Subtraction structures for an overlapping OAR
When the target overlaps something you must spare, split the target rather than asking the optimizer to satisfy both in the same voxels.
Built in·updated 2026-08-03
Where a target overlaps an organ you must protect, no weighting resolves it: the same voxel cannot be at prescription and under a limit. The standard answer is to make the conflict explicit in the structures.
The prostate SBRT pattern, as published: expand the urethra, bladder and rectum by 3 mm, subtract that expansion from the prostate, and prescribe the high dose to what is left. The overlap region gets its own, lower goal — a urethral planning-risk volume at 32.5 Gy where the rest of the target takes 36.25 Gy in 5 fractions, or a Dmax < 39 Gy on the contoured urethra.
The general form. PTV_high = PTV − (OAR + margin), and a second target structure for the overlap with a goal you can actually meet. Now the DVH tells you what happened in each region instead of averaging the conflict.
Where it goes wrong on a scored case. The criteria are written against the published structure names. If you invent PTV_minus_rectum and optimize on it beautifully, the scorer still reads PTV, including the part you deliberately underdosed. Optimize on your own structures; check the score on theirs.
Where the numbers come from
- Long-term results of prostate SBRT with real-time tracking and novel urethral- and rectal-sparing techniques
Int J Radiat Oncol Biol Phys · 2025
3 mm expansion of urethra/bladder/rectum subtracted from prostate to define the high-dose PTV
- Optimizing prostate cancer SBRT: acute GU toxicity outcomes of urethral delineation methods
PMC · 2025
Urethral steering with Dmax < 39 Gy; urethra PRV 32.5 Gy against 36.25 Gy in 5 fractions