Session Item

While the MR linear accelerator (MRL) enables soft tissue visualization to aid in daily online adaptive re-planning, such activities increase radiation treatment delivery times. The purpose of this study was to quantify bladder volume changes and their impact on the target position for patients undergoing MR-guided prostate stereotactic body radiation therapy (SBRT).

Patients were requested to come hydrated for their MR-simulation appointments, asked to void prior to the session and given 600 mL of water to drink. Based on their filling rate at MR-simulation, patients treated on the MRL over a 3000cGy/5 SBRT schedule were asked to void and drink either 300 mL or 600 mL of water within 10 minutes prior to treatment. A total of 3 T2 MR scans were acquired: at the start of the session (localization MR), at the conclusion of online plan adaptation (verification MR) and during beam delivery (beam-on MR). Clinical target volume (CTV)/prostate positional changes ≥ 5 mm noted on the verification MR initiated a second online plan adaptation process prior to treatment delivery. The entire bladder volume and CTV on each of the MR scans for all treated fractions were retrospectively contoured for each patient. The CTV’s center of mass position during each of the MR scans was used to quantify target positional changes in the left/right (X), superior/inferior (Y), and anterior/posterior (Z) directions. The Spearman coefficient (ρ) was used to correlate changes in CTV position to bladder volume. 

A total of 125 localization, 125 verification and 125 beam-on MRs were contoured from 25 patients. The average time from localization to verification was 24:10 minutes, and from localization to beam-on, 35:07 minutes. The mean (standard deviation [SD]) bladder volume change from localization to verification was 63.5(54.5) cc (range 3.9 – 279.8), and from localization to beam-on, 113.2(91.0) cc (range 12.5 – 520.7). CTV mean (SD) positional changes on verification in the X, Y, and Z directions were -0.019(0.70) mm, 1.68(2.4) mm, and -0.86(1.8) mm, respectively.  CTV mean (SD) positional changes during beam-on in the X, Y, and Z directions were -0.031(0.96) mm, 1.96(2.7) mm, and -1.15(1.9) mm, respectively. Changes in CTV position was plotted against changes in bladder volume in Figure 1; the ρ was 0.016 (p=0.803), 0.247 (p=0.0001), and 0.123 (p=0.057) in X, Y and Z, respectively.

Despite consistent timing and drinking instructions, bladder volume changes varied within and between patients during prostate SBRT on the MRL. Bladder volume changes in-session were primarily associated with target displacement in the posterior and inferior directions. Future work will identify bladder fill characteristics that will have the greatest impact on target position.

Figure 1: Positional changes in prostate (CTV) position relative to changes in bladder volume in the a) left/right, b) superior/inferior, and c) anterior/posterior directions.