Mm In Cranio-caudal Direction Research Articles

An offline technique to evaluate residual motion of the diaphragm during deep inspiratory breath-hold from cone-beam CT datasets.

In radiation therapy, the computer-assisted deep inspiration breath-hold (DIBH) technique is one approach to deal with respiratory motion of tumors in the lung, liver, or upper abdomen. However, inter- and intra-breath-hold deviations from an optimal static tumor position might occur. Anovel method is presented to noninvasively measure the diaphragm position and thus estimate its residual deviation (as surrogate for the tumor position) based on cone-beam computed tomography (CBCT) projection data using active breathing control during acquisition. The diaphragm dome (DD) position relative to the isocenter of alinear accelerator is known from the static (DIBH) planning CT. A ball-bearing phantom (BB) is placed at this position, aCBCT dataset is acquired, and in each projection the position of the projected BB is determined automatically based on thresholding. The position of the DD is determined manually in CBCT projections of apatient. The distance between DD and BB (ideal static setting) in craniocaudal direction is calculated for agiven angle based on the distance in the projection plane and the relative position of the BB referring to the source and the detector. An angle-dependent correction factor is introduced which takes this geometrical setting into account. The accuracy of the method is assessed. The method allows aCBCT projection-based estimation of the deviation between the DD and its optimal position as defined in the planning CT, i.e., the residual motion of the DD can be assessed. The error of this estimation is 2.2 mm in craniocaudal direction. The developed method allows an offline estimation of the inspiration depth (inter- and intra-breath-hold) over time. It will be useful as areference for comparison to other methods of residual motion estimation, e.g., surface scanning.

227P Stereotactic body radiotherapy with helical tomotherapy for lung tumors: A single-center experience

Background: Several studies report favorable outcomes for stereotactic body radiotherapy (SBRT) in the treatment of early stage NSCLC or lung metastases. Here we introduce our experience of Helical TomoTherapy (HT) SBRT for primary and secondary lung tumors. Methods: Between March 2014 and June 2017, 57 lung lesions in 56 patients (pts), median age 72 years (range 26–91) underwent ablative HT-SBRT: 34 pts (60.7%) for primary tumors and 22 pts (39.3%) for lung metastases. Staging was performed by contrast-enhanced chest CT and PET-CT scans. Immobilization was obtained with breast board and abdominal pressure mould mask. The planning target volume (PTV) included a margin of 10 mm in cranio-caudal direction and 5 mm in all other directions around the internal target volume (ITV), defined basing on the volumetric sum of the clinical target volumes of a free breathing planning CT and a pretreatment MVCT scan acquired in treatment position. According to tumor site, different schedules of RT were used: 60–70 Gy in 8–10 alternate fractions for peripheral lesions (n = 47) and 50 Gy and 60 Gy in 10 daily fractions for central (n = 10). Median BED10 was 96 Gy (range 75–119). Treatment-related toxicity was evaluated using CTCAE v4.0 scale. For the first year of follow-up, physical evaluation and chest CT were conducted every 2–3 months. Results: Median GTV and PTV sizes were respectively 6.14 and 22.86 cc (range, 0.87–75.1 and 65.15–131.03). Median duration of RT was 15 days (range 10–20) and all pts were successfully treated with mild acute adverse events. Concurrent EGF-R-targeting TKIs were administered in 4 cases. With a median follow-up of 17 months (range 6–39) no ≥ G3 radiation pneumonitis or ≥ G2 esophagitis occurred; only 1 patient showed G2 noncardiac chest pain, 2 pts G2 radiation pneumonitis. At the time of the analysis, 4 local failures were registered, resulting in Local Control (LC) rates of 97.1% for primary tumors and 84.8% for metastatic lesions; 49 pts are alive and 7 dead: 1y- and 2y- Overall survival rates were 96.6% and 87.8% for primary lesions and 94.1% and 79% for metastases, respectively. Conclusions: HT-SBRT in primary or metastatic lung tumors demonstrates low risk of normal tissue complications and high rates of LC. Legal entity responsible for the study: ARNAS Civico Hospital, Palermo, Italy Funding: Has not received any funding Disclosure: All authors have declared no conflicts of interest.

Liquid fiducial marker performance during radiotherapy of locally advanced non small cell lung cancer

Background and purposeWe analysed the positional and structural stability of a long-term biodegradable liquid fiducial marker (BioXmark) for radiotherapy in patients with locally advanced lung cancer. Material and methodsMarkers were injected via endoscopic- or endobronchial ultrasound in lymph nodes and reachable primary tumours. Marker volume and Hounsfield Units (HU) changing rates were estimated using breath-hold CBCT. Inter-fraction variation in marker position relative to gross tumour volume (GTV) position was established, as well as the inter-fraction variation in mediastinal marker registration relative to a carina registration through the treatment. ResultsFifteen patients were included and 29 markers analysed. All markers that were in situ at planning were visible through the treatment. Mean HU was 902±165HU for lymph node and 991±219HU for tumour markers. Volume degradation rates were −5% in lymph nodes and −23% in primary tumours. Three-dimensional inter-fraction variation for marker position relative to the GTV position was −0.1±0.7mm in lymph nodes and −1.5±2.3mm in primary tumours. Inter-fraction variations in marker registration relative to carina registration were −0.4±1.2mm in left–right, 0.2±2.0mm in anterior–posterior and −0.5±2.0mm in cranio-caudal directions. ConclusionsThe liquid fiducial markers were visible and stable in size and position throughout the treatment course.

Plastic surgery for breast conservation therapy: How to define the volume of the tumor bed for the boost?

PurposeTo describe the procedure of definition of the boost volume using pre- and post-operative computed tomography (CT) and surgical clips in the tumor bed after oncoplastic surgical procedure. Patients and methodsThirty-one consecutive breast cancer patients who underwent simple lumpectomy or oncoplastic surgery were studied. All of them underwent pre- and post-operative CT scan in treatment position to evaluate the planning target volume (PTV) boost volume and define the primary tumor (gross tumor volume (GTV)) and tumor bed zones (CTV), with an overall margin of 5 mm in lateral and 10 mm in craniocaudal directions, corresponding to localization and setup uncertainties. ResultsThirteem patients underwent simple lumpectomy and 18 oncoplastic surgery. The volumetric analysis showed that the intersection between GTV and CTV clips was significantly higher in patients with three and more clips (28.4% vs 3.14%; p < 0.001). In the case of patients with oncoplastic surgery, more than three clips were needed to define the tumor bed volume with accuracy. The number of clips was directly related to the exact definition of the boost volume. ConclusionsThe use of more than three clips allows better definition of the PTV boost volume after oncoplastic surgical procedure.

Carbon ion radiotherapy in a hypofractionation regimen for stage I non-small-cell lung cancer

Introduction: In 1994, we started carbon-ion radiotherapy (CIRT) for peripheral stage I non-small-cell lung cancer (NSCLC). First, two phase I/II clinical trials demonstrated the optimal doses of 90.0 GyE in 18 fractions over 6 weeks (Protocol 9303) and 72.0 GyE in 9 fractions over 3 weeks (Protocol 9701) for achieving more than 95% local control with minimal pulmonary toxicity. As a next step, we conducted two successive phase II trials. The first trial (Protocol 9802) used a regimen of 72 GyE per 9 fractions over 3 weeks and the second trial (Protocol 0001) used a regimen of 4 fractions over 1 week, at a fixed dose of 52.8 GyE for stage IA and 60 GyE for IB. In these Phase II trials, the local control rate (LCR) for all patients was 91.5%, and those for T1 and T2 tumors were 96.3 and 84.7%, respectively. The 5-year cause-specific survival rate (CSS) was 67.0% (IA: 84.4, IB: 43.7), and overall survival (OS) was 45.3% (IA: 53.9, IB: 34.2). No adverse events greater than grade 2 occurred in the lung.In 2003, we also started a phase I/II clinical trial (Protocol 0201) as a dose escalation study using single fraction. The initial total dose was 28.0 GyE administered and escalated in increments of 2.0 GyE each, up to 50.0 GyE. This clinical trial ended in February 2012 and is still followed up. In this article, we investigated the preliminary results of this phase I/II trial.Materials and methods: In this prospective study, 151 primary stage I NSCLC were treated by CIRT monotherapy using a total dose of 36.0 GyE (n = 18), 38.0 GyE (n = 14), 40.0 GyE (n = 20), 42.0 GyE (n = 15), 44.0 GyE (n = 44), 46.0 GyE (n = 20), 48.0 GyE (n = 10) and 50.0 GyE (n = 10) using single fractionation. Mean age was 73.9 years, and size of tumor included T1 (n = 91) and T2 (n = 60). By type (cancer type was determined by biopsy), there were 104 adenocarcinomas, 46 squamous cell carcinomas and 1 large cell carcinoma. Medical inoperability was 55.6%.The patient is fixed on the rotational couch by using a custom-made immobilization device. Under free breathing conditions, planning CT images were acquired for treatment planning. The clinical target volume (CTV) was determined by adding >10-mm margin to the gross tumor volume (GTV). The planning target volume (PTV) was created by adding an internal margin to CTV as 5 mm in craniocaudal direction. The prescribed dose was delivered to PTV with different coplanar four beam angles. A respiratory-gated irradiation system was used in all irradiation sessions.Results: The median follow-up time was 45.6 months (range, 1.6–88.4 months). For 151 patients, the 5-year overall LCR was 79.2%, and those for T1 (n = 91) and T2 (n = 60) tumors were 83.6 and 72.2%, respectively. Also, local control in T1a, T1b, T2a and T2b were 96.8, 84.4, 80.2 and 20.0%, respectively. The OS was 55.1% and the CSS was 73.1%. No toxicity greater than grade 2 was observed in the lung and the skin.Conclusions: In patients with stage I NSCLC, CIRT using single fraction is considered as a promising curative modality. Especially for elderly and inoperable cases, CIRT could be a minimally invasive therapeutic option as a valid alternative to surgical resection (Fig. 1).Fig 1.Overall survival and local control in 151 patients with stage I non-small-cell lung cancer.

Deep inspiration breath‐hold technique guided by an opto‐electronic system for extracranial stereotactic treatments

The purpose of this work was to evaluate the intrapatient tumor position reproducibility in a deep inspiration breath‐hold (DIBH) technique based on two infrared optical tracking systems, ExacTrac and ELITETM, in stereotactic treatment of lung and liver lesions. After a feasibility study, the technique was applied to 15 patients. Each patient, provided with a real‐time visual feedback of external optical marker displacements, underwent a full DIBH, a free‐breathing (FB), and three consecutive DIBH CT‐scans centered on the lesion to evaluate the tumor position reproducibility. The mean reproducibility of tumor position during repeated DIBH was 0.5±0.3mm in laterolateral (LL), 1.0±0.9mm in anteroposterior (AP), and 1.4±0.9mm in craniocaudal (CC) direction for lung lesions, and 1.0±0.6mm in LL, 1.1±0.5mm in AP, and 1.2±0.4mm in CC direction for liver lesions. Intra‐and interbreath‐hold reproducibility during treatment, as determined by optical markers displacements, was below 1 mm and 3 mm, respectively, in all directions for all patients. Optically‐guided DIBH technique provides a simple noninvasive method to minimize breathing motion for collaborative patients. For each patient, it is important to ensure that the tumor position is reproducible with respect to the external markers configuration.PACS numbers: 87.53.Ly, 87.55.km

Accuracy evaluation of CatalystTM, a 3d surface imaging device designed for set up guidance and on-line target monitoring for tomotherapy treatments

Introduction Highly conformal helical treatments require a daily MV image for patient repositioning. Additional dose related to MV image guidance is evaluated at 1 to 3cGy per fraction. Over the last years, non-irradiant optical imaging technics have been developed such as Catalyst TM . This system designed for handling patient positioning and intra-fraction motion detection is based on principles of photogrammetry and optical triangulation. With knowledge of the surface movements, a volumetric deformable model is used to predict the influence on the actual target position. Our study aims to compare accuracy of Catalyst TM system against MVCT one and to develop an efficient procedure for clinical routine. Material and methods In this study, repositioning deviations were measured by Catalyst TM on 2 groups of 10 patients treated by Tomotherapy, for pelvic and thoracic locations. For each patient, positioning accuracy using Catalyst TM was evaluated comparing deviations calculated by Catalyst TM to MVCT versus reference scan registration results. Regarding to the validation of the on-line tracking, deviations between pre- and post-treatment images were compared against the results of the on-line motion tracking system. Systematic residual error was also investigated and quantified using a thoracic phantom. Results A clinical efficient procedure has been developed for surface image acquisition. The process takes less than 5 min per treatment fraction. Mean deviations for patient setup between MVCT and Catalyst TM system obtained for 2 patients during 3 fractions are: 2.5 mm in left-right direction, 2.8 mm in cranio-caudal direction, 3.8 mm in antero-posterior and less than 1.5° for all rotations. Conclusion This study will allow us to determine localization that benefits the most of Catalyst TM 3D surface imaging device and for which it may be possible to avoid MV daily imaging keeping the same PTV margins.

PP-173 LEFT VENTRICULAR PSEUDOANEURYSM FORMATION DUE TO FREE WALL RUPTURE

A 66 year old male patient admitted with chest pain on exertion. His coronary angiography revealed total occlusion of first obtuse marginal branch. Cardiac CT and MRI were performed and demonstrated left ventricular free wall rupture leading to pseudoaneurysm formation (Figure 1, Video 1). Cardiac CT image (Figure 1B) illustrates left ventricular free wall rupture on posterolateral aspect with pseudoaneurysm formation. Cardiac MRI (Figure 1C) revealed the defect on left ventricular free wall. The size of the defect was 20.4 mm in cranio-caudal direction. The dimensions of the pseudoaneurysm sac were measured as 49.5X42.4X46.7 mm. The patient refused surgical treatment. Figure1. Chest X-ray image (A), Cardiac CT image (B), Cardiac MRI image (C) Video 1. Cardiac MRI demonstrating the defect on left ventricular free wall and pseudoaneurysm formation.

Frame-based radiosurgery: Is it relevant in the era of IGRT?

To assess the setup errors and intrafraction motion in patients treated with frame-based and frameless stereotactic radiosurgery (SRS). Ten patients treated with frame-based and six patients treated with frameless radiosurgery were prospectively enrolled in the study. Leksell frame was used for frame-based and a customized uniframe orfit cast for frameless techniques. Cone beam computed tomography (CBCT) scans were taken immediately before and after each treatment to evaluate the positional accuracy and corrections applied with the use of hexapod couch for both groups. The mean translational shifts with frame-based SRS were 1.00 ± 0.30 mm in the lateral direction (X), 0.20 ± 1.20 mm in craniocaudal direction (Y) and -0.10 ± 0.31 mm in the anteroposterior direction (Z). The rotational shifts for frame-based treatments were as follows: roll 0.32 ± 0.70, pitch 0.44 ± 0.66 and yaw 0.20 ± 0.4. For frameless SRS, translational shifts were -0.40 ± 0.90, 1.10 ± 1.10, and 0.50 ± 1.30 mm in X, Y, and Z directions, respectively, and rotational shifts were -0.11 ± 0.78, 0.20 ± 0.44, and 0.29 ± 0.35 in roll, pitch, and yaw, respectively. Intrafraction shifts with frame-based SRS were: X = 0.60 ± 1.80 mm, Y = 0.20 ± 0.60 mm, and Z = 0.00 ± 0.05 mm; and rotational shifts were: roll 0.01 ± 0.27, pitch 0.06 ± 0.15, and yaw 0.01 ± 0.09. For frameless SRS, these were: X = 0.11 ± 0.20 mm, Y = 0.20 ± 0.40 mm, and Z = 0.20 ± 0.20 mm and rotational shifts were: roll 0.09 ± 0.23, pitch 0.00 ± 0.12, and yaw 0.00 ± 0.09. In our experience, set up accuracy of frameless SRS is as good as frame-based SRS. With availability of verification methods such as CBCT and hexapod couch, an accurate and precise treatment delivery is feasible with frameless techniques.

How to Boost the Breast Tumor Bed After Oncoplastic Surgical Procedure Using Surgical Clips and Image Registration?

How to Boost the Breast Tumor Bed After Oncoplastic Surgical Procedure Using Surgical Clips and Image Registration?

Navigated laparoscopy – liver shift and deformation due to pneumoperitoneum in an animal model

Background: Precise laparoscopic liver resection requires accurate planning and visualization of important anatomy such as vessels and tumors. Combining laparoscopic ultrasound with navigation technology could provide this. Preoperative images are valuable for planning and overview of the procedure, while intraoperative images provide an updated view of the surgical field. Purpose: To validate the accuracy of navigation technology based on preoperative images, we need to understand how much the liver shifts and deforms due to heartbeat, breathing, surgical manipulation and pneumoperitoneum. In this study, we evaluated liver tumor shift and deformation due to pneumoperitoneum in an animal model. Methods: Tumor models were injected into the liver of the animal, and 3D CT images were acquired before and after insufflation. Tumor shifts and deformation were determined. Results: The results showed significant tumor position shift due to pneumoperitoneum, with a maximum of 28 mm in cranio-caudal direction. No significant tumor deformation was detected. Small standard deviations suggest rigid body transformation of the liver as a whole, but this needs further investigation. Conclusion: The findings indicate a need for anatomic shift correction of preoperative images before they are used in combination with LUS guidance during a laparoscopic liver resection procedure.

Dosimetric consequences of inter-fraction breathing-pattern variation on radiotherapy with personalized motion-assessed margins

The data from eight patients who had undergone stereotactic body radiotherapy were selected due to their 4D-CT planning scans showing that their tumours had respiratory induced motion trajectories of large amplitude (greater than 9 mm in cranio-caudal direction). Radiotherapy plans with personalized motion-assessed margins were generated for these eight patients. The margins were generated by inverse 4D planning on an eight-bin phase-sorted 4D-CT scan. The planning was done on an in-house software system with a non-rigid registration stage being completed using freely available software. The resultant plans were then recalculated on a 4D-CT scan taken later during the course of treatment. Simulated image-guided patient set-up was used to align the geometric centres of the tumour region and minimize any misalignment between the two reconstructions. In general, the variation in the patient breathing patterns was found to be very small. Consequently, the degradation of the mean dose to the tumour region was found to be around a few percent (<3%) and hence was not a large effect.

SU-E-T-558: Intrafractional Target Displacement during Stereotactic Body Radiation Therapy in Lung for Patients Immobilized Using BodyFix System

Puropose: To investigate intrafractional target displacement and assess the adequacy of margins used for early stage lungcarcinoma patients treated to 50Gy in five fractions and immobilized using BodyFix (Elekta, Stockholm, Sweden). Methods: Patients selected are medically inoperable or refuse surgery with T1 or T2 tumors <5cm. Patients are immobilized head first supine using BodyFix system. If target motion exceeds 1cm, as seen on fluoroscopic imaging, abdominal compression is used. The abdominal compression device consists of a rigid belt and pressure cuff. Treatments are performed on Tomotherapy (Tomotherapy Inc, Madison, WI) using megavoltage (MV) CTimage guidance. MVCT images are slow, thus, representing an average image of the moving target. Each fraction was divided into two parts to permit assessment of intrafraction movement and allow for an interruption if long treatment is too difficult to tolerate. In many cases, patients are willing to continue with the second part of the delivery without a break. Based on our clinical protocol, the second part is still preceded by the imaging. Comparison of the images prior to the first and second portions of radiation provided us with target shifts during treatment. 23 treatments of 5 patients have been studied. Image registration was performed using Eclipse (Varian Medical Systems, Palo Alto, CA). Results: Average of absolute target displacement during treatment was 2mm in lateral, 1mm in anterior‐posterior, and 2mm in craniocaudal directions. Maximum displacement was 4mm. Average target displacements were below 0.3mm for all directions. Per patient, average absolute shifts were similar to the population average, however, averages shifts were about 1mm in anterior posterior and 2mm in craniocaudal directions. Conclusions: Keeping in mind that hardware error is of the order of 1mm, and target delineation error in lung is about 1–2mm, 5mm margins, currently accepted in our institution, are sufficient for adequate coverage.

Evaluation of Possible Prostate Displacement Induced by Pressure Applied during Transabdominal Ultrasound Image Acquisition

For accurate positioning of the prostate in external radiotherapy, transabdominal ultrasound localization and positioning systems are available. Reports have stated that probe pressure applied during image acquisition causes clinically relevant prostate displacement. The aim of this study was to investigate the prostate displacement due to the pressure applied during transabdominal ultrasound image acquisition with the BAT ultrasound system. For ten patients who had undergone iodine-125 seed implantation for brachytherapy of prostate cancer, X-ray simulations were performed before and during ultrasound image acquisition. The iodine seeds are visible on the X-ray images, representing the position of the prostate. The simulator's crosshair, indicating the isocenter, was used as reference coordinate system. For each patient the change in prostate position was calculated based on the seed positions during and after ultrasound examination. A maximum displacement of the prostate of 2.3 mm in anteroposterior and 1.9 mm in craniocaudal direction and a rotational change of up to 2.5 degrees were observed. If the system was not handled correctly and too much pressure was applied, a shift of the prostate of up to 10 mm could be induced. Compared to the prostate displacement due to changes in rectal filling, which according to Crook et al. can be as much as 1.7 cm, the maximum displacement of less than 0.3 cm caused by the probe pressure is negligible. However, proper education of the staff and preparation of the patient are essential for the safe use of the system.

A new irradiation system for lung cancer: patient’s self-breath-hold and self-switching radiation-beam on and off without any respiratory monitoring devices

Purpose: It is helpful to irradiate a lung tumor under a breath-hold for a reduction of planning target volume(PTV). To develop a simple and efficient breath-hold radiation technique for lung cancer, we made three purposes. 1) To compare the reproducibility of tumor-position under a breath-hold by a patient’s self-estimation with that under a breath-hold by a radiation technologist’s instruction. 2) To develop a switch which enable a patient to turn the radiation-beam on and off. 3) To evaluate the reproducibility of a tumor-position in the radiation field with a patient’s self-breath-hold and self-switching radiation-beam. Materials and Methods: Twenty patients with lung cancer were enrolled in this study. All patients were taught sufficiently how to hold the breath at a same inspiration phase with showing a fluoroscopy of respiratory motion before evaluation. CT images were obtained with 2mm-thickness in the vicinity of the tumor under a series of the breath-hold by a radiation technologist’s instruction and another series of the breath-hold by a patient’s self-estimation. CT was repeated three times with each breath-hold method. Differences of tumor position on CT images were measured in three dimensions with a CT analyzing menu. We have newly developed a switch which was connected directly to the console box of linac ( EXL-15DP; Mitsubishi Electric, Tokyo, Japan). It enables a patient to turn radiation beam on and off freely and repeatedly during a term designated by a radiation technologist. Ten real-time portal images were taken for each patient during irradiation under a patient’s self-breath-hold and self-switching radiation-beam, and the accuracy of reproducibility of the tumor position in the radiation field was visually evaluated. Results: The average of maximum differences of the tumor position in three series of 20 patients under the breath-hold by a radiation technologist’s instruction was 3.0 mm in cranio-caudal direction, 2.1mm in antero-posterior direction and 2.3mm in right-left direction. And that in three series under the breath-hold by a patient’s self-estimation was 2.3mm in cranio-caudal direction, 1.4mm in antero-posterior direction and 1.4mm in right-left direction, respectively. A standard deviation of these differences was larger in the method of breath-hold by a radiation technologist’s instruction than in the method of breath-hold by a patient’s self-estimation. And these differences were larger in tumors of lower lung field than upper lung field. There was statistically significant difference between two methods of breath-hold. The actual switching of the radiation beam was delayed less than 0.1 second behind the patient’s switching. All portal images of 20 patients had a visually sufficient accuracy of tumor position in the radiation field with a difference less than 3mm between a planned position and actual tumor positions. Conclusion: The reproducibility of tumor position under patient’s self breath-hold without any respiratory monitoring devices had a satisfactory accuracy with a difference of tumor position less than 3mm in all directions. The method of breath-hold by a patient’s self-estimation was more accurate than that by a radiation technologist’s instruction. Our newly developed switch which enabled patients to turn the radiation-beam on and off was useful with a good reproducibility under a breath-hold at patients’ pace. And it has an excellent time-efficiency because the irradiation starts at the initiation of patients’ breath-hold and continues irradiation as long as patients keep the breath-hold. This new irradiation system is simple (no need for a respiratory monitoring device) and useful for irradiation of lung cancer with reduced PTV and a sufficient reproducibility.

Reduction of irradiated small bowel volume and accurate patient positioning by use of a bellyboard device in pelvic radiotherapy of gynecological cancer patients

Purpose: To reduce the volume of small bowel within pelvic treatment fields for gynecological cancer using a bellyboard device and to determine the accuracy of the prone treatment position. Materials and methods: Fifteen consecutive patients with a gynecologic malignancy who were treated with postoperative pelvic radiotherapy were selected for this study. The volume of small bowel within the treatment fields was calculated for both the supine and prone treatment positions. The patients were treated in the prone position in a so-called bellyboard device. During treatment sessions electronic portal images were obtained. An off-line setup verification and correction protocol was used and the setup accuracy of the positioning in the bellyboard was determined. Results: The average volume of small bowel within the treatment fields was 229 cm 3 and 66 cm 3 in the supine and prone treatment, respectively, which means an average volume reduction in the prone position of 64% (95% CI 56–72%), as compared with the supine position. For the position of the patient in the field, the systematic error defined by the standard deviation (SD) of the mean difference per patient between simulation and treatment images was 1.7 mm in the lateral direction, 2.1 mm in the craniocaudal direction and 1.7 mm in the ventrodorsal direction. On average, only 0.4 setup correction per patient was required to achieve this accuracy. The random day-to-day variations were 1.9 (1SD), 2.6 and 2.3 mm, respectively. Standard deviations of the systematic differences between patient positioning relative to the bellyboard were 6.2 mm in lateral direction and 9.1 mm in craniocaudal direction. Conclusions: Treatment of gynecological cancer patients in the prone position using a bellyboard reduces the volume of irradiated small bowel. An off-line verification and correction protocol ensures accurate patient positioning. Daily setup variations using the bellyboard were small (1 SD<3 mm). Therefore for pelvic radiotherapy in patients with a gynecological malignancy, the use of a bellyboard is recommended.