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.
OBJECTIVE
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).


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.
Perna
Lucia
L
Medical Physics, S. Raffaele Institute, Milano, Italy.
Fiorino
Claudio
C
Cozzarini
Cesare
C
Broggi
Sara
S
Cattaneo
Giovanni Mauro
GM
De Cobelli
Francesco
F
Mangili
Paola
P
Di Muzio
Nadia
N
Calandrino
Riccardo
R
D015331:Cohort Studies; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010413:Penis / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control; D011834:Radiation Monitoring / Q000379:methods; D018787:Radiation Oncology / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D036542:Tomography, Spiral Computed; D014057:Tomography, X-Ray Computed / Q000379:methods