3D geometry of prostate treatment. The prostate lies inferior/posterior to the bladder and anterior to the rectal wall. The ERB expands the rectal wall and exerts force against the prostate in the anterior direction. The OAR for this treatment include the bladder and anterior rectal wall.
Sagittal slices of the pelvis showing the effect of ERB intervention. Two structures are contoured: bladder (superior/anterior) and prostate (inferior/posterior). A) pCT image showing radiotherapy planning position. B) CBCT image showing pre-intervention ERB position. The ERB was not inserted to the same depth as it was for the pCT scan, and also exhibits a relative tilt. The shape of the prostate is noticeably different. C) CBCT image showing post-intervention ERB position, which more closely matches the pCT image.
Axial slice of the prostate and ERB demonstrating displacement and deformation. Left: The ERB is placed incorrectly at time of treatment, causing the prostate to shift in the AP direction from the plan position. A deformable algorithm is used to determine the displacement of each voxel in the prostate in order to determine dose received during treatment. Right: The same algorithm is applied after rigid alignment of the two prostate structures in order to quantify the deformation (i.e., difference in shape) of each voxel.
3D view of patient-specific changes in dose to and displacement of the prostate and ARW, where each row represents the prostate/ARW of a different patient. For each patient with more than one intervention, the dose and displacement of each voxel were averaged within all pre- and postintervention treatments [denoted as columns (a) and (b), respectively], based on the voxel-wise displacements computed from CBCT contours. Doses on the surface of the prostate are displayed as the difference from the prescription dose of 10 Gy/fx; thus, any nonzero data points represent underdosing of the prostate. Doses to the anterior rectal wall are displayed as the average difference per fraction from the treatment plan dose. In the prostate, the cumulative effect of intervening was an increase in target coverage and a decrease in surface displacement from the planning prostate shape. In the ARW, the cumulative effect of intervention was that the resultant doses more closely resembled the plan dose and that there was a decrease in surface displacement near the ERB–prostate interface. Note that the perspective of this figure is identical to Fig. 1.
Effect of ERB interventions on the position of each voxel in the prostate, anterior rectal wall, and bladder. Deformation represents the changes in shape, while displacement represents changes in shape in addition to any rigid-body misalignment. In all cases (a)–(c), the displacement of each organ relative to the radiotherapy plan was reduced as a result of interventions. Additionally, the deformation of the prostate (d) was reduced.
Cumulative DVH for pre-/postintervention and planned dose based on all 35 treated fractions. In fractions where no intervention occurred (n = 11), the doses based on the CBCT for that fraction were used for both pre- and postinterventions. In other words, “Post-Intv” represents the total doses delivered to the patients, and “Pre-Intv” represents the total doses that would have been delivered, had no interventions occurred.
Effect of ERB position on prostate deformation in one sample treatment fraction. A patient is simulated with an ERB, and the prostate and ERB occupy the plan positions. During treatment, the ERB is misplaced, and the anterior edge of the ERB is displaced toward the base of the prostate. This causes the prostate to be displaced/deformed superiorly to the prostate treatment position.
Single-fraction dosimetric effect of manual ERB adjustment (n = 24).
Effect of manual ERB adjustment on prostate shape (n = 24).
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