Correlation between patients’ anatomical characteristics and interfractional internal prostate motion during intensity modulated radiation therapy for prostate cancer

Maruoka et al. SpringerPlus (2015) 4:579 DOI 10.1186/s40064-015-1382-z Open Access RESEARCH Correlation between patients’ anatomical characteristics and interfractional internal prostate motion during intensity modulated radiation therapy for prostate cancer Shintaroh Maruoka1*, Yasuo Yoshioka1, Fumiaki Isohashi1, Osamu Suzuki1, Yuji Seo1, Yuki Otani2, Yuichi Akino1, Yutaka Takahashi1, Iori Sumida1 and Kazuhiko Ogawa1 Abstract Intensity modulated radiation therapy (IMRT) is one of a standard treatment for localized prostate cancer. Although lower complication is expected for smaller target margin, determination of optimal margin is important. For bonystructure based registration, internal prostate motion is the main factor determining the margin from clinical target volume to planning target volume. The purpose of this study was to measure interfractional internal motion of the prostate and to identity the factors which enlarge or reduce the margin, with special focus on patients’ anatomical characteristics. The 586 image sets of 16 patients acquired with megavoltage cone beam computed tomography were analyzed. For each patient, prostate shift in three directions was recorded for each fraction to calculate the required margin. Correlations between these values and patients’ anatomical characteristics were evaluated. The posteriorly required margin correlated positively with rectal volume and rectal mean area (p = 0.015 and p = 0.008), while random error in lateral, craniocaudal and anteroposterior direction correlated negatively (p = 0.014, 0.04 and 0.0026, respectively) with body mass index (BMI). In addition to the previously identified factor of distended rectum, BMI was newly identified as another significant factor influencing interfractional internal prostate motion. Keywords: Prostate cancer, Internal motion, Interfractional motion, Megavoltage cone beam computed tomography, Body mass index Background External beam radiotherapy (EBRT) is a main therapeutic modality for localized prostate cancer (Mohler et al. 2012). Some randomized studies have demonstrated the efficacy of dose-escalated EBRT for the treatment of localized prostate cancer (Dearnaley et al. 2007; AlMamgani et al. 2008; Kuban et al. 2011), but such efficacy often involves a trade-off in the form of heightened rectal toxicity. Intensity modulated radiation therapy (IMRT) is an improved version of EBRT that produces a steep dose gradient between the prostate and the surrounding risk *Correspondence: ‑u.ac.jp 1 Department of Radiation Oncology, Osaka University Graduate School of Medicine, 2‑2 Yamadaoka, Suita, Osaka 565‑0871, Japan Full list of author information is available at the end of the article organs such as rectum and bladder. EBRT using IMRT can deliver a higher dose to the prostate while keeping the dose to risk organs low, but the steep dose gradient results in a higher risk of setup error and internal prostate motion than with previous procedures. In fact, the National Comprehensive Cancer Network (NCCN) guidelines require image guided radiation therapy (IGRT) if the prescription dose is 78 Gy or more. The common procedures for image guidance include trans-abdominal ultrasonography (US), in-room helical computed tomography (CT), on-board cone beam CT (megavoltage or kilovoltage), and electric portal imaging devices (EPID) with or without implanted fiducial markers (Stephans et al. 2010; Soete et al. 2008). Although these modalities are helpful for accurate patient set up, each procedure has some disadvantages. The accuracy of © 2015 Maruoka et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Maruoka et al. SpringerPlus (2015) 4:579 Page 2 of 7 trans-abdominal US is affected by the technical capability of the performer, and organ displacement may occur due to image acquisition (Soete et al. 2008). Image guidance by in-room or cone beam CT requires extra medical staff and generates increased radiation exposure for the patients, especially for prostate based registration. EPID is an older technique but it is still widely used for bony structures-based registration. Although target position can be identified with fiducial markers, implantation of fiducial markers is invasive. Although the prostate-based registration may preferable for IMRT, bony structuresbased registration is still majority of the image-guided radiotherapy because of its simplicity, convenience, and less invasiveness. For bone-matching registration, internal prostate motion is the main factor which defines the margin from clinical target volume (CTV) to planning target volume (PTV). The purpose of this study was to measure interfractional internal motion of the prostate and to identify the factors which enlarge or reduce such a margin, with special focus on patients’ anatomical characteristics. Methods Patients This study was performed with permission of the Institutional Review Board of our hospital. Between October 2010 and May 2011, 16 patients with localized prostate cancer participated in this clinical trial. All of them were informed of this clinical trial and agreed by document to the participation in this study. All of them completed the study regimen. The patient distribution for T-stage was T1: T2: T3 = 5: 7: 4, for the Gleason score ≤6: 7: 8≤ = 6: 4: 6, for the pretreatment prostate-specific antigen level <10 ng/ml: 10–20 ng/ml: 20 ng/ml< = 7: 5: 4, and for low: intermediate: high risk = 3: 6: 7. The definitions of these factors were derived from the NCCN guidelines. Table 1 shows the patients’ anatomical characteristics which we considered to be candidates for affecting interfractional internal motion of the prostate. Table 1 Patients’ anatomical characteristics Variables Means (range) Age (years old) 71.5 (60–81) BMI (kg/m2) 23.8 (14.8–30.7) Prostate volume (cc) 22.5 (14.2–51.1) Bladder volume (cc) 144.1 (34.7–645.1) Rectal volume (cc)a 42.0 (28.9–55.2) Rectal mean area (cm2)b 5.86 (4.12–7.46) a Rectal volume was measured from the 2-cm above the prostate base level to the 2-cm below the prostate apex level b Rectal mean area was calculated as rectal volume divided by its length in craniocaudal direction Radiotherapy, acquisition of registry image, and measurement of prostate shift Oncor Impression PLUS™ with a megavoltage cone beam CT (MV-CBCT) system (MVisionTM; Siemens Medical Solutions, Concord, CA, USA) was used. Patients emptied their rectum in the morning of CT simulation as well as their bladder 30 min before CT simulation. Patients were immobilized in supine position using Vac-Lok™ Cushions (CIVCO Medical Solutions, Orange City, (...truncated)