Image Guidance Shifts Research Articles

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.

Dosimetric impact of placement errors in optically stimulated luminescent in vivo dosimetry in radiotherapy.

IntroductionStudies have suggested that optically stimulated luminescent dosimeters (OSLDs) can be used for in vivo dosimetry of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Clinical uncertainties such as placement error have not been thoroughly investigated. The purpose of this work was to measure OSLD placement error in a clinical sample and analyze its dosimetric impact.Methods and materialsThe analysis consisted of three parts: first, quantification of placement error in a clinical sample of 128 patients yielding 293 cone-beam CT (CBCT) with visible OSLDs registered to the treatment plan; Second, correlation of placement error and clinical OSLD measurements; third, simulation of dosimeter placement in the treatment plan and correlation of recalculated dose with placement error.ResultsIn the first analysis, average placement error was 9.7 ± 9.5 mm. In the second analysis, placement error and measured-to-planned dose agreement yielded no correlation (R2 = 0.02) for a subsample of 77 CBCTs of 55 head-and-neck patients. Average placement error was 7.0 ± 6.0 mm. Several factors, including image-guided shifts, introduced uncharacterized uncertainty to the measured-to-planned dose agreement. The third analysis isolated placement error from these other effects. Average dosimetric error was −2.4 ± 19.3%. Simulated dosimetric impact was weakly correlated with placement error (R2 = 0.39). Removing outliers reduced the average dosimetric error to −2.1 ± 10.9%, marginally improving the correlation (R2 = 0.44).ConclusionPlacement error can substantially impact measured-to-planned dose agreement of OSLDs in high gradient regions, demonstrating the criticality of accurate dosimeter placement for IMRT and VMAT treatments.

Comparison of 2 transabdominal ultrasound image guidance techniques for prostate and prostatic fossa radiation therapy

PurposeOur clinic is a long-term user of a first-generation transabdominal (TA) biplanar (2.5-dimensional [2.5D]) ultrasound image guidance (USIG) system for prostate cancer treatments. We are also an early adopter and development partner for a new, second-generation, fully 3D USIG system that allows for volumetric TA localization of the prostate. This new system has been evaluated at our institution by direct comparison with the previously established first-generation TA method for prostate alignment. Methods and materialsWe compared the 2 TA-USIG methods on the same subjects and same treatment sessions. A total of 1428 fractions delivered to 41 treated patients (16 intact prostate, 25 fossa) were analyzed regarding the agreement of alignments between the 2 US positioning systems. Patients were first aligned to tattoos using treatment room lasers. TA-USIG using the 3D system was then performed to align contours derived during the computed tomography simulation process to their corresponding daily US-visualized structures. The US-3D system image guidance shifts were performed and recorded as the “initial” shifts. A 2.5D system alignment was then immediately performed using the same computed tomography derived reference contours and the indicated shifts, relative to the 3D system, were recorded as the difference between the 2 alignment methods. ResultsThe average difference between the 2 TA-USIG alignments for all patients was 0.4 ± 0.7 mm, 0.7 ± 0.9 mm, and 0.5 ± 0.9 mm in the left-right, anteroposterior, and superoinferior directions, respectively. No significant difference in system agreement between intact prostate versus fossa patients was observed. ConclusionOur comparison of an established 2.5D USIG method with a newer, fully 3D approach for prostate alignment of 41 different patients (1428 fractions) shows excellent agreement with each other, despite the nontrivial difference in imaging approaches. This shows that the 2 specific USIG approaches yield results that are consistent with each other, and that the USIG modality yields consistent results within the modality.

SU‐F‐T‐320: Assessing Placement Error of Optically Stimulated Luminescent in Vivo Dosimeters Using Cone‐Beam Computed Tomography

Purpose:Optically‐stimulated luminescent dosimeters (OSLDs) are increasingly utilized for in vivo dosimetry of complex radiation delivery techniques such as intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT). Evaluation of clinical uncertainties such as placement error has not been performed. This work retrospectively investigates the magnitude of placement error using conebeam computed tomography (CBCT) and its effect on measured/planned dose agreement.Methods:Each OSLD was placed at a physicist‐designated location on the patient surface on a weekly basis. The location was given in terms of a gantry angle and two‐dimensional offset from central axis. The OSLDs were placed before daily image guidance. We identified 77 CBCTs from 25 head‐and‐neck patients who received IMRT or VMAT, where OSLDs were visible on the CT image. Grossly misplaced OSLDs were excluded (e.g. wrong laterality). CBCTs were registered with the treatment plan and the distance between the planned and actual OSLD location was calculated in two dimensions in the beam's eye view. Distances were correlated with measured/planned dose percent differences.Results:OSLDs were grossly misplaced for 5 CBCTs (6.4%). For the remaining 72 CBCTs, average placement error was 7.0±6.0 mm. These errors were not correlated with measured/planned dose percent differences (R2=0.0153). Generalizing the dosimetric effect of placement errors may be unreliable.Conclusion:Correct placement of OSLDs for IMRT and VMAT treatments is critical to accurate and precise in vivo dosimetry. Small placement errors could produce large disagreement between measured and planned dose. Further work includes expansion to other treatment sites, examination of planned dose at the actual point of OSLD placement, and the influence of imageguided shifts on measured/planned dose agreement.

TH-CD-304-02: Clinical Uncertainty of in Vivo Dosimetry for Intensity-Modulated Radiation Therapy Using Optically-Stimulated Luminescent Dosimeters

Purpose: Several studies have reported the physical properties of optically-stimulated luminescent dosimeters (OSLDs) and suggest their efficacy for clinical in vivo dosimetry, but few publications have assessed the clinical uncertainties associated with OSLD-based in vivo dosimetry for conformal and intensity-modulated treatment. The purpose of the current work is to identify and characterize clinical uncertainties for OSLD-based in vivo dosimetry. Methods: OSLDs are placed in dosimetrically-appropriate locations on a weekly basis, covered with small 5 mm bolus squares, exposed during patient treatment, read, and compared with doses predicted from the treatment planning system. Six (6) parameters were identified as significant contributors to uncertainty in the process: Inherent physical OSLD uncertainty (σ_ OSLD), OSLD reader uncertainty (σ_reader), dose calculation uncertainty in the build-up region (σ _calc), uncertainty of depth for planned dose (σ_d), dosimetric uncertainty due to daily image-guided shifts (σ_ s), and placement uncertainty (σ_p). σ_d, σ_ s, and σ_p were estimated by analyzing clinical OSLD dosimetry for inverse-planned intensity-modulated treatment plans (prostate, lung, and head-and-neck) and field-in-field intensity-modulated treatment plans (breast). Total uncertainty was estimated by summing the 6 components in quadrature. Results: σ_OSLD was defined by the manufacturer (±3.0%). σ_calc was assumed to be approximately ±5.0% at 5 mm depth from other works in the literature. σ_reader and σ_s were measured to be ±1.0 and ±5.8% at 5 mm depth respectively. σ_p was found to be ±3.6% for breast, ±4.2% for prostate, ±4.4% for lung, and ±10.6% for head-and-neck. Total uncertainty was ±9.0% for breast, ±11.5% for prostate, ±13.2% for lung, and ±16.1% for head-and-neck. Conclusion: Site-specific clinical uncertainty for a limited selection of sites ranged from ±9.0% to ±16.1%. The largest components were image-guided shifts and estimated placement uncertainty. A wider selection of anatomical sites, site-specific correction factors, and clinical tolerance/action level guidelines will be presented.

SU-E-U-09: Direct Clinical Comparison of 2D Versus 3D Transabdominal Ultrasound Image Guidance Methods for Prostate Radiotherapy Treatment

Purpose: Direct clinical comparison of 2D versus 3D transabdominal ultrasound image guidance (USIG) methods for prostate radiotherapy Methods: Image guidance shifts were compared on the same subject, same treatment session between Nomos BAT and Elekta Clarity ultrasound methods for 20 patients and 662 prostate treatments. Both systems use unique approaches to USIG. BAT system acquires two static 2D ultrasound images (axial and sagittal) and compares them to contours derived from simulation CT (i.e. intermodality comparison). Clarity system acquires a 3D ultrasound data set at CT simulation and compares to treatment‐day 3D ultrasound image set for deriving image guidance shifts (i.e. intramodality comparison). In this study, transabdominal Clarity image guidance was performed and the shifts were implemented first. Then transabdominal ultrasound images were acquired approximately 1minute later with BAT to derive image guidance shifts. These BAT‐reported shifts were recorded as the difference between BAT and Clarity USIG. The same experienced operator performed both BAT and Clarity paired alignments. Results: The average differences between the two systems were less than 1.0 mm in all three principle directions. The average standard deviation of these differences in lateral, vertical, and longitudinal direction were 1.2, 1.7, and 1.4 mm, respectively. Average Max difference (across all patients) was 1.9 mm; absolute Max across all image sessions was 10.2 mm. Differences between the two USIG approaches can be partially explained by prostate motion as a function of the time elapsed between imaging sessions, and 10.2 mm Max is not inconsistent with prostate motion we have observed during a similar time frame when using Calypso RF tracking. Results were further stratified into two groups: intact prostate and post‐prostatectomy, and no systematic differences were observed between these two groups. Conclusion: Good agreement was observed in this head‐to‐head comparison study between BAT and Clarity USIG methods for prostate localization.

SU-E-J-60: Comparison of CT-On-Rails and a 3D Surface Imaging System for Image Guided Pelvic Radiation Therapy for Both Supine and Prone Patient Positions.

We investigate the feasibility of using AlignRT for pelvic radiation image guidance. The uniqueness of our study is that all patients have multiple CT-on-rails (CTOR) scans to compare corresponding AlignRT images to. 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. The patient was aligned to treatment isocenter with room lasers, and then scanned with both CTOR and AlignRT in both supine and prone positions. Image guidance shifts were calculated for both modalities by comparison to the simulation CT and the differences between them were analyzed. These procedures were performed for each patient once per week for five weeks. The average and maximum difference of displacement between AlignRT and CTOR were calculated for each patient. For supine position, there are 4 patients who had the average difference of displacement between AlignRT and CTOR along any direction (vertical, longitudinal, and lateral) greater than 0.5cm, and 1 patient greater than 1cm. For prone position, there are 7 patients who had the average difference greater than 0.5cm, and 3 patients greater than 1cm. For supine position, there are 4 patients who had the maximum difference great than 1cm. For prone position, there are 9 patients who had the maximum difference greater than 1 cm. The difference of displacement between AlignRT and CTOR is greater for prone position than for supine position. AlignRT 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 AlignRT and CTOR.

SU-E-T-404: A Prototype Program for Analyzing 4D Image Guidance Shifts for Lung SBRT.

A prototype in-house program was developed to assist deriving image guidance shifts clinically using our 4DCT method for lung SBRT. Our in-house prototype program was implemented in MATLAB. The 4D DICOM CT dataset of interest is imported by the program from AW and subsequently accepts input from the user regarding the desired relative IGRT shifts. The DICOM header information defining the relative 4D dataset origin is then read in and the desired relative IGRT shifts are applied to re-define the origin for the dataset that is to be shifted. Lastly, the redefined (shifted) origin coordinates are written to the header of the 4D dataset and the dataset is then exported to AW as DICOM files. We used this program to process 4DCT datasets for four patients with 3-5 fractions each. The clinical shifts that were derived previously, without this program, were applied to the images and the acceptability of these shifts were evaluated. All slices and planes of the shifted 4D dataset were reviewed and, in general, demonstrated acceptable re-centering of the target within the ITV. However, it is noted that some small adjustments to the shift coordinates would likely have been made had we had the opportunity to visualize the shifted 4D dataset as facilitated by the new in-house software. The additional adjustments for the four patient cases studied here had an average of lmm and maximum of 2mm in three principle directions. We look forward to exploring the potential for implementing this software into our clinical workflow, and we believe that such a workflow is worthy of consideration by vendors for implementation as an available feature in future versions of their software. We presented a prototype program to shift 4DCT image set for lung SBRT applications and demonstrated four clinical cases.

SU-E-T-527: Comparison of CT-On-Rails and a 3D Surface Imaging System for Image Guided Partial Breast Radiation Therapy

Purpose: We evaluate the feasibility of using a 3D surface imaging system (AlignRT) for image guidance for partial breast radiation treatments through comparison with the CT-on-rails image guidance currently clinically employed at our institution. Methods/Materials: Four patients receiving external beam partial breast radiation therapy with multiple image guidance datasets were analyzed retrospectively. A total of 51 fractions were studied, each with a CT-on-rails dataset and an AlignRT body surface contour snapshot. For each fraction, the patient was aligned to treatment isocenter with room lasers and scanned with both CT-on-rails and AlignRT. Image guidance shifts were calculated for both modalities by comparison to the simulation CT and the differences between the two sets of shifts were analyzed. Results: Two patients showed significant change in surgical cavity volume and shape between simulation and fraction 1. For these patients it was found that the discrepancy between CT-on-rails based shifts and AlignRT shifts was significant (up to 11.1 mm) indicating that use of AlignRT for image guidance of these cases would be inadvisable. The remaining two patients did not show noticeable change in surgical cavity and the discrepancy between the two sets of shifts for these patients was minimal, thus indicating that AlignRT is potentially valuable for these cases. Conclusion: AlignRT appears to be a reasonable and potentially valuable image guidance approach for partial breast radiation therapy for patients with small changes in surgical cavity between CT simulation and treatment. However, there may be potential for alignment inaccuracies when using AlignRT for patients with dramatic changes in surgical cavity.

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.PACS number: 87.55.ne

Reduction of Dose Delivered to Organs at Risk in Prostate Cancer Patients via Image-Guided Radiation Therapy

To determine whether image guidance can improve the dose delivered to target organs and organs at risk (OARs) for prostate cancer patients treated with intensity-modulated radiotherapy (IMRT). Eight prostate cancer patients were treated with IMRT to 76 Gy at 2 Gy per fraction. Daily target localization was performed via alignment of three intraprostatic fiducials and weekly kV-cone beam computed tomography (CBCT) scans. The prostate and OARs were manually contoured on each CBCT by a single physician. Daily patient setup shifts were obtained by comparing alignment of skin tattoos with the treatment position based on fiducials. Treatment fields were retrospectively applied to CBCT scans. The dose distributions were calculated using actual treatment plans (an 8-mm PTV margin everywhere except for 6-mm posteriorly) with and without image guidance shifts. Furthermore, the feasibility of margin reduction was evaluated by reducing planning margins to 4 mm everywhere except for 3 mm posteriorly. For the eight treatment plans on the 56 CBCT scans, the average doses to 98% of the prostate (D98) were 102% (range, 99-104%) and 99% (range, 45-104%) with and without image guidance, respectively. Using margin reduction, the average D98s were 100% (range, 84-104%) and 92% (range, 40-104%) with and without image guidance, respectively. Currently, margins used in IMRT plans are adequate to deliver a dose to the prostate with conventional patient positioning using skin tattoos or bony anatomy. The use of image guidance may facilitate significant reduction of planning margins. Future studies to assess the efficacy of decreasing margins and improvement of treatment-related toxicities are warranted.

SU-FF-J-88: A Novel Workflow for SBRT Image Guidance Using 4D CT Method

Purpose: We presented a novel clinical workflow to determine interfractional image guidance shifts for SBRT treatment of lung using 4D CT scans, and validated this workflow in phantom studies. Method and Materials: An ITV was first generated on the planning 4D CT image set and later used for treatment planning. The ITV and the 4D CT image sets on both the planning and treatment days were loaded into GE AW software. A manual registration was then performed. Using the measurement tool on AW, image guidance shifts were measured based on the same phase of the two CT image sets. Phantom studies were performed to simulate the entire SBRT process. The target was programmed to be shift for three scenarios: a) L/R and A/P shift; b) Longitudinal shift; and c) combination of L/R, A/P and longitudinal shift. All these phantom studies were repeated for two breathing traces: a sinusoidal and a real patient breathing trace. Results: The phantom studies showed that the differences were within a CT voxel size for all three shifting scenarios between the programmed and the measured shifts using the 4D CT method, for both the sinusoidal and patient breathing traces. We also performed two other validation studies retrospectively for one patient. An average was calculated for the tumor centroid position on 10 phases using a segmentation program. The difference between the simulation day and the treatment day gave the image guidance shifts. The second method used the average intensity projection image on the AW software. The image guidance shifts determined by these two retrospective methods agreed within 2mm with our clinically-used method. Conclusion: The workflow to determine image guidance shifts for SBRT treatment was validated by phantom studies, and it was further verified retrospectively by two independent methods for one patient with a lung tumor.

SU‐FF‐J‐53: Experience and Comparison of Two Image Guidance Methods for SBRT Treatment: 4D CT and ExacTrac Respiratory‐Triggered Imaging

Purpose: For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4D CT and ExacTrac respiratory triggered images. Method and Materials: 5 liver lesions and 1 lung lesion for a total of 21 fractions were studied. For the 4D CT method, a manual registration method was used between the 4D CT image sets from initial simulation and treatment day. 4D CT derived image guidance shifts were ascertained from comparison to the same breathing phase at simulation. We also used ExacTrac respiratory triggered imaging to verify the target positioning and calculated the differences in image guidance shifts for the two methods. Results: The mean of the observed differences in image guided shifts between 4D CT image guidance and ExacTrac respiratory triggered image guidance was x = 1.6 mm, y = 1.9 mm, z = 2.8 mm, with no difference larger than 5.0 mm noted in any given direction for any individual case. As expected, the largest error occurred in S/I direction, with a mean of 2.8 mm for the six patients. This is expected because respiratory motion and the resulting uncertainties are most pronounced in this S/I direction. The ExacTrac images triggered at two extreme breathing phase, e.g., full exhale vs. inhale, agreed well (< 2.0mm) with each other. Conclusion: The two very promising image guidance methods of 4D CT and ExacTrac respiratory triggered imaging have been presented and compared for SBRT of lung and liver. The mean differences in image guided shifts between the two methods were less than 3 mm, with no individual direction difference greater than 5 mm for the six SBRT patients, 21 delivered fractions evaluated in this study. Potential contributing factors to the disagreement were presented and will be studied further in future work.