Implications of CT imaging for postplan quality assessment in prostate brachytherapy

Area of Science:

• Radiology
• Oncology
• Medical Physics

Background:

• Computed tomography (CT) is used for post-treatment quality assurance in prostate brachytherapy.
• Significant interobserver variability exists in CT-based contouring.
• Magnetic resonance (MR) imaging offers superior soft-tissue contrast for prostate delineation.

Purpose of the Study:

• To compare CT-determined prostate volumes with MR-determined volumes.
• To assess the accuracy of CT postplans in prostate brachytherapy quality assurance.
• To evaluate dosimetric differences arising from CT versus MR contouring.

Main Methods:

• Seventy-five patients underwent both CT and MR scans one month after Iodine-125 (I-125) prostate brachytherapy.
• Prostate contours were delineated on CT by the treating physician and independently on MR by an experienced observer.
• Scans were fused, and dosimetric parameters were compared between CT and MR contours.

Main Results:

• Mean prostate volume was larger on CT (38.3 cc) than on MR (33.3 cc), averaging 16.1% larger.
• CT contours included only 90% of the MR-defined prostate on average, while also encompassing normal tissue (mean 8.3 cc).
• Significant craniocaudal discordance was observed, with CT contours extending beyond MR base/apex and vice versa. The average difference in D(90) was 10.0 Gy.

Conclusions:

• CT-based prostate contouring in brachytherapy is inaccurate, underestimating the true prostate volume and including adjacent normal tissues.
• This contour variability leads to a false sense of treatment conformity and can result in suboptimal dose delivery.
• Awareness of these CT contouring deficiencies is crucial to prevent potential side effects and improve treatment planning.