Implications of CT imaging for postplan quality assessment in prostate brachytherapy.
OBJECTIVE
Postplan quality assurance using CT shows considerable interobserver contour variability. We examined CT postplans of four experienced brachytherapists for comparison with MR-determined prostate volumes.


METHODS
Seventy-five patients had CT and MR scans 1 month post-(125)I prostate brachytherapy. CT scans were contoured by the treating physician and dosimetry calculated. The prostate was contoured independently on MR by one observer with extensive MR experience, the scans were fused and dosimetric parameters compared.


RESULTS
The mean prostate volume on CT was 38.3 cc (17.5-78.6 cc), on MR 33.3 cc (16.3-66.1 cc). On average, the volume on CT was 16.1% larger than on MR (range, 8% smaller to 64% larger). Craniocaudal discordance of the CT vs. MR prostate contours ranged from 4 mm cranial to 10 mm caudal to MR base and from 6 mm cranial to 14 mm caudal to MR apex. The CT prostate volume not only included an average of 90% of the MR prostate (range, 75-99%) but also included normal tissue (mean, 8.3 cc; range, 2.9-17.1 cc). The average difference between the calculated D(90) from CT contours vs. MR contours was 10.0 Gy (standard deviation, 8.8; range, -37.6 to +41.6 Gy).


CONCLUSIONS
On average, only 90% of the MR-defined prostate is included in CT contours, while a volume of normal tissue is erroneously designated as prostate. Lack of awareness of this deficiency in planning and/or operative technique gives a false sense of appreciation of the true conformality, delays implementation of corrective measures, and risks unnecessary side effects.
Department of Radiation Oncology, British Columbia Cancer Agency, Center for the Southern Interior, Kelowna, BC, Canada.
David
D
Petrik
Cynthia
C
Araujo
David
D
Kim
Ross
R
Halperin
Juanita M
JM
Crook
D000368:Aged; D000369:Aged, 80 and over; D001918:Brachytherapy; D001955:British Columbia; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D011471:Prostatic Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D011785:Quality Assurance, Health Care; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D061089:Radiotherapy, Image-Guided / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods* / Q000592:standards; D016896:Treatment Outcome; D047368:Tumor Burden