Shawn McNeeley

A comparison of daily CT localization to a daily ultrasound-based system in prostate cancer.

PURPOSE Daily CT localization has been demonstrated to be a precise method of correcting radiation field placement by reducing setup and organ motion variations to facilitate dose escalation in prostate carcinoma. The purpose of this study was to evaluate the feasibility and accuracy of daily ultrasound-guided localization utilizing daily CT as a standard. The relatively simple computer-assisted ultrasound-based system is designed to be an efficient means of achieving daily accuracy. METHODS AND MATERIALS After five weeks of conformal external beam radiation therapy, 23 patients underwent a second CT simulation. Prostate-only fields based on this scan were created with no PTV margin. On each of the final conedown treatment days, a repeat CT simulation and isocenter comparison was performed. Ten of the above patients also underwent prostate localization with a newly developed ultrasound-based system (BAT) that is designed to facilitate patient positioning at the treatment machine. The portable system, which electronically imports the CT simulation target contours and isocenter, is situated adjacent to the treatment couch. Transverse and sagittal suprapubic ultrasound images are captured, and the system overlays the corresponding CT contours relative to the machine isocenter. The CT contours are maneuvered in three dimensions by a touch screen menu to match the ultrasound images. The system then displays the 3-D couch shifts required to produce field alignment. RESULTS The BAT ultrasound system produced good quality images with minimal operator training required. The localization process was completed in less than 5 min. The absolute magnitude difference between CT and ultrasound was small (A/P range 0 to 5.9 mm, mean 3 mm +/- 1.8; Lat. range 0 to 7.9 mm, mean 2.4 mm +/- 1.8; S/I range 0 to 9 mm, mean 4.6 mm +/- 2.8). Analysis confirmed a significant correlation of isocenter shifts (A/P r = 0.66, p < 0.0001; Lat. r = 0.58, p < 0.003; S/I r = 0.78, p < 0.0001) in all dimensions, and linear regression confirmed the equivalence of the two modalities. CONCLUSIONS Daily CT localization is a precise method to improve daily target localization in prostate carcinoma. However, it requires significant human and technical resources that limit its widespread applicability. Conversely, localization with the BAT ultrasound system is simple and expeditious by virtue of its ability to image the prostate at the treatment machine in the treatment position. Our initial evaluation revealed ultrasound targeting to be functionally equivalent to CT. This ultrasound technology is promising and warrants further investigation in more patients and at other anatomical sites.

Ultrasound-based stereotactic guidance of precision conformal external beam radiation therapy in clinically localized prostate cancer.

OBJECTIVES Use of external beam radiation fields that conform to the shape of the target improves biochemical control in prostate cancer by facilitating dose escalation through increased sparing of normal tissue. By correcting potential organ motion and setup errors, ultrasound-directed stereotactic localization is a method that may improve the accuracy and effectiveness of current conformal technology. The purpose of this study was to quantify the precision of the transabdominal ultrasound-based approach using computed tomography (CT) as a standard. METHODS Thirty-five consecutive men participated in a prospective comparison of daily CT and ultrasound-guided localization at Fox Chase Cancer Center. Daily CT prostate localization was completed before the delivery of each final boost field. In the CT simulation suite, transabdominal ultrasound-based stereotactic localization was also performed. The main outcome measure was a three-dimensional comparison of prostate position as determined by CT versus ultrasound. RESULTS Sixty-nine daily CT and ultrasound prostate position shifts were recorded for 35 patients. The magnitude of difference between the CT and ultrasound localization ranged from 0 to 7.0 mm in the anterior/posterior, 0 to 6.4 mm in the lateral, and 0 to 6.7 mm in the superior/inferior dimension. The corresponding directed average disagreements were extremely small: anterior/posterior, -0.09 +/- 2.8 mm SD; lateral, -0.16 +/- 2.4 mm SD; and superior/inferior, -0.03 +/- 2.3 mm SD). Analysis of the paired CT-ultrasound shifts revealed a high correlation between the two modalities in all three dimensions (anterior/ posterior r = 0.88; lateral r = 0.91; and superior/inferior r = 0.87). CONCLUSIONS Ultrasound-directed stereotactic localization is safe and as accurate as CT scanning in targeting the prostate for conformal external beam radiation therapy. The application of this technology to current conformal techniques will allow the reduction of treatment margins in all dimensions. This should diminish treatment-related morbidity and facilitate further dose escalation, resulting in improved cancer control.

Ultrasound-Based Stereotactic Guidance in Prostate Cancer—Quantification of Organ Motion and Set-Up Errors in External Beam Radiation Therapy

OBJECTIVE A mobile transabdominal ultrasound-based targeting system (BAT(R)) has been developed which can stereotactically localize the position of the prostate each treatment day and directly integrate this information into the treatment planning system. Daily target verification facilitates a marked reduction in planning treatment margins by correcting potential organ-motion and set-up errors. Previous studies have been performed to establish the precision of ultrasound localization. This report quantifies the magnitude of the patient isocenter shift parameters encountered during clinical implementation of this system. MATERIAL AND METHODS After five weeks of conformal external beam radiation therapy, 54 patients underwent a second CT simulation. Prostate-only fields based on this scan were created with no PTV margin beyond the CTV. For each of the final conedown treatments (2-4 fractions), patients underwent ultrasound-based stereotactic prostate localization at the treatment machine. The portable system, which electronically imports the CT simulation target-contour and isocenter information, is situated adjacent to the treatment couch. Transverse and sagittal suprapubic ultrasound images are captured, and the system electronically couples this data to the baseline isocenter. The CT contours are maneuvered in three dimensions by a touch-screen menu to visually overlay the ultrasound images. The system then displays the three-dimensional (3D) couch shifts required to produce field alignment. RESULTS One hundred and eighty-nine daily ultrasound prostate position shifts were recorded for 54 patients. The isocenter field misalignment between the baseline CT and ultrasound ranged from -26.8 to 33.8 mm in the anterior/posterior (A/P) dimension, -10.2 to 30.9 mm in the lateral dimension, and -24.6 to 9.0 mm in the superior/inferior (S/I) dimension. The corresponding directed average disagreements were -3.0 mm (SD 8.3 mm) A/P, 1.86 mm (SD 5.7 mm) lateral, and -2.6 mm (SD 6.5 mm) S/I. The magnitudes of undirected misalignments were frequently larger than 5 mm (51% of A/P, 31% of lateral, and 35% of superior measurements) and oftentimes larger than 10 mm (21% of A/P, 7% of lateral, and 12% of superior measurements). CONCLUSIONS Organ motion and set-up uncertainties limit optimization of 3D treatment planning by expanding the width of PTV margins required to ensure target coverage. Transabdominal ultrasound-based stereotactic guidance is a safe and direct method for correcting patient positioning. Our experience with the BAT system in a large cohort of prostate cancer patients revealed that substantial daily isocenter corrections were encountered in a large percentage of cases. This data would suggest that daily clinical isocenter misalignments are greater than would be expected from published data on organ motion and set-up variations encountered in the study setting.

Beam characteristics of a retrofitted double-focused multileaf collimator.

Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.

Monte Carlo based IMRT dose verification using MLC log files and R/V outputs

Conventional IMRT dose verification using film and ion chamber measurements is useful but limited with respect to the actual dose distribution received by the patient. The Monte Carlo simulation has been introduced as an independent dose verification tool for IMRT using the patient CT data and MLC leaf sequence files, which validates the dose calculation accuracy but not the plan delivery accuracy. In this work, we propose a Monte Carlo based IMRT dose verification method that reconstructs the patient dose distribution using the patient CT, actual beam data based on the information from the record and verify system (R/V), and the MLC log files obtained during dose delivery that record the MLC leaf positions and MUs delivered. Comparing the Monte Carlo dose calculation with the original IMRT plan using these data simultaneously validates the accuracy of both the IMRT dose calculation and beam delivery. Such log file based Monte Carlo simulations are expected to be employed as a useful and efficient IMRT QA modality to validate the dose delivered to the patient. We have run Monte Carlo simulations for eight IMRT prostate plans using this method and the results for the target dose were consistent with the original CORVUS treatment plans to within 3.0% and 2.0% with and without heterogeneity corrections in the dose calculation. However, significant dose deviations in nearby critical structures have been observed. The results showed that up to 9.0% of the bladder dose and up to 38.0% of the rectum dose, to which leaf position errors were found to contribute <2%, were underestimated by the CORVUS treatment planning system. The concept of average leaf position error has been defined to analyze MLC leaf position errors for an IMRT plan. A linear correlation between the target dose error and the average position error has been found based on log file based Monte Carlo simulations, showing that an average position error of 0.2 mm can result in a target dose error of about 1.0%.

Dosimetric verification of IMRT treatment planning using Monte Carlo simulations for prostate cancer

The purpose of this work is to investigate the accuracy of dose calculation of a commercial treatment planning system (Corvus, Normos Corp., Sewickley, PA). In this study, 30 prostate intensity-modulated radiotherapy (IMRT) treatment plans from the commercial treatment planning system were recalculated using the Monte Carlo method. Dose-volume histograms and isodose distributions were compared. Other quantities such as minimum dose to the target (D(min)), the dose received by 98% of the target volume (D98), dose at the isocentre (D(iso)), mean target dose (D(mean)) and the maximum critical structure dose (D(max)) were also evaluated based on our clinical criteria. For coplanar plans, the dose differences between Monte Carlo and the commercial treatment planning system with and without heterogeneity correction were not significant. The differences in the isocentre dose between the commercial treatment planning system and Monte Carlo simulations were less than 3% for all coplanar cases. The differences on D98 were less than 2% on average. The differences in the mean dose to the target between the commercial system and Monte Carlo results were within 3%. The differences in the maximum bladder dose were within 3% for most cases. The maximum dose differences for the rectum were less than 4% for all the cases. For non-coplanar plans, the difference in the minimum target dose between the treatment planning system and Monte Carlo calculations was up to 9% if the heterogeneity correction was not applied in Corvus. This was caused by the excessive attenuation of the non-coplanar beams by the femurs. When the heterogeneity correction was applied in Corvus, the differences were reduced significantly. These results suggest that heterogeneity correction should be used in dose calculation for prostate cancer with non-coplanar beam arrangements.

Target localization for post-prostatectomy patients using CT and ultrasound image guidance.

We conducted a study comparing B‐mode acquisition and targeting (BAT) ultrasound alignments based on CT data in the postoperative setting. CT scans were obtained with a Primatom CT‐on‐rails on nine patients. Two CT scans were obtained each week, while setup error was minimized by BAT ultrasounds. For the first three patients, a direct comparison was performed. For the next six patients, a template based on the shifts from the week 1 CT during treatment was used for subsequent setup. Comparison of isocenter shifts between the BAT ultrasound and CT was made by the difference, absolute difference, and improvement (using CT alignments as the reference technique). A total of 90 image comparisons were made. The average interfraction motion was 3.2 mm in the lateral, 3.0 mm in the longitudinal, and 5.1 mm in the AP direction. The results suggest that the CT‐based ultrasound templates can improve the localization of the prostate bed when the initial displacements are greater than 4 mm. For initial displacements smaller than 4 mm, the technique neither improved nor worsened target localization. However, ultrasound alignments performed without the use of a template deteriorated patient positioning for two out of three patients, demonstrating that the use of a CT template was beneficial even at small initial displacements. PACS numbers: 87.53.‐j, 87.53.Kn, 87.53.Xd

A method for increased dose conformity and segment reduction for SMLC delivered IMRT treatment of the prostate.

PURPOSE The focus of this work is to develop a practical planning method that results in increased dose conformity and reduced treatment time for segmental multileaf collimation (sMLC) based intensity-modulated radiation therapy (IMRT) delivery. METHODS AND MATERIALS Additional regions for dose constraint are introduced within the normal tissue during the planning process by designing a series of concentric ellipsoids around the target. A dose gradient is then defined by assigning dose constraints to each concentric region. The technique was tested at two centers and data for 26 and 10 patients, respectively, are presented allowing for differences in treatment technique, beam energy, ellipsoid definition, and prescription dose. At both centers, a series of patients previously treated for prostate cancer with IMRT were selected, and comparisons were made between the original and new plans. RESULTS While meeting target dose specifications and normal tissue constraints, the average number of beam directions decreased by 1.6 with a standard error (SE) of 0.1. The average time for delivery at center 1 decreased by 29.0% with an SE of 2.0%, decreasing from 17.5 min to 12.3 min. The average time for delivery at center 2 decreased by 29.9% with an SE of 3.8%, decreasing from 11 min to 7.7 min. The amount of nontarget tissue receiving D(100) decreased by 15.7% with an SE of 2.4%. Nontarget tissue receiving D(95), D(90), and D(50) decreased by 16.3, 15.1, and 19.5%, respectively, with SE values of approximately 2% at center 1. Corresponding values for D(100), D(95), D(90), and D(50) decreased by 13.5, 16.7, 17.1, and 5.1%, respectively, with SE values of less than 3% at center 2. CONCLUSION By designating subsets of tissue as concentric regions around the target(s) and carefully defining each regions dose constraints, we have gained an increased measure of control over the region outside the target boundaries. This increased control manifests as two distinct endpoints that are beneficial to the IMRT process: increased dose conformity and decreased treatment time.

Quality assurance of U.S.-guided external beam radiotherapy for prostate cancer: Report of AAPM Task Group 154

Task Group 154 (TG154) of the American Association of Physicists in Medicine (AAPM) was created to produce a guidance document for clinical medical physicists describing recommended quality assurance (QA) procedures for ultrasound (U.S.)-guided external beam radiotherapy localization. This report describes the relevant literature, state of the art, and briefly summarizes U.S. imaging physics. Simulation, treatment planning and treatment delivery considerations are presented in order to improve consistency and accuracy. User training is emphasized in the report and recommendations regarding peer review are included. A set of thorough, yet practical, QA procedures, frequencies, and tolerances are recommended. These encompass recommendations to ensure both spatial accuracy and image quality.

Daily target localization for prostate patients based on 3D image correlation

There are several localization techniques that have been used for prostate treatment. Recently, the potential use of a variety of CT-based equipment in the treatment room has been discussed. The goal of our study was to develop an automated procedure for daily treatment table shift calculation based on two CT data sets: simulation CT data and localization CT data. The method suggested in this study is a 3D image cross-correlation of small regions of interest (ROI) within the two data sets. The relative position of the two ROIs with respect to each other is determined by the maximum value of the normalized cross-correlation function, calculated for all possible relative locations of the two ROIs. After the best match is found the shifts are given by the vector connecting the treatment isocentre and the planning isocentre (both determined by the radio opaque fiducial markers on the patients skin). The results have been compared with shifts calculated through manual fusion. The shift differences, averaged over 17 statistically independent shift calculations, are less then 1 mm in the lateral and longitudinal directions, and about 1 mm in the AP direction. The impact of image noise on the performance of the algorithm has been tested. The results show that the algorithm accurately adjusts for target positional changes even with Gaussian noise levels as high as 20% inserted.