WE-E-213CD-09: Multi-Atlas Fusion Using a Tissue Appearance Model

PURPOSE

To improve multi-atlas based auto-segmentation by integrating a tissue appearance model with the Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm to perform multi-atlas fusion.

METHODS

Ten head-and-neck planning CT images were acquired (resolution: 1.0×1.0×2.5mm³ ) and the parotid glands were contoured manually by a head-and-neck oncologist. We performed 10 leave-one-out tests by using one patient as test patient and the rest of 9 patients as atlases. Deformable registration was first applied to transform the atlas parotid contours to the test image one by one. The STAPLE algorithm was initialized by a parotid tissue appearance model, which was estimated from the test image and encoded the intensity information of parotid glands. The individual deformed contours were then fused using the STAPLE algorithm to produce a best approximation of the true contour. The tissue appearance model was also applied to a deformable model segmentation to further refine the fused contours.

RESULTS

The multi-atlas fusion using the tissue appearance model produced an average Dice coefficient of 85.2%±3.1% (left parotid) and 84.9%±3.9% (right parotid) over the 10 tests between the auto-contour and the manual contour, and an average mean surface distance of 1.6±0.3mm and 1.6±0.4mm for left and right parotids respectively. This demonstrated a good agreement between the manual contours and the auto- delineated contours. Our results also showed that, without using the tissue appearance model, the auto-delineated parotid contours might include nearby bony structures; however, using the appearance model was able to correct this problem.

CONCLUSIONS

Including the intensity information using a tissue appearance model into STAPLE algorithm for multi-atlas fusion showed improvement in refining the anatomical boundaries in the multi- atlas based auto-segmentation.

SU-E-T-86: Development and Implementation of the Use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms

PURPOSE

To study the angular dependence of optically stimulated luminescence dosimeters (OSLD) in the Radiological Physics Center anthropomorphic quality assurance pelvic phantom to provide accurate dosimetric measurements as a replacement for TLD.

METHODS

A spherical phantom was constructed to investigate the angular response of the OSLD as oriented in the RPC pelvic phantom. Three OSLD per irradiation angle, placed at the center of the spherical phantom, were irradiated with 100 cGy from six different angles. The angular response at each angle was determined relative to the OSLD response when the beam was incident normally on the OSLD surface. A pelvic phantom dosimetry insert was modified to include both TLD and OSLD. Three treatment plans were developed in Pinnacle v9.0 and one in Accuray's Multiplan, each with increasing angular beam delivery (4 field, IMRT, SmartArc, CyberKnife) for the pelvic phantom using a common dose prescription and constraints. Each plan was delivered to the phantom three times, containing two TLD and two OSLD, oriented in the transverse plane, at the center of the PTV. The dose delivered to the TLD and OSLD was calculated for each treatment and then compared.

RESULTS

The angular dependence correction factor for the spherical phantom was found to be uniformly 1.041 ± 0.003 from single beam edge-on irradiations. The angular dependence correction in the pelvic phantom from multiple beam orientation irradiations was 1.024 ± 0.002, such that the OSLD dose agreed with the TLD dose. Applying the OSLD pelvic phantom correction factor, the RPC measured dose to planning system calculated dose ratio was 0.995 ± 0.009. The established RPC phantom TLD dose to calculated dose ratio was 0.995 ± 0.010.

CONCLUSIONS

An anthropomorphic phantom OSLD angular dependence correction factor was established such that the final OSLD dose measurements agreed with RPC's TLD dose measurements to within 1%. Work supported by grant CA 10953, awarded by NCI, DHHS.

SU-F-BRCD-05: Mean Regional Dose to the Esophagus Predicts Acute Toxicity Rate for Lung Cancer Patients

PURPOSE

The relationship between spatial aspects of the dose distribution and incidence of acute radiation esophagitis for non-small-cell lung cancer (NSCLC) patients is not well understood. Specifically, the location of dose along the superior-inferior (SI) axis of the esophagus has not been previously considered. We introduce the concept of mean regional dose (MRD) calculated for esophageal subvolumes, and test for significance for prediction of acute esophagitis (AE).

METHODS

The 3D dose distribution within the esophagus was extracted for 541 NSCLC patients treated with definitive photon therapy. The esophagus contour was divided into equal geometric halves, thirds, and fourths along the SI direction of the structure. MRD in each subvolume was calculated. Univariate logistic regression was performed to determine the correlation between MRD and CTCAE3.0 AE grade = 2 (medical intervention). The MRD was incorporated into an existing NTCP model (based on mean dose for the total esophageal volume) as a separate additive factor.

RESULTS

Univariate analysis indicated a significant correlation between AE grade = 2 and MRD in each of the esophageal subvolumes except for the inferior third and inferior-most quarter. There was a statistically significant improvement when including the additive MRD factor for the superior/inferior halves, superior/inferior thirds, and superior-most/inferior-most quarters into the NTCP model.

CONCLUSIONS

This study investigates previously unexplored regional differences in delivered dose to the esophagus of patients treated for NSCLC. There is evidence to suggest that dose to the superior portions of the esophagus is more important as it relates to the potential for acute toxicity. The 541 patient cohort is the largest database used to investigate AE in patients treated for NSCLC, strengthening the power of the statistical results. Additional methods to incorporate dose in individual esophagus voxels (along the SI axis) into the NTCP model are also being explored.