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. 2013 May 6;14(3):4262.
doi: 10.1120/jacmp.v14i3.4262.

Prostate rotation detected from implanted markers can affect dose coverage and cannot be simply dismissed

Qingyang Shang  1 , Lawrence J Sheplan Olsen, Kevin Stephans, Rahul Tendulkar, Ping Xia
Affiliations

Prostate rotation detected from implanted markers can affect dose coverage and cannot be simply dismissed

Qingyang Shang et al. J Appl Clin Med Phys. .

Abstract

With implanted markers, daily prostate displacements can be automatically detected with six degrees of freedom. The reported magnitudes of the rotations, however, are often greater than the typical range of a six-degree treatment couch. The purpose of this study is to quantify geometric and dosimetric effects if the prostate rotations are not corrected (ROT_NC) and if they can be compensated with translational shifts (ROT_C). Forty-three kilovoltage cone-beam CTs (KV-CBCT) with implanted markers from five patients were available for this retrospective study. On each KV-CBCT, the prostate, bladder, and rectum were manually contoured by a physician. The prostate contours from the planning CT and CBCT were aligned manually to achieve the best overlaps. This contour registration served as the benchmark method for comparison with two marker registration methods: (a) using six degrees of freedom, but rotations were not corrected (ROT_NC); and (b) using three degrees of freedom while compensating rotations into the translational shifts (ROT_C). The center of mass distance (CMD) and overlap index (OI) were used to evaluate these two methods. The dosimetric effects were also analyzed by comparing the dose coverage of the prostate clinical target volume (CTV) in relation to the planning margins. According to our analysis, the detected rotations dominated in the left-right axis with systematic and random components of 4.6° and 4.1°, respectively. When the rotation angles were greater than 10°, the differences in CMD between the two registrations were greater than 5 mm in 85.7% of these fractions; when the rotation angles were greater than 6°, the differences of CMD were greater than 4 mm in 61.1% of these fractions. With 6 mm/4 mm posterior planning margins, the average difference between the dose to 99% (D99) of the prostate in CBCTs and the planning D99 of the prostate was -8.0 ± 12.3% for the ROT_NC registration, and -3.6 ± 9.0% for the ROT_C registration (p = 0.01). When the planning margin decreased to 4 mm/2 mm posterior, the average difference in D99 of the prostate was -22.0 ± 16.2% and -15.1 ± 15.2% for the ROT_NC and ROT_C methods, respectively (p < 0.05). In conclusion, prostate rotation cannot be simply dismissed, and the impact of the rotational errors depends on the distance between the isocenter and the centroid of implanted markers and the rotation angle.

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Figures

Figure 1
Figure 1
Prostate contours from different registration methods in (a) transverse, (b) coronal, and (c) sagittal views. Prostate ROT_NC is shown in blue, ProstateROT_C in red, ProstateContourT in purple, and ProstateCBCT in yellow.
Figure 2
Figure 2
Prostate rotations about the left–right (LR), anterior–posterior (AP), and superior–inferior (SI) axes for 43 fractions.
Figure 3
Figure 3
Center of mass distance (CMD): (a) between the ProstateContourT and ProstateCBCT; (b) between the Prostate ContourT and ProstateROT_NC, between the ProstateContourT and ProstateROT_C, and the difference.
Figure 4
Figure 4
Overlap index (OI): (a) the average OI for ProstateContourT and ProstateCBCT for each patient; (b) the average OI for each patient compared between the two marker‐based registration methods. Error bar shows one standard deviation.
Figure 5
Figure 5
Average D99 of the Prostate‐CTV as a function of CTV expansion margins for the three registration methods. D99 is expressed as a ratio of the daily dose to the planned daily D99 of the prostate. Error bar represents one standard deviation.
Figure 6
Figure 6
Average D5 and D50 of the bladder and rectum. Column represents mean value (normalized to the planned dose) and error bar corresponds to one standard deviation.
Figure 7
Figure 7
The magnitude of measured and estimated errors associated with the ROT_NC method.
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