Abstract
PurposeWith 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 methodsT2‐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.ResultsBladder 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%).ConclusionsVariations 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.
Highlights
Prostate cancer is the most common type of cancer in men, with over 160 000 cases reported in 2017 in the United States.[1]
Two MR‐only packages are currently clinically available for prostate cancer treatment planning with synthetic computed tomography (CT) [synCTs, or CTs generated from magnetic resonance imaging (MRI) input(s)]
In a recent study by Farjam et al.,[12] pelvic Magnetic Resonance for Calculating Attenuation (MR‐CAT) images of 23 patients with prostate cancer underwent deformable registration to the planning CT images and found good overall agreement over the entire pelvis volume [mean absolute error (MAE) values of 65 ± 5 HU] with even smaller difference observed in the fat and muscle (~40 HU) across all subjects
Summary
Prostate cancer is the most common type of cancer in men, with over 160 000 cases reported in 2017 in the United States.[1]. One FDA‐approved software package, the Philips Magnetic Resonance for Calculating Attenuation (MR‐CAT), is based on a dual echo three‐dimensional (3D) mDixon fast field echo sequence with synCTs generated on the scanner.[10,11] In a recent study by Farjam et al.,[12] pelvic MR‐CAT images of 23 patients with prostate cancer underwent deformable registration to the planning CT images and found good overall agreement over the entire pelvis volume [mean absolute error (MAE) values of 65 ± 5 HU] with even smaller difference observed in the fat and muscle (~40 HU) across all subjects It is not currently known how synthetic CT generation performs over a variety of internal conditions nor has the dosimetric impact of this been characterized. This work characterizes the temporal, spatial, and dosimetric intra‐fractional changes between the prostate, seminal vesicles (SVs), and other organs at risk (OARs) in response to bladder filling conditions for MR‐only prostate cancer radiation therapy planning
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