Target coverage and organs at risk dose in hypofractionated salvage radiotherapy after prostatectomy

TL;DR

This study compares hypofractionated and conventional radiotherapy after prostate surgery, finding similar organ changes and optimal target coverage with specific margin sizes.

Contribution

The study introduces optimal planning target volume margins for hypofractionated salvage radiotherapy after prostatectomy.

Findings

A 6 mm margin for the prostate bed and 8 mm for the vesicle bed ensured excellent target coverage.

Bladder and rectum volume changes were similar between hypofractionated and conventional radiotherapy.

Bladder and rectum dose constraints were not fully met in some fractions despite optimal margins.

Abstract

•Hypofractionation had similar interfractional changes as conventional fractionation.•Bladder volume changed significantly, yet target coverage remained excellent.•A planning target volume margin of 6 mm to the prostate bed seems sufficient.•A planning target volume margin of 8 mm to the vesicle bed is needed.

Hypofractionation had similar interfractional changes as conventional fractionation.

Bladder volume changed significantly, yet target coverage remained excellent.

A planning target volume margin of 6 mm to the prostate bed seems sufficient.

A planning target volume margin of 8 mm to the vesicle bed is needed.

Introducing moderately hypofractionated salvage radiotherapy (SRT) following prostatectomy obligates investigation of its effects on clinical target volume (CTV) coverage and organ-at-risk (OAR) doses. This study assessed interfractional volume and dose changes in OARs and CTV in moderately hypofractionated SRT and evaluated the 8-mm planning target volume (PTV) margin.

Twenty patients from the PERYTON-trial were included; 10 received conventional SRT (35 × 2 Gy) and 10 hypofractionated SRT (20 × 3 Gy). OARs were delineated on 539 pre-treatment Cone Beam CT (CBCT) scans to compare interfractional OAR volume changes. CTVs for the hypofractionated group were delineated on 199 CBCTs. Dose distributions with 4 and 6 mm PTV margins were generated using voxel-wise minimum robustness evaluation of the original 8-mm PTV plan, and dose changes were assessed.

Median volume changes for bladder and rectum were −26 % and −10 %, respectively. OAR volume changes were not significantly different between the two treatment schedules. The 8-mm PTV margin ensured optimal coverage for prostate bed and vesicle bed CTV (V95 = 100 % in >97 % fractions). However, bladder V60 <25 % was not achieved in 5 % of fractions, and rectum V60 <5 % was unmet in 33 % of fractions. A 6-mm PTV margin resulted in CTV V95 = 100 % in 92 % of fractions for prostate bed, and in 86 % for vesicle bed CTV.

Moderately hypofractionated SRT yielded comparable OAR volume changes to conventionally fractionated SRT. Interfractional changes remained acceptable with a PTV margin of 6 mm for prostate bed and 8 mm for vesicle bed.

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