Assessment of the residual error in soft tissue setup in patients undergoing partial breast irradiation: results of a prospective study using cone-beam computed tomography.
OBJECTIVE
On-board cone-beam computed tomography (CBCT) provides soft tissue information that may improve setup accuracy in patients undergoing accelerated partial breast irradiation (APBI). We used CBCT to assess the residual error in soft tissue after two-dimensional kV/MV alignment based on bony anatomy. We also assessed the dosimetric impact of this error.


METHODS
Ten patients undergoing APBI were studied as part of an institutional review board-approved prospective trial. Patients were aligned based on skin/cradle marks plus orthogonal kV/MV images registered based on bony landmarks to digitally reconstructed radiographs from the planning CT. A subsequent CBCT was registered to the planning CT using soft tissue information. This "residual error" and its dosimetric impact was measured.


RESULTS
The root-mean-square of the residual error was 3, 4, and 4 mm, in the right-left, anterior-posterior, and superior-inferior directions, respectively. The average vector sum was 6+/-2 mm. Average reductions in mean dose to the lumpectomy cavity, clinical target volume (CTV), and planning target volume were 0.1%, 0.4%, and 1%, respectively. The mean difference in the clinical target and planning target volumes that received 95% of the prescribed dose (V95) were 1% and 4%.


CONCLUSIONS
In this initial study with a modest number of patients, the residual error in soft tissue was typically <5 mm, and with the field margins used, the resultant dosimetric consequences were modest. In patients immobilized in a customized cradle, setup using orthogonal kV images thus appears accurate and reproducible. The CBCT technique may have particular utility in patients with larger breast volumes or breast deformations. Further studies involving larger numbers of patients are needed to further assess the utility of CBCT.
Fatunase
Toyosi
T
Department of Radiation Oncology, Duke University Medical Center, Durham, NC 27710, USA.
Wang
Zhiheng
Z
Yoo
Sua
S
Hubbs
Jessica L
JL
Prosnitz
Robert G
RG
Yin
Fang-Fang
FF
Marks
Lawrence B
LB
D001943:Breast Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D002138:Calibration; D054893:Cone-Beam Computed Tomography; D005260:Female; D006801:Humans; D008327:Mammography / Q000379:methods*; D010315:Particle Accelerators; D011446:Prospective Studies; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results