Program Information
Estimation of Mean Bladder Volume Based On Cone-Beam Computed Tomography Modeling for Post-Operative Prostate Cancer Patients
B Housawi*, E Moros , P Johnstone , K Latifi , H. Lee Moffitt Cancer Center, Tampa, FL
Presentations
SU-I-GPD-J-18 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Evaluate bladder volume (BV) variation during radiotherapy in patients following a bladder filling protocol and to evaluate a Cone-Beam Computed Tomography (CBCT) based model for mean bladder volume (mBV) estimation.
Methods: Eleven post-operative prostate cancer patients underwent daily CBCT scans prior to daily fractions. To maintain full bladder, patients were to urinate 30-60 min prior to treatment, then consume 500ml of water. Current dosing consists of 60 Gy in 30 fractions, with boost of up to 6 fractions based on mBV. From 330 CBCTs, BVs were delineated and dimensions in craniocaudal (D1), lateral (D2) and anterior-posterior (D3) directions were measured. The ellipsoid model (EM), V=0.52 X D1 X D2 X D3 used to estimate mBV. For each patient, CBCT with BV equal to calculated mBV was registered to planning CT, including contours. D25% to mBV was compared to simulation BV. Daily CBCTs were divided into two groups, A:1-15 and B:16-30 fractions.
Results: BVs varied highly. The mBV was 227.6 ml, range 51-542 ml, SD 112.1 ml. Calculated mBV from model was correlated significantly to true mBV on Pearson’s (r=0.82, p<0.01). Errors of 7% and 10% were obtained in mBV from the first 15 and 20 CBCTs, respectively. Compared to simulation BV, mBV of 2/11 patients had comparable D25%. In 7/11 patients, the mBV was larger and received less D25% than at simulation, providing a favorable DVH and potential to increase dose using adaptive re-planning. BV in group B had less variation on f-test than group A though it was not significant (p=0.18).
Conclusion: High BV variation indicates that the used bladder filling protocol does not render reproducible BV. The model showed good mBV estimation. Further research is planned to study the effectiveness of mBV estimation for adaptive treatment strategy.
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