Dosimetric comparison of high-dose-rate brachytherapy and intensity-modulated radiation therapy as a boost to the prostate.

PURPOSE: We compared the dose conformity of two radiation modalities: high-dose-rate brachytherapy (HDR BT) and intensity-modulated radiation therapy (IMRT) to deliver a boost to the prostate after external beam radiotherapy (EBRT).

METHODS AND MATERIALS: Ten successive patients with prostate adenocarcinoma treated with a single 10-Gy HDR BT boost after EBRT were investigated. Four theoretical IMRT plans were computed: (a) 32.85 Gy IMRT and (b) 26 Gy IMRT with CTV-PTV expansions, doses corresponding to the equivalent dose in 2-Gy fractions (EQD2) of one 10-Gy fraction calculated with a prostate alpha/beta ratio of respectively 1.5 and 3 Gy; and (c) 32.85 Gy IMRT and (d) 26 Gy IMRT without CTV-PTV expansions. The dose-volume histogram values converted in EQD2 with an alpha/beta ratio of 3 Gy for the organs at risk were compared.

RESULTS: The HDR BT plan delivered higher mean doses to the PTV compared with IMRT plans. In all, 33% of the rectal volume received a mean dose of 5.32 +/- 0.65 Gy and 20% of bladder volume received 4.61 +/- 1.24 Gy with HDR BT. In comparison, doses delivered with IMRT were respectively 13.4 +/- 1.49 Gy and 10.81 +/- 4 Gy, even if only 26 Gy was prescribed to the PTV with no CTV-PTV expansion (p < 0.0001). The hot spots inside the urethra were greater with HDR BT but acceptable.

CONCLUSIONS: Use of HDR BT produced a more conformal plan for the boost to the prostate than IMRT even without CTV-PTV expansions.