An Effective Strategy for Set-up Error Estimation and Reduction in Image-Guided Prostate Radiotherapy
Date Issued
2011
Date
2011
Author(s)
Wang, Chun-Wei
Abstract
Radiotherapy dose escalation may achieve higher biochemical control rates for prostate cancer. However, a higher radiation dose is associated with a higher risk of radiation toxicity. Improving the accuracy of dose delivery is an important issue in order to safely escalate the radiation dose. Therefore, we conducted an investigation to estimate the set-up errors in prostate radiotherapy using different image-guidance tools. Moreover, we designed a correction protocol and modified it step-by-step to effectively reduce the set-up error and improve treatment accuracy.
Eighty-five patients with prostate cancer undergoing intensity-modulated radiation therapy (IMRT) were enrolled. In the first phase of the study, electronic portal images (EPIs) were collected weekly to estimate displacements in 3 orthogonal directions. In the second phase, cone-beam computed tomography (CBCT) images were collected daily and compared with the pretreatment simulation CT to estimate the set-up errors and time-dependent changes over the course of treatment. Patients were then divided into 3 groups: (1) protocol-A: weekly corrections (a total of 7 corrections) of body re-alignment throughout the entire course; (2) protocol-B: weekly corrections in the second and third weeks of treatment (a total of 2 corrections); and (3) protocol-C: weekly corrections in the second, third, and sixth weeks of treatment. The serial set-up errors were compared to evaluate the effectiveness and workload of the 3 correction strategies.
With the use of an electronic portal image device (EPID), the mean displacements of the isocenter relative to the bony landmark were determined as 1.9 mm in the superior-inferior (SI), 3.4 mm in the anterior-posterior (AP), and 2.5 mm in the left-right (LR) directions. Balloon deformations of 2.8 mm, 2.5 mm, and 2.6 mm occurred in the SI, AP, and LR directions, respectively. The use of CBCT to estimate the set-up error revealed that the average three-dimensional (3D) displacement increased from 3.2 (first week) to 7.3 mm (eighth week, p = 0.027). Using different correction strategies, the mean displacements for protocols A, B, and C were 0.4/0.4/0.3, 0.9/0.4/0.7, and 0.2/0.3/0.3 mm in the SI, AP, and LR directions, respectively. The effectiveness of reducing set-up errors (3D mean displacements) was more remarkable in groups A (3.4 mm) and C (4.0 mm) compared to group B (5.2 mm, p < 0.05). The numbers of CBCT images required for each correction were 35, 10, and 15 for groups A, B, and C, respectively.
Radiotherapy with CBCT was effective for estimating and reducing the set-up errors. Although protocol-A had the least set-up errors, the greater amount of CBCT images required for correction resulted in a notably increased workload. Protocol-C achieved similar effectiveness with an acceptable workload.
Subjects
Prostate cancer
radiotherapy
image guidance
set-up error
SDGs
Type
thesis
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