Geometric and dosimetric impact of anatomical changes for MR‐only radiation therapy for the prostate

Abstract

Purpose With the move towards magnetic resonance imaging (MRI) as a primary treatment planning modality option for men with prostate cancer, it becomes critical to quantify the potential uncertainties introduced for MR‐only planning. This work characterized geometric and dosimetric intra‐fractional changes between the prostate, seminal vesicles (SVs), and organs at risk (OARs) in response to bladder filling conditions. Materials and methods T2‐weighted and mDixon sequences (3–4 time points/subject, at 1, 1.5 and 3.0 T with totally 34 evaluable time points) were acquired in nine subjects using a fixed bladder filling protocol (bladder void, 20 oz water consumed pre‐imaging, 10 oz mid‐session). Using mDixon images, Magnetic Resonance for Calculating Attenuation (MR‐CAT) synthetic computed tomography (CT) images were generated by classifying voxels as muscle, adipose, spongy, and compact bone and by assignment of bulk Hounsfield Unit values. Organs including the prostate, SVs, bladder, and rectum were delineated on the T2 images at each time point by one physician. The displacement of the prostate and SVs was assessed based on the shift of the center of mass of the delineated organs from the reference state (fullest bladder). Changes in dose plans at different bladder states were assessed based on volumetric modulated arc radiotherapy (VMAT) plans generated for the reference state. Results Bladder volume reduction of 70 ± 14% from the final to initial time point (relative to the final volume) was observed in the subject population. In the empty bladder condition, the dose delivered to 95% of the planning target volume (PTV) (D95%) reduced significantly for all cases (11.53 ± 6.00%) likely due to anterior shifts of prostate/SVs relative to full bladder conditions. D15% to the bladder increased consistently in all subjects (42.27 ± 40.52%). Changes in D15% to the rectum were patient‐specific, ranging from −23.93% to 22.28% (−0.76 ± 15.30%). Conclusions Variations in the bladder and rectal volume can significantly dislocate the prostate and OARs, which can negatively impact the dose delivered to these organs. This warrants proper preparation of patients during treatment and imaging sessions, especially when imaging required longer scan times such as MR protocols.

Dosimetric effects of adaptive prostate cancer radiotherapy in an MR-linac workflow

“…Nejad-Davarani et al [19] studied the dosimetric effect of anatomical changes in prostate cancer patients for an MRI-only workflow. MR images of volunteers with both full and empty bladder were acquired, with 45 min apart.…”

“…Intrafractional prostate motion is a well-known phenomenon [12][13][14][15][16][17][18][19] and can have dosimetric impact. Nejad-Davarani et al [19] found a 20.2% decrease in the dose to 95% of the volume (D 95% ) in the PTV due to prostate displacement occurring during approximately 45 minutes (min). There are various reasons for prostate displacements, such as bladder filling, rectum activity and patient motion due to muscle tension or leg motion [12].…”

“…The prostate can be displaced by more than 10 mm during a conventional radiotherapy treatment session [14,15,17], and the nature of the motion is often a combination of large, sudden shifts and a slow drift [15]. It has also been shown that the probability of motion increases with time [13][14][15][16][17][18][19], mainly in the longitudinal and vertical direction [14-16, 18, 19] .…”

VAPOR: Visual Analytics for the Exploration of Pelvic Organ Variability in Radiotherapy

“…Both are shown in Figure 2. First, spatial 2D/3D views (Figure (a)) allow the experts to see how the dose affects the tumor and its surrounding organs for a given point in the treatment period [19]. This approach does not support an easy exploration of multiple patients or multiple fractions at the same time-an important aspect for judging the robustness of treatment strategies.…”

“…Instead, if a specific group 17is selected, it is brought forward with a F+C strategy in the 2DFigure 7 (c)) and the 3D views. On demand, the 3D view can19 show the explicit encoding of the surface variations(Figure 7…”