Dosimetric effect of external beam planning preceding combined high-dose-rate brachytherapy of the prostate.
OBJECTIVE
The sequencing of external beam radiotherapy (EBRT) and a high-dose-rate brachytherapy (HDRB) boost is often interchangeable in clinical practice. When given before EBRT, HDRB could induce volume alterations in the prostate, which may have significant implications for EBRT dosimetry. We aimed to assess the influence of HDRB on prostate volume and, hence, prostate dosing via subsequent EBRT.


METHODS
Fifteen men had both pre- and post-HDRB CT performed followed by EBRT. After deidentification, the clinical target volume (CTV) was defined on each CT by a single-blinded observer. Volumes were compared for the pre- and post-HDRB scans in each patient. Radiotherapy planning was performed using the prebrachytherapy volumes aiming for the planning target volume (PTV) to be covered by 43.7Gy. After soft-tissue coregistration, this plan was also applied to the postbrachytherapy volumes.


RESULTS
Median volume increase was 35.4% for the CTV after HDRB. No patient experienced a decrease in CTV volume (range, 0-79% volume increase; p-value<0.001). Median volume increase was 26.1% for the PTVs, with no volume decrease observed (range, 8-56%; p<0.001). PTV proportion achieving dose target (V43.7Gy) decreased by median of 7% (range, 0-21.5%; p=0.004). The minimum dose to the PTV (D(100)%) decreased by a median of 6Gy (range, 0.5-16Gy; p<0.001).


CONCLUSIONS
Insertion of HDRB catheters exerts substantial acute volumetric distortion on the prostate. EBRT planning performed on the basis of pre-HDRB imaging only inherently risks underdosing tumor. Planning adjustments based on repeat CT, or dedicated post-HDBT planning, is warranted for men managed with HDRB before EBRT.
Division of Oncology, Cancer Care Services, Royal Brisbane and Women's Hospital, Brisbane, Queensland, Australia. jarad.martin@roq.net.au
Jarad M
JM
Martin
Richard
R
Brett
Jemma
J
Blyth
Stewart
S
Morrison
Daniel
D
Bryant
Ashley
A
Plank
Robyn
R
Cheuk
Michael
M
Fay
Graeme
G
Dickie
John
J
Yaxley
D006801:Humans; D008297:Male; D009929:Organ Size; D011467:Prostate / Q000473:pathology*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011878:Radiotherapy / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal