Dosimetric effect of external beam planning preceding combined high-dose-rate brachytherapy of the prostate

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.

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.

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).

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.