Navigating the medical physics education and training landscape.

PURPOSE: The education and training landscape has been profoundly reshaped by the ABR 2012/2014 initiative and the MedPhys Match. This work quantifies these changes and summarizes available reports, surveys, and statistics on education and training. METHODS: We evaluate data from CAMPEP-accredited program websites, annual CAMPEP graduate and residency program reports, and surveys on the MedPhys Match and Professional Doctorate degree (DMP). RESULTS: From 2009-2015, the number of graduates from CAMPEP-accredited graduate programs rose from 210 to 332, while CAMPEP-accredited residency positions rose from 60 to 134. We estimate that approximately 60% of graduates of CAMPEP-accredited graduate programs intend to enter clinical practice, however, only 36% of graduates were successful in acquiring a residency position in 2015. The maximum residency placement percentage for a graduate program is 70%, while the median for all programs is only 22%. Overall residency placement percentage for CAMPEP-accredited program graduates from 2011-2015 was approximately 38% and 25% for those with a PhD and MS, respectively. The disparity between the number of clinically oriented graduates and available residency positions is perceived as a significant problem by over 70% of MedPhys Match participants responding to a post-match survey. Approximately 32% of these respondents indicated that prior knowledge of this situation would have changed their decision to pursue graduate education in medical physics. CONCLUSION: These data reveal a substantial disparity between the number of residency training positions and graduate students interested in these positions, and a substantial variability in residency placement percentage across graduate programs. Comprehensive data regarding current and projected supply and demand within the medical physics workforce are needed for perspective on these numbers. While the long-term effects of changes in the education and training infrastructure are still unclear, available survey data suggest that these changes could negatively affect potential entrants to the profession.

Dosimetric effects of jaw tracking in step-and-shoot intensity-modulated radiation therapy.

The purpose of this work was to determine the dosimetric benefit to normal tissues by tracking the multi-leaf collimator (MLC) apertures with the photon jaws in step-and-shoot intensity-modulated radiation therapy (IMRT) on the Varian 2100 platform. Radiation treatment plans for ten thoracic, three pediatric, and three head and neck cancer patients were converted to plans with the jaws tracking each segment's MLC apertures, and compared to the original plans in a commercial radiation treatment planning system (TPS). The change in normal tissue dose was evaluated in the new plan by using the parameters V5, V10, and V20 (volumes receiving 5, 10 and 20 Gy, respectively) in the cumulative dose-volume histogram for the following structures: total lung minus gross target volume, heart, esophagus, spinal cord, liver, parotids, and brainstem. To validate the accuracy of our beam model, MLC transmission was measured and compared to that predicted by the TPS. The greatest changes between the original and new plans occurred at lower dose levels. In all patients, the reduction in V20 was never more than 6.3% and was typically less than 1%; the maximum reduction in V5 was 16.7% and was typically less than 3%. The variation in normal tissue dose reduction was not predictable, and we found no clear parameters that indicated which patients would benefit most from jaw tracking. Our TPS model of MLC transmission agreed with measurements with absolute transmission differences of less than 0.1% and, thus, uncertainties in the model did not contribute significantly to the uncertainty in the dose determination. We conclude that the amount of dose reduction achieved by collimating the jaws around each MLC aperture in step-and-shoot IMRT is probably not clinically significant.

Thoracic target volume delineation using various maximum-intensity projection computed tomography image sets for radiotherapy treatment planning.

PURPOSE: Four-dimensional computed tomography (4D-CT) is commonly used to account for respiratory motion of target volumes in radiotherapy to the thorax. From the 4D-CT acquisition, a maximum-intensity projection (MIP) image set can be created and used to help define the tumor motion envelope or the internal gross tumor volume (iGTV). The purpose of this study was to quantify the differences in automatically contoured target volumes for usage in the delivery of stereotactic body radiation therapy using MIP data sets generated from one of the four methods: (1) 4D-CT phase-binned (PB) based on retrospective phase calculations, (2) 4D-CT phase-corrected phase-binned (PC-PB) based on motion extrema, (3) 4D-CT amplitude-binned (AB), and (4) cine CT built from all available images. METHODS: MIP image data sets using each of the four methods were generated for a cohort of 28 patients who had prior thoracic 4D-CT scans that exhibited lung tumor motion of at least 1 cm. Each MIP image set was automatically contoured on commercial radiation treatment planning system. Margins were added to the iGTV to observe differences in the final simulated planning target volumes (PTVs). RESULTS: For all patients, the iGTV measured on the MIP generated from the entire cine CT data set (iGTVcine) was the largest. Expressed as a percentage of iGTVcine, 4D-CT iGTV (all sorting methods) ranged from 83.8% to 99.1%, representing differences in the absolute volume ranging from 0.02 to 4.20 cm3; the largest average and range of 4D-CT iGTV measurements was from the PC-PB data set. Expressed as a percentage of PTVcine (expansions applied to iGTVeine), the 4D-CT PTV ranged from 87.6% to 99.6%, representing differences in the absolute volume ranging from 0.08 to 7.42 cm3. Regions of the measured respiratory waveform corresponding to a rapid change of phase or amplitude showed an increased susceptibility to the selection of identical images for adjacent bins. Duplicate image selection was most common in the AB implementation, followed by the PC-PB method. The authors also found that the image associated with the minimum amplitude measurement did not always correlate with the image that showed maximum tumor motion extent. CONCLUSIONS: The authors identified cases in which the MIP generated from a 4D-CT sorting process under-represented the iGTV by more than 10% or up to 4.2 cm3 when compared to the iGTVcine. They suggest utilization of a MIP generated from the full cine CT data set to ensure maximum inclusive tumor extent.

Comparing the accuracy of four-dimensional photon dose calculations with three-dimensional calculations using moving and deforming phantoms.

PURPOSE: Four-dimensional (4D) dose calculation algorithms, which explicitly incorporate respiratory motion in the calculation of doses, have the potential to improve the accuracy of dose calculations in thoracic treatment planning; however, they generally require greater computing power and resources than currently used for three-dimensional (3D) dose calculations. The purpose of this work was to quantify the increase in accuracy of 4D dose calculations versus 3D dose calculations. METHODS: The accuracy of each dose calculation algorithm was assessed using measurements made with two phantoms. Specifically, the authors used a rigid moving anthropomorphic thoracic phantom and an anthropomorphic thoracic phantom with a deformable lung insert. To incorporate a clinically relevant range of scenarios, they programed the phantoms to move and deform with two motion patterns: A sinusoidal motion pattern and an irregular motion pattern that was extracted from an actual patient's breathing profile. For each combination of phantom and motion pattern, three plans were created: A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan. Doses were calculated using 4D dose calculation methods as well as conventional 3D dose calculation methods. The rigid moving and deforming phantoms were irradiated according to the three treatment plans and doses were measured using thermoluminescent dosimeters (TLDs) and radiochromic film. The accuracy of each dose calculation algorithm was assessed using measured-to-calculated TLD doses and a gamma analysis. RESULTS: No significant differences were observed between the measured-to-calculated TLD ratios among 4D and 3D dose calculations. The gamma results revealed that 4D dose calculations had significantly greater percentage of pixels passing the 5%/3 mm criteria than 3D dose calculations. CONCLUSIONS: These results indicate no significant differences in the accuracy between the 4D and the 3D dose calculation methods inside the gross tumor volume. On the other hand, the film results demonstrated that the 4D dose calculations provided greater accuracy than 3D dose calculations in heterogeneous dose regions. The increase in accuracy of the 4D dose calculations was evident throughout the planning target volume.

Verification of four-dimensional photon dose calculations.

Recent work in the area of thoracic treatment planning has been focused on trying to explicitly incorporate patient-specific organ motion in the calculation of dose. Four-dimensional (4D) dose calculation algorithms have been developed and incorporated in a research version of a commercial treatment planning system (Pinnacle3, Philips Medical Systems, Milpitas, CA). Before these 4D dose calculations can be used clinically, it is necessary to verify their accuracy with measurements. The primary purpose of this study therefore was to evaluate and validate the accuracy of a 4D dose calculation algorithm with phantom measurements. A secondary objective was to determine whether the performance of the 4D dose calculation algorithm varied between different motion patterns and treatment plans. Measurements were made using two phantoms: A rigid moving phantom and a deformable phantom. The rigid moving phantom consisted of an anthropomorphic thoracic phantom that rested on a programmable motion platform. The deformable phantom used the same anthropomorphic thoracic phantom with a deformable insert for one of the lungs. Two motion patterns were investigated for each phantom: A sinusoidal motion pattern and an irregular motion pattern extracted from a patient breathing profile. A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan were created. Doses were calculated in the treatment planning system using the 4D dose calculation algorithm. Then each plan was delivered to the phantoms and delivered doses were measured using thermoluminescent dosimeters (TLDs) and film. The measured doses were compared to the 4D-calculated doses using a measured-to-calculated TLD ratio and a gamma analysis. A relevant passing criteria (3% for the TLD and 5% /3 mm for the gamma metric) was applied to determine if the 4D dose calculations were accurate to within clinical standards. All the TLD measurements in both phantoms satisfied the passing criteria. Furthermore, 42 of the 48 evaluated films fulfilled the passing criteria. All films that did not pass the criteria were from the rigid phantom moving with irregular motion. The author concluded that if patient breathing is reproducible, the 4D dose calculations are accurate to within clinically acceptable standards. Furthermore, they found no statistically significant differences in the performance of the 4D dose calculation algorithm between treatment plans.

Influence of source parameters on large-field electron beam profiles calculated using Monte Carlo methods.

The purpose of this paper was to study the source model for a Monte Carlo simulation of electron beams from a medical linear accelerator. In a prior study, a non-divergent Gaussian source with a full-width at half-maximum (FWHM) of 0.15 cm was successful in predicting relative dose distributions for electron beams with applicators. However, for large fields with the applicator removed, discrepancies were found between measured and calculated profiles, particularly in the shoulder region. In this work, the source was changed to a divergent Gaussian spatial distribution and the FWHM parameter was varied to produce better agreement with measured data. The influence of the FWHM source parameter on profiles was observed at multiple locations in the simulation geometry including in-air fluence profiles at a 95 cm source-to-surface distance (SSD), percent depth dose profiles and off-axis profiles (OARs) in a water phantom for two SSDs, 80 and 100 cm. For a 6 MeV 40 x 40 cm(2) OAR profile, discrepancies in the shoulder region were reduced from 15% to 4% using a FWHM value of 0.45 cm. The optimal FWHM values for the other energies were 0.45 cm for 9 MeV, 0.22 for 12 MeV, 0.25 for 16 MeV and 0.2 cm for 20 MeV. Although this range of values was larger than measured focal spot sizes reported by other researchers, using the increased FWHM values improved the fit at most locations in the simulation geometry, giving confidence that the model could be used with a variety of SSDs and field sizes.

Respiratory gating with EPID-based verification: the MDACC experience.

We have investigated the feasibility and accuracy of using a combination of internal and external fiducials for respiratory-gated image-guided radiotherapy of liver tumors after screening for suitable patients using a mock treatment. Five patients were enrolled in the study. Radio-opaque fiducials implanted adjacent to the liver tumor were used for daily online positioning using either electronic portal or kV images. Patient eligibility was assessed by determining the degree of correlation between the external and internal fiducials as analyzed during a mock treatment. Treatment delivery was based on the modification of conventional amplitude-based gating. Finally, the accuracy of respiratory-gated treatment using an external fiducial was verified offline using the cine mode of an electronic portal imaging device. For all patients, interfractional contribution to the random error was 2.0 mm in the supero-inferior direction, which is the dominant direction of motion due to respiration, while the interfractional contribution to the systematic error was 0.9 mm. The intrafractional contribution to the random error was 1.0 mm. One of the significant advantages to this technique is improved patient set-up using implanted fiducials and gated imaging. Daily assessment of images acquired during treatment verifies the accuracy of the delivered treatment and uncovers problems in patient set-up.

Validation of a model-based segmentation approach to propagating normal anatomic regions of interest through the 10 phases of respiration.

PURPOSE: To validate a model-based segmentation (MBS) algorithm in a commercial radiation treatment planning system for use in propagating the contours of normal anatomic regions of interest (ROIs) through the respiratory phases that constitute a four-dimensional (4D) computed tomography (CT) image data set. METHODS AND MATERIALS: The 4D CT data sets for 12 patients treated for non-small-cell lung cancer were acquired. Five ROIs were selected for delineation: right and left lungs, spinal cord, heart, and esophagus. These ROIs were manually delineated on the CT data set corresponding to the end-inspiration respiratory phase (0%). An MBS algorithm implemented on the treatment planning system propagated the ROIs sequentially through the respiratory phases that constituted the 4D CT data sets, concluding with the 0% phase data set, which was propagated from the 90% phase data set. The propagated ROIs on the 0% phase were compared with the original ROIs on that phase by using visual assessment and a quantitative measure of coincidence. RESULTS: Acceptable propagation accuracy within 1 mm of uncertainty was achieved for lungs and spinal cord. Propagation of the heart produced slightly larger contours that were similar to interphysician variations in contouring the heart. The esophagus was poorly propagated because of lack of tissue contrast and definitive shape. CONCLUSIONS: The MBS propagation is a promising tool for efficiently propagating contours through the different phases of respiration. However, propagating the esophagus through this technique may be difficult because of the lack of definitive shape and clearer boundaries from surrounding tissue.

Utility of four-dimensional computed tomography for analysis of intrafractional and interfractional variation in lung volumes.

PURPOSE: To assess the viability of four-dimensional (4D) computed tomography (CT) in describing intrafractional and interfractional changes in lung volumes and to determine which breathing phase, if any, produces the most highly reproducible lung volumes among fractions. METHODS AND MATERIALS: Weekly 4D CT scans were acquired for 13 patients with non-small-cell lung cancer during a course of radiotherapy. Contours delineating the right lung, left lung, and total lung were obtained by adapting library models of the anatomic structures to the CT images and propagating them to all 10 respiratory phases represented in the 4D CT image data set. Lung volumes were calculated using software tools in a commercial radiation treatment-planning system and analyzed for interfractional volume reproducibility using t tests and for phase reproducibility using a phase-dependent uncertainty curve across all patients. Probability (p) values of <0.05 were considered to indicate significant differences in all comparisons. RESULTS: The average mean coefficient of variation of tidal volume across all patients was 25.0%. The average standard deviation of tidal volumes was 5.7% relative to the lung volume at end-expiration. Total volumes measured at the 30% phase were 15% more consistent than those measured at end-inspiration (p = 0.03). CONCLUSIONS: Four-dimensional CT assesses lung volume with acceptable precision; but the technique was unable to accurately predict interfractional changes in lung volume because wide variations in intra- and interfractional breathing cause high uncertainties in 4D CT data acquisition. The most reproducible breathing phase seems to be at the 30-40% phase (just before end-expiration).

Comparison of rigid and adaptive methods of propagating gross tumor volume through respiratory phases of four-dimensional computed tomography image data set.

PURPOSE: To compare three different methods of propagating the gross tumor volume (GTV) through the respiratory phases that constitute a four-dimensional computed tomography image data set. METHODS AND MATERIALS: Four-dimensional computed tomography data sets of 20 patients who had undergone definitive hypofractionated radiotherapy to the lung were acquired. The GTV regions of interest (ROIs) were manually delineated on each phase of the four-dimensional computed tomography data set. The ROI from the end-expiration phase was propagated to the remaining nine phases of respiration using the following three techniques: (1) rigid-image registration using in-house software, (2) rigid image registration using research software from a commercial radiotherapy planning system vendor, and (3) rigid-image registration followed by deformable adaptation originally intended for organ-at-risk delineation using the same software. The internal GTVs generated from the various propagation methods were compared with the manual internal GTV using the normalized Dice similarity coefficient (DSC) index. RESULTS: The normalized DSC index of 1.01 +/- 0.06 (SD) for rigid propagation using the in-house software program was identical to the normalized DSC index of 1.01 +/- 0.06 for rigid propagation achieved with the vendor's research software. Adaptive propagation yielded poorer results, with a normalized DSC index of 0.89 +/- 0.10 (paired t test, p <0.001). CONCLUSION: Propagation of the GTV ROIs through the respiratory phases using rigid- body registration is an acceptable method within a 1-mm margin of uncertainty. The adaptive organ-at-risk propagation method was not applicable to propagating GTV ROIs, resulting in an unacceptable reduction of the volume and distortion of the ROIs.

Long-term results of the M. D. Anderson randomized dose-escalation trial for prostate cancer.

PURPOSE: To report the long-term results of a randomized radiotherapy dose escalation trial for prostate cancer. METHODS AND MATERIALS: From 1993 to 1998, a total of 301 patients with stage T1b to T3 prostate cancer were accrued to a randomized external beam dose escalation trial using 70 Gy versus 78 Gy. The median follow-up is now 8.7 years. Kaplan-Meier analysis was used to compute rates of prostate-specific antigen (PSA) failure (nadir + 2), clinical failure, distant metastasis, disease-specific, and overall survival as well as complication rates at 8 years post-treatment. RESULTS: For all patients, freedom from biochemical or clinical failure (FFF) was superior for the 78-Gy arm, 78%, as compared with 59% for the 70-Gy arm (p = 0.004, and an even greater benefit was seen in patients with initial PSA >10 ng/ml (78% vs. 39%, p = 0.001). The clinical failure rate was significantly reduced in the 78-Gy arm as well (7% vs. 15%, p = 0.014). Twice as many patients either died of prostate cancer or are currently alive with cancer in the 70-Gy arm. Gastrointestinal toxicity of grade 2 or greater occurred twice as often in the high dose patients (26% vs. 13%), although genitourinary toxicity of grade 2 or greater was less (13% vs. 8%) and not statistically significantly different. Dose-volume histogram analysis showed that the complication rate could be significantly decreased by reducing the amount of treated rectum. CONCLUSIONS: Modest escalation in radiotherapy dose improved freedom from biochemical and clinical progression with the largest benefit in prostate cancer patients with PSA >10 ng/ml.

Anniversary paper: roles of medical physicists and health care applications of informatics.

Over the past 100 years, both diagnostic radiology and radiation therapy have grown from infancy to maturity. Accompanying this growth, the discipline of medical physics has evolved and advanced accordingly. New diagnostic and therapeutic procedures continue to be developed, for example, multidetector computed tomography, multileaf collimation, magnetic resonance imaging, dual-source computed tomography, and intensity-modulated radiation therapy. These are now incorporated in health care facilities throughout the world. Modern technologies such as these provide information on underlying pathology at increasingly higher resolutions, generating more information; thus requiring complex methods of image recording and storage. The management of the storage and retrieval of accumulated information is a domain of informatics. In this short review, we describe the different roles of medical physicists and the effective contribution of the American Association of Physicists in Medicine in the evolution of informatics. Medical physicists have contributed to the development of informatics in numerous ways, such as designing hospital information systems and infrastructures that better serve radiologists and other physicians. In addition, the positive exploitation of knowledge gathered in medical settings and effective interdisciplinary collaborations between scientists of different backgrounds have increased. These developments provide future medical physicists the opportunity to develop strategic roles in information technology and thus better contribute to health care.

Monte Carlo calculations and measurements of absorbed dose per monitor unit for the treatment of uveal melanoma with proton therapy.

The treatment of uveal melanoma with proton radiotherapy has provided excellent clinical outcomes. However, contemporary treatment planning systems use simplistic dose algorithms that limit the accuracy of relative dose distributions. Further, absolute predictions of absorbed dose per monitor unit are not yet available in these systems. The purpose of this study was to determine if Monte Carlo methods could predict dose per monitor unit (D/MU) value at the center of a proton spread-out Bragg peak (SOBP) to within 1% on measured values for a variety of treatment fields relevant to ocular proton therapy. The MCNPX Monte Carlo transport code, in combination with realistic models for the ocular beam delivery apparatus and a water phantom, was used to calculate dose distributions and D/MU values, which were verified by the measurements. Measured proton beam data included central-axis depth dose profiles, relative cross-field profiles and absolute D/MU measurements under several combinations of beam penetration ranges and range-modulation widths. The Monte Carlo method predicted D/MU values that agreed with measurement to within 1% and dose profiles that agreed with measurement to within 3% of peak dose or within 0.5 mm distance-to-agreement. Lastly, a demonstration of the clinical utility of this technique included calculations of dose distributions and D/MU values in a realistic model of the human eye. It is possible to predict D/MU values accurately for clinical relevant range-modulated proton beams for ocular therapy using the Monte Carlo method. It is thus feasible to use the Monte Carlo method as a routine absolute dose algorithm for ocular proton therapy.

Determination of an optimal organ set to implement deformations to support four-dimensional dose calculations in radiation therapy planning.

Surface-based deformable image registration to generate a four-dimensional (4D) dose calculation in radiation treatment planning requires the selection of a set of organ contours to represent a basis set to generate anatomic deformation. The purpose of the present work was to determine the optimal set of organs needed to generate a basis set for deformation in treatment planning for thoracic tumors such that the required computations are minimized but dose accuracy is high. Using retrospectively reviewed records, we calculated 4D dose distributions based on treatment plans for 10 patients with thoracic tumors using a deformable model algorithm in a research version of a commercial radiation treatment planning system. Various combinations of organs (total lungs, heart, spinal cord, external body surface) were used to generate the basis set for deformations used in the calculations. The external surface contour did not have a noticeable effect on the dose calculation. Total lung, heart, and spinal cord together provided an adequate set of deformation organs to generate accurate dose deformations. The magnitude of calculated dose differences had no obvious relationship to tumor parameters, including site, histologic type, disease stage, extent of motion, or degree of centralization.

Comparison of breath-hold and free-breathing positions of an external fiducial by analysis of respiratory traces.

An internal target volume (ITV) accounting for respiratory-induced tumor motion is best obtained using 4DCT. However, when 4DCT is not available, inspiratory/expiratory breath-hold (BH insp, BH exp) CT images have been suggested as an alternative. In such cases, an external fiducial on the abdomen can be used as a substitute for tumor motion and CT images are acquired when the marker position matches - as judged by the therapist/physicist - its positions at previously determined free-breathing (FB) respiratory extrema (FB insp, FB exp). In this study we retrospectively determined the accuracy of these matches. Free breathing 4DCT images were acquired, followed by BH insp and BH exp CT images for 25 patients with non-small-cell lung cancer. Respiration was monitored using a commercial external fiducial system, which generates positional information while CT studies are conducted. Software was written for statistically analyzing the displacement of the external fiducial during BH insp and BH exp CT acquisition and comparing these displacements with corresponding mean FB extrema positions (FB insp and FB exp, respectively) using a Student's t-test. In 72% of patients, mean positions at BH insp differed significantly from mean positions at FB insp (p < 0.05: 0.13 - 1.40 cm). In 92% of patients, mean positions at BH exp differed significantly from mean positions at FB exp (p < 0.05: 0.03 - 0.70 cm), although this difference was smaller than 0.5 cm in many cases (median = 0.34 cm). Our findings indicate that relying solely on abdominal external markers for accurate BH CT imaging in order to accurately estimate FB extrema positions may be subject to significant error.

Use of three-dimensional (3D) optical flow method in mapping 3D anatomic structure and tumor contours across four-dimensional computed tomography data.

A 3-dimensional (3D) optical flow program that includes a multi-resolution feature has been developed and applied to 3D anatomical structure and gross tumor volume (GTV) contour mapping for 4-dimensional (4D) CT data. The study includes contour mapping for 3 real patient CT data sets, and also for a thoracic phantom in which the displacement for each voxel is known. Of the real patient CT data sets, one set has been used to map contours of lung and GTV over all the respiration phases, while the others were studied using only the end inspiration and end expiration phases, in which the displacement between the phases were the largest. Including the residual motion in the 4D CT data and motion table shaking, the optical flow calculation agrees to within 1 mm with the known displacement. Excluding those errors that are not introduced by optical flow algorithm, the agreement can be within 0.1 mm with a displacement magnitude of 24 mm. The mapped contours of lungs, liver, esophagus, GTV, etc. in real patient 4D CT images were acceptable to clinicians. The 3D optical flow program is a good tool for anatomical structure and tumor volume contour mapping across 4D CT scans.

A technique for reducing patient setup uncertainties by aligning and verifying daily positioning of a moving tumor using implanted fiducials.

This study aimed to validate and implement a methodology in which fiducials implanted in the periphery of lung tumors can be used to reduce uncertainties in tumor location. Alignment software that matches marker positions on two-dimensional (2D) kilovoltage portal images to positions on three-dimensional (3D) computed tomography data sets was validated using static and moving phantoms. This software also was used to reduce uncertainties in tumor location in a patient with fiducials implanted in the periphery of a lung tumor. Alignment of fiducial locations in orthogonal projection images with corresponding fiducial locations in 3D data sets can position both static and moving phantoms with an accuracy of 1 mm. In a patient, alignment based on fiducial locations reduced systematic errors in the left-right direction by 3 mm and random errors by 2 mm, and random errors in the superior-inferior direction by 3 mm as measured by anterior-posterior cine images. Software that matches fiducial markers on 2D and 3D images is effective for aligning both static and moving fiducials before treatment and can be implemented to reduce patient setup uncertainties.

Assessment of lung tumor motion and setup uncertainties using implanted fiducials.

PURPOSE: The purpose of this work was to assess the magnitude of setup uncertainties and respiratory-induced motion of lung tumors by monitoring the location of fiducials implanted in the vicinity of the tumors. METHODS AND MATERIALS: Gold fiducials were implanted in the periphery of lung tumors in 5 patients who had Stage III non-small-cell lung cancer. Fiducial motion was measured using weekly repeated four-dimensional computed tomography (4DCT) imaging and during gated treatment each day using an electronic portal imaging device (EPID). Setup uncertainties were quantified using both the EPID images and the 4DCT data sets. RESULTS: We observed a reduction in fiducial motion (left/right and superior/inferior directions) during gated treatment; however, large gated motion was present (>1 cm). Systematic and random uncertainties based on patient setup ranged from 4 to 6 mm in all three directions as measured using fiducials on gated EPID images and repeat 4DCTs, and using bony anatomy on repeat 4DCTs. CONCLUSIONS: Respiratory gating may be an effective method of reducing average motion during the course of treatment, but large motion is still possible when delivering gated treatment. Setup uncertainties were on the order of, if not larger than, residual gated motion. We recommend careful consideration of all sources of error before reducing margins on the basis of respiratory motion management alone without a strategy for accurate patient setup on a daily basis.