Sparing the penile bulb in the radical irradiation of clinically localised prostate carcinoma: A comparison between MRI and CT prostatic apex definition in 3DCRT, Linac-IMRT and Helical Tomotherapy.

摘要

BACKGROUND AND PURPOSE: To assess the impact of using MRI and Helical Tomotherapy (HT) compared to 3DCRT and dynamic IMRT on the dose to the penile bulb (PB). MATERIALS AND METHODS: Eight patients diagnosed with prostate cancer entered a treatment protocol including CT and MRI simulation. The prostate apex was defined on both MRI and CT. Treatment plans (HT, Linac-IMRT, 3DCRT and conventional technique), were elaborated on both MRI and CT images. A dose of 71.4Gy (2.55Gy/fraction) was prescribed; it was requested that PTVs be covered by 95% isodose line. The mean dose and V50 of PB were evaluated. RESULTS: PTV-MRI plans reduced PB mean dose and V50 compared to PTV-CT plans. This improvement, deriving also from the treatment modality, was 89% for 3DCRT, 99% for Linac-IMRT and 97% for HT (p<0.01), considering V50. Conventional plans resulted in a significantly higher mean PB dose/V50 compared to 3DCRT-PTV-CT (+27%/+38%), Linac-IMRT-PTV-CT (+42%/+57%) and HT-PTV-CT (+32%/+48%) (p<0.01). The comparison between conventional and PTV-MRI techniques showed a still larger increase: +73%/+93% 3DCRT; +86%/+99% Linac-IMRT; +56%/+99% HT (p<0.01). The PB mean dose reduction with Linac-IMRT compared to 3DCRT was 24% (p=0.034) and 40% (p=0.027) for PTV-CT and PTV-MRI, respectively. This gain remained significant even when comparing Linac-IMRT to HT: 21% (p=0.07) PTV-CT and 68% (p=0.00002) PTV-MRI. HT was superior to 3DCRT with respect to PTV-CT (average gain 4%, p=0.044), whereas it resulted to be detrimental considering PTV-MRI (26Gy vs 16.5Gy), possibly due to the helical delivery of HT; however, in a patient where the distance bulb-PTV <1cm, HT provided better PB sparing than 3DCRT (29.5Gy vs 45.2Gy). CONCLUSIONS: MRI allowed efficient sparing of PB irrespective of the treatment modality. Linac-IMRT was shown to further reduce the dose to the bulb compared to 3DCRT and HT.