Couch Velocity Research Articles

Characteristics and clinical application of the TomoDose diode array for quality control of Tomotherapy

Objective To evaluate the dosimetric characteristics of the TomoDose diode and its application in detecting the couch velocity and assessing the beam Profile stability for convenient and efficient quality control of Tomotherapy. Methods Fundamental properties of the TomoDose detector, such as dose linearity, dose per pulse dependence, directional dependence and field size dependence were tested by the water-equivalent phantom and the results were statistically compared with those measured by the ion-chamber (0.057 cm3, A1SL). Five different couch velocities with a time interval of 0.0005 s were designed to test the sensitivity of TomoDose for detecting the couch velocity, and then the clinical data of 7 patients with different couch velocities were selected to verify the test accuracy. The beam profile of three different jaws of 1.0, 2.5 and 5.0 cm were measured using TomoDose with water-equivalent phantom in the x and y directions under the same conditions as water tank with an underwater depth of 15, 50, 100, 150 and 200 mm, respectively. The Profile data obtained by water tank and TomoDose were evaluated using the Gamma (γ) evaluation method with the 2%/1 mm criterion. Results The dose linearity of the TomoDose and ion chamber was in a linear pattern within the beam-on-time≤30 s. The difference between the TomoDose and ion chamber was less than 2% for the dose per pulse dependence, and the response trends of them were consistent. TomoDose showed angular response dependence with a maximum difference of 2.53% for the gantry angle of ±60°.For the field size dependence, the response difference between the TomoDose and ion chamber was increased with the decreasing field size when the Jaw was 5.0 cm, and the maximum deviation was 0.78% when the field size was 5.0 cm×2.5 cm. TomoDose could detect the couch distance error of 0.5 mm, and determine the couch velocity deviation of 200 mm), the value of γ was larger and even with γ>1.In the y direction, all comparison results of Profile under three field width demonstrated that γ index was larger on the edge of beam, whereas γ<1 was found in all cases. Conclusions TomoDose is suitable f or the quality control of Tomotherapy, which can accurately measure the couch velocity, precisely monitor the stability of beam Profile of Tomotherapy and complete the quality control process in a convenient and efficient manner. Key words: Semiconductor detector; Couch velocity; Profile; Tomotherapy

Multi-institutional Feasibility Study of a Fast Patient Localization Method in Total Marrow Irradiation With Helical Tomotherapy: A Global Health Initiative by the International Consortium of Total Marrow Irradiation

To develop, characterize, and implement a fast patient localization method for total marrow irradiation. Topographic images were acquired using megavoltage computed tomography (MVCT) detector data by delivering static orthogonal beams while the couch traversed through the gantry. Geometric and detector response corrections were performed to generate a megavoltage topogram (MVtopo). We also generated kilovoltage topograms (kVtopo) from the projection data of 3-dimensional CT images to reproduce the same geometry as helical tomotherapy. The MVtopo imaging dose and the optimal image acquisition parameters were investigated. A multi-institutional phantom study was performed to verify the image registration uncertainty. Forty-five MVtopo images were acquired and analyzed with in-house image registration software. The smallest jaw size (front and backup jaws of 0) provided the best image contrast and longitudinal resolution. Couch velocity did not affect the image quality or geometric accuracy. The MVtopo dose was less than the MVCT dose. The image registration uncertainty from the multi-institutional study was within 2.8 mm. In patient localization, the differences in calculated couch shift between the registration with MVtopo-kVtopo and MVCT-kVCT images in lateral, cranial-caudal, and vertical directions were 2.2 ± 1.7 mm, 2.6 ± 1.4 mm, and 2.7 ± 1.1 mm, respectively. The imaging time in MVtopo acquisition at the couch speed of 3 cm/s was <1 minute, compared with ≥15 minutes in MVCT for all patients. Whole-body MVtopo imaging could be an effective alternative to time-consuming MVCT for total marrow irradiation patient localization.

SU‐E‐J‐25: Analysis of Commercial 4DCT Flaws and the Potential Benefits of a New Technique for Irregular Breathing Patients

Purpose:To determine the distribution of inhalation to exhalation breathing amplitudes as measured using a commercial 4DCT and a proposed 4DCT protocol for irregular breathing patients.Methods:External breathing surrogate signals correlating with lung tidal volume were collected using an abdominal bellows for a set of 50 patients. Breathing traces over 5‐minute durations were evaluated and the peak inhalation and exhalation peaks identified. An arbitrary time point was identified and the nearest exhalation peak located within a ±180° gantry rotation period. The times at which the follow‐up inhalation scans occurred were correlated with the couch velocity to determine whether the tumor would be within 180° of that specific peak inhalation given the CT scanner coverage, pitch, rotation period, tumor size, and tumor motion amplitude (defined in units of centimeters per bellows signal). The imaged tumor motion amplitude was based on which inhalation peak would be imaged and the couch location at that time relative to the exhalation peak. This analysis provided a series of measured breathing motion amplitudes, which were compared against the 85th percentile motion, consistent with the proposed 4DCT protocol. Because the 4DCT scans would yield a single amplitude determination for each patient, errors in the amplitude measurement were considered systematic. Therefore, the largest 5% and smallest 5% of the measured amplitudes were selected to identify over‐and under‐determined motion amplitudes.Results:30% of the patients exhibited breathing amplitude overestimations of at least 27%, while 30% showed underestimations of at least 34%.Conclusion:For the majority of patients, the current system provided a high probability of correct breathing motion amplitude measurements. 30% had a significant chance of a systematic over‐ or under‐estimation of tumor motion. The proposed protocol will be immune to such irregularities because the entire breathing trace is used for breathing motion amplitude assessments.

SU‐E‐J‐38: Fast Patient Localization Method in Total Marrow Irradiation with Helical Tomotherapy

Purpose: Reducing imaging and treatment times is a crucial issue in total marrow irradiation (TMI) with helical tomotherapy. The purpose of this study was to develop the fast patient localization method for TMI patients with helical tomotherapy. Methods: The topographic images were acquired by getting the megavoltage CT (MVCT) after running calibration procedures with the couch traversing through the gantry at static orthogonal beam angles. Geometric and detector response corrections were performed to generate a MV topogram (MVtopo). We also generated kilovoltage topogram (kVtopo) from the projection data of 3D CT images datasets to reproduce the same geometry as MVtopo. Imaging dose and the optimal image acquisition parameters including jaw size, couch velocity, and dose rate were investigated. Geometric uncertainty and image quality of MVtopo were analyzed. In‐house image registration software was developed and verified with a rando phantom. Finally, MVtopo was acquired for a TMI patient. Results: The longitudinal geometric uncertainty was 0 mm, 10.2 mm, and 23.6 mm, in the jaw size of 0.3 cm, 1 cm, and 2.5 cm, respectively. No jaw size dependency in the lateral geometric uncertainty was observed. Couch velocity did not have an effect on the geometric uncertainty. The doses of MVCT and MVtopo with the jaw size of 3 mm and the couch velocity of 0.8cm/sec were 1.5cGy, and 1.7 cGy, respectively. Contrast‐Noise‐ratio, noise, and MTF were improved by using lower dose rate (55MU/min) and smaller jaw size (3 mm). The image registration accuracy with our in‐house software was within 1 mm. In a clinical TMI case, MVtopo with couch speed of 1.0cm/s reduced imaging time by 80% compared with MVCT. The couch shifts derived from KVCT with MVCT registration, and kVtopo with MVtopo registrations agreed within 4 mm. Conclusion: The MVtopo could be useful for TMI treatment with helical tomotherapy. This work was supported by the National Institute of Health grants (1R01CA154491‐01).This work was also supported by PHS Cancer Center Support Grant P30CA77398.

SU-E-T-173: TomoTherapy Treatment Couch Velocity Verification Using TomoDose.

Accurate TomoTherapy treatment is dependent on multiple parameters, including the accuracy of couch velocity during beam delivery and gantry rotation. The recent TG-148 protocol recommends quarterly film tests to verify the constancy of couch velocity. We determined the feasibility of using TomoDose, a quality assurance device, to detect the variation in couch velocity from baseline by a deviation of one or two percent. The TomoDose unit was rotated 90 degrees clockwise so that the long axis, consisting of 107 detectors, would translate across the TomoTherapy 'Y' jaws. The collimator field size used was the 1 cm mode and the table velocity was set to 0.5 mm/sec to establish a baseline. The velocity was then varied by 1% or 2% from nominal. The five tested velocities were 0.495, 0.490, 0.500, 0.505 and 0.510 mm/sec. The total translation time was kept at a constant 400 sec. The field was progressively painted during the scans and was shortened if table velocity was decreased, and lengthened as velocity was increased. The baseline field size was 20.03 cm (400 sec × 0.05 cm/sec). A 2% increase in velocity translated to an increase in the field size to 20.39 cm and a 2% decrease in velocity resulted in a decrease in field size to 19.64 cm. Similarly, a 1% increase in velocity increased the field to 20.18 cm, and a 1% decrease velocity decreased the field to 19.76 cm. An increase or decrease in the couch velocity, for a fixed beam delivery time, resulted in an altered radiation field size. The field size results are approximately as expected for both the baseline scans as well as the modified velocity scans. This study demonstrates that, under these conditions, TomoDose can be used to determine small variations in table velocity as small as 1 %.

A quality assurance tool for helical tomotherapy using a step‐wedge phantom and the on‐board MVCT detector

The purpose of this study was to develop and evaluate filmless quality assurance (QA) tools for helical tomotherapy by using the signals from the on‐board megavoltage computed tomography (MVCT) detector and applying a dedicated step‐wedge phantom. The step‐wedge phantom is a 15 cm long step‐like aluminum block positioned on the couch. The phantom was moved through the slit beam and MVCT detector signals were analyzed. Two QA procedures were developed, with gantry fixed at 0°: 1) step‐wedge procedure: to check beam energy consistency, field width, laser alignment with respect to the virtual isocenter, couch movement, and couch velocity; and 2) completion procedure: to check the accuracy of a field abutment made by the tomotherapy system after a treatment interruption. The procedures were designed as constancy tool and were validated by measurement of deliberately induced variations and comparison with a reference method. Two Hi‐Art II machines were monitored over a period of three years using the step‐wedge procedures. The data acquisition takes 5 minutes. The analysis is fully automated and results are available directly after acquisition. Couch speed deviations up to 2% were induced. The mean absolute difference between expected and measured couch speed was 0.2% ±0.2% (1 standard deviation SD). Field width was varied around the 10 mm nominal size, between 9.7 and 11.1 mm, in steps of 0.2 mm. Mean difference between the step‐wedge analysis and the reference method was <0.01 mm ±0.03 mm (1 SD). Laser (mis)alignment relative to a reference situation was detected with 0.3 mm precision (1SD). The step‐wedge profile was fitted to a PDD in water. The PDD ratio D20/D10, measured at depths of 20 cm and 10 cm, was used to check beam energy consistency. Beam energy variations were induced. Mean difference between step‐wedge and water PDD ratios was 0.2% ±0.3% (1SD). The completion procedure was able to reveal abutment mismatches with a mean error of ‐0.6 mm ±0.2 mm (1SD). The trending data over a period of three years showed a mean deviation of 0.4% ±0.1% (1 SD) in couch speed. The spread in field width was 0.15 mm (1 SD). The sagittal and transverse lasers showed a variation of 0.5 mm (1 SD). Beam energy varied 1.0% (1 SD). A mean abutment mismatch was found of −0.4 mm ±0.2 mm (1 SD) between interrupted treatments. The on‐board MVCT detector, in combination with the step‐wedge phantom, is a suitable tool for a QA program for helical tomotherapy. The method allowed frequent monitoring of machine behavior for the past three years.PACS number: 87.55.Qr

Development of cylindrical stepwedge phantom for routine quality controls of a helical tomotherapy machine

The aim of this study was to design a cylindrical stepwedge phantom and an appropriate treatment procedure, based on which parameters of tomotherapy machine and generated beam of radiation will be defined. The accuracy of parameter determination, which can be defined with the aid of the measurement system, was also evaluated.The cylindrical phantom that we developed and manufactured (stepwedge phantom) consists of four cylinders with different diameters made of polycaprolactam-PA-6, i.e. material with high mechanical strength, low water absorption (making measurements repeatable) and a density comparable to that of human soft tissues. The appropriate treatment procedure is carried out in a dynamic mode, which is focused on specific properties of the tomotherapy machine. It means that a phantom situated on the couch moves to the inside of the rotating linear accelerator.A total of 18 procedures were implemented in order to calculate the following parameters: couch velocity, dose rate value at a depth, Dose Ratio coefficients, dose variation (so-called Dose Flatness) coefficients, and the time of gantry rotation. Reference intervals for these parameters were determined to be as follows: for the couch velocity: ±1.2%, the average dose rate measured at depth: ±1.8%, the calculated values of the coefficients Dose Ratio: ±0.5% and Dose Flatness: (0.53–0.65)%, the time of gantry rotation: ±3%.The final results showed that during a single irradiation procedure, which lasts 5 min, the cylindrical stepwedge phantom allows to precisely determine the values of the above-mentioned parameters. Its use in the daily dosimetric measurements can ensure better control of the work of the tomotherapy machine.

SU-GG-T-108: Optimal Management of Respiratory Motion without Motion Management: A Tomotherapy Phantom Demonstration

Purpose: To demonstrate a potential alternative scenario for accurate dose painting (non-homogeneous planned dose) delivery at the 1 cm beam width with helical tomotherapy (HT) in the presence of 1 cm, three dimensional, intra-fraction respiratory motion, but without any active motion management. Method and Materials: A model dose painting experiment was planned and delivered to the average position (proper phase of a 4DCT scan) with three spherical PTV levels to approximate dose painting to compensate for hypothetical hypoxia in a model lung tumor. Realistic but regular motion was produced with the Washington University 4D Motion Phantom. A small spherical Virtual Water™ phantom was used to simulate a moving lung tumor inside of the LUNGMAN™ anthropomorphic chest phantom to simulate realistic heterogeneity uncertainties. A piece of 4 cm Gafchromatic EBT™ film was inserted into the 6 cm diameter sphere. The TomoTherapy, Inc. DQA™ software was used to verify the delivery performed on a TomoTherapy Hi-Art II™ device. Results: The dose uncertainty in the purposeful absence of motion management and in the absence of large, low frequency drifts (periods greater than the beam width divided by the couch velocity) or randomness in the breathing displacement yields very favorable results. Instead of interference effects, only small blurring is observed because of the averaging of many breathing cycles and beamlets and no other frequency coherence to the modulation, likely in most cases. Conclusion: Dose painting during respiration with helical tomotherapy is feasible in certain situations without motion management. A simple recommendation is to make respiration as regular as possible without low frequency drifting. The blurring is just small enough to suggest that it may be acceptable to deliver without motion management in many situations when registered to the average position.

A phantom model demonstration of tomotherapy dose painting delivery, including managed respiratory motion without motion management

The aim of the study was to demonstrate a potential alternative scenario for accurate dose-painting (non-homogeneous planned dose) delivery at 1 cm beam width with helical tomotherapy (HT) in the presence of 1 cm, three-dimensional, intra-fraction respiratory motion, but without any active motion management. A model dose-painting experiment was planned and delivered to the average position (proper phase of a 4DCT scan) with three spherical PTV levels to approximate dose painting to compensate for hypothetical hypoxia in a model lung tumor. Realistic but regular motion was produced with the Washington University 4D Motion Phantom. A small spherical Virtual Water™ phantom was used to simulate a moving lung tumor inside of the LUNGMAN™ anthropomorphic chest phantom to simulate realistic heterogeneity uncertainties. A piece of 4 cm Gafchromic EBT™ film was inserted into the 6 cm diameter sphere. TomoTherapy, Inc., DQA™ software was used to verify the delivery performed on a TomoTherapy Hi-Art II™ device. The dose uncertainty in the purposeful absence of motion management and in the absence of large, low frequency drifts (periods greater than the beam width divided by the couch velocity) or randomness in the breathing displacement yields very favorable results. Instead of interference effects, only small blurring is observed because of the averaging of many breathing cycles and beamlets and the avoidance of interference. Dose painting during respiration with helical tomotherapy is feasible in certain situations without motion management. A simple recommendation is to make respiration as regular as possible without low frequency drifting. The blurring is just small enough to suggest that it may be acceptable to deliver without motion management if the motion is equal to the beam width or smaller (at respiration frequencies) when registered to the average position.

TU‐EE‐A3‐01: Breathing‐Motion Induced Tomotherapy Dose Delivery Errors in the Presence of Beam Modulation

Purpose: To determine breathing‐motion induced Tomotherapy dose delivery errors in the presence of beam modulation. Method and Materials: Previous studies have shown that dose delivery errors are possible due to breathing motion. While these studies have systematically investigated the roles of field size, couch velocity, and breathing motion magnitude on the dose errors, they have neglected the role of intensity modulation. This study used measured breathing patterns and clinically realistic delivery parameters to simulate the breathing‐motion induced errors. Modulation was included by varying the simulated delivered dose on two timescales; the short timescale simulating the 51 angular subsets that subdivide the delivered fluence patterns. A step function was used that varied the leaves open and close time every 0.4 seconds, or every 7.0°, simulating a modulation factor of 2.0. Modulation was also conducted on a longer timescale corresponding to four beam intensity directions (two of high dose and two of low dose). The earlier studies using 52 patients' breathing patterns were repeated with the addition of fluence modulation. Results: The impact of breathing motion on Tomotherapy delivery results in delivery errors of greater than 10%, even for relatively small breathing motions. This is due to the subtle variation in the breathing patterns, including changes in breathing waveform and drifting. The addition of fluence modulation varied the delivered dose patterns slightly, but the magnitude of dose delivery errors was unchanged. Conclusion: These results indicate that previous simulations indicating the challenges of using Tomotherapy dose delivery due to breathing motion are valid in the presence of beam modulation. Treatment planners should take care when planning treatments for mobile tumors.This work supported in part by Tomotherapy.

TH-D-352-03: Quality Assurance Procedures for Helical Tomotherapy

Purpose: To develop simple, accurate, and preferably filmless Quality Assurance (QA) procedures for Helical Tomotherapy, by using the signals from the systems' built‐in MegaVolt CT‐detector (MVCT). Method and Materials: We developed three new QA procedures, all using data files imported from the Tomotherapy machines, containing 85 control signals and signals from all 640 MVCT channels, at a sample rate of 30Hz. An important role in this QA program is taken by the StepWedge, a step‐like aluminium block positioned over the couch end in the gantry. 1) StepWedge procedure: to check laser alignment, couch movement and velocity, beam energy and field width. This 5min procedure is performed with a static beam, while couch and StepWedge move 20cm into the gantry. 2) Completion procedure: to check the correctness of a completion made by Tomotherapy software after an interruption. This procedure is based on the StepWedge procedure, interrupted by the user and completed by Tomotherapy software, of which the correctness is tested. 3) MLC‐Gantry‐Synchrony procedure: to check synchronization of MLC and gantry rotation. During this procedure the couch is static while the gantry rotates 40× 20s, opening the middle two leaves at gantry angles 0°, 120° and 240°. Results: From the StepWedge procedure the position of the transversal and sagittal lasers can be checked with an accuracy of ∼0.5mm, the field width with an accuracy of ∼0.5mm, beam energy consistency with an accuracy of ∼1%, and couch movement and velocity with an accuracy of ∼0.1mm resp. ∼0.5%. The accuracy of the Completion procedure, in terms of a possible additional couch shift, is ∼0.1mm. MLC synchronization with gantry rotation can be checked with an accuracy of ∼0.1°. Conclusion: The StepWedge combined with MVCT offer a fast, accurate and filmless QA program, allowing monitoring of almost all relevant parameters of Helical Tomotherapy.

SU-FF-T-294: Modeling of Tomotherapy Delivery to Predict Dose Errors Due to Tumor Motion in Lung

Purpose: To develop an approximate model to the convolution technique simulations to predict tomotherapy dose delivery errors in the presence of respiratory motion using one-dimensional breathing motion and unmodulated delivery models. Catalog the “space” of safe treatments as defined by the model. Method: The patient breathing motion data, acquired as a function of tidal volume (ml) is converted to distance (mm) with time. Moving-average of the motion data is determined where the average is taken over the time span of time taken for a point at the edge of the field to cross other edge with constant couch velocity. The time axis is converted into distance by multiplying by the couch speed. Moving difference of the moving-average of the motion data is determined over the same time span to determine the drift of the point during the treatment. The dose error ratio is then found by dividing the drift by the field size. Results: Dose error ratio for combination of delivery parameters (couch speed, field size) and breathing magnitude for 42 clinical patient motions was studied. The dose error prediction by this approximate model is compared with convolution technique simulation is presented for different delivery scenarios for two different field size. Conclusion: The results show the proposed approximate model predicts the dose errors ratio reasonably close to convolution dose error prediction approach for clinically used tomotherapy delivery parameters. The approximate prediction model breaks down for extreme conditions of breathing magnitude and couch velocity. It will allow the treatment planner to determine if Tomotherapy treatment delivery is appropriate for specific patients whose tumors move due to breathing.

Modelling simple helically delivered dose distributions**This work was funded by NIH grant P01 CA088960.

In a previous paper, we described quality assurance procedures for Hi-Art helical tomotherapy machines. Here, we develop further some ideas discussed briefly in that paper. Simple helically generated dose distributions are modelled, and relationships between these dose distributions and underlying characteristics of Hi-Art treatment systems are elucidated. In particular, we describe the dependence of dose levels along the central axis of a cylinder aligned coaxially with a Hi-Art machine on fan beam width, couch velocity and helical delivery lengths. The impact on these dose levels of angular variations in gantry speed or output per linear accelerator pulse is also explored.

A translational couch technique for total body irradiation

We have constructed a computer controlled translational couch to administer total body irradiation reproducibly and safely. The system has replaced the previous stationary anterior-posterior technique in our institution and 30 plus patients have been treated with it so far. In this technique, patients comfortably lie on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in an upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry, and machine dose rate. In our design, the couch velocity is continuously updated to compensate for machine dose rate fluctuations. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents a special challenge for dosimetry calculations. A dosimetry parameter is introduced that converts the moving beam output to the fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been developed that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Multiple point dose measurements within a phantom confirmed the validity of the calculation method. PACS number(s): 87.53.–j, 87.66.–a

A translational couch technique for total body irradiation.

We have constructed a computer controlled translational couch to administer total body irradiation reproducibly and safely. The system has replaced the previous stationary anterior‐posterior technique in our institution and 30 plus patients have been treated with it so far. In this technique, patients comfortably lie on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in an upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry, and machine dose rate. In our design, the couch velocity is continuously updated to compensate for machine dose rate fluctuations. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents a special challenge for dosimetry calculations. A dosimetry parameter is introduced that converts the moving beam output to the fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been developed that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Multiple point dose measurements within a phantom confirmed the validity of the calculation method.PACS number(s): 87.53.–j, 87.66.–a

A translating-bed technique for total-body irradiation

Total-body irradiation (TBI) is a therapy modality that is being used with increasing frequency, in conjunction with chemotherapy, for patients undergoing bone-marrow transplantation. At the Ottawa Regional Cancer centre a technique has been developed for the delivery of TBI to patients prior to bone-marrow transplantation. In this technique patients are treated on a mobile couch at approximately 195 cm SSD with a field size of 66.5 cm wide by 57 cm long. A computer-controlled stepping motor drives the patient couch at a user-selectable speed. The total dose delivered to the patient is a function of couch velocity, field size and patient separation. Treatment times are of the order of 10 min for each of the anterior and posterior fields for a 400 cGy fraction. It has been found that the conventional central axis tissue maximum ratio (TMR) and percentage depth dose (PDD) functions are not appropriate for describing dose delivered during dynamic treatment. To this end the authors have developed dynamic TMR and PDD functions. Extensive measurements have been performed in an anthropomorphic water phantom to determine the dose distributions in three dimensions and the efficacy of polymethyl methacrylate (PMMA) beam spoilers as a replacement for anterior and lateral bolus. It has been found that 2.4 cm PMMA spoilers do provide full skin dose and negate the requirement for lateral bolus. This TBI procedure is simple, rapid and appears to be well tolerated by the patients. 55 patients have been treated since the introduction of this technique in 1991.