Room: AAPM ePoster Library
Purpose: appropriate target coverage of prostate is achievable with IGRT, simultaneously constraining rectal doses that are consistent with planned values is challenging, due to rectal shape/filling variability. We investigated if rigid on-board alignment can fulfill rectum planning goals.The delivered rectal dose range varies between doses corresponding to prostate alignment (“P”) and anterior rectal wall alignment (“R”).
Methods: CBCTs(dCBCT) and planning images (8 cases) were aligned by “P” and “R”. Rectal doses were estimated by rigidly mapping the planned dose on dCBCT according to the respective alignment shift. Rectum V95% was used for analyses; daily values similar to planning values demonstrate reproducible anatomy, larger indicate a non-reproducibile/less favorable anatomy, smaller indicate a non-reproducible/more favorable anatomy. The AP-shifts from “P” to “R” were used to evaluate %number-of-fractions with the shift magnitude larger than PTV margin (indicating potential target underdosage) and %number-of-fractions for which “R” alignment moved the rectum into higher isodose regions.
Results: rectum V95% over all patients/fractions was less in “R”(6.2cc) than “P”(7.5cc), and exceeded the average planning value (5.3cc). 1/8 (12.5%) cases had a reproducible planning anatomy. Of the patients with non-reproducible anatomy, 2 had dosimetrically more favorable, while 5 had less favorable anatomies. On average, “P” V95% increased by 6% from “R” [range:-87.6% to +43.5%] and by 100.2% from planning [range:-37.8% to +503.5%]. 18.9% [range:0 to 62.5%] of the shifts from “P” to “R” were larger than the PTV margin.
Conclusion: interfraction separation between prostate and rectal wall translates into deformations that cannot be mitigated with rigid alignment. If systematic errors exist due to a non-reproducible planning anatomy, attempts to restore the planned rectal doses through rectal wall alignment produce rather small improvements and may result in unacceptable target underdosage. While CBCT can provide snapshots of the daily anatomy, its usefulness in achieving planning goals for rectum is questionable.
Not Applicable / None Entered.