Quantitative evaluation of cone-beam computed tomography in target volume definition for offline image-guided radiation therapy of prostate cancer

Purpose: To quantitatively evaluate cone-beam CT (CBCT) in target volume definition in an offline image guidance environment. Methods and materials: Fifteen patients each with five helical CTs (HCT) and eight CBCTs were included. A single physician manually delineated prostate and seminal vesicles (SVs) on each CT. The clinical target volume (CTV) was prostate for low risk group (G1), plus SVs for intermediate risk group (G2). The internal target volumes (ITVs) on CBCT (ITVCBCT) were constructed and compared with ITVHCT. The following comparisons were performed: CTV and ITV in HCT and CBCT; similarity of ITVs using overlap index (OI); surface differences between ITVs; quality assurance of ITVCBCT using CTV from weekly CBCT; and dosimetric evaluations of ITVHCT coverage on plans from ITVCBCT. Results: There was no statistical significant difference of CTV or ITV. The ITV OIs were 91%/88% for G1/G2 patients. They improved significantly with 1-2 mm margins. Therefore, the ITVs were mostly within 2 mm. The CTVs from weekly CBCT had >95% overlap with ITVCBCT. The ITV dose differences (D95, and Dmean) were <0.3%. Conclusions: It is feasible to use CBCT for target definition in offline image guidance, thereby eliminating the separate helical CT scan process.