A magnetic resonance imaging study of prostate deformation relative to implanted gold fiducial markers

PURPOSE: To describe prostate deformation during radiotherapy and determine the margins required to account for prostate deformation after setup to intraprostatic fiducial markers (FM). METHODS AND MATERIALS: Twenty-five patients with T1c-T2c prostate cancer had three gold FMs implanted. The patients presented with a full bladder and empty rectum for two axial magnetic resonance imaging (MRI) scans using a gradient recalled echo (GRE) sequence capable of imaging the FMs. The MRIs were done at the time of radiotherapy (RT) planning and a randomly assigned fraction. A single observer contoured the prostate surfaces. They were entered into a finite element model and aligned using the centroid of the three FMs. RESULTS: During RT, the prostate volume decreased by 0.5%/fraction (p = 0.03) and the FMs in-migrated by 0.05 mm/fraction (p < 0.05). Prostate deformation was unrelated to differential bladder and bowel filling, but was related to a transurethral resection of the prostate (TURP) (p = 0.003). The standard deviation for systematic uncertainty of prostate surface contouring was 0.8 mm and for FM centroid localization was 0.4 mm. The standard deviation of random interfraction prostate deformation was 1.5 mm and for FM centroid variability was 1.1 mm. These uncertainties from prostate deformation can be incorporated into a margin recipe to determine the total margins required for RT. CONCLUSIONS: During RT, the prostate exhibited: volume decrease, deformation, and in-migration of FMs. Patients with TURPs were prone to prostate deformation.     

X-ray-assisted positioning of patients treated by conformal arc radiotherapy for prostate cancer: comparison of setup accuracy using implanted markers versus bony structures

PURPOSE: The aim of this study was to compare setup accuracy of NovalisBody stereoscopic X-ray positioning using implanted markers in the prostate vs. bony structures in patients treated with dynamic conformal arc radiotherapy for prostate cancer. METHODS AND MATERIALS: Random and systematic setup errors (RE and SE) of the isocenter with regard to the center of gravity of three fiducial markers were measured by means of orthogonal verification films in 120 treatment sessions in 12 patients. Positioning was performed using NovalisBody semiautomated marker fusion. The results were compared with a control group of 261 measurements in 15 patients who were positioned with NovalisBody automated bone fusion. In addition, interfraction and intrafraction prostate motion was registered in the patients with implanted markers. RESULTS: Marker-based X-ray positioning resulted in a reduction of RE as well as SE in the anteroposterior, craniocaudal, and left-right directions compared with those in the control group. The interfraction prostate displacements with regard to the bony pelvis that could be avoided by marker positioning ranged between 1.6 and 2.8 mm for RE and between 1.3 and 4.3 mm for SE. Intrafraction random and systematic prostate movements ranged between 1.4 and 2.4 mm and between 0.8 and 1.3 mm, respectively. CONCLUSION: The problem of interfraction prostate motion can be solved by using implanted markers. In addition, the NovalisBody X-ray system performs more accurately with markers compared with bone fusion. Intrafraction organ motion has become the limiting factor for margin reduction around the clinical target volume.