treatment.

PURPOSE: To apply failure mode and effect analysis (FMEA) to generate an effective and efficient initial physics plan checklist. METHODS: A team of physicists, dosimetrists, and therapists was setup to reconstruct the workflow processes involved in the generation of a treatment plan beginning from simulation. The team then identified possible failure modes in each of the processes. For each failure mode, the severity (S), frequency of occurrence (O), and the probability of detection (D) was assigned a value and the risk priority number (RPN) was calculated. The values assigned were based on TG 100. Prior to assigning a value, the team discussed the values in the scoring system to minimize randomness in scoring. A local database of errors was used to help guide the scoring of frequency. RESULTS: Twenty-seven process steps and 50 possible failure modes were identified starting from simulation to the final approved plan ready for treatment at the machine. Any failure mode that scored an average RPN value of 20 or greater was deemed "eligible" to be placed on the second checklist. In addition, any failure mode with a severity score value of 4 or greater was also considered for inclusion in the checklist. As a by-product of this procedure, safety improvement methods such as automation and standardization of certain processes (e.g., dose constraint checking, check tools), removal of manual transcription of treatment-related information as well as staff education were implemented, although this was not the team's original objective. Prior to the implementation of the new FMEA-based checklist, an in-service for all the second checkers was organized to ensure further standardization of the process. CONCLUSION: The FMEA proved to be a valuable tool for identifying vulnerabilities in our workflow and processes in generating a treatment plan and subsequently a new, more effective initial plan checklist was created.

Calculation of delivered composite dose from Calypso tracking data for prostate cancer: And subsequent evaluation of reasonable treatment interruption tolerance limits.

PURPOSE: In this study we calculate composite dose delivered to the prostate by using the Calypso tracking -data- stream acquired during patient treatment in our clinic. We evaluate the composite distributions under multiple simulated Calypso tolerance level schemes and then recommend a tolerance level. MATERIALS AND METHODS: Seven Calypso-localized prostate cancer patients treated in our clinic were selected for retrospective analysis. Two different IMRT treatment plans, with prostate PTV margins of 5 and 3 mm respectively, were computed for each patient. A delivered composite dose distribution was computed from Calypso tracking data for each plan. Additionally, we explored the dosimetric implications for "worst case" scenarios by assuming that the prostate position was located at one of the eight extreme corners of a 3 or 5 mm "box." To characterize plan quality under each of the studied scenarios, we recorded the maximum, mean, and minimum doses and volumetric coverage for prostate, PTV, bladder, and rectum. RESULTS AND DISCUSSIONS: Calculated composite dose distributions were very similar to the original plan for all patients. The difference in maximum, mean, and minimum doses as well as volumetric coverage for the prostate, PTV, bladder, and rectum were all < 4.0% of prescription dose. Even for worst scenario cases, the results show acceptable isodose distribution, with the exception for the combination of a 3 mm PTV margin with a 5 mm position tolerance scheme. CONCLUSIONS: Calculated composite dose distributions show that the vast majority of dosimetric metrics agreed well with the planned dose (within 2%). With significant/detrimental deviations from the planned dose only occurring with the combination of a 3 mm PTV margin and 5 mm position tolerance, the 3 mm position tolerance strategy appears reasonable, confirming that further reducing prostate PTV margins to 3 mm is possible when using Calypso with a position tolerance of 3 mm.

A systematic evaluation of the error detection abilities of a new diode transmission detector.

Transmission detectors meant to measure every beam delivered on a linear accelerator are now becoming available for monitoring the quality of the dose distribution delivered to the patient daily. The purpose of this work is to present results from a systematic evaluation of the error detection capabilities of one such detector, the Delta4 Discover. Existing patient treatment plans were modified through in-house-developed software to mimic various delivery errors that have been observed in the past. Errors included shifts in multileaf collimator leaf positions, changing the beam energy from what was planned, and a simulation of what would happen if the secondary collimator jaws did not track with the leaves as they moved. The study was done for simple 3D plans, static gantry intensity modulated radiation therapy plans as well as dynamic arc and volumetric modulated arc therapy (VMAT) plans. Baseline plans were delivered with both the Discover device and the Delta4 Phantom+ to establish baseline gamma pass rates. Modified plans were then delivered using the Discover only and the predicted change in gamma pass rate, as well as the detected leaf positions were evaluated. Leaf deviations as small as 0.5 mm for a static three-dimensional field were detected, with this detection limit growing to 1 mm with more complex delivery modalities such as VMAT. The gamma pass rates dropped noticeably once the intentional leaf error introduced was greater than the distance-to-agreement criterion. The unit also demonstrated the desired drop in gamma pass rates of at least 20% when jaw tracking was intentionally disabled and when an incorrect energy was used for the delivery. With its ability to find errors intentionally introduced into delivered plans, the Discover shows promise of being a valuable, independent error detection tool that should serve to detect delivery errors that can occur during radiotherapy treatment.

Comparison of surface matching and target matching for image-guided pelvic radiation therapy for both supine and prone patient positions.

We investigate the difference between surface matching and target matching for pelvic radiation image guidance. The uniqueness of our study is that all patients have multiple CT-on-rails (CTOR) scans to compare to corresponding AlignRT images. Ten patients receiving pelvic radiation were enrolled in this study. Two simulation CT scans were performed in supine and prone positions for each patient. Body surface contours were generated in treatment planning system and exported to AlignRT to serve as reference images. During treatment day, the patient was aligned to treatment isocenter with room lasers, and then scanned with both CTOR and AlignRT. Image-guidance shifts were calculated for both modalities by com-parison to the simulation CT and the differences between them were analyzed for both supine and prone positions, respectively. These procedures were performed for each patient once per week for five weeks. The difference of patient displace-ment between AlignRT and CTOR was analyzed. For supine position, five patients had an average difference of displacement between AlignRT and CTOR along any direction (vertical, longitudinal, and lateral) greater than 0.5 cm, and one patient greater than 1 cm. Four patients had a maximum difference greater than 1 cm. For prone position, seven patients had an average difference greater than 0.5 cm, and three patients greater than 1 cm. Nine patients had a maximum difference greater than 1 cm. The difference of displacement between AlignRT and CTOR was greater for the prone position than for the supine position. For the patients studied here, surface matching does not appear to be an advisable image-guidance approach for pelvic radiation therapy for patients with either supine or prone position. There appears to be a potential for large alignment discrepancies (up to 2.25 cm) between surface matching and target matching.

Planning for mARC treatments with the Eclipse treatment planning system.

While modulated arc (mARC) capabilities have been available on Siemens linear accelerators for almost two years now, there was, until recently, only one treatment planning system capable of planning these treatments. The Eclipse treatment planning system now offers a module that can plan for mARC treatments. The purpose of this work was to test the module to determine whether it is capable of creating clinically acceptable plans. A total of 23 plans were created for various clinical sites and all plans delivered without anomaly. The average 3%/3 mm gamma pass rate for the plans was 98.0%, with a standard deviation of 1.7%. For a total of 14 plans, an equivalent static gantry IMRT plan was also created to compare delivery time. In all but two cases, the mARC plans delivered significantly faster than the static gantry plan. We have confirmed the successful creation of mARC plans that are deliverable with high fidelity on an ARTISTE linear accelerator, thus demonstrating the successful implementation of the Eclipse mARC module.

Dosimetric impact of the 160 MLC on head and neck IMRT treatments.

The purpose of this work is to investigate if the change in plan quality with the finer leaf resolution and lower leakage of the 160 MLC would be dosimetrically significant for head and neck intensity-modulated radiation therapy (IMRT) treat- ment plans. The 160 MLC consisting of 80 leaves of 0.5 cm on each bank, a leaf span of 20 cm, and leakage of less than 0.37% without additional backup jaws was compared against the 120 Millennium MLC with 60 leaves of 0.5 and 1.0 cm, a leaf span of 14.5 cm, and leakage of 2.0%. CT image sets of 16 patients previously treated for stage III and IV head and neck carcinomas were replanned on Prowess 5.0 and Eclipse 11.0 using the 160 MLC and the 120 MLC. IMRT constraints for both sets of 6 MV plans were identical and based on RTOG 0522. Dose-volume histograms (DVHs), minimum dose, mean dose, maximum dose, and dose to 1 cc to the organ at risks (OAR) and the planning target volume, as recommended by QUANTEC 2010, were compared. Both collimators were able to achieve the target dose to the PTVs. The dose to the organs at risk (brainstem, spinal cord, parotids, and larynx) were 1%-12% (i.e., 0.5-8 Gy for a 70 Gy prescription) lower with the 160 MLC compared to the 120 MLC, depending on the proximity of the organ to the target. The large field HN plans generated with the 160 MLC were dosimetrically advantageous for critical structures, especially those located further away from the central axis, without compromising the target volume. 

Four-dimensional tissue deformation reconstruction (4D TDR) validation using a real tissue phantom.

Calculation of four-dimensional (4D) dose distributions requires the remapping of dose calculated on each available binned phase of the 4D CT onto a reference phase for summation. Deformable image registration (DIR) is usually used for this task, but unfortunately almost always considers only endpoints rather than the whole motion path. A new algorithm, 4D tissue deformation reconstruction (4D TDR), that uses either CT projection data or all available 4D CT images to reconstruct 4D motion data, was developed. The purpose of this work is to verify the accuracy of the fit of this new algorithm using a realistic tissue phantom. A previously described fresh tissue phantom with implanted electromagnetic tracking (EMT) fiducials was used for this experiment. The phantom was animated using a sinusoidal and a real patient-breathing signal. Four-dimensional computer tomography (4D CT) and EMT tracking were performed. Deformation reconstruction was conducted using the 4D TDR and a modified 4D TDR which takes real tissue hysteresis (4D TDR(Hysteresis)) into account. Deformation estimation results were compared to the EMT and 4D CT coordinate measurements. To eliminate the possibility of the high contrast markers driving the 4D TDR, a comparison was made using the original 4D CT data and data in which the fiducials were electronically masked. For the sinusoidal animation, the average deviation of the 4D TDR compared to the manually determined coordinates from 4D CT data was 1.9 mm, albeit with as large as 4.5 mm deviation. The 4D TDR calculation traces matched 95% of the EMT trace within 2.8 mm. The motion hysteresis generated by real tissue is not properly projected other than at endpoints of motion. Sinusoidal animation resulted in 95% of EMT measured locations to be within less than 1.2 mm of the measured 4D CT motion path, enabling accurate motion characterization of the tissue hysteresis. The 4D TDR(Hysteresis) calculation traces accounted well for the hysteresis and matched 95% of the EMT trace within 1.6 mm. An irregular (in amplitude and frequency) recorded patient trace applied to the same tissue resulted in 95% of the EMT trace points within less than 4.5 mm when compared to both the 4D CT and 4D TDR(Hysteresis) motion paths. The average deviation of 4D TDR(Hysteresis) compared to 4D CT datasets was 0.9 mm under regular sinusoidal and 1.0 mm under irregular patient trace animation. The EMT trace data fit to the 4D TDR(Hysteresis) was within 1.6 mm for sinusoidal and 4.5 mm for patient trace animation. While various algorithms have been validated for end-to-end accuracy, one can only be fully confident in the performance of a predictive algorithm if one looks at data along the full motion path. The 4D TDR, calculating the whole motion path rather than only phase- or endpoints, allows us to fully characterize the accuracy of a predictive algorithm, minimizing assumptions. This algorithm went one step further by allowing for the inclusion of tissue hysteresis effects, a real-world effect that is neglected when endpoint-only validation is performed. Our results show that the 4D TDR(Hysteresis) correctly models the deformation at the endpoints and any intermediate points along the motion path.

Technical note: Clinical evaluation of a newly released surface-guided radiation therapy system on DIBH for left breast radiation therapy.

BACKGROUND: It has been shown that a significant reduction of mean heart dose and left anterior descending artery (LAD) dose can be achieved through the use of DIBH for left breast radiation therapy. Surface-guided DIBH has been widely adopted during the last decade, and there are mainly three commercially available SGRT systems. The reports of the performance of a newly released SGRT system for DIBH application are currently very limited. PURPOSE: To evaluate the clinical performance of a newly released SGRT system on DIBH for left breast radiation therapy. METHODS: Twenty-five left breast cancer patients treated with DIBH utilizing Varian's Identify system were included (total 493-fraction treatments). Four aspects of the clinical performance were evaluated: Identify offsets of free breathing post patient setup from tattoos, Identify offsets during DIBH, Identify agreement with radiographic ports during DIBH, and DIBH reference surface re-capture post patient shifts. The systematic and random errors of free breathing Identify offsets post patient setup were calculated for each patient, as well as for offsets during DIBH. Radiographic ports were taken when the patient's DIBH position was within the clinical tolerance of (± 0.3 cm, ± 30 ), and these were then compared with treatment field DRRs. If the ports showed that the patient alignment did not agree with the DRRs within 3 mm, a patient shift was performed. A new reference surface was captured and verification ports were taken. RESULTS: The all-patient average systematic and random errors of Identify offsets for free breathing were within (0.4 cm, 1.50 ) post tattoo setup. The maximum per-patient systematic and random errors were (1.1 cm, 6.20 ) and (0.9 cm, 20 ), and the maximum amplitude of Identify offsets were (2.59 cm, 90 ). All 493-fraction DIBH treatments were delivered and successfully guided by the Identify SGRT system. The systematic and random errors of Identify offsets for DIBH were within (0.2 cm, 2.30 ). Seven patients needed re-captured surface references due to surface variation or position shifts based on the ports. All patient DIBH verification ports guided by Identify were approved by attending physicians. CONCLUSION: This evaluation showed that the Identify system performed effectively for surface-guided patient setup and surface-guided DIBH imaging and treatment delivery. The feature of color-coded real-time patient surface matching feedback facilitated the evaluation of the patient alignment accuracy and the adjustment of the patient position to match the reference.

Initial clinical evaluation of a novel combined biometric, radio-frequency identification, and surface imaging system.

PURPOSE: To evaluate and characterize the overall clinical functionality and workflow of the newly released Varian Identify system (version 2.3). METHODS: Three technologies included in the Varian Identify system were evaluated: patient biometric authentication, treatment accessory device identification, and surface-guided radiation therapy (SGRT) function. Biometric authentication employs a palm vein reader. Treatment accessory device verification utilizes two technologies: device presence via Radio Frequency Identification (RFID) and position via optical markers. Surface-guidance was evaluated on both patient orthopedic setup at loading position and surface matching and tracking at treatment isocenter. A phantom evaluation of the consistency and accuracy for Identify SGRT function was performed, including a system consistency test, a translational shift and rotational accuracy test, a pitch and roll accuracy test, a continuous recording test, and an SGRT vs Cone-Beam CT (CBCT) agreement test. RESULTS: 201 patient authentications were verified successfully with palm reader. All patient treatment devices were successfully verified for their presences and positions (indexable devices). The patient real-time orthopedic pose was successfully adjusted to match the reference surface captured at simulation. SGRT-reported shift consistency against couch readout was within (0.1 mm, 0.030). The shift accuracy was within (0.3 mm, 0.10). In continuous recording mode, the maximum variation was 0.2 ± 0.12 mm, 0.030 ± 0.020. The difference between Identify SGRT offset and CBCT was within (1 mm, 10). CONCLUSIONS: This clinical evaluation confirms that Identify accurately functions for patient palm identification and patient treatment device presence and position verification. Overall SGRT consistency and accuracy was within (1 mm, 10), within the 2 mm criteria of AAPM TG302.

Tissue characterization using a phantom to validate four-dimensional tissue deformation.

PURPOSE: This project proposes using a real tissue phantom for 4D tissue deformation reconstruction (4DTDR) and 4D deformable image registration (DIR) validation, which allows for the complete verification of the motion path rather than limited end-point to end-point of motion. METHODS: Three electro-magnetic-tracking (EMT) fiducials were implanted into fresh porcine liver that was subsequently animated in a clinically realistic phantom. The animation was previously shown to be similar to organ motion, including hysteresis, when driven using a real patient's breathing pattern. For this experiment, 4DCTs and EMT traces were acquired when the phantom was animated using both sinusoidal and recorded patient-breathing traces. Fiducial were masked prior to 4DTDR for reconstruction. The original 4DCT data (with fiducials) were sampled into 20 CT phase sets and fiducials' coordinates were recorded, resulting in time-resolved fiducial motion paths. Measured values of fiducial location were compared to EMT measured traces and the result calculated by 4DTDR. RESULTS: For the sinusoidal breathing trace, 95% of EMT measured locations were within 1.2 mm of the measured 4DCT motion path, allowing for repeatable accurate motion characterization. The 4DTDR traces matched 95% of the EMT trace within 1.6 mm. Using the more irregular (in amplitude and frequency) patient trace, 95% of the EMT trace points fitted both 4DCT and 4DTDR motion path within 4.5 mm. The average match of the 4DTDR estimation of the tissue hysteresis over all CT phases was 0.9 mm using a sinusoidal signal for animation and 1.0 mm using the patient trace. CONCLUSIONS: The real tissue phantom is a tool which can be used to accurately characterize tissue deformation, helping to validate or evaluate a DIR or 4DTDR algorithm over a complete motion path. The phantom is capable of validating, evaluating, and quantifying tissue hysteresis, thereby allowing for full motion path validation.

4D CT image acquisition errors in SBRT of liver identified using correlation.

In the AAPM Report 80, the imaging modality of 4D CT and respiration-correlated CT was declared a "promising solution for obtaining high-quality CT data in the presence of respiratory motion". To gather anatomically correct data over time, the existence of correlation between the internal organ movement and an external surrogate has to be assumed. For the in-house evaluation of such correlation, we retrospectively analyzed 21 four-dimensional computer tomography (4D CT) scans of five patients, out of which the artifacts experienced in three patients are shown here. To provide context and a baseline for the analysis of patient motion, a real-tissue liver phantom was used with a solid water block and liver tissue. The superior-inferior motion of fiducials in phantom and patients was correlated to the recorded anterior-posterior motion of an external surrogate marker on the chest. The use of a solid water block yielded a measurable correlation coefficient of 0.98 or better using a sinusoidal animation pattern. With sinusoidally-animated liver tissue, the minimum correlation observed was 0.96. Comparing this to retrospective patient data, we found three cases of a change in the correlation coefficient, or simply a low correlation. The source of this low correlation was investigated by careful examination of the breathing traces and the CT-phase assignments used to reconstruct the datasets. Consequences of nonregular breathing are elaborated on. We demonstrate the impact of wrong phase assignments and missing image information in the 4D CT phase sampling processes. We also show how daily patient-based correlation analysis can indicate changes in breathing traces, which can be significant enough to decrease, or completely eliminate, previously existing correlation.

Surface-Guided Patient Setup Versus Traditional Tattoo Markers for Radiation Therapy: Is Tattoo-Less Setup Feasible for Thorax, Abdomen and Pelvis Treatment?

PURPOSE: In this study, patient setup accuracy was compared between surface guidance and tattoo markers for radiation therapy treatment sites of the thorax, abdomen and pelvis. METHODS AND MATERIALS: A total of 608 setups performed on 59 patients using both surface-guided and tattoo-based patient setups were analyzed. During treatment setup, patients were aligned to room lasers using their tattoos, and then the six-degree-of-freedom (6DOF) surface-guided offsets were calculated and recorded using AlignRT system. While the patient remained in the same post-tattoo setup position, target localization imaging (radiographic or ultrasound) was performed and these image-guided shifts were recorded. Finally, surface-guided vs tattoo-based offsets were compared to the final treatment position (based on radiographic or ultrasound imaging) to evaluate the accuracy of the two setup methods. RESULTS: The overall average offsets of tattoo-based and surface-guidance-based patient setups were comparable within 3.2 mm in three principal directions, with offsets from tattoo-based setups being slightly less. The maximum offset for tattoo setups was 2.2 cm vs. 4.3 cm for surface-guidance setups. Larger offsets (ranging from 2.0 to 4.3 cm) were observed for surface-guided setups in 14/608 setups (2.3%). For these same cases, the maximum observed tattoo-based offset was 0.7 cm. Of the cases with larger surface-guided offsets, 13/14 were for abdominal/pelvic treatment sites. Additionally, larger rotations (>3°) were recorded in 18.6% of surface-guided setups. The majority of these larger rotations were observed for abdominal and pelvic sites (~84%). CONCLUSIONS: The small average differences observed between tattoo-based and surface-guidance-based patient setups confirm the general equivalence of the two potential methods, and the feasibility of tattoo-less patient setup. However, a significant number of larger surface-guided offsets (translational and rotational) were observed, especially in the abdominal and pelvic regions. These cases should be anticipated and contingency setup methods planned for.

Individualized margins for prostate patients using a wireless localization and tracking system.

This study investigates the dosimetric benefits of designing patient-specific margins for prostate cancer patients based on 4D localization and tracking. Ten prostate patients, each implanted with three radiofrequency transponders, were localized and tracked for 40 fractions. "Conventional margin" (CM) planning target volumes (PTV) and PTVs resulting from uniform margins of 5 mm (5M) and 7 mm (7M) were explored. Through retrospective review of each patient's tracking data, an individualized margin (IM) design for each patient was determined. IMRT treatment plans with identical constraints were generated for all four margin strategies and compared. The IM plans generally created the smallest PTV volumes. For similar PTV coverage, the IM plans had a lower mean bladder (rectal) dose by an average of 3.9% (2.5%), 8.5% (5.7%) and 16.2 % (9.8%) compared to 5M, 7M and CM plans, respectively. The IM plan had the lowest gEUD value of 23.8 Gy for bladder, compared to 35.1, 28.4 and 25.7, for CM, 7M and 5M, respectively. Likewise, the IM plan had the lowest NTCP value for rectum of 0.04, compared to 0.07, 0.06 and 0.05 for CM, 7M and 5M, respectively. Individualized margins can lead to significantly reduced PTV volumes and critical structure doses, while still ensuring a minimum delivered CTV dose equal to 95% of the prescribed dose.

Initial experience and clinical comparison of two image guidance methods for SBRT treatment: 4DCT versus respiratory-triggered imaging.

For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4DCT and ExacTrac respiratory-triggered imaging. Five different patients with five liver lesions and one lung lesion for a total of 19 SBRT delivered fractions were studied. For the 4DCT method, a manual registration process was used between the 4DCT image sets from initial simulation and treatment day to determine the required daily image-guided corrections. We also used the ExacTrac respiratory-triggered imaging capability to verify the target positioning, and calculated the differences in image guidance shifts between these two methods. The mean (standard deviation) of the observed differences in image-guided shifts between 4DCT and ExacTrac respiratory-triggered image guidance was left/right (L/R) = 0.4 (2.0) mm, anterior/posterior (A/P) = 1.4 (1.7) mm, superior/inferior (S/I) = 2.2 (2.0) mm, with no difference larger than 5.0 mm in any given direction for any individual case. The largest error occurred in the S/I direction, with a mean of 2.2 mm for the six lesions. This seems reasonable, because respiratory motion and the resulting imaging uncertainties are most pronounced in this S/I direction. Image guidance shifts derived from ExacTrac triggered imaging at two extreme breathing phases (i.e., full exhale vs. full inhale), agreed well (less than 2.0 mm) with each other. In summary, two very promising image guidance methods of 4DCT and ExacTrac respiratory-triggered imaging were presented and the image guidance shifts were comparable for the patients evaluated in this study.

Evaluation of the dosimetric impact of changes in shoulder position on target coverage for spine SBRT to metastases in the lower cervical spine region.

For patients treated with SBRT for spinal metastases in the cervical area, a thermoplastic mask is the usual immobilization technique. This project investigates the impact of shoulder position variability on target coverage for such cases. Eight HN patients treated in a suite equipped with a CT-on-rails system (CTOR) were randomly chosen. Of these, three were treated with shoulder depressors. For each patient, their planning CT was used to contour spine targets at the C5, C6 and C7 levels for which two VMAT plans were developed to deliver 18 Gy to each target per the RTOG 0631 protocol. One plan used full arcs while the other used avoidance sectors around the lateral positions. For each patient, IGRT CTOR images were used to recalculate doses that would have been delivered from these plans. Target coverage and dose to the spinal cord were compared for four scenarios: full and partial arcs, with or without depressors. A Dunn test showed significant differences between groups with and without shoulder depressors, but not between those with full versus partial arcs. For most of the investigated cases, the coverage ended up being higher than planned due to the shoulder position being inferior at treatment compared to simulation. In some cases, this led to higher spinal cord doses than allowed per protocol. The results of this study confirm that, when treating lower cervical spine lesions with SBRT, special care should be taken to ensure that the shoulders are positioned as they were during planning CT acquisition.

Prevention of Radiation Therapy Treatment Deviations by a Novel Combined Biometric, Radiofrequency Identification, and Surface Imaging System.

PURPOSE: To evaluate the impact of Varian Identify, a novel combined radiofrequency identification, biometric and surface-matching technology, on its potential for patient safety and prevention of radiation therapy treatment deviations. METHODS AND MATERIALS: One hundred eight radiation therapy treatment deviation reports at our facility over the past 8 years were analyzed. Three major categories were defined based on the time point of occurrence: physician order deviations (19.4%), treatment-planning deviations (24.1%), and machine treatment deviations (56.5%). The impact of Identify on potential prevention of machine treatment deviations was analyzed. A failure mode and effects analysis was performed on the 5 most frequently occurring errors preventable with Identify. Safety analysis of the Identify system was reported based on 3.5 years of clinical data post-Identify system installation on 3 treatment vaults. RESULTS: Of the 61 machine treatment deviations, 47 (77%) were interpreted as being preventable by using Identify. Our failure mode and effects analysis showed reductions in all risk priority numbers post-Identify application. Safety analysis of the Identify system from our direct observation that for approximately 7 cumulative years of Identify use in 3 different treatment vaults, where 9 deviations would have been expected to occur over this combined period, zero machine treatment events occurred. CONCLUSIONS: The combination of Identify biometric, radiofrequency identification, and surface-matching technologies was observed to enable an effective process for enhancing safety and efficiency of radiation therapy treatment. A significant reduction in machine-related deviations was observed.

Comparison of transperineal ultrasound image guidance technique to transabdominal technique for prostate radiation therapy.

INTRODUCTION: Ultrasound (US) guidance of the prostate has long been conducted using a transabdominal (TA) approach. More recently, a transperineal (TP) approach has been made available for image guidance. Our aim was to determine if both methods produced similar alignments within the same patients. MATERIALS AND METHODS: We utilized two clinical US image guidance (IG) systems (Elekta Clarity and Best BAT). The B-mode Acquisition and Targeting USIG system is a bi-planar, so-called 2.5D USIG system, that is acquired TA. Clarity is a 3D US system that generates a volumetric 3D US data set and US-derived IG contours that are coregistered to the planning CT images. The probe is oriented in the sagittal plane against the perineum (TP). After positioning the patient for treatment using the TP USIG, we maintained the position defined by Clarity tracking and then acquired a TA-based USIG. The two US-based methods of localizing the prostate (TA vs TP) were compared via Bland-Altman (BA) statistical analysis to determine if there was alignment agreement between methods. RESULTS: The BA test for all 101 patients, 2093 fractions resulted in 95% confidence intervals (upper and lower limits of the BA test) of 0.6 mm in LR, 0.9 mm in AP and 1.0 mm in SI. The bias between the two systems was calculated as 0.03, 0.02, and 0.03 mm in LR, AP, and SI. CONCLUSIONS: Both systems resulted in statistically equivalent targeting positions for the prostate. Because of the unique intrafraction, real-time motion tracking capability of the TP system, this solution represents a unique extension to the previously reported clinical benefits of a TA approach by providing assurance of the prostate remaining in the treatment field during beam-on.