Effect of planning margin on dosimetric quality in 131Cs permanent prostate brachytherapy

To investigate the dosimetric effect of planning margin in (131)Cs prostate seed implants.

The transrectal ultrasonography images are obtained intraoperatively in 5-mm steps from base to apex. The prostate is contoured as clinical target volume (CTV). The CTV is enlarged with 3mm expansion except the posterior. The CTV and planning target volume (PTV) are then used as planning target for treatment planning, respectively. Dose calculations are performed using VariSeed treatment planning system using AAPM TG-43 formalism. The total activity implanted, target coverage (the percent of the prostate volume covered by the prescription dose, V(100); the dose that covers 90% of the prostate volume, D(90)) for CTV and PTV, dose inhomogeneity (the percentage volume of the prostate receiving 150% of the prescription dose [V(150)]), and the critical organ dose (the dose that covers 10% of the urethra volume [UD(10)] for urethra and the dose that covers 50% of the rectum volume [RD(50)] for rectum) are compared.

When CTV is used as target for planning, compared with PTV as planning target, the total activity implanted is decreased by 5.6%. Integral dose is thus lower by 5.6%. Coverage for CTV (CTV(100)) is increased by 0.3%. Coverage for PTV (PTV(100)) is lower by 2.1%. CTV(150) is increased by 13.6%. PTV(150) is higher by 2.5% with a standard deviation of 10.2%. Rectum dose (RD(50)) is lower by 4.5%. Urethra dose (UD(10)) is higher by 10.0%.

It is shown that the planning margin has minimal effect on dosimetric quality because of (131)Cs's gradual dose fall-off. Thus, it is possible to reduce or even eliminate planning margin using (131)Cs. The modest benefits in reducing the planning margin, such as lower total activity (lower integral dose), dose reduction to surrounding healthy tissues and reduced likelihood of seeds migration, can be achieved while adequate coverage is maintained.