Impact Of Anti-3-[18f]Facbc Positron Emission Tomography On Target Volume Definition After Prostatectomy: Initial Findings From A Randomized Trial

Anti-1-amino-3-[18F]fluorocyclobutane-1-carboxylic acid (FACBC) is a novel radiotracer that has demonstrated high sensitivity and specificity for detection of prostate cancer. The goal of this study was to evaluate the role of anti-3-[18F]FACBC in modifying postprostatectomy clinical and planning target volume (CTV and PTV) definition and the resulting dosimetric consequences to surrounding organs at risk (OARs). The records of 24 postprostatectomy patients (on the experimental arm of a randomized controlled trial) who received external beam radiation therapy after prostatectomy and who underwent FACBC registration were reviewed. Patients were treated either to the prostate fossa alone (CTV; 66.6 Gy, n = 10) or pelvis (CTV1; 45.0 Gy) followed by prostate fossa boost (CTV2; 21.6 Gy, n = 14). In each case the corresponding PTV expansion was 0.8 cm (0.6 cm posterior). For each patient, the target volume that would have been treated prior to (PRE) was compared to that after the FACBC registration (POST), and the V40 Gy and V65 Gy of the OARs (rectum, bladder [minus CTV], and penile bulb) PRE and POST were also compared. In all cases statistical comparisons were made using the paired t test. Radiation Therapy Oncology Group acute genitourinary (GU) and gastrointestinal (GI) toxicity were also tabulated and compared to a control group (n = 27) using the chi-square test. CTV/PTV, CTV1/PTV1, and CTV2/PTV2 PRE and POST volumes are shown in the Table. In all cases, with the exception of PTV2, the POST volumes were significantly larger than the PRE volumes. Rectum, bladder (minus CTV), and penile bulb V40 Gy and V60 Gy PRE and POST percentages are also shown in the Table. As shown, only penile bulb dose endpoints were significantly higher after registration. No significant differences in acute GU (P = .140) or acute GI (P = .153) toxicity were observed. Inclusion of FACBC information in the treatment planning process leads to significant differences in target definition with higher doses to the penile bulb but no significant differences in bladder or rectal dose or acute GU/GI toxicity. Longer follow-up is needed to determine the impact of FACBC on cancer control and late toxicity endpoints.Oral Scientific Abstracts 308; Table 1Target volumesCTV (cc)PTV (cc)CTV1 (cc)PTV1 (cc)CTV2 (cc)PTV2 (cc)PRE137.26340.83480.181130.6135.52334.47POST138.74344.91497.801167.4139.05341.00P-value0.01690.01450.00100.00140.01700.1020OAR’sRectum-V40 (%)Rectum-V65 (%)Bladder-CTV-V40 (%)Bladder-CTV-V65 (%)Penile-Bulb V40 (%)Penile-Bulb V65 (%)PRE51.06320.72761.62932.5252.8226.738POST50.59620.50161.1333.01860.60535.321P value.4571.2795.2919.2682.0078.0064 Open table in a new tab