Simplified material assignment for cone beam computed tomography-based dose calculations of prostate radiotherapy with hip prostheses

Abstract

ObjectiveCone beam computed tomography (CBCT) images contain more scatter than a conventional computed tomography (CT) image and therefore provide inaccurate Hounsfield units (HUs). Consequently, CBCT images cannot be used directly for dose calculation. The aim of this study is to enable dose calculations to be performed with the use of CBCT images taken during radiotherapy and potentially avoid the necessity of re-planning.MethodologyA phantom and prostate cancer patient with a metallic prosthetic hip replacement were imaged using both CT and CBCT. The multilevel threshold algorithm was used to categorise pixel values in the CBCT images into segments of homogeneous HU. The variation in HU with position in the CBCT images was taken into consideration and the benefit of using a larger number of materials than typically used in previous work has been explored. This segmentation method relies upon the operator dividing the CBCT data into a set of volumes where the variation in the relationship between pixel values and HUs is small. A field-in-field treatment plan was generated from the CT of the phantom. An intensity-modulated radiation therapy plan was generated from CT images of the patient. These plans were then copied to the segmented CBCT datasets with identical settings and the doses were recalculated and compared.ResultsIn the phantom study,γevaluation showed that the percentage of points falling in planning target volume, rectum and bladder withγ<1 (3%/3 mm) was 100%. In the patient study, increasing the number of bins to define the material type from seven materials to eight materials required 50% more operator time to improve the accuracy by 0·01% using pencil beam and collapsed cone and 0·05% when using Monte Carlo algorithms.ConclusionThe segmentation of CBCT images using the method in this study can be used for dose calculation. For a simple phantom, 2 values of HU were needed to improve dose calculation accuracy. In challenging circumstances such as that of a prostate patient with hip prosthesis, 5 values of HU were found to be needed, giving a reasonable balance between dose accuracy and operator time.

Enhancement of Hounsfield unit distribution in cone-beam CT images for adaptive radiation therapy: Evaluation of a hybrid correction approach

“…Furthermore, to provide a stronger validation for the proposed method we are looking forward to study the impact of HU error reduction on the dose calculation. Finally, for the use of MLT, five different tissue types were chosen in this study, while in [38] it was shown that the higher the number of classes the more the correction is accurate. Considering that increasing the number of classes may consequently increase the computation time, the automation of the proposed method and its testing under more efficient hardware architecture are currently being investigated for an online replanning process.…”

Automated algorithm for CBCT-based dose calculations of prostate radiotherapy with bilateral hip prostheses

“…Based on Almatani et al, 15 the binning of CBCT images of a patient with hip prosthesis into five HU values results in sufficiently accurate and clinically acceptable dose distribution. Considering more than five HU values provides more anatomical information and improves dose calculation accuracy (by 0.23%) but would require more operator time (58%), as the sensitivity increases when increasing the number of HU bins to define the material type.…”

“…In fact, none of the above studies used a patient with prostheses, which would provide a more general assessment of dose calculation using CBCT. Almatani et al 15 studied CBCT-based dose calculations of a patient with prostate cancer with a single hip prosthesis using the MLT algorithm. The work showed that it was necessary to extend the MLT algorithm to categorize pixel values into segments on a region-by-region basis, with the region size changing depending on the anatomical features.…”

Dosimetric feasibility of magnetic resonance (MR)-based dose calculation of prostate radiotherapy using multilevel threshold algorithm

“…However, despite its major improvement in image guided radiotherapy (IGRT), the image quality of CBCT images makes it hard to accurately identify the prostate, due to the increased amount of scatter (5)(6)(7). More importantly, the CBCT imaging dose limits the frequency with which this technique can be used (8).…”

“…The MLT algorithm has been used to enable dose calculation to be performed on CBCT images by the authors previously, even for more challenging circumstances as for a prostate patient with hip prostheses (6,7). This method does not require database, as for the atlas approach, nor extra non-clinical MR sequence.…”