Original paperComputational analysis of interfractional anisotropic shape variations of the rectum in prostate cancer radiation therapy
Introduction
Prostate cancer was ranked as the fifth leading cause of death from cancer for men worldwide in 2012 [1]. Incidence rates are increasing every year in the developed countries such as United Kingdom and Japan [2]. Several options are available to treat the prostate cancer including radiation therapy which allowed the prostate to be treated with high dose of radiation while sparring surrounding normal tissues [3].
The quality of radiation therapy in prostate cancer treatment is affected by high dose regions which could be induced by patient movement, internal motion of the organ, and patient set-up errors [4], [5]. Fig. 1 illustrates the anatomical regions of a rectum, bladder, and planning target volume (PTV) determined by radiation oncologists. Anterior parts of the rectum may overlap with the PTV due to large internal margins and/or rectal displacements as shown in Fig. 1. The rectal position uncertainties, which could cause toxicities (e.g., rectal bleeding, fecal incontinence), mainly comes from the rectal motion due to the changes in rectal filling [6], [7], [8], [9], [10]. The two common methods used to study the rectal motion were tracking the changes in rectal volume and evaluating the translation and rotation errors of the rectum [5], [11], [12], [13]. Fontenla et al. [14], however, noted that the more complex problem of internal organ motion involve changes in the shape (shape variations) of the organ especially along the anterior direction of the rectum [5], [15]. Therefore, the shape variations of the rectum, especially along the anterior direction, need to be investigated.
In order to dealt with the position uncertainties of the organs at risks (OARs), the International Commission on Radiation Units and Measurements (ICRU) reports no. 62 [16] and 83 [17] introduced the concept of planning risk volume (PRV) margins. In the case of prostate cancer radiation therapy, the use of PRV dose-volume histograms (DVHs) is recommended to predict acute rectal toxicity [15], [18], [19]. “Recipes” to determine the uniform PRV margins have been developed by McKenzie et al. and Stroom and Heijmen [20], [21]. However, the uniform PRV margins are inadequate to represent the actual rectal variations during treatment, as noted by McKenzie et al. [20] and Prabhakar et al. [22]. Therefore, an application of anisotropic PRV margins of the rectum should be considered.
There have been three studies that dealt with the shape variations of the rectum. Hoogeman et al. [23] analyzed the quantification of local rectal wall displacements by calculating local systematic and random errors of the rectum along three directions where they unfolded the outer surface of the delineated rectal wall and projected the 3-space coordinates of each surface element to a 2D map. Sohn et al. [24] investigated the correlated motion of adjacent organ structures between prostate, bladder and rectum which were parametrized by using sets of corresponding surface points and calculated the displacements between surface points at each fraction. They did not calculate the systematic and random errors that could be used in determining anisotropic PRV margins. Brierley et al. [25] investigated the determination of the PTV based on the rectal shape variations by using finite element modeling. They did not investigate the geometric errors related to the determination of PRV margins.
None of the previously mentioned studies, including ICRU, investigated directly the shape variations of the rectum along each anatomical direction separately (anterior, posterior, superior, inferior, left and right). The investigation along separate anatomical directions is indispensable for determining the anisotropic PRV margins. There have been also no studies on the systematic and random errors of the region in which the rectum overlapped with the PTV along the anterior wall (ROP regions), even though the shape variations of the ROP regions may cause the regions to be included in high dose distributions which can lead to rectum toxicities. Therefore, this study aims to investigate the anisotropic shape variations of the rectum and the ROP regions for prostate cancer radiation therapy along separate anatomical direction (anterior, posterior, superior, inferior, left and right).
Section snippets
Clinical study
This study was performed with the approval of the Institutional Review Board of our university hospital. The clinical data used in this study were obtained from 11 patients (range: 60–75 years; median age: 64 years; stage: T1-T3a, N0, M0), who had undergone intensity modulated radiation therapy (IMRT) for prostate cancer. The planning CT images were acquired from a CT scanner (Mx 8000, Philips, Amsterdam, Netherlands) with 512 × 512-pixel dimensions, 0.98 mm in-plane pixel size, and 2.0 mm
Results
Fig. 8 shows the population SDs of the systematic and random errors of the rectum due to shape variations along each anatomical direction of all patients. The population SDs for systematic errors were 0.6 mm along the left direction, 0.3 mm along the right direction, 1.0 mm along the anterior direction, 0.7 mm along the posterior direction, 2.1 mm along the inferior direction and 2.4 mm along the superior direction. The population SDs for random errors were 1.2 mm along the left direction,
Discussion
The deviation along the superior, inferior, and anterior directions were dominant for systematic and random errors. Brierley et al. and Nuyttens et al. [25], [47] noted similar observations of large deviations along superior and inferior directions of the rectum. The deviations were affected largely by the variabilities of other organ proximal to the rectum such as small bowel [47].
Fig. 11, Fig. 12 illustrate the SDs of the local systematic and random errors visualized on the reference rectum
Conclusions
An analysis of interfractional anisotropic rectum shape variations using a statistical PDM in a computational framework has been presented. The population SDs for the whole rectum calculated by the proposed method were larger than 1.0 mm along all directions for random errors, while for systematic errors the population SDs were smaller than 1.0 mm along the posterior, left, and right directions. The population SDs of systematic errors for ROP regions calculated by the proposed method were
Conflict of interest statement
None.
Acknowledgements
The authors would like to send the utmost gratitude to all members in Arimura laboratory, which had contribute a great deal of efforts in the performance of this study.
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