Dosimetric effect of online image-guided anatomical interventions for postprostatectomy cancer patients.
OBJECTIVE
To assess daily variations in delivered doses in postprostatectomy patients, using kilovoltage cone-beam CT (CBCT) datasets acquired before and after interventions to correct for observed distortions in volume/shape of rectum and bladder.


METHODS
Seventeen consecutive patients treated with intensity-modulated radiotherapy to the prostate bed were studied. For patients with large anatomical variations, quantified by either a rectal wall displacement of >5 mm or bladder volume change of >50% on the CBCT compared with the planning CT, an intervention was performed to adjust the rectum and/or bladder filling. Cumulative doses over the pre- and post-intervention fractions were calculated by tracking the position of the planning CT voxels on different CBCTs using a deformable surface-mapping algorithm. Dose and displacements vectors were projected on two-dimensional maps, the minimal dose received by the highest 95% of the planing target volume (PTV D95) and the highest 10% of the rectum volume (D10) as well as the bladder volume receiving >2 Gy (V2) were evaluated.


RESULTS
Of 544 fractions, 96 required intervention. Median (range) number of interventions per patient was 5 (2-12). Compared with the planning values, the mean (SD) pre- vs. postintervention value for PTV D95 was -2% (2%) vs. -1% (2%) (p < 0.12), for rectum D10 was -1% (4%) vs. +1% (4%) (p < 0.24), and for bladder V2 was +6% vs. +20% (p < 0.84).


CONCLUSIONS
Interventions to reduce treatment volume deformations due to bladder and rectum fillings are not necessary when patients receive daily accurate CBCT localization, and the frequency of those potential interventions is low. However, for hypofractionated treatments, the relative frequency can significantly increase, and interventions can become more dosimetrically beneficial.
Diot
Quentin
Q
Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, CO 80045, USA. quentin.diot@ucdenver.edu
Olsen
Christine
C
Kavanagh
Brian
B
Raben
David
D
Miften
Moyed
M
D000368:Aged; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement; D009929:Organ Size; D058958:Organs at Risk / Q000528:radiation effects; D011468:Prostatectomy; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery; D011832:Radiation Injuries / Q000517:prevention & control; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D012189:Retrospective Studies; D047368:Tumor Burden; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging*