Novalis System Research Articles

Stereotactic Radiosurgery for Trigeminal Neuralgia Utilizing the BrainLAB Novalis System

Stereotactic radiosurgery (SRS) is one of the least invasive treatments for trigeminal neuralgia (TN). To date, most reports have been about Cobalt-based treatments (i.e., Gamma Knife) with limited data on image-guided stereotactic linear accelerator treatments. We describe our initial experience of using BrainLAB Novalis stereotactic system for the radiosurgical treatment of TN. A total of 20 patients were treated between July 2004 and February 2007. Each SRS procedure was performed using the BrainLAB Novalis System. Thin cuts MRI images of 1.5 mm thickness were acquired and fused with the simulation CT of each patient. Majority of the patients received a maximum dose of 90 Gy. The median brainstem dose to 1.0 cc and 0.1 cc was 2.3 Gy and 13.5 Gy, respectively. In addition, specially acquired three-dimensional fast imaging sequence employing steady-state acquisition (FIESTA) MRI was utilized to improve target delineation of the trigeminal proximal nerve root entry zone. Barrow Neurological Index (BNI) pain scale for TN was used for assessing treatment outcome. At a median follow-up time of 14.2 months, 19 patients (95%) reported at least some improvement in pain. Eight (40%) patients were completely pain-free and stopped all medications (BNI Grade I) while another 2 (10%) patients also stopped medications but reported occasional pain (BNI Grade II). Another 2 (10%) patients reported no pain and 7 (35%) patients only occasional pain while continuing medications, BNI Grade IIIA and IIIB, respectively. Median time to pain control was 8.5 days (range: 1-70 days). No patient reported severe pain, worsening pain or any pain not controlled on their previously taken medication. Intermittent or persistent facial numbness following treatments occurred in 35% of patients. No other complications were reported. Stereotactic radiosurgery using the BrainLAB Novalis system is a safe and effective treatment for TN. This information is important as more centers are obtaining image-guided stereotactic-based linear accelerators capable of performing radiosurgery.

The geometric accuracy of frameless stereotactic radiosurgery using a 6D robotic couch system

The aim of this paper is to assess the overall geometric accuracy of the Novalis system using the Robotic Tilt Module in terms of the uncertainty in frameless stereotactic radiotherapy. We analyzed the following three metrics: (1) the correction accuracy of the robotic couch, (2) the uncertainty of the isocenter position with gantry and couch rotation, and (3) the shift in position between the isocenter and central point detected with the ExacTrac x-ray system. Based on the concept of uncertainty, the overall accuracy was calculated from these values. The accuracy in positional correction with the robotic couch was 0.07 ± 0.22 mm, the positional shift of the isocenter associated with gantry rotation was 0.35 mm, the positional shift of the isocenter associated with couch rotation was 0.38 mm and the difference in position between the isocenter and the ExacTrac x-ray system was 0.30 mm. The accuracy of intracranial stereotactic radiosurgery with the Novalis system in our clinic was 0.31 ± 0.77 mm. The overall geometric accuracy based on the concept of uncertainty was 0.31 ± 0.77 mm, which is within the tolerance given in the American Association of Physicists in Medicine report no. 54.

Assessment of Spatial Uncertainties in the Radiotherapy Process With the Novalis System

The purpose of this study was to evaluate the accuracy of a new version of the ExacTrac X-ray (ETX) system with statistical analysis retrospectively in order to determine the tolerance of systematic components of spatial uncertainties with the Novalis system. Three factors of geometrical accuracy related to the ETX system were evaluated by phantom studies. First, location dependency of the detection ability of the infrared system was evaluated. Second, accuracy of the automated calculation by the image fusion algorithm in the patient registration software was evaluated. Third, deviation of the coordinate scale between the ETX isocenter and the mechanical isocenter was evaluated. From the values of these examinations and clinical experiences, the total spatial uncertainty with the Novalis system was evaluated. As to the location dependency of the detection ability of the infrared system, the detection errors between the actual position and the detected position were 1% in translation shift and 0.1 degrees in rotational angle, respectively. As to the accuracy of patient verification software, the repeatability and the coincidence of the calculation value by image fusion were good when the contrast of the X-ray image was high. The deviation of coordinates between the ETX isocenter and the mechanical isocenter was 0.313 +/- 0.024 mm, in a suitable procedure. The spatial uncertainty will be less than 2 mm when suitable treatment planning, optimal patient setup, and daily quality assurance for the Novalis system are achieved in the routine workload.

Curative Stereotactic Radiotherapy for Stage I Lung Cancer Arising after Previous Pneumonectomy

Curative Stereotactic Radiotherapy for Stage I Lung Cancer Arising after Previous Pneumonectomy

Evaluation of residual patient position variation for spinal radiosurgery using the Novalis image guided system

The Novalis system has been demonstrated to achieve accurate target localization on anthropomorphic phantoms. However, other factors, such as rotational deviation, patient intrafraction motion, and image fusion uncertainty due to patient body deformation, could contribute additional position uncertainty for actual patients. This study evaluates such position uncertainty for spinal radiosurgery patients. Fifty-two consecutive spinal radiosurgery patients were included in the study. Rotational deviation was evaluated from 6-deg of freedom (6D) fusion results for all patients. The combined uncertainty of patient motion and image fusion was determined from fusion results of additional kV x-ray images acquired before, during, and after treatment for 25 of the 52 patients. The uncertainty of image fusion was also evaluated by performing 6D fusion ten different times with various regions of interest in the images selected for fusion. This was performed for two patients with L3 and T2 lesions, respectively, for comparison. The mean rotational deviation was 0.7 +/- 1.8, 0.7 +/- 1.5, and 0.7 +/- 1.6 deg along the yaw, roll, and pitch directions, respectively. The combined uncertainty from patient motion and image fusion was 0.1 +/- 0.9, 0.2 +/- 1.2, and 0.2 +/- 1.0 mm in the anteroposterior (AP), longitudinal, and lateral directions, respectively. The uncertainty (standard deviation) due to image fusion was less than 0.28 mm in any direction for the L3 lesion and 0.8 mm in the AP direction for the T2 lesion. Overall position uncertainty for spinal radiosurgery patients has been evaluated. Rotational deviation and patient motion were the main factors contributed to position uncertainty for actual patient treatment.

Clinical assessment and characterization of a dual‐tube kilovoltage X‐ray localization system in the radiotherapy treatment room

Although flat‐panel kilovoltage X‐ray imaging devices have been well tested for clinical use in diagnostic radiology, their use as a part of an image‐guided radiation therapy (IGRT) system in a treatment room is new and requires systematic assessment.We used the Novalis IGRT system (BrainLAB, Feldkirchen, Germany) for the present study. The system consists of two floor‐mounted kilovoltage X‐ray tubes projecting obliquely onto two flat‐panel detectors mounted on the ceiling. The system can automatically fuse two‐dimensional localization images with three‐dimensional simulation computed tomography images to provide positioning guidance. We evaluated these system parameters: Overall performance of the IGRT system, including isocenter correlation between the IGRT system and the linear accelerator (LINAC)Image quality of the systemExposure received by patients for a pair of imagesLinearity, uniformity, and repeatability of the system The Novalis system uses a daily isocenter calibration procedure to ensure consistency of isocenters between the IGRT and the LINAC systems. The localization accuracy was about 1 mm. We measured the relative modulation transfer function (RMTF) to quantify the spatial resolution of the imaging device, with f50 being 0.7 – 0.9 line pairs per millimeter. The maximal exposure of an image was 95 mR. We derived an empirical relationship between the exposure and the X‐ray technical settings. The other parameters of the system were quantitatively measured and generally met the requirements.The IGRT system is safe and reliable for clinical use as a target localization device. The measured data can be used as a benchmark for a quality assurance procedure.PACS number: 87.56‐v

Stereotactic Radiosurgery (SRS) for Trigeminal Neuralgia With BrainLab Novalis System: The Initial Baylor College of Medicine/The Methodist Hospital Experience

Stereotactic radiosurgery (SRS) is one of the least invasive treatments for trigeminal neuralgia (TN). It has become increasingly used in hopes of providing durable responses with low complication rates compared to more invasive neurosurgical approaches. To date, mostly all data has been investigated using Cobalt-based machines such as gamma knife treatments with only a few reports publishing data on image-guided stereotactic-based linear accelerators. We describe our initial experience of using Brainlab Novalis for the radiosurgical treatment of TN.

Versatility of the Novalis System to Deliver Image-Guided Stereotactic Body Radiation Therapy (SBRT) for Various Anatomical Sites

Stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (FSRT) programs to treat brain tumors were implemented when we first acquired the Brainlab Novalis system in 2003. Two years later, we started an extra-cranial stereotactic radio-ablation or more appropriately a stereotactic body radiation therapy (SBRT) program using the Brainlab Novalis image-guided system at The Methodist Hospital in Houston, Texas. We hereby summarize our initial experience with this system in delivering image-guided SBRT to a total of 80 patients during our first year of clinical implementation, from February 2005 to January 2006. Over 100 lesions in more than 20 distinct anatomical sites were treated. These include all levels of spine from cervical, thoracic, lumbar, and sacral lesions. Spinal lesions encompass intramedullary, intradural, extradural, or osseous compartments. Also treated were lesions in other bony sites including orbit, clavicle, scapula, humerus, sternum, rib, femur, and pelvis (ilium, ischium, and pubis). Primary or metastatic lesions located in the head and neck, supraclavicular region, axilla, mediastinum, lung (both central and peripheral), abdominal wall, liver, kidney, para-aortic lymph nodes, prostate, and pelvis were also treated. In addition to primary radiotherapy, SBRT program using the Brainlab Novalis system allows re-irradiation for recurrence and "boost" after conventional treatment to various anatomical sites. Treating these sites safely and efficaciously requires knowledge in radiation tolerance, fraction size, total dose, biologically equivalent dose (BED), prior radiotherapy, detailed dose volume histograms (DVH) of normal tissues, and the radiosensitive/radioresistant nature of the tumor. Placement of radio-opaque markers (Visicoil, Radiomed) in anatomical sites not in close proximity to bony landmarks (e.g., kidney and liver) helps in measuring motion and providing image guidance during each treatment fraction. Tumor/organ motion data obtained using 4D-CT while the patient is immobilized in the body cast aids in planning treatment margin and determining the need for respiratory motion control, e.g., abdominal compressor, gating, or active breathing control. The inclusion of PET/CT to the Brainlab treatment planning system further refines the target delineation and possibly guides differential fraction size prescription and delivery. The majority of the patients tolerated the SBRT treatment well despite the longer daily treatment time when compared to that of conventional treatment. All patients achieved good pain relief after SBRT. Compared to conventional standard radiotherapy of lower daily fraction size, we observed that the patients achieved faster pain relief and possibly more durable symptom control. Very high local control with stable disease on imaging was observed post SBRT. Our initial experience shows that the Brainlab Novalis system is very versatile in delivering image-guided SBRT to various anatomical sites. This SBRT approach can be applied to either primary or metastatic lesions in the primary, "boost," or re-irradiation settings. The understanding of fraction size, total dose, BED, and DVH of normal tissues is very important in the treatment planning. Appropriate use of immobilization devices, radio-opaque markers for image-guidance, 4D-CT for tumor/organ motion estimates, and fusion of planning CT scans with biological/functional imaging will further improve the planning and delivery of SBRT, hopefully leading to better treatment outcome.

Breathing-synchronized irradiation using stereoscopic kV-imaging to limit influence of interplay between leaf motion and organ motion in 3D-CRT and IMRT: Dosimetric verification and first clinical experience

Purpose: To verify the technical feasibility of a prototype developed for breathing-synchronized irradiation by phantom measurement and report on the first clinical experience of 3 patients. Methods and Materials: Adaptations to a commercially available image-guidance technique (Novalis Body/ExacTrac4.0; BrainLAB AG, Heimstetten, Germany) were implemented, allowing breathing-synchronized irradiation with the Novalis system. A simple phantom simulating a breathing pattern of 16 cycles per minute and covering a distance of 4 cm was introduced to assess the system's performance to: (<i>1</i>) trigger the linac at the right moment (using a hidden target in the form of a 3-mm metal beads mounted to the phantom); (<i>2</i>) assess the delivered dose in nongated and gated mode (using an ionization chamber mounted to the phantom); (<i>3</i>) evaluate dose blurring and interplay between organ motion and leaf motion when applying dynamic multileaf collimation (DMLC) intensity-modulated radiation therapy (IMRT) techniques (using radiographic film mounted to the phantom). The effect of motion was evaluated by importing the measured fluence maps generated by the linac into the treatment planning system and recalculating the resulting dose distribution from DMLC IMRT fluence patterns acquired in nongated and gated mode. The synchronized-breathing technique was applied to three clinical cases: one liver metastasis, one lung metastasis, and one primary lung tumor. Results: No measurable delay in the triggering of the linac can be observed based on the hidden target test. The ionization chamber measurements showed that the system is able to improve the dose absorption from 44% (in nongated mode) to 98% (in gated mode) for a small field irradiation (3 × 3 cm²) of a moving target. Importing measured fluence maps generated for a realistic patient treatment and actually delivered by the linac into the treatment planning system yielded highly disturbed dose distributions in nongated delivery, whereas the gated delivery showed good agreement with the original theoretical dose distribution. These findings were confirmed by dose–volume histograms. The three different clinical cases revealed three different practical problems that could easily be resolved by minor adjustments to the system. The applied breathing-synchronization technique introduced an increased treatment time by a factor of 3 to 4. Conclusions: Initial tests with the prototype for breathing-synchronized irradiation showed promising results. The use of measured fluence fields, delivered by the linac in nongated and gated mode as imported fluence maps for the treatment planning system, revealed the dramatic impact of dose blurring and interplay between leaf motion and organ motion, as well as the advantage of breathing synchronization to resolve this issue. The latter should, however, be weighed against the increased treatment time.

Comparative dosimetric study of three-dimensional conformal, dynamic conformal arc, and intensity-modulated radiotherapy for brain tumor treatment using Novalis system

Purpose: To investigate the dosimetric differences among three-dimensional conformal radiotherapy (3D-CRT), dynamic conformal arc therapy (DCAT), and intensity-modulated radiotherapy (IMRT) for brain tumor treatment. Methods and Materials: Fifteen patients treated with Novalis were selected. We performed 3D-CRT, DCAT, and IMRT plans for all patients. The margin for the planning target volume (PTV) was 1 mm, and the specific prescription dose was 90% for all plans. The target coverage at the prescription dose, conformity index (CI), and heterogeneity index were analyzed for all plans. Results: For small tumors (PTV ≤2 cm 3 ), the three dosimetric parameters had approximate values for both 3D-CRT and DCAT plans. The CI for the IMRT plans was high. For medium tumors (PTV >2 to ≤100 cm 3 ), the three plans were competitive with each other. The IMRT plans had a greater CI, better target coverage at the prescription dose, and a better heterogeneity index. For large tumors (PTV >100 cm 3 ), the IMRT plan had good target coverage at the prescription dose and heterogeneity index and approximate CI values as those in the 3D-CRT and DCAT plans. Conclusion: The results of our study have shown that DCAT is suitable for most cases in the treatment of brain tumors. For a small target, 3D-CRT is useful, and IMRT is not recommended. For larger tumors, IMRT is superior to 3D-CRT and very competitive in sparing critical structures, especially for big tumors.

212 Experience with the Novalis system

212 Experience with the Novalis system

SU‐FF‐T‐152: Independent Dosimetric Validation of Novalis IMRT and Dynamic Conformal Plans Using PHILIPS Pinnacle3Treatment Planning System

Purpose: To study and validate the calculated dose distributions generated from Novalis inverse treatment planning system using PHILIPS Pinnacle3 treatment planning system. Method &amp; Materials: A total of 60 clinical Novalis plans comprising of 41 Dynamic Arc and 19 IMRT plans were selected in this study. The CT images for the individual cases were exported to Pinnacle3 treatment planning system via network. A software script was developed to extract beam segments including MLC jaws, gantry, collimator and couch angles and monitor units from the RTP file generated by Novalis inverse planning system. The segmentation of MLC‐based IMRT plans from Novalis system was imported into the Pinnacle3 using the above software. The prescription and beam weights were set to reflect the treatment plan computed by the Novalis system. In all the above clinical cases, the dose distributions were computed and analyzed for homogeneous dose calculations. Results: The dosimetric results of all the above clinical cases have been analyzed. It was found that the agreement between the two systems at the isocenter dose was 1.00 ± 0.028. In addition, the shapes of isodose lines in the three principal planes(axial, coronal and sagittal) were qualitatively agreed in both the systems Conclusion: This study helps in validating the Novalis IMRT and Dynamic Conformal plans on Pinnacle3 which is well tested and documented on its convolution algorithm. These results indicate that the approach is robust and valuable for routine clinical IMRT plan validations as it takes account of entire clinical data set for the individual cases.

SU-FF-T-414: Transfer of Brain Tumor Patients From Leksell Gamma Knife to BrainLAB Novalis

Purpose: To identify and resolve issues associated with transferring braintumor patients, after placement of Leksell frame (LF) and the completion of MRI and CT scans for Gamma Knife (GK) radiosurgery, to linac‐based stereotactic radiosurgery(SRS) with BrainLAB Novalis. Method and Materials: Three patients were studied. These patients could not technically be treated with GK radiosurgery given the technical limitations posed by frame placement or tumor location. CT and MRI scans were transferred to the BrainLAB treatment planning system (TPS), BrainSCAN. Treatment plans were then generated and evaluated. Systematic and potential issues were identified. Compatibility of the LF and the BrainLab system was evaluated. Imaging issues were investigated, and time restriction considerations to have the patient treated the same day were addressed. Results: The LF is compatible with the Novalis system using a special adapter. The MRI scans presented no issues. CTimaging may constitute an issue since the Leksell localizer box is shorter than the BrainLab box. Depending on the frame placement on the patient head, part of the anatomy may not be localized by BrainSCAN and cannot be used. While the GK TPS can extrapolate parts of the anatomy, BrainSCAN cannot. As a result, planning options are limited (novertex beams/arcs). In this case, a new CT scan with the LF and the BrainLAB localizer is required. Once imaging issues are resolved, a plan, quality assurance, evaluation and treatment can be completed the same day. Conclusion: Patients scheduled for GK radiosugery can be efficiently transferred to Novalis the same day of the procedure when tumor location or frame placement does not allow GK radiosurgery. The issues associated with the transfer are manageable and allow same day treatment with the Novalis system.

SU-FF-T-413: The Impact of Heterogeneity Correction On Tumor Dosimetry for Lung Cancer Stereotactic Body Radiation Therapy

Purpose: Stereotactic body radiation therapy(SBRT) for non‐small cell lungcancer has been shown to limit toxicity. Heterogeneity correction on lungcancerradiotherapy has not been recommended by RTOG. In this study, dosimetric difference between the SBRT plans with/without heterogeneity correction is analyzed. Method and Materials: Nine lungcancer patients treated with SBRT techniques using a 6 MV Novalis system were selected. Using the path length algorithm in BrainLAB treatment planning system, all the treatment plans were applied the heterogeneity correction. With same beam parameters, we performed the plans without the heterogeneity correction, and compared the dosimetric difference to the treatment plans. The heterogeneity correction factors (Kc) at iso‐center, target coverage, heterogeneity index (HI), and conformity index (CI) were used in the comparison. Results: The average of Kc at isocenter for 14 planning target volumes (PTV) was 1.002±0.02, only three Kc values (1.07, 0.985, 0.981) were relatively off to the average. Except one case, the other 13 target coverage values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the target coverage was ∼7%. All the HI values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the HI reached 300%. The difference of the CI between the compared plans was within ∼10%. The CIs of the plans without correction were better than those corrected. For one case — the tumor located in the lung base, the impact of heterogeneity correction was significant. Conclusion: The impact of heterogeneity correction for tumordosimetry on SBRT for lungcancer is case depended. For most primary lungtumors the difference between the plans with/without heterogeneity correction is clinically insignificant. For the tumors near the interface of different mass density, the heterogeneity correction is necessary.

Spinal lesions treated with Novalis shaped beam intensity-modulated radiosurgery and stereotactic radiotherapy

Object. Spinal radiosurgery was implemented to improve quality of life (QOL) in patients with malignancies. It may also be applicable to the treatment of benign lesions. Methods. Between July 2002 and January 2004, 14 patients harboring 22 lesions were treated; 13 received single-dose stereotactic radiosurgery. Six were women. The mean age was 60.2 years (range 48–82 years). There were 11 metastases, two neurofibromas, and one meningioma. Six lesions were cervical, 10 thoracic, and six lumbar. Ten patients suffered pain, three paresthesias, two weakness, and three were asymptomatic. Seven patients underwent spinal surgery, with four receiving instrumentation. Twelve patients underwent conventional irradiation before stereotactic radiosurgery/stereotactic radiotherapy. A mean dose of 12 ± 2.7 Gy (range 8–21 Gy) was prescribed to the 91% isodose line (range 85–97%). The mean tumor volume was 25 ± 27.1 ml (range 0.75–91.8 ml). Treatment was planned using intensity-modulated radiosurgery (IMRS) fields in 15 cases, dynamic arcs in five, and conformal beams in two. The mean follow-up period was 6.1 ± 3.9 months (range 1–16 months). Three patients became pain free and four experienced considerable relief. Weakness improved in the two patients with this preoperative symptom and the asymptomatic patients remained so. Four lesions decreased in size, five remained stable, seven progressed, and six were not followed up (two patients died before follow up). Four patients in all died, three of systemic disease and one of thoracic lesion progression. No complications due to shaped beam and IMRS/intensity-modulated radiotherapy (IMRT) techniques were observed. Conclusions. Shaped beam and IMRS/IMRT involving the Novalis system may delay neurological deterioration, improving QOL. The lack of complication suggests that higher doses can be delivered to improve the control rate in patients with metastases.

Spinal lesions treated with Novalis shaped beam intensity-modulated radiosurgery and stereotactic radiotherapy

Object. Spinal radiosurgery was implemented to improve quality of life (QOL) in patients with malignancies. It may also be applicable to the treatment of benign lesions. Methods. Between July 2002 and January 2004, 14 patients harboring 22 lesions were treated; 13 received single-dose stereotactic radiosurgery. Six were women. The mean age was 60.2 years (range 48–82 years). There were 11 metastases, two neurofibromas, and one meningioma. Six lesions were cervical, 10 thoracic, and six lumbar. Ten patients suffered pain, three paresthesias, two weakness, and three were asymptomatic. Seven patients underwent spinal surgery, with four receiving instrumentation. Twelve patients underwent conventional irradiation before stereotactic radiosurgery/stereotactic radiotherapy. A mean dose of 12 ± 2.7 Gy (range 8–21 Gy) was prescribed to the 91% isodose line (range 85–97%). The mean tumor volume was 25 ± 27.1 ml (range 0.75–91.8 ml). Treatment was planned using intensity-modulated radiosurgery (IMRS) fields in 15 cases, dynamic arcs in five, and conformal beams in two. The mean follow-up period was 6.1 ± 3.9 months (range 1–16 months). Three patients became pain free and four experienced considerable relief. Weakness improved in the two patients with this preoperative symptom and the asymptomatic patients remained so. Four lesions decreased in size, five remained stable, seven progressed, and six were not followed up (two patients died before follow up). Four patients in all died, three of systemic disease and one of thoracic lesion progression. No complications due to shaped beam and IMRS/intensity-modulated radiotherapy (IMRT) techniques were observed. Conclusions. Shaped beam and IMRS/IMRT involving the Novalis system may delay neurological deterioration, improving QOL. The lack of complication suggests that higher doses can be delivered to improve the control rate in patients with metastases.

Dosimetric evaluation of partially overlapping intensity modulated beams using dynamic mini-multileaf collimation.

The dose distribution resulting from partially overlapping intensity modulated beams (IMBs) assigned to different isocenters for the treatment of the same planning target volume (PTV) was evaluated. These partially overlapping IMBs are used in static intensity modulated radiation therapy (IMRT) treatments with the Novalis system using the mini-MultiLeaf Collimator (mini-MLC) in Dynamic MultiLeaf Collimation (DMLC) mode. The resultant dose distribution was verified dosimetrically for a cylindrical target defined in a homogeneous cubic phantom. The phantom positioning can introduce dose nonuniformities in the resultant dose distribution by nonperfect positioning of the isocenters in accordance with each other. The dose inhomogeneities are quantified mathematically by summation of the dose profiles of the used IMBs and experimentally by measurement of the resulting dose profiles with radiographic film and thermoluminescent detectors (TLD). The mathematical estimation of the resulting dose profile of the treatment with a perfect positioning of the isocenters showed a good agreement with the planned dose profile. The magnitude of the maximum dose inhomogeneities introduced by the simulated supplementary shifts between the isocenters decreases by -8.54% mm(-1) as the shift changes from -0.30 +/- 0.10 cm to +0.30 +/- 0.10 cm. The TLD measurements showed a similar variation of the magnitude of the maximum dose inhomogeneities: -8.77% mm(-1). The amount of dose variation was underestimated with the radiographic film measurements, which showed a variation of -7.17% mm(-1). The film measurements demonstrated that the magnitude of the introduced maximum dose inhomogeneities did not alter significantly throughout the PTV. The approach of using partially overlapping IMBs assigned to different isocenters to enlarge the treatment region introduces smaller dose inhomogeneities in the resultant dose distribution than when abutting treatment fields are used. The resultant dose distribution of this treatment technique is less sensitive to positioning errors of the used treatment isocenters.