Schematic illustration of beam placement for (a) FTP and (b) STP. The transversal cross section of the planning target volume is outlined in green. Two opposed lateral beams were used: right lateral (RL), and left lateral (LL). Open circles represent the pencil beam spots: red for the RL beam and blue for the LL beam. In (a) FTP, both beams targeted the entire gland. In (b) STP, each beam only targeted the respective proximal hemisphere. As the result of expansion of the scanned volume, the spots from RL and LL beams overlap near the midplane.
Workflow used to generate and evaluate IFD plans.
Schematic illustration of construction of intensity maps for two consecutive fractions delivering IFD, which boosted (a) right and (b) left hemisphere of the prostate. Target contour is shown in white; contours of the bladder and rectum are shown in yellow. Intensities of spots within the rectum or bladder (blue) were not modified. Spots outside of organs at risk were doubled on the proximal side of the target, and suppressed on the distal side. The spots within 10 mm of the plane of splitting were further enhanced by 25%.
IFD distributions of two consecutive fractions for a representative patient case. (a) and (b) m = 1 (FTP-sourced IFD); (c) and (d) m = 0 (STP-sourced IFD). In each fraction both fields are used. The summed dose of two fractions is approximately uniform on the whole target. The contour for PTV1 is shown in white. Contours for OARs are shown in yellow. The colorscales are identical in all three subfigures.
Percentage EUD gain in IFD plans constructed with different mixing factors m (i.e., relative weight of FTP) for a representative patient case. The physical dose of the IFD plans was renormalized to eliminate any increase of rectal or bladder EUD. A range of α/β-values was considered. The solid lines correspond to α/β of 1.5 Gy for GTV, 8 Gy for rectum, and 3 Gy for bladder, and nominal beam range. The shadowed bands correspond to the likely interval of the ΔEUD% due to (a) the uncertainty of α/β (±0.5 Gy for GTV, ±2 Gy for rectum, ±1 Gy for bladder) and (b) range shift (2.5 mm SD).
Dose-volume histograms (DVHs) of the prostate (GTV) for the standard uniform-dose FTP course of 78 Gy in 39 fractions (red), the hypofractionated FTP course of 60 Gy in 20 fractions (green), and the 20-fraction IFD course ( , blue), for a representative patient case. (a) Physical dose; (b) ED2Gy.
LET distributions calculated with Monte Carlo for (a) FTP, (b) STP, and (c) IFD course ( ), for a representative patient case. Contours for GTV and PTV1 are shown in white. Contours for OARs are shown in yellow. The colorscales are identical in all three subfigures.
Maximum target EUD gain for 13 patients and the corresponding mixing factor m, (i.e., relative weight of FTP contribution). The physical dose of the IFD plans was renormalized to eliminate any increase of rectal or bladder EUD. The data points corresponding to the three patients treated with endorectal balloon for immobilization are circled.
Dose constraints for the treatment plan optimization.
Tumor control and NTCP model parameters.
EUD gain in IFD plans for a representative patient case, with physical dose normalized subject to the rectal and bladder EUD constraints (ΔEUD ⩽ 0 for rectum and bladder).
The gain in EUD and TCP for IFD plan of a representative patient case ( ) compared to conventional uniform-dose FTP, for a range of a-values.
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