Stereotactic body radiotherapy to the prostate and pelvic lymph nodes: A detailed dosimetric analysis of a phase II prospective trial

Objective: To evaluate the dosimetric ramifications of simultaneously irradiating the prostate and pelvic lymph nodes (PLNs) with a stereotactic body radiotherapy approach based on rigid registration to intraprostatic markers (IPMs). Methods and materials: Nineteen patients received concurrent SBRT to the prostate and PLNs on a phase II clinical trial. The translational and rotation shifts required for rigid registration to bony anatomy and changes in bladder and rectal anatomy were compared between patients with > 90% and < 90% coverage of the nodal clinical target volume (CTVN) as drawn on fractional kilovoltage cone-beam CTs. Stepwise multivariable regression models evaluated relationships between these anatomical parameters and the change in V100%CTVN. Results: The average V100%CTVN per patient was 92.4 % (IQR, 90.2 - 96.4 %). For five patients (26.3%), the average was 85,0 % (IQR, 82.4-88.3 %), The left-right and superior-inferior translational shifts, sagittal rotational shift, and change in bladder volume were significantly different ( p < 0.05 for all via Student's t-test). Changes in bladder height, left/right shift, superior/inferior shift, 3-D shift, and axial rotation as significant predictors of change in dosing of V100%CTVN. Conclusion: While simultaneous SBRT to the prostate and PLNs based on rigid registration to IPMs provides adequate PLN coverage in most instances, overall coverage may be lower than anticipated if anatomy is unstable. Careful evaluation of bladder filling on kV-CBCT before treatment may be the most practical method for estimating accuracy prior to treatment. Advances in knowledge: Simultaneous SBRT to the prostate and PLNs based on rigid registration to IPMs provides adequate PLN coverage in most instances.