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

Abstract

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 × 5 or 500cGy × 5 to the prostate and 500cGy × 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 ~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.

Maintaining consistent bladder filling during external beam radiotherapy for prostate cancer

“…The challenge of having the patients on the treatment table with the bladder consistently full for each treatment visit and with bladder filling the same as during the CT planning scan, has been investigated in many studies which suggest that bladder filling is not consistent [4,5,15,16,19]. Even with established drinking protocols for bladder filling, the volume of urine remained inconsistent between treatment visits [6,19]. The rate of urine production could be affected by the hydration status of the patient before radiation, as well as other factors such as background diseases (diabetes, renal failure, etc.)…”

“…Moreover, the continuous accumulation of urine in the bladder from the initial patient setup and cone beam imaging until the end of radiation delivery, could result in intrafraction error, with potentially lower doses to the clinical target volumes and higher doses to the organs at risk, as a result from the change in bladder filling. Variations in bladder filling have been shown to affect target coverage is several studies [3,4,[15][16][17][18][19].…”