Adapt or die.
Adaptive planning has intriguing potential utility for radiation to the prostate, but what about…dun dun dun…to the prostate fossa? Instead of theorizing, this diligent group prospectively obtained not only standard daily pretreatment cone beam CT (CBCT) imaging but also post-treatment CBCT imaging immediately after treatment delivery on 20 consecutive patients receiving conventionally fractionated postoperative radiation for prostate cancer for 248 paired daily images. Clinical target volumes (CTV) were retrospectively (post course completion) drawn on every single one. After all was said and done, besides developing wicked carpal tunnel syndrome, they determined that a PTV margin of 3 mm accounted for all intrafraction movement in 97% of treatments. In other words, when obviating the need to account for for interfraction movement with daily adaptive replanning, PTV margins can effectively be reduced from the standard 7 mm to 3 mm. What does this translate to dosimetrically? The average volume of rectum receiving 40 Gy was decreased from 50 → 20 cc and 65 Gy from 18 → 4 cc.
- Morgan, Pract Radiat Oncol 2022