Dosimetric Uncertainties in Dominant Intraprostatic Lesion Simultaneous Boost Using Intensity Modulated Proton Therapy

Abstract

Purpose: While intensity modulated proton therapy can deliver simultaneous integrated boost (SIB) to the dominant intraprostatic lesion (DIL) with high precision, it is sensitive to anatomic changes. We investigated the dosimetric effects from these changes based on pretreatment cone-beam computed tomographic (CBCT) images and identified the most important factors using a multilayer perceptron neural network (MLPNN).

Methods and materials: DILs were contoured based on coregistered multiparametric magnetic resonance images for 25 previously treated prostate cancer patients. SIB plans were created with (1) prostate clinical target volume - V70 Gy = 98%; (2) DIL - V98 Gy > 95%; and (3) all organs at risk (OARs)"?> within clinical constraints. SIB plans were applied to daily CBCT-based deformed planning computed tomography (CT)"?>. DIL - V98 Gy, bladder/rectum maximum dose (Dmax) and volume changes, femur shifts, and the distance from DIL to organs at riskOARs"?> in both planning computed tomogramsCT"?> and CBCT were calculated. Wilcoxon signed-ranks tests were used to compare the changes. MLPNNs were used to model the change in ΔDIL - V98 Gy > 10% and bladder/rectum Dmax > 80 Gy, and the relative importance factors for the model were provided. The performances of the models were evaluated with receiver operating characteristic curves.

Results: Comparing initial plan to the average from evaluation plans, respectively, DIL - V98 Gy was 89.3% ± 19.9% versus 86.2% ± 21.3% (P = .151); bladder Dmax 71.9 ± 0.6 Gy versus 74.5 ± 2.9 Gy (P < .001); and rectum Dmax 70.1 ± 2.4 Gy versus 74.9 ± 9.1Gy (P = .007). Bladder and rectal volumes were 99.6% ± 39.5% and 112.8% ± 27.2%, respectively, of their initial volume. The femur shift was 3.16 ± 2.52 mm. In the modeling of ΔDIL V98 Gy > 10%, DIL to rectum distance changes, DIL to bladder distance changes, and rectum volume changes ratio are the 3 most important factors. The areas under the receiver operating characteristic curves were 0.89, 1.00, and 0.99 for the modeling of ΔDIL - V98 Gy > 10%, and bladder and rectum Dmax > 80 Gy, respectively.

Conclusions: Dosimetric changes in DIL SIB with intensity modulated proton therapy can be modeled and classified based on anatomic changes on pretreatment images by an MLPNN.

Figure 1

Axial (top) and coronal (middle) view of dose distribution of a prostate dominant intraprostatic lesion simultaneous integrated boost plan on the initial planning CT (left) and an evaluation deformed CT (right). The dose-volume histograms for the dominant intraprostatic lesion and prostate clinical target volume, bladder, rectum, and urethra in the initial plan (solid lines) and evaluation plan (dashed lines) are shown at the bottom. The contours in the deformed CT were drawn based on the cone-beam computed tomographic image. Abbreviations: CT = computerized tomogram.

Figure 2

Flowchart of the evaluation process. Abbreviations: DIL = dominant intraprostatic lesion; DIR = deformable image registration; OAR = organ at risk.

Figure 3

(a) Boxplot for bladder volume. (b) Boxplot for rectum volume. (c) Boxplot for the mean femur deformation vector. (d) Mean bladder and rectum volume ratio (= volume in cone-beam computed tomography/volume in planning computed tomography). (e) Mean distance from the dominant intraprostatic lesion to bladder and rectum. (f) Their corresponding changes at cone-beam computed tomography.

Figure 4

(a) Boxplot for clinical target volume D99 in evaluation plan and initial plans. (b) Boxplot for DIL V98 Gy in evaluation plan and initial plans. (c) Histogram of DIL V98 Gy changes for all evaluation fractions. (d) Normalized importance factors in the multilayer perceptron neural network for predicting change in DIL V98 Gy > 10%. (e) Boxplot of the bladder maximum doses in both the evaluation and the initial plans are shown at the bottom. (f) Boxplot of rectum maximum doses in both the evaluation and the initial plans are shown at the bottom. (g) Normalized importance factors for the bladder maximum dose > 80 Gy. (h) Normalized importance factors for the rectum maximum dose > 80 Gy. The stars and circles are outlier cases. Abbreviations: D = dose; DIL = dominant intraprostatic lesion.

Figure 5

(a) Receiver operating characteristic curve for the modeling of bladder maximum dose > 80 Gy, using the multilayer perceptron neural networks. (b) Receiver operating characteristic curve for the modeling of rectum maximum dose > 80 Gy, using the multilayer perceptron neural networks. (c) Receiver operating characteristic curve for the modeling of change in dominant intraprostatic lesion V98 Gy > 10%, using the multilayer perceptron neural networks.