Isocenter Verification Research Articles

On the Accuracy of Isocenter Verification with kV Imaging in Stereotactic Radiosurgery

Modern medical linear accelerators (linacs) are equipped with X-ray systems, which allow to check the patient's position just prior to treatment. Their usefulness for stereotactic radiosurgery (SRS) depends on how accurately they allow to determine the deviation between the actual and planned isocenter positions. This accuracy was investigated with measurements using two different phantoms (Figures 1 and 2). After precisely aligning a phantom onto the linac isocenter, two perpendicular X-rays or a cone-beam CT (CBCT) are taken, and the isocenter position is deduced from this data. The deviation of the thereby gained position from the setup isocenter is taken as a measure for the uncertainty of this method. Isocenter verification with two orthogonal X-rays (Figure 4) achieves accuracies of better than 1 mm (Table 3). The distance between the isocenters of the CBCT and the linac (Figure 3) is in the order of 1 mm, but remains constant on the time scale of 1 week (Table 1) and may therefore be taken into account. The uncertainty after correction is below 0.2 mm. kV imaging with the patient in treatment position allows to verify the isocenter position with submillimeter precision, and therefore offers a supplemental test, suitable for SRS, which takes all positional uncertainties into account.

Early Clinical Experience With Kilovoltage Image-Guided Radiation Therapy for Interfraction Motion Management

Interest in image-guided radiation therapy (IGRT) reflects the desire to minimize interfraction positioning variability. Using a kilovoltage (kV) imaging unit mounted to a traditional LINAC allows daily matching of kV images to planning digitally reconstructed radiographs (DRRs). We quantify and evaluate the significance of calculated deviation from the intended isocenter. Since September 2004, 117 patients with various malignancies were treated using the On-Board Imaging (OBI) system, with 2088 treatment sessions. Patients were positioned by the treating therapist; orthogonal images were then obtained with the OBI unit. Couch shifts were made, aligning bony anatomy to the initial simulation image. Routine port films were performed weekly (after that day's OBI session). Ninety percent of all lateral, longitudinal, and vertical shifts were less than 0.8 cm, 0.6 cm, and 0.7 cm, respectively. The median vector shift for each anatomic site was: 0.42 cm for head and neck, 0.40 cm for CNS, 0.59 cm for GU/prostate, and 0.73 cm for breast; shift magnitude did not change with successive OBI sessions. The use of OBI effectively corrects setup variability. These shifts are typically small and random. The use of OBI likely can replace weekly port films for isocenter verification; however, OBI does not provide field shape verification.

SU‐GG‐T‐457: Gamma Knife Quality Assurance Technique for Treatment of Functional Disorders

Purpose: Present the technique used for commissioning and validation of Leksell Gamma Knife Radiosurgery machine for treatment of functional disorders. Method and Materials: Leksell Gamma Knife C Radiosurgery machine was commissioned and validated for use for treatment of functional disorders such as Essential Tremor and Parkinson Disease. These patients are treated with 4 mm collimator to a much higher radiation dose of 120 Gy to the nerve. Due to the proximity of the target with vital structure, internal capsule it is very critical that the target localization and delivery accuracy is maintained to less than 1 mm. The isocenter verification of the machine was done using the Elekta target localizer using Gafchromic film, which was calibrated for dose range from 5 Gy to 120 Gy. The isocenter was verified along X,Y and Z orientations. The beam profile for the 4 mm collimator was determined using films exposed along X‐Y and X‐Z plane. The FWHM along both planes were determined from the scans converted to dose, and compared with the data in the Leksell Gamma Plan software. The absolute output for 18 mm collimator was determined using a micro chambers in a spherical phantom. The film from the same batch of calibration was irradiated with 18mm and 4 mm collimator. The ratio of the doses was used for output factor of 4mm relative to 18 mm collimator for dosimetry. The computer generated plan was verified in a phantom with ion chamber, and TLD measurements. Results:The root mean square deviation of target isocenter was 0.6mm. The beam profiles agreed within planning system to 0.8 mm. The absolute dose agreed with computer value to 1.2% Conclusion: Using these tests we have established confidence in the dosimetry and localization for use on patient.

Simulation of Dose to Surrounding Normal Structures in Tangential Breast Radiotherapy Due to Setup Error

Setup error plays a significant role in the final treatment outcome in radiotherapy. The effect of setup error on the planning target volume (PTV) and surrounding critical structures has been studied and the maximum allowed tolerance in setup error with minimal complications to the surrounding critical structure and acceptable tumor control probability is determined. Twelve patients were selected for this study after breast conservation surgery, wherein 8 patients were right-sided and 4 were left-sided breast. Tangential fields were placed on the 3-dimensional–computed tomography (3D-CT) dataset by isocentric technique and the dose to the PTV, ipsilateral lung (IL), contralateral lung (CLL), contralateral breast (CLB), heart, and liver were then computed from dose-volume histograms (DVHs). The planning isocenter was shifted for 3 and 10 mm in all 3 directions (X, Y, Z) to simulate the setup error encountered during treatment. Dosimetric studies were performed for each patient for PTV according to ICRU 50 guidelines: mean doses to PTV, IL, CLL, heart, CLB, liver, and percentage of lung volume that received a dose of 20 Gy or more (V20); percentage of heart volume that received a dose of 30 Gy or more (V30); and volume of liver that received a dose of 50 Gy or more (V50) were calculated for all of the above-mentioned isocenter shifts and compared to the results with zero isocenter shift. Simulation of different isocenter shifts in all 3 directions showed that the isocentric shifts along the posterior direction had a very significant effect on the dose to the heart, IL, CLL, and CLB, which was followed by the lateral direction. The setup error in isocenter should be strictly kept below 3 mm. The study shows that isocenter verification in the case of tangential fields should be performed to reduce future complications to adjacent normal tissues.

SU-FF-T-14: A Filmless Verification of the Radiation Isocenter for a Micromultileaf-Based Radiosurgery System

Purpose: To assess the feasibility of a commercial electronic portal image device (EPID) along with an in‐house developed software, in order to verify the alignment of the radiation isocenter. Method and Materials: The system used for the radiosurgery delivery consists of a Varian Clinac 2100C/D (Varian Inc, Palo Alto, CA) modified by an attachable micromultileaf collimator (m3, BrainLAB AG, Heimstetten, Germany) and equipped with a Varian Mark II EPID. A 5mm diameter tungsten ball centered in the room laser isocenter is shot and imaged with the EPID for several gantry, collimator and couch angles combinations by a 30×30mm field size shaped with m3. The software Rodeo1.1 (written using Fortran) detects the centers of the radiation field and the ball shadow in every 2D image taken. From several projections, the 3D position of the radiation isocenter can be obtained and compared with the laser isocenter The distance between them is a measure of the alignment error which includes also the effects due to possible displacement of the portal itself that could depend on the gantry value. The accuracy of the procedure described was investigated by applying known shifts to the ball and recording the displacements detected by the mentioned software. For comparison purpose with the standard procedure which uses film and analysis by visual inspection, several pointer images were acquired and the reproducibility of two methods was stated. Results: The verification system composed of the EPID and Rodeo 1.1 shows an accuracy better than 0.2mm. Differences inter‐observer up to 0.3mm were found when we compared the results of the film test against a perfect reproducibility with the EPID‐based method. Conclusions: The EPID and Rodeo 1.1 set is a reliable tool for isocenter verification, with not observer dependence and time saving in relation to the film procedure.

SU‐FF‐T‐161: Development of a More Objective and Accurate Film Verification Tool for Stereotactic Radiosurgery

Purpose: To develop a more objective and accurate quality assurance (QA) tool for use in spatial verification of patient treatment isocenter(s) to be used in conjunction with a commercial linear accelerator based stereotactic radiosurgery (SRS) system. Method and Materials: A plastic positioning device was constructed to house an open‐ended cylindrical lead foil ring approximately 0.3 [mm] thick and 1.0 [cm] in diameter. The cylindrical design was used for comparison versus the vendor supplied radio‐opaque ball design. Verification films were acquired using a 1.75 [cm] SRS cone with the same set‐up used for a SRS patient treated at our clinic. Films were acquired at gantry angles of 0°, 90°, and 270° for both the radio‐opaque ball and cylindrical device and repeated three (3) times. The objectivity of the film QA evaluation was determined by calculating the variance in the measurement of the radial offset distance for each gantry angle. In addition, five (5) additional sets of verification film data were acquired to evaluate the detection of a range of deliberate erroneous incremental offsets (0.0, 0.5, 1.0, 1.5, and 2.0 [mm]). The accuracy of the film QA evaluation was determined by measuring the isocenter offsets in the QA films and comparing those results to the known offset using linear regression analysis. Results: Calculated variance values from the QA films acquired at three (3) gantry angles measuring radial offset showed a reduction in user variability from 28% to 21% when the cylindrical device was used. Similarly, linear regression analyses of the incremental offset QA films demonstrated a slightly better correlation to the expected values when using the cylindrical device. Conclusion: The cylindrical SRS positioning device improves the objectivity of the QA procedure for isocenter verification and detects critical set‐up errors more accurately thus improving the ability of the physicist to assess patient set‐up.

Intra- and inter-radiation therapist reproducibility of daily isocenter verification using prostatic fiducial markers

BackgroundWe sought to determine the intra- and inter-radiation therapist reproducibility of a previously established matching technique for daily verification and correction of isocenter position relative to intraprostatic fiducial markers (FM).Materials and methodsWith the patient in the treatment position, anterior-posterior and left lateral electronic images are acquired on an amorphous silicon flat panel electronic portal imaging device. After each portal image is acquired, the therapist manually translates and aligns the fiducial markers in the image to the marker contours on the digitally reconstructed radiograph. The distances between the planned and actual isocenter location is displayed. In order to determine the reproducibility of this technique, four therapists repeated and recorded this operation two separate times on 20 previously acquired portal image datasets from two patients. The data were analyzed to obtain the mean variability in the distances measured between and within observers.ResultsThe mean and median intra-observer variability ranged from 0.4 to 0.7 mm and 0.3 to 0.6 mm respectively with a standard deviation of 0.4 to 1.0 mm. Inter-observer results were similar with a mean variability of 0.9 mm, a median of 0.6 mm, and a standard deviation of 0.7 mm. When using a 5 mm threshold, only 0.5% of treatments will undergo a table shift due to intra or inter-observer error, increasing to an error rate of 2.4% if this threshold were reduced to 3 mm.ConclusionWe have found high reproducibility with a previously established method for daily verification and correction of isocenter position relative to prostatic fiducial markers using electronic portal imaging.

TU-C-T-617-06: Importance of Pre-Fraction Helical CT Isocenter Verification in Extracranial Stereotactic Radiosurgery

Purpose: To quantify the impact of pre-fraction helical CT isocenter verification vs. setup based on planning CT in fractionated extracranial stereotactic radiosurgery. Method and Materials: Treatment plans (Elekta PrecisePlan) and pre-fraction isocenter verification helical CT scans for 12 patients (40 fractions) were recovered from treatment plan archives. All structures were contoured by a single physician at the time of treatment. Each plan was imported into a customizable treatment plan analysis suite (CERR). Using CERR, pre-fraction isocenter verification CT scans were fused with the original treatment plan using the external body frame as a reference. The original planned dose distribution was then translated from original treatment plan isocenter to pre-fraction verification isocenters in each fraction. Dose and volume parameters for pertinent structures were automatically extracted using both registration methods (planned or pre-fraction scans) for the original treatment plan and for all subsequent fractions. All patients were treated using the pre-fraction verified isocenter rather than pre-calculated body frame fiducials as per our institutional policies. Results: GTV volumes on pre-fraction CTs varied from original planned GTV volume (64%–203%, mean=101.8+/−26.5%) largely due to helical sampling of a mobile target. Using the external body frame as the only setup reference would have resulted in geographic misses (<80% coverage of 95% of GTV) in 7/40 (17.5%) fractions. Pre-fraction isocenter verification resulted in improved D95 GTV coverage (88–102%, mean=99.3% +/−2.4%) with no geographic misses. Conclusion: The current RTOG protocol (0236) evaluating extracranial stereotactic radiosurgery does not require pre-fraction CT tumor position verification. Our institutional policy is to verify isocenter/tumor position prior to each fraction via CT. Although helical scanning artifacts are present, pre-fraction CT-based isocenter verification may provide more consistent tumor coverage than setup to planned body frame fiducials. Conflict of Interest: Support for this research was provided in part by Elekta, Inc.

SU-FF-J-82: Use of CT Based IGRT for Daily Set-Up of Frameless Fractionated Stereotactic Patients

Purpose: To compare orthogonal port films against CT based Image Guided Radiation Therapy(IGRT) for patient isocenter verification and assess the use of CT based IGRT for daily patient set‐up. Method and Materials: Eleven patients with brain metastases were CT'd for each of their fractionated radiosurgerytreatments. The patients were immobilized in the supine position with a stereotactic head cast. Two of the patients had a bite block in their cast to minimize rotational movement. The daily CT was taken on a SOMATON Emotion Duo CT on rails and the treatment was delivered by a SIEMENS PRIMUS linear accelerator(linac). The daily CTimages were then fused to the planning CT using fiducial markers and anatomy as guides. The shifts in the X, Y, and Z directions were computed by the CoherenceDosimetry Workstation and the patient was shifted accordingly to within 1 millimeter accuracy. Results: We assessed the accuracy of the orthogonal port film technique against the CT based IGRT for 8 frameless stereotactic treatments. The implementation of IGRT increased the importance of patient immobilization while decreasing the need for accurate initial patient set‐up since the patient would be imaged immediately prior to treatment and corrections could then be made to the set‐up. Furthermore it was observed that using a bite block inside the head cast significantly improved patient immobilization and decreased the magnitude of IGRT shifts. Conclusion: Orthogonal port film validation of the isocenter is adequate for an accuracy of 5mm for frameless stereotactic treatments.CT based IGRT provides an adequate verification of the isocenter for frameless stereotactic treatments to within 2–3mm accuracy. Furthermore, CT proves to be of clinical benefit to the patient since the mean dose used for each CT was only .7cGy where port films use 3cGy each for the two orthogonal images.

Multifractionated image-guided and stereotactic intensity-modulated radiotherapy of paraspinal tumors: A preliminary report

The use of image-guided and stereotactic intensity-modulated radiotherapy (IMRT) techniques have made the delivery of high-dose radiation to lesions within close proximity to the spinal cord feasible. This report presents clinical and physical data regarding the use of IMRT coupled with noninvasive body frames (stereotactic and image-guided) for multifractionated radiotherapy. The Memorial Sloan-Kettering Cancer Center (Memorial) stereotactic body frame (MSBF) and Memorial body cradle (MBC) have been developed as noninvasive immobilizing devices for paraspinal IMRT using stereotactic (MSBF) and image-guided (MBC) techniques. Patients were either previously irradiated or prescribed doses beyond spinal cord tolerance (54 Gy in standard fractionation) and had unresectable gross disease involving the spinal canal. The planning target volume (PTV) was the gross tumor volume with a 1 cm margin. The PTV was not allowed to include the spinal cord contour. All treatment planning was performed using software developed within the institution. Isocenter verification was performed with an in-room computed tomography scan (MSBF) or electronic portal imaging devices, or both. Patients were followed up with serial magnetic resonance imaging every 3-4 months, and no patients were lost to follow-up. Kaplan-Meier statistics were used for analysis of clinical data. Both the MSBF and MBC were able to provide setup accuracy within 2 mm. With a median follow-up of 11 months, 35 patients (14 primary and 21 secondary malignancies) underwent treatment. The median dose previously received was 3000 cGy in 10 fractions. The median dose prescribed for these patients was 2000 cGy/5 fractions (2000-3000 cGy), which provided a median PTV V100 of 88%. In previously unirradiated patients, the median prescribed dose was 7000 cGy (5940-7000 cGy) with a median PTV V100 of 90%. The median Dmax to the cord was 34% and 68% for previously irradiated and never irradiated patients, respectively. More than 90% of patients experienced palliation from pain, weakness, or paresthesia; 75% and 81% of secondary and primary lesions, respectively, exhibited local control at the time of last follow-up. No cases of radiation-induced myelopathy or radiculopathy have thus far been encountered. Precision stereotactic and image-guided paraspinal IMRT allows the delivery of high doses of radiation in multiple fractions to tumors within close proximity to the spinal cord while respecting cord tolerance. Although preliminary, the clinical results are encouraging.

The impact of daily setup variations on head-and-neck intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) in the treatment of head-and-neck (H&N) cancer provides the opportunity to diminish normal tissue toxicity profiles and thereby enhance patient quality of life. However, highly conformal treatment techniques commonly establish steep dose gradients between tumor and avoidance structures. Daily setup variations can therefore significantly compromise the ultimate precision of idealized H&N IMRT delivery. This study provides a detailed analysis regarding the potential impact of daily setup variations on the overall integrity of H&N IMRT. A series of 10 patients with advanced H&N cancer were prospectively enrolled in a clinical trial to examine daily H&N radiation setup accuracy. These patients were treated with conventional shrinking field design using three-dimensional treatment planning techniques (not IMRT). Immobilization and alignment were performed using modern H&N practice techniques including conventional thermoplastic masking, baseplate fixation to the treatment couch, three-point laser alignment, and weekly portal film evaluation. After traditional laser alignment, setup accuracy was assessed daily for each patient by measuring 3 Cartesian and 3 angular deviations from the specified isocenter using a high-precision, optically guided patient localization system, which affords submillimeter setup accuracy. These positional errors were then applied to a distinct series of 10 H&N IMRT plans for detailed analysis regarding the impact of daily setup variation (without optical guidance) on the ultimate integrity of IMRT plans over a 30-day treatment course. Dose-volume histogram (DVH), equivalent uniform dose (EUD), mean total dose (mTd), and maximal total dose (MTD) for normal structures were analyzed for IMRT plans with and without incorporation of daily setup variation. Using conventional H&N masking and laser alignment for daily positioning, the mean setup error in any single dimension averaged 3.33 mm. However, when all six degrees of freedom were accounted for, using the optically guided patient localization system, the mean composite vector offset was 6.97 mm with a standard deviation of 3.63 mm. Superimposition of mean offset vectors on idealized H&N IMRT treatment plans enabled evaluation of resultant shifts in DVH, EUD, mTd, and MTD calculations. Partial geographic tumor miss (GTV underdosing) and normal tissue overdosing was common when these mean positional offsets were incorporated. The decrease in EUD for defined tumor volumes ranged up to 21% when the largest offset histories were applied, and 3-14% for plans when the least and median offset histories were applied. The successful implementation of H&N IMRT requires accurate and reproducible treatment delivery over a 6- to 7-week treatment course. The adverse impact of daily setup variation, which occurs routinely with conventional H&N masking techniques, may be considerably greater than recognized. Isocenter verification checks on two-dimensional orthogonal films may not sufficiently alert the clinician to the magnitude of three-dimensional offset vectors and the resultant impact on the quality of overall IMRT delivery. Unrecognized geographic miss and resultant target underdose may occur. Similarly, selected normal structures such as parotid glands may receive higher doses than intended. The results of this study suggest that more rigorous immobilization techniques than conventional masking and routine patient setup tracking methodologies are important for the accurate monitoring and successful delivery of high-quality IMRT for H&N cancer.

Commissioning and implementation of a stereotactic conformal radiotherapy technique using a general‐purpose planning system

The purpose of this paper is to report on commissioning and clinical implementation of a customized system for pediatric stereotactic conformal radiotherapy (SCRT). The system is based on the Pinnacle treatment‐planning system and its interfaces with other equipment: (1) Beam models were optimized for our compact blocking system and a new LINAC. (2) Three CT‐to‐density conversion tables were evaluated, one using tabulated data for a commercial phantom, the second including additional points from the manufacturer's data for the inserts in an in‐house phantom, and the third using measured densities for the in‐house phantom materials combined with tabulated data for the commercial phantom. (3) Blocks were transferred to a computerized block cutter using in‐house software that extracted the block shape from the export file and custom‐fitted the additional necessary shapes. (4) In the absence of a DICOM RT Image link, a method based on screen data capture was used to export digitally reconstructed radiographs (DRRs) to two portal imaging systems for treatment verification. Lens shielding by multileaf collimation in the anterior‐posterior isocenter verification field was investigated. (1) Computed dose distributions using the beam models agreed with measurements well within published acceptability criteria. A difference of up to 1.0 mm was measured between the beam's eye views of aperture blocks and computed 50% isodose contours for a 2×2×2 mm dose calculation grid. (2) The third table, which included measured densities, improved the accuracy of the calculated isocenter dose by up to 0.5% in typical patient SCRT treatments and up to 1.0% in a phantom with 5‐cm diameter inhomogeneity inserts. (3) The block export and customization process was shown to introduce no additional uncertainty. A 1‐mm block production uncertainty was measured using film dosimetry on six blocks. (4) The DRR transfer method did not introduce uncertainty into the process. Verification field shielding reduced lens dose by 12 to 15 times. In conclusion, this customized system for planning and verification of pediatric SCRT provides a high level of precision as well as reasonable practical efficiency.PACS numbers: 87.53.Kn, 87.53.Ly, 87.53.Oq, 87.53.Tf, 87.53.Uv

521 The use of electronic portal image device (EPID) in the isocenter verification in stereotactic radiosurgery

The biophysical aspects of stereotaxic treatment are summarized, emphasizing the complex interplay between time-dose-fractionation, partial volume effect, physical dose distribution, and set-up uncertainties in influencing treatment outcome. The radiobiological advantage of dose-fractionation for normal tissue sparing is estimated using the linear-quadratic model and viewed in the context of clinical situations and results. Counterbalancing this benefit is the larger geometrical uncertainty associated with multiple fraction treatment, which would require increased treatment volume and an associated decrease in treatment dose. This last consideration requires knowledge of volume-dose relation of radiation-induced normal brain toxicity, the current status of which is also outlined.

343 poster Isocenter verification in fractionated stereotactic radiotherapy with CT image fusion

343 poster Isocenter verification in fractionated stereotactic radiotherapy with CT image fusion

Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame

Background and purpose: Evaluation of set-up accuracy and analysis of target reproducibility in the stereotactic body frame (SBF), designed by Blomgren and Lax from Karolinska Hospital, Stockholm. Different types of targets were analyzed for the risk of target deviation. The correlation of target deviation to bony structures was analyzed to evaluate the value of bones as reference structures for isocenter verification. Materials and methods: Thirty patients with 32 targets were treated in the SBF for primary or metastatic peripheral lung cancer, liver metastases, abdominal and pelvic tumor recurrences or bone metastases. Set-up accuracy and target mobility were evaluated by CT-simulation and port films. The contours of the target at isocenter level, bony structures and body outline were compared by matching the CT-slices for treatment planning and simulation using the stereotactic coordinates of the SBF as external reference system. The matching procedure was performed by using a 3D treatment planning program. Results: Set-up accuracy represented by bony structures revealed standard deviations (SD) of 3.5 mm in longitudinal, 2.2 mm in anterior–posterior and 3.9 mm in lateral directions. Target reproducibility showed a SD of 4.4 mm in longitudinal, 3.4 mm ap and 3.3 mm in lateral direction prior to correction. Correlation of target deviation to bones ranged from 33% (soft tissue targets) to 100% (bones). Conclusion: A security margin of 5 mm for PTV definition is sufficient, if CT simulation is performed prior to each treatment to correct larger target deviations or set-up errors. Isocenter verification relative to bony structures is only safe for bony targets but not for soft tissue targets.

Portal Imaging Protocol for Radical Dose-Escalated Radiotherapy Treatment of Prostate Cancer

Purpose: The use of escalated radiation doses to improve local control in conformal radiotherapy of prostatic cancer is becoming the focus of many centers. There are, however, increased side effects associated with increased radiotherapy doses that are believed to be dependent on the volume of normal tissue irradiated. For this reason, accurate patient positioning, CT planning with 3D reconstruction of volumes of interest, clear definition of treatment margins and verification of treatment fields are necessary components of the quality control for these procedures. In this study electronic portal images are used to (a) evaluate the magnitude and effect of the setup errors encountered in patient positioning techniques, and (b) verify the multileaf collimator (MLC) field patterns for each of the treatment fields. Methods and Materials: The Phase I volume, with a planning target volume (PTV) composed of the gross tumour volume (GTV) plus a 1.5 cm margin is treated conformally with a three-field plan (usually an anterior field and two lateral or oblique fields). A Phase II, with no margin around the GTV, is treated using two lateral and four oblique fields. Portal images are acquired and compared to digitally reconstructed radiographs (DRR) and/or simulator films during Phase I to assess the systematic (CT planning or simulator to treatment error) and the daily random errors. The match results from these images are used to correct for the systematic errors, if necessary, and to monitor the time trends and effectiveness of patient imobilization systems used during the Phase I treatment course. For the Phase II, portal images of an anterior and lateral field (larger than the treatment fields) matched to DRRs (or simulator images) are used to verify the isocenter position 1 week before start of Phase II. The Portal images are acquired for all the treatment fields on the first day to verify the MLC field patterns and archived for records. The final distribution of the setup errors was used to calculate modified dose–volume histograms (DVHs). This procedure was carried out on 36 prostate cancer patients, 12 with vacuum-molded (VacFix) bags for immobilization and 24 with no immobilization. Results: The systematic errors can be visualized and corrected for before the doses are increased above the conventional levels. The requirement for correction of these errors (e.g., 2.5 mm AP shift) was demonstrated, using DVHs, in the observed 10% increase in rectal volume receiving at least 60 Gy. The random (daily) errors observed showed the need for patient fixation devices when treating with reduced margins. The percentage of fields with displacements of ≤5.0 mm increased from 82 to 96% with the use of VacFix bags. The rotation of the pelvis is also minimized when the bags are used, with over 95% of the fields with rotations of ≤2.0° compared to 85% without. Currently, a combination of VacFix and thermoplastic casts is being investigated. Conclusion: The systematic errors can easily be identified and corrected for in the early stages of the Phase I treatment course. The time trends observed during the course of Phase I in conjunction with the isocenter verification at the start of Phase II give good prediction of the accuracy of the setup during Phase II, where visibility of identifiable structures is reduced in the small fields. The acquisition and inspection of the portal images for the small Phase II fields has been found to be an effective way of keeping a record of the MLC field patterns used. Incorporation of the distribution of the setup errors into the planning system also gives a clearer picture of how the prescribed dose was delivered. This information can be useful in dose–escalation studies in determining the relationship between the local control or morbidity rates and prescribed dose.