PURPOSEThe purpose of this article was to determine the suitability of the prostate and seminal vesicle volumes as factors to consider patients for treatment with image-guided 3D-conformal radiation therapy (3D-CRT) or intensity-modulated radiation therapy (IMRT), using common dosimetry parameters as comparison tools.METHODSDosimetry of 3D and IMRT plans for 48 patients was compared. Volumes of prostate, SV, rectum, and bladder, and prescriptions were the same for both plans. For both 3D and IMRT plans, expansion margins to prostate+SV (CTV) and prostate were 0.5 cm posterior and superior and 1 cm in other dimensions to create PTV and CDPTV, respectively. Six-field 3D plans were prepared retrospectively. For 3D plans, an additional 0.5 cm margin was added to PTV and CDPTV. Prescription for both 3D and IMRT plans was the same: 45 Gy to CTV followed by a 36 Gy boost to prostate. Dosimetry parameters common to 3D and IMRT plans were used for comparison: Mean doses to prostate, CDPTV, SV, rectum, bladder, and femurs; percent volume of rectum and bladder receiving 30 (V30), 50 (V50), and 70 Gy (V70), dose to 30% of rectum and bladder, minimum and maximum point dose to CDPTV, and prescription dose covering 95% of CDPTV (D95).RESULTSWhen the data for all patients were combined, mean dose to prostate and CDPTV was higher with 3D than IMRT plans (P < 0.01). Mean D95 to CDPTV was the same for 3D and IMRT plans (P > 0.2). On average, among all cases, the minimum point dose was less for 3D-CRT plans and the maximum point dose was greater for 3D-CRT than for IMRT (P < 0.01). Mean dose to 30%, rectum with 3D and IMRT plans was comparable (P > 0.1). V30 was less (P < 0.01), V50 was the same (P > 0.2), and V70 was more (P < 0.01) for rectum with 3D than IMRT plans. Mean dose to bladder was less with 3D than IMRT plans (P < 0.01). V30 for bladder with 3D plans was less than that of IMRT plans (P < 0.01). V50 and V70 for 3D plans were the same for 3D and IMRT plans (P > 0.2). Mean dose to femurs was more with 3D than IMRT plans (P < 0.01). For a given patient, mean dose and dose to 30% rectum and bladder were less with 3D than IMRT plans for prostate or prostate+SV volumes <65 (38/48) and 85 cm3 (39/48), respectively (P < 0.01). The larger the dose to rectum or bladder with 3D plans, the larger also was the dose to these structures with IMRT (P < 0.001). For both 3D and IMRT plans, dose to rectum and bladder increased with the increase in the volumes of prostate and seminal vesicles (P < 0.02 to 0.001).CONCLUSIONSVolumes of prostate and seminal vesicles provide a reproducible and consistent basis for considering patients for treatment with image-guided 3D or IMRT plans. Patients with prostate and prostate+SV volumes <65 and 85 cm3, respectively, would be suitable for 3D-CRT. Patients with prostate and prostate+SV volumes >65 and 85 cm3, respectively, might get benefit from IMRT.