Purpose: Image-guided radiation therapy (IGRT) was suggested to improve outcomes and reduce toxicity for localized prostate cancer treatment. We generated virtual treatment plans to evaluate the dosimetric impacts with and without IGRT. Materials and Methods: Daily position setup errors from 10 patients treated with volumetric modulated arc therapy (VMAT) were acquired with cone-beam computed tomography (CBCT). A margin of 5 mm was added to the clinical target volume (CTV) to generate a planned target volume (PTV). We shifted the isocenters in the planning system with errors, fraction by fraction, to simulate the situation without IGRT. Dosimetric indices were compared between the initial plans (pretreatment CT plans) and the shifted plans. Radiobiological evaluations of an equivalent uniform dose (EUD) and normal tissue complication probability (NTCP) were also performed. Results : The system errors (Σ) and random errors (σ) in the left-right (LR), superiorinferior (SI), and anterior-posterior (AP) directions were Σ LR=1.4 mm, σ LR= 1.95 mm, Σ SI =3.22 mm, σ SI = 2.46 mm, Σ AP=3.17 mm and σ AP = 2.79 mm. Coverage of the CTV for 100% and 95% of the prescribed doses were lower in the shifted plans (V100%: 99.99% vs. 92.37%, p ＜ 0.05 and V95%: 100% vs. 99.46%, p ＜ 0.05). There was no difference in the dosimetric indices for the bladder and rectum. The EUD value of the CTV was also lower in the shifted plan (76.8 Gy vs. 75.4 Gy, p ＜ 0.05). The EUD and NTCP for the bladder and rectum did not statistically differ. Conclusions: Using CBCT-acquired VMAT for treating prostate cancer not only shortened the treatment time but also decreased the setup uncertainty. In the setting of reducing the PTV margin to 5 mm, the dose to the prostate was compromised without IGRT but the estimated toxicity to the rectum and bladder exhibited no difference.
- Volumetric modulated arc therapy (VMAT)
- Cone-beam computed tomography (CBCT)
- Equivalent uniform dose (EUD)
- Normal tissue complication probability (NTCP)