On correlated sources of uncertainty in four dimensional computed tomography data sets

Analysis of the variability among radiation oncologists in delineation of the postsurgical tumor bed based on 4D-CT

OBJECTIVE

This study investigated interobserver and intraobserver variability in radiation oncologists' definition of the tumor bed (TB) after breast-conserving surgery (BCS).

RESULTS

The TB volume, CVS and number of surgical clips were not significantly related to intraobserver variability. Moreover, no correlation was noted between CT slice thickness and interobserver variability (Δinter, DSCinter) in TB delineation, and no significant difference was noted among the three groups. The TB volume was negatively correlated with Δinter. DSCinter improved significantly with increased TB volume and decreased Δinter. DSCinter also increased significantly in patients with a CVS of 3 to 5 compared with patients with a CVS of 1 to 2. DSCinter was thus positively correlated with the CVS, with a correlation coefficient of 0.451. The use of 7 to 9 surgical clips neither decreased Δinter nor increased DSCinter.

MATERIALS AND METHODS

Five or more surgical clips were placed at the TB during lumpectomy. The TB was delineated on the end expiration scan. The data were stratified based on the cavity visualization score (CVS), CT slice thickness and surgical clip number. The Dice similarity coefficient (DSC) and inter(intra)observer variability (Δinter and Δintra) in different groups were evaluated and compared.

CONCLUSIONS

Inter(intra)observer variability in TB delineation was decreased for breast cancer patients implanted with 5 or more surgical clips in the cohort with a higher CVS and a larger TB. The use of more than 6 surgical clips did not significantly improve TB delineation, so 5 to 6 surgical clips are likely adequate to delineate the TB.

Comparison of planning target volumes based on three-dimensional and four-dimensional CT imaging of thoracic esophageal cancer

Background and purpose

To investigate the definition of planning target volumes (PTVs) based on four-dimensional computed tomography (4DCT) compared with conventional PTV definition and PTV definition using asymmetrical margins for thoracic primary esophageal cancer.

Materials and methods

Forty-three patients with esophageal cancer underwent 3DCT and 4DCT simulation scans during free breathing. The motions of primary tumors located in the proximal (group A), middle (group B), and distal (group C) thoracic esophagus were obtained from the 4DCT scans. PTV_3D was defined on 3DCT using the tumor motion measured based on 4DCT, PTV conventional (PTV_conv) was defined on 3DCT by adding a 1.0 cm margin to the clinical target volume, and PTV_4D was defined as the union of the target volumes contoured on the ten phases of the 4DCT images. The centroid positions, volumetric differences, and dice similarity coefficients were evaluated for all PTVs.

Results

The median centroid shifts between PTV_3D and PTV_4D and between PTV_conv and PTV_4D in all three dimensions were <0.3 cm for the three groups. The median size ratios of PTV_4D to PTV_3D were 0.80, 0.88, and 0.71, and PTV_4D to PTV_conv were 0.67, 0.73, and 0.76 (χ²=−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The dice similarity coefficients were 0.87, 0.90, and 0.81 between PTV_4D and PTV_3D and 0.80, 0.84, and 0.83 between PTV_4D and PTV_conv (χ² =−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The difference between the degree of inclusion of PTV_4D in PTV_3D and that of PTV_4D in PTV_conv was <2% for all groups. Compared with PTV_conv, the amount of irradiated normal tissue for PTV_3D was decreased by 11.81% and 11.86% in groups A and B, respectively, but was increased by 2.93% in group C.

Conclusion

For proximal and middle esophageal cancer, 3DCT-based PTV using asymmetrical margins provides good coverage of PTV_4D; however, for distal esophageal cancer, 3DCT-based PTV using conventional margins provides ideal conformity with PTV_4D.