B White

A novel fast helical 4D-CT acquisition technique to generate low-noise sorting artifact-free images at user-selected breathing phases.

PURPOSE To develop a novel 4-dimensional computed tomography (4D-CT) technique that exploits standard fast helical acquisition, a simultaneous breathing surrogate measurement, deformable image registration, and a breathing motion model to remove sorting artifacts. METHODS AND MATERIALS Ten patients were imaged under free-breathing conditions 25 successive times in alternating directions with a 64-slice CT scanner using a low-dose fast helical protocol. An abdominal bellows was used as a breathing surrogate. Deformable registration was used to register the first image (defined as the reference image) to the subsequent 24 segmented images. Voxel-specific motion model parameters were determined using a breathing motion model. The tissue locations predicted by the motion model in the 25 images were compared against the deformably registered tissue locations, allowing a model prediction error to be evaluated. A low-noise image was created by averaging the 25 images deformed to the first image geometry, reducing statistical image noise by a factor of 5. The motion model was used to deform the low-noise reference image to any user-selected breathing phase. A voxel-specific correction was applied to correct the Hounsfield units for lung parenchyma density as a function of lung air filling. RESULTS Images produced using the model at user-selected breathing phases did not suffer from sorting artifacts common to conventional 4D-CT protocols. The mean prediction error across all patients between the breathing motion model predictions and the measured lung tissue positions was determined to be 1.19 ± 0.37 mm. CONCLUSIONS The proposed technique can be used as a clinical 4D-CT technique. It is robust in the presence of irregular breathing and allows the entire imaging dose to contribute to the resulting image quality, providing sorting artifact-free images at a patient dose similar to or less than current 4D-CT techniques.

Accuracy of Routine Treatment Planning 4-Dimensional and Deep-Inspiration Breath-Hold Computed Tomography Delineation of the Left Anterior Descending Artery in Radiation Therapy

PURPOSE To assess the feasibility of radiation therapy treatment planning 4-dimensional computed tomography (4DCT) and deep-inspiration breath-hold (DIBH) CT to accurately contour the left anterior descending artery (LAD), a primary indicator of radiation-induced cardiac toxicity for patients undergoing radiation therapy. METHODS AND MATERIALS Ten subjects were prospectively imaged with a cardiac-gated MRI protocol to determine cardiac motion effects, including the displacement of a region of interest comprising the LAD. A series of planar views were obtained and resampled to create a 3-dimensional (3D) volume. A 3D optical flow deformable image registration algorithm determined tissue displacement during the cardiac cycle. The measured motion was then used as a spatial boundary to characterize motion blurring of the radiologist-delineated LAD structure for a cohort of 10 consecutive patients enrolled prospectively on a breast study including 4DCT and DIBH scans. Coronary motion-induced blurring artifacts were quantified by applying an unsharp filter to accentuate the LAD structure despite the presence of motion blurring. The 4DCT maximum inhalation and exhalation respiratory phases were coregistered to determine the LAD displacement during tidal respiration, as visualized in 4DCT. RESULTS The average 90th percentile heart motion for the region of interest was 0.7 ± 0.1 mm (left-right [LR]), 1.3 ± 0.6 mm (superior-inferior [SI]), and 0.6 ± 0.2 mm (anterior-posterior [AP]) in the cardiac-gated MRI cohort. The average relative increase in the number of voxels comprising the LAD contour was 69.4% ± 4.5% for the DIBH. The LAD volume overestimation had the dosimetric impact of decreasing the reported mean LAD dose by 23% ± 9% on average in the DIBH. During tidal respiration the average relative LAD contour increase was 69.3% ± 5.9% and 67.9% ± 4.6% for inhalation and exhalation respiratory phases, respectively. The average 90th percentile LAD motion was 4.8 ± 1.1 mm (LR), 0.9 ± 0.4 mm (SI), and 1.9 ± 0.6 mm (AP) for the 4DCT cohort, in the absence of cardiac gating. CONCLUSIONS An anisotropic margin of 2.7 mm (LR), 4.1 mm (SI), and 2.4 mm (AP) was quantitatively determined to account for motion blurring and patient setup error while placing minimum constraint on the plan optimization.

Initial Report of Pencil Beam Scanning Proton Therapy for Posthysterectomy Patients With Gynecologic Cancer

PURPOSE To report the acute toxicities associated with pencil beam scanning proton beam radiation therapy (PBS) for whole pelvis radiation therapy in women with gynecologic cancers and the results of a dosimetric comparison of PBS versus intensity modulated radiation therapy (IMRT) plans. METHODS AND MATERIALS Eleven patients with posthysterectomy gynecologic cancer received PBS to the whole pelvis. The patients received a dose of 45 to 50.4 Gy relative biological effectiveness (RBE) in 1.8 Gy (RBE) daily fractions. Acute toxicity was scored according to the Common Terminology Criteria for Adverse Events, version 4. A dosimetric comparison between a 2-field posterior oblique beam PBS and an IMRT plan was conducted. The Wilcoxon signed rank test was used to assess the potential dosimetric differences between the 2 plans and PBS target coverage robustness relative to setup uncertainties. RESULTS The median patient age was 55 years (range 23-76). The primary site was cervical in 7, vaginal in 1, and endometrial in 3. Of the 11 patients, 7 received concurrent cisplatin, 1 each received sandwich carboplatin and paclitaxel chemotherapy, both sandwich and concurrent chemotherapy, and concurrent and adjuvant chemotherapy, and 1 received no chemotherapy. All patients completed treatment. Of the 9 patients who received concurrent chemotherapy, the rate of grade 2 and 3 hematologic toxicities was 33% and 11%, respectively. One patient (9%) developed grade 3 acute gastrointestinal toxicity; no patient developed grade ≥3 genitourinary toxicity. The volume of pelvic bone marrow, bladder, and small bowel receiving 10 to 30 Gy was significantly lower with PBS than with intensity modulated radiation therapy (P<.001). The target coverage for all PBS plans was robust relative to the setup uncertainties (P>.05) with the clinical target volume mean dose percentage received by 95% and 98% of the target volume coverage changes within 2% for the individual plans. CONCLUSIONS Our results have demonstrated the clinical feasibility of PBS and the dosimetric advantages, especially for the low-dose sparing of normal tissues in the pelvis with the target robustness maintained relative to the setup uncertainties. Future studies with larger patient numbers are planned to further validate our preliminary findings.

Is there an ideal set of prospective scan acquisition phases for fast-helical based 4D-CT?

The article aims to determine if a prospective acquisition algorithm can be used to find the ideal set of free-breathing phases for fast-helical model-based 4D-CT. A retrospective five-patient dataset that consisted of 25 repeated free breathing CT scans per patient was used. The sum of the square root amplitude difference between all the breathing phases was defined as an objective function to determine the optimality of sets of breathing phases. The objective function was intended to determine if a specific set of breathing phases would yield a motion model that could accurately predict the motion in all 25 CT scans. Voxel specific motion models were calculated using all combinations of N scans from 25 breathing trajectories, (3  ⩽  N  ⩽  25), and the minimum number of scans required to absolutely characterize the motion model was analyzed. This analysis suggests that the number of scans could potentially be reduced to as few as five scans. When the objective function was large, the resulting motion model provided an excellent approximation to the motion model created using all 25 scans.

Geometric validation of MV topograms for patient localization on TomoTherapy.

Our goal was to geometrically validate the use of mega-voltage orthogonal scout images (MV topograms) as a fast and low-dose alternative to mega-voltage computed tomography (MVCT) for daily patient localization on the TomoTherapy system. To achieve this, anthropomorphic head and pelvis phantoms were imaged on a 16-slice kilo-voltage computed tomography (kVCT) scanner to synthesize kilo-voltage digitally reconstructed topograms (kV-DRT) in the Tomotherapy detector geometry. MV topograms were generated for couch speeds of 1-4 cm s(-1) in 1 cm s(-1) increments with static gantry angles in the anterior-posterior and left-lateral directions. Phantoms were rigidly translated in the anterior-posterior (AP), superior-inferior (SI), and lateral (LAT) directions to simulate potential setup errors. Image quality improvement was demonstrated by estimating the noise level in the unenhanced and enhanced MV topograms using a principle component analysis-based noise level estimation algorithm. Average noise levels for the head phantom were reduced by 2.53 HU (AP) and 0.18 HU (LAT). The pelvis phantom exhibited average noise level reduction of 1.98 HU (AP) and 0.48 HU (LAT). Mattes Mutual Information rigid registration was used to register enhanced MV topograms with corresponding kV-DRT. Registration results were compared to the known rigid displacements, which assessed the MV topogram localizations sensitivity to daily positioning errors. Reduced noise levels in the MV topograms enhanced the registration results so that registration errors were <1 mm. The unenhanced head MV topograms had discrepancies < 2.1 mm and the pelvis topograms had discrepancies < 2.7 mm. Result were found to be consistent regardless of couch speed. In total, 64.7% of the head phantom MV topograms and 60.0% of the pelvis phantom MV topograms exactly measured the phantom offsets. These consistencies demonstrated the potential for daily patient positioning using MV topogram pairs in the context bony-anatomy based procedures such as total marrow irradiation, total body irradiation, and cranial spinal irradiation.

A beam angle optimization technique for proton pencil beam scanning treatment planning of lower pelvis targets

The optimal proton pencil beam scanning (PBS) beam angle was calculated for a population of patients with lower pelvis targets. 10 patients were planned with a single PBS beam in the left lateral, right lateral, and posterior directions. A beam direction was considered to be optimal if it satisfied two metrics: shortest path length and least Hounsfield Unit (HU) variation. To determine these metrics, a ray-trace approach was adopted where the length of a ray represented the path length and the variation of HUs across the ray represented the HU variation. A Kolmogorov-Smirnov test determined the normalcy of the path length and HU variation at the 95% confidence level. Results showed that both the path length and HU variation were normally distributed. The path length was shortest for the posterior beam, and the left lateral beam had the least HU variation. Combining both optimization metrics, the posterior beam was more optimal since i) the path length was much shorter than the lateral beam’s path length, and ii) the HU variation along the posterior beam’s path length was only slightly less homogeneous than the HU variation along the lateral beam’s path length, but statistically similar. This study found that a posterior beam is optimal for lower pelvis targets when a single PBS beam is used.

Quantitative early decision making metric for identifying irregular breathing in 4DCT

PURPOSE To develop a quantitative early decision making metric for prediction of breathing pattern and irregular breathing and validate the metric in a large patient population receiving clinical phase-sorted four-dimensional computed tomography (4DCT). METHODS This study employed three patient cohorts. The first cohort contained 47 patients, imaged with a nonclinical tidal volume metric. The second cohort contained a sample of 256 patients who received a clinical 4DCT. The third cohort contained 86 patients who received three 4DCT scans at 1-week increment during the course of radiotherapy. The second and third cohorts did not have tidal volume measurements, as per standard radiation oncology clinical practice. Based on a previously published technique that used a single abdominal surrogate, the ratio of extreme inhalation tidal volume to normal inhalation tidal volume (κ) metric was calculated and the patient breathing pattern was characterized. The use of a single surrogate precluded the use of a κ determined by tidal volume, so a κ(rel) was defined based on the amplitude of the surrogate. Patients were classified as either Type 1 or Type 2, based on a previously published technique, where Type 1 patients were apneic at end of exhalation and Type 2 patients exhibited forced respiration. The Ansari-Bradley test was used to determine the statistical similarity between the Type 1 and Type 2 distributions. A Kruskal-Wallis one way analysis of variance was used to determine the statistical similarities among the classified breathing types, κ(rel), and the qualified medical physicist denoted breathing classification (regular or irregular). Receiver operator characteristic curves were used to quantitatively determine optimal cutoff value j(κ) and efficiency cutoff value (τ(κ)) κ(rel) to provide a quantitative early warning of irregular breathing during 4DCT procedures. RESULTS The statistical tests show a significant consistency for the breathing pattern classifications between the physiologically measured cohort #1 and the remaining cohorts. The classification types were statistically different between Type 1 and Type 2 patients over all cohorts. Values of κ(rel) in excess of 1.72 indicated a substantial presence of irregular breathing that could negatively affect the quality of a 4DCT image dataset. Values of κ(rel) in lower than 1.45 indicated minimal presence of irregular breathing. For values of κ(rel) such that j(κ) ≤ κ(rel) ≤ τ(κ), the decision to reacquire the 4DCT would be at the discretion of the physician. This accounted for only 11.9% of the patients in this study. The magnitude of κ(rel) held consistent over three weeks of treatment for 73% of the patients in cohort #3. CONCLUSIONS The decision making metric based on κ was shown to be an accurate classifier of regular and irregular breathing patterns in a large patient population. Breathing type, as defined in a previous published work, was accurately classified by κ(rel) with the use of a single respiratory surrogate compared to the physiological use of multiple respiratory surrogates. This work provided a quantitative early decision making metric to quickly and accurately assess breathing patterns as well as the presence and magnitude of irregular breathing during 4DCT.

Dosimetric feasibility of single-energy proton modulated arc therapy for treatment of chordoma at the skull base

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Dosimetric impact of accurately delineating of the left anterior descending artery in photon and proton radiotherapy

The dosimetric impact of accurately delineating the left anterior descending artery (LAD) was investigated for routine treatment planning deep inspiration breath-hold (DIBH) CT where cardiac motion was not accounted for. The LAD was contoured in the routine DIBH CT images by an expert radiologist in a population of 10 patients with cancer of the left breast. The motion blurring of the LAD in the DIBH CT images was extracted from the contoured LAD to create a corrected LAD volume. This was used to compare the maximum and mean LAD dose over 3D conformal radiotherapy (3DCRT), uniform scattering (US) proton, and pencil beam scanning (PBS) proton plans. Using a corrected LAD volume reduced the maximum dose LAD DVH indicator by 2% (3DCRT), 4% (US), and 25% (PBS). A corrected LAD volume increased the mean dose LAD DVH indicator by 25% (3DCRT), 61% (US), and 35% (PBS). In terms of absolute dose the impact of contouring on LAD is higher for photon therapy due to the higher doses delivered using this modality. Overall, the results demonstrate that the LAD volume could potentially be the source of inconsistencies in correlation between dose and radiation-induced cardiac toxicity when uncompensated motion is present in the treatment planning images.

TU-EF-304-04: A Heart Motion Model for Proton Scanned Beam Chest Radiotherapy

Purpose: To model fast-moving heart surface motion as a function of cardiac-phase in order to compensate for the lack of cardiac-gating in evaluating accurate dose to coronary structures. Methods: Ten subjects were prospectively imaged with a breath-hold, cardiac-gated MRI protocol to determine heart surface motion. Radial and planar views of the heart were resampled into a 3-dimensional volume representing one heartbeat. A multi-resolution optical flow deformable image registration algorithm determined tissue displacement during the cardiac-cycle. The surface of the heart was modeled as a thin membrane comprised of voxels perpendicular to a pencil beam scanning (PBS) beam. The membrane’s out-of-plane spatial displacement was modeled as a harmonic function with Lame’s equations. Model accuracy was assessed with the root mean squared error (RMSE). The model was applied to a cohort of six chest wall irradiation patients with PBS plans generated on phase-sorted 4DCT. Respiratory motion was separated from the cardiac motion with a previously published technique. Volumetric dose painting was simulated and dose accumulated to validate plan robustness (target coverage variation accepted within 2%). Maximum and mean heart surface dose assessed the dosimetric impact of heart and coronary artery motion. Results: Average and maximum heart surface displacements were 2.54±0.35mm and 3.6mm from the end-diastole phase to the end-systole cardiac-phase respectively. An average RMSE of 0.11±0.04 showed the model to be accurate. Observed errors were greatest between the circumflex artery and mitral valve level of the heart anatomy. Heart surface displacements correspond to a 3.6±1.0% and 5.1±2.3% dosimetric impact on the maximum and mean heart surface DVH indicators respectively. Conclusion: Although heart surface motion parallel to beam’s direction was substantial, its maximum dosimetric impact was 5.1±2.3%. Since PBS delivers low doses to coronary structures relative to photon radiotherapy, it is unknown whether this variation would be clinically significant for late effects.