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

Background and purposeTo 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 methodsEight 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 71.4 Gy (2.55 Gy/fraction) was prescribed; it was requested that PTVs be covered by 95% isodose line. The mean dose and V50 of PB were evaluated.ResultsPTV-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 (26 Gy vs 16.5 Gy), possibly due to the helical delivery of HT; however, in a patient where the distance bulb-PTV <1 cm, HT provided better PB sparing than 3DCRT (29.5 Gy vs 45.2 Gy).ConclusionsMRI 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.