Assessment of a quantitative metric for 4D CT artifact evaluation by observer consensus

The benefits of four-dimensional computed tomography (4D CT) are limited by the presence of artifacts that remain difficult to quantify. A correlation-based metric previously proposed for ciné 4D CT artifact identification was further validated as an independent artifact evaluator by using a novel qualitative assessment featuring a group of observers reaching a consensus decision on artifact location and magnitude. The consensus group evaluated ten ciné 4D CT scans for artifacts over each breathing phase of coronal lung views assuming one artifact per couch location. Each artifact was assigned a magnitude score of 1-5, 1 indicating lowest severity and 5 indicating highest severity. Consensus group results served as the ground truth for assessment of the correlation metric. The ten patients were split into two cohorts; cohort 1 generated an artifact identification threshold derived from receiver operating characteristic analysis using the Youden Index, while cohort 2 generated sensitivity and specificity values from application of the artifact threshold. The Pearson correlation coefficient was calculated between the correlation metric values and the consensus group scores for both cohorts. The average sensitivity and specificity values found with application of the artifact threshold were 0.703 and 0.476, respectively. The correlation coefficients of artifact magnitudes for cohort 1 and 2 were 0.80 and 0.61, respectively, (p < 0.001 for both); these correlation coefficients included a few scans with only two of the five possible magnitude scores. Artifact incidence was associated with breathing phase (p < 0.002), with presentation less likely near maximum exhale. Overall, the correlation metric allowed accurate and automated artifact identification. The consensus group evaluation resulted in efficient qualitative scoring, reduced interobserver variation, and provided consistent identification of artifact location and magnitudes.

Evaluation of IsoCal geometric calibration system for Varian linacs equipped with on-board imager and electronic portal imaging device imaging systems

The purpose of this study is to evaluate the accuracy and reproducibility of the IsoCal geometric calibration system for kilovoltage (kV) and megavoltage (MV) imagers on Varian C‐series linear accelerators (linacs). IsoCal calibration starts by imaging a phantom and collimator plate using MV images with different collimator angles, as well as MV and kV images at different gantry angles. The software then identifies objects on the collimator plate and in the phantom to determine the location of the treatment isocenter and its relation to the MV and kV imager centers. It calculates offsets between the positions of the imaging panels and the treatment isocenter as a function of gantry angle and writes a correction file that can be applied to MV and kV systems to correct for those offsets in the position of the panels. We performed IsoCal calibration three times on each of five Varian C‐series linacs, each time with an independent setup. We then compared the IsoCal calibrations with a simplified Winston‐Lutz (WL)‐based system and with a Varian cubic phantom (VC)‐based system. The maximum IsoCal corrections ranged from 0.7 mm to 1.5 mm for MV and 0.9 mm to 1.8 mm for kV imagers across the five linacs. The variations in the three calibrations for each linac were less than 0.2 mm. Without IsoCal correction, the WL results showed discrepancies between the treatment isocenter and the imager center of 0.9 mm to 1.6 mm (for the MV imager) and 0.5 mm to 1.1 mm (for the kV imager); with IsoCal corrections applied, the differences were reduced to 0.2 mm to 0.6 mm (MV) and 0.3 mm to 0.6 mm (kV) across the five linacs. The VC system was not as precise as the WL system, but showed similar results, with discrepancies of less than 1.0 mm when the IsoCal corrections were applied. We conclude that IsoCal is an accurate and consistent method for calibration and periodic quality assurance of MV and kV imaging systems.

PACS numbers: 87.55.Qr, 87.56.Fc

Stereotactic ablative radiation therapy for centrally located early stage or isolated parenchymal recurrences of non-small cell lung cancer: how to fly in a "no fly zone"

PURPOSE

We extended our previous experience with stereotactic ablative radiation therapy (SABR; 50 Gy in 4 fractions) for centrally located non-small cell lung cancer (NSCLC); explored the use of 70 Gy in 10 fractions for cases in which dose-volume constraints could not be met with the previous regimen; and suggested modified dose-volume constraints.

METHODS AND MATERIALS

Four-dimensional computed tomography (4DCT)-based volumetric image-guided SABR was used for 100 patients with biopsy-proven, central T1-T2N0M0 (n=81) or isolated parenchymal recurrence of NSCLC (n=19). All disease was staged with positron emission tomography/CT; all tumors were within 2 cm of the bronchial tree, trachea, major vessels, esophagus, heart, pericardium, brachial plexus, or vertebral body. Endpoints were toxicity, overall survival (OS), local and regional control, and distant metastasis.

RESULTS

At a median follow-up time of 30.6 months, median OS time was 55.6 months, and the 3-year OS rate was 70.5%. Three-year cumulative actuarial local, regional, and distant control rates were 96.5%, 87.9%, and 77.2%, respectively. The most common toxicities were chest-wall pain (18% grade 1, 13% grade 2) and radiation pneumonitis (11% grade 2 and 1% grade 3). No patient experienced grade 4 or 5 toxicity. Among the 82 patients receiving 50 Gy in 4 fractions, multivariate analyses showed mean total lung dose >6 Gy, V20 >12%, or ipsilateral lung V30 >15% to independently predict radiation pneumonitis; and 3 of 9 patients with brachial plexus Dmax >35 Gy experienced brachial neuropathy versus none of 73 patients with brachial Dmax <35 Gy (P=.001). Other toxicities were analyzed and new dose-volume constraints are proposed.

CONCLUSIONS

SABR for centrally located lesions produces clinical outcomes similar to those for peripheral lesions when normal tissue constraints are respected.

Statistical modeling approach to quantitative analysis of interobserver variability in breast contouring

PURPOSE

To develop a new approach for interobserver variability analysis.

METHODS AND MATERIALS

Eight radiation oncologists specializing in breast cancer radiation therapy delineated a patient's left breast "from scratch" and from a template that was generated using deformable image registration. Three of the radiation oncologists had previously received training in Radiation Therapy Oncology Group consensus contouring for breast cancer atlas. The simultaneous truth and performance level estimation algorithm was applied to the 8 contours delineated "from scratch" to produce a group consensus contour. Individual Jaccard scores were fitted to a beta distribution model. We also applied this analysis to 2 or more patients, which were contoured by 9 breast radiation oncologists from 8 institutions.

RESULTS

The beta distribution model had a mean of 86.2%, standard deviation (SD) of ±5.9%, a skewness of -0.7, and excess kurtosis of 0.55, exemplifying broad interobserver variability. The 3 RTOG-trained physicians had higher agreement scores than average, indicating that their contours were close to the group consensus contour. One physician had high sensitivity but lower specificity than the others, which implies that this physician tended to contour a structure larger than those of the others. Two other physicians had low sensitivity but specificity similar to the others, which implies that they tended to contour a structure smaller than the others. With this information, they could adjust their contouring practice to be more consistent with others if desired. When contouring from the template, the beta distribution model had a mean of 92.3%, SD ± 3.4%, skewness of -0.79, and excess kurtosis of 0.83, which indicated a much better consistency among individual contours. Similar results were obtained for the analysis of 2 additional patients.

CONCLUSIONS

The proposed statistical approach was able to measure interobserver variability quantitatively and to identify individuals who tended to contour differently from the others. The information could be useful as feedback to improve contouring consistency.

A new respiratory monitoring and processing system based on Wii remote: proof of principle

PURPOSE

To create a patient respiratory management system and patient self-practice tool using the Wii remote, a widely available consumer hardware product.

METHODS

The Wii remote (Wiimote) (Nintendo, Redmond, WA) contains an infrared (IR) camera that can track up to four spots whose coordinates are reported to a host computer via Bluetooth. The Wiimote is capable of tracking a fiducial box currently used by a commercial monitoring system [Real-time Position Management(TM) (RPM) system, Varian Associates, Palo Alto, CA], if the correct IR source is used. The authors validated the Wiimote tracking by comparing the amplitude and frequency of signals among those reported by Wiimote with known movements from an inhouse servo-driven respiratory simulator, as well as with those measured using the RPM. The simulator comparison was done using standard sinusoid signals with amplitude of 2.0 cm as well as recorded patient respiratory traces. The RPM comparisons were done by simultaneously recording the RPM reflective box position with the Wiimote and the RPM. Timing was compared between these two systems by using the digital beam-on signal from the CT scanner, for the 4DCT to synchronize these acquisitions.

RESULTS

The data acquisition rate from the Wiimote was 100.0 ± 0.4 Hz with a version 2.1 Bluetooth adaptor. The standard deviation of the height of the motion extrema was 0.06 and 1.1 mm when comparing those measured by the Wiimote and the servomotor encoder for standard sinusoid signal and prerecorded patient respiratory signal, respectively. The standard deviation of the amplitude of motion extrema between the Wiimote and RPM was 0.9 mm and the timing difference was 253 ms.

CONCLUSION

The performance of Wiimote shows promise for respiratory monitoring for its faster sampling rate as well as the potential optical and GPU abilities. If used with care it can deliver reasonable spatial and temporal accuracy.

Capsular contracture of subcutaneous breast implant following hypofractionated stereotactic body radiotherapy for early stage lung cancer

BACKGROUND

Development of capsular contracture around subcutaneously implanted breast prostheses, producing poor cosmetic outcome and pain, has been reported following standard fractionated external beam radiotherapy to whole implants for breast cancer. We report capsular contracture following partial implant irradiation from hypofractionated stereotactic body radiotherapy (SBRT) for lung cancer in a 64 year-old female with augmentation mammaplasty.

METHODS

The patient had biopsy-proven, T1 non-small cell lung carcinoma, adjacent to the implant. She received 50 Gy in 4 fractions to 91% of planning target volume using a 7-field, 3D-conformal plan with 6 MV photons and daily CT-guided target localization. The implant received 9.3 Gy mean dose, 51.7 Gy maximum point dose, with V₁₀ 41%, V₂₀ 15% and V₃₀ 4%.

RESULTS

At seven months, the patient reported left breast pain requiring narcotic analgesics and demonstrated modified Baker/Palmer grade 4 capsular contracture. Breast retraction assessment measurement increased from baseline 10.4 mm to 19.8 mm.

CONCLUSIONS

This represents the first reported case of capsular contracture from partial breast implant radiation following SBRT for lung cancer. Further investigation to elucidate maximum tolerated dose of radiation given to breast implants in this setting is needed.

Evaluation of dose variation to normal and critical structures for lung hypofractionated stereotactic body radiation therapy

PURPOSE

To quantify the dose received by normal and critical structures during lung stereotactic body radiation therapy (SBRT) when registered to tumor or bone.

METHODS AND MATERIALS

Sixteen patients with lung cancer receiving a total dose of 50 Gy in 4fractions for lung SBRT were retrospectively studied. Cone-beam computed tomography (CT) was performed for all fractions, and the images obtained were registered with planning CT with respect tosoft tissue for target localization. Isocenter shifts were determined for each fraction from differences between the bony and tumor alignments; doses were then recalculated based on the new isocenters and summed over all 4 fractions to compare against the planned normal and critical tissue dose. The normal and critical structures evaluated were total and ipsilateral lung, spinal cord, and esophagus. The first data collected were isocenter coordinate shifts in all 3 Cartesian coordinates for both tumor andbony alignments. The second were the dose differences to the normal and critical structures fromthe planned and recalculated doses for alignment based on the tumor.

RESULTS

The study showed that while the maximum isocenter coordinate shifts in any direction couldbe as much as 1.60 cm, the normal and critical structure dose variations between the original plans and the simulated plans showed almost no change. The mean volume of total lung that receivedat least 20Gy difference for total lung and ipsilateral lung were 0.01% and -0.04%, respectively. For the esophagus, spinal cord, and heart the maximum and mean dose differences were 0.25 Gy and -0.04 Gy, -0.08 Gy and -0.02 Gy, and 0.02 Gy and 0.05 Gy, respectively.

CONCLUSIONS

Target localization using daily cone-beam CT with soft tissue registration was appropriate for minimizing the dose to the normal and critical structures without the need to re-plan due to the changes in the tumor position. For tumors located close to a critical structure, daily cone-beam CT is recommended to determine the appropriate isocenter shifts.

A comparison of tumor motion characteristics between early stage and locally advanced stage lung cancers

PURPOSE

With the increasing use of conformal radiation therapy methods for non-small cell lung cancer (NSCLC), it is necessary to accurately determine respiratory-induced tumor motion. The purpose of this study is to analyze and compare the motion characteristics of early and locally advanced stage NSCLC tumors in a large population and correlate tumor motion with position, volume, and diaphragm motion.

METHODS AND MATERIALS

A total of 191 (94 early stage, 97 locally advanced) non-small cell lung tumors were analyzed for this study. Each patient received a four-dimensional CT scan prior to receiving radiation treatment. A soft-tissue-based rigid registration algorithm was used to track the tumor motion. Tumor volumes were determined based on the gross tumor volume delineated by physicians in the end of expiration phase. Tumor motion characteristics were correlated with their standardized tumor locations, lobe location, and clinical staging. Diaphragm motion was calculated by subtracting the diaphragm location between the expiration and the inspiration phases.

RESULTS

Median, max, and 95th percentile of tumor motion for early stage tumors were 5.9 mm, 31.0 mm, and 20.0 mm, which were 1.2 mm, 12 mm, and 7 mm more than those in locally advanced NSCLC, respectively. The range of motion at 95th percentile is more than 50% larger in early stage lung cancer group than in the locally advanced lung cancer group. Early stage tumors in the lower lobe showed the largest motion with a median motion of 9.2mm, while upper/mid-lobe tumors exhibited a median motion of 3.3mm. Tumor volumes were not correlated with motion.

CONCLUSION

The range of tumor motion differs depending on tumor location and staging of NSCLC. Early stage tumors are more mobile than locally advanced stage NSCLC. These factors should be considered for general motion management strategies when 4D simulation is not performed on individual basis.