Impact of daily soft-tissue image guidance to prostate on pelvic lymph node (PLN) irradiation for prostate patients receiving SBRT.

Purpose: To determine the impact of using fiducial match for daily image-guidance on pelvic lymph node (PLN) coverage for prostate cancer patients receiving stereotactic body radiation therapy (SBRT). Methods: Thirty patients underwent SBRT treatment to the prostate and PLN from 2014 to 2016. Each patient received either 800cGy x 5 or 500cGy x 5 to the prostate and 500cGy x 5 to the PLN. A 5 mm clinical target volume (CTV)-to-planning target volume (PTV) margin around the PLN was used for planning. Two registrations with planning computed tomography (PCT) for each of the daily cone beam CTs (CBCTs) were performed: a rigid registration to fiducials and to the bony anatomy. The average translational difference between fiducial and bony match as well as percentage of fractions with differences > 5mm were calculated. Changes in bladder and rectal volume as well as center-of-mass (COM) position from simulation parameters, and their correlation with translational difference were also evaluated. The dosimetric impact of the translational differences was calculated by shifting the plan isocenter. Results: The average translational difference between fiducial and bony match was 0.06 +/- 0.82, 2.1 +/- 4.1, -2.8 +/- 4.3, and 5.5 +/- 4.2 mm for lateral, vertical, longitudinal, and vector directions. The average change in bladder and rectal volume from simulation was -67.2 +/- 163.04 cc (-12 +/- 52%) and -1.6 +/- 18.75 (-2 +/- 30%) cc. The average change in COM of bladder from the simulation position was 0.34 +/- 2.49, 4.4 +/- 8.1, and -3.9 +/- 7.5 mm along the LR, AP, and SI directions. The corresponding COM change for the rectum was 0.17 +/- 1.9, 1.34 +/- 3.5, and -0.6 +/- 5.2 mm. Conclusions: The 5 mm margin covered similar to 75% of fractions receiving PLN irradiation with SBRT, daily CBCT and fiducial-guided setup. The dosimetric impact on PLN coverage was significant in 19% of fractions or 25% of patients. A larger translational shift was due to variation in rectal volume and changes in COM position of the bladder and rectum. A consistent bladder positioning and/or rectum filling compared with presimulation volume were essential for adequate coverage of PLN in a hypofractionated treatment regime.