A head immobilization system for radiation simulation, CT, MRI, and PET imaging

An aquaplast mask/marker immobilization system for the routine radiation therapy treatment of head and neck disease is described. The system utilizes a commercially available thermoplastic mesh indexed and mounted to a rigid frame attached to the therapy couch. The apparatus is designed to permit CT, MRI, and PET diagnostic scans of the patient to be performed in the simulation and treatment position utilizing the same mask, thereby facilitating image correlation. Studies employing weekly simulation indicate that patient treatment position movement can be restricted to 3 mm over the course of treatment. This easily constructed system permits rapid mask formation to be performed on the treatment simulator, resulting in an immobilization device comparable to masks produced with vacuum-forming techniques. Details of construction, verification, and central axis CT, MRI, PET markers are offered.     

Erste Erfahrungen mit einem nichtinvasiven Patientenfixieungssystem für die stereotaktische Strahlentherapie der Prostata

PURPOSE: Highly conformal radiotherapy techniques require precise patient positioning. We report our first experience with a new cast system for fixation of the pelvis during stereotactically guided intensity modulated radiotherapy (IMRT) of the prostate with respect to positioning accuracy of the prostate. MATERIAL AND METHODS: The immobilization device consists of a custom-made wrap-around body cast that extends from the abdomen to the thighs and a separate head mask, both made from Scotchcast, and attaches to a frame for extracranial stereotaxy. Sixteen CT-studies (> or = 25 slices, thickness: 3 mm) of 2 patients who were immobilized for IMRT of prostate tumors were evaluated with respect to set-up accuracy of bony structures and the prostate itself. CT-studies were performed immediately before or after a treatment fraction. Deviations of bony landmarks and anatomical landmarks inside the planning target volume were measured in all 3 dimensions. RESULTS: Mean patient movements of 0.15 +/- 0.3 mm (latero-lateral), 0.9 +/- 1 mm (anterior-posterior), 1 +/- 1 mm (tranversal vectorial error) and < 3 mm slice thickness (craniocaudal) were recorded using bony landmarks and 0.9 +/- 0.9 mm (latero-lateral), 1.8 +/- 1.5 mm (anterior-posterior), 2.2 +/- 1.5 mm (transversal vectorial error) and < 3 mm (craniocaudal) using the confines of, or landmarks within the prostate. Standard deviations of absolute positioning error as an often used metric for positioning accuracy ranged between 0.3 and 1.7 mm in the transversal plane. The worst case transversal vectorial deviation for the prostate was 4.4 mm. Figure 4 summarizes the set-up accuracy of bony landmarks and the prostate. CONCLUSION: The presented combination of a body cast and head mask system in a rigid stereotactic body frame ensures reliable noninvasive patient fixation for fractionated extracranial stereotactic radiotherapy. It provides precise and reliable positioning of the prostate and meets the requirements for highly conformal radiotherapy such as IMRT. No further improvement of repositioning can be achieved with external immobilization devices since the positioning error of the target relative to the skeleton exceeds the accuracy of the positioning of the skeleton itself.     

Nichtinvasive Kopffixation für externe Bestrahlung von Tumoren im Kopf-Hals-Bereich

Purpose

To fully utilize the technical capabilities of radiation diagnostics and planning, a precise and reproducible method of head fixation is a prerequisite.

Method

We have adapted the Vogele-Bale-Hohner (VBH) head holder (Wellhöfer Dosimetrie, Schwarzenbruck, Germany), originally designed for frameless stereotactic operations, to the requirements of external beam radiotherapy. A precise and reproducible head fixation is attained by an individualized vacuum upper-dental cast which is connected over 2 hydraulic arms to an adjustable head- and rigid base-plate. Radiation field and patient alignment lasers are marked on a relocatable clear PVC localization box.

Results

The possibility of craniocaudal adjustment of the head plate on the base plate allows the system to adapt to the actual position of the patient on the radiotherapy couch granting tensionless repositioning. The VBH head holder has proven itself to be a precise yet practicable method of head fixation. Duration of mouthpiece production and daily repositioning is comparable to that of the thermoplastic mask.

Conclusion

The new head holder is in routine use at our hospital and quite suitable for external beam radiation of patients with tumors of the head and neck.     

A comparative evaluation of two head and neck immobilization devices using electronic portal imaging

A study was performed to compare the positioning reproducibility and the cost efficiency for two head and neck immobilization devices: the Uvex (Uvex Safety, Smithfield, USA) plastic mask system and the Finesse Frame with Ultraplast System (PLANET Medical, Svendborg, Denmark). 20 patients treated with 3D conformal radiation therapy for head and neck cancers were randomly selected (10 for each of the two different immobilization systems) and electronic portal images acquired during their course of treatment were saved and used in this study. The anatomical landmark coordinates and their shifts in the anteroposterior (AP) and craniocaudal (CC) directions with respect to the digitized simulator films for lateral fields were analysed using an in-house developed portal image registration system. Statistically, no evidence was found to indicate that the systematic components of the displacement for the Uvex system and the Finesse Frame with Ultraplast System were different from each other or from zero. The random component of displacement was slightly smaller in the AP direction for the Uvex than the Ultraplast system (sigma = 1.9 mm and 2.9 mm, respectively, p = 0.007), but larger in the CC direction (sigma = 3.8 mm and 2.2 mm, respectively, p<10(-9)). Production time and required materials for a radiation therapy department were also quantified to assess costs for each system. The overall costs per patient were estimated at 141.50 dollars (CAD) and 82.10 dollars for the Uvex and Ultraplast systems, respectively. The Finesse Frame with Ultraplast System of immobilization for head and neck cancer treatment provides a field placement reproducibility that is equal to, or greater than, that of the Uvex plastic mask immobilization system and, while it requires more expensive materials, the workload and consequently overall cost is greatly reduced.     

A simple device for positioning and fixing the head in radiotherapy of brain tumors in frontal and lateral position

In radiotherapy of cerebral tumors in the occipital, central, and parietal area it can be favourable to fix the patient's head in frontal or lateral position. It is true that this problem can be solved by manufacturing individual face masks of PVC with the vacuum deep-drawing method, but this method is very time- and cost-consuming. The present study suggests a simplified method using the thermoplastic polyester "Orfit". Such a mask provides a good immobilization and can be manufactured in 10 to 15 minutes without bothering much the patient. Simulation and computed tomography are not substantially affected by the material.     

Simple and effective immobilization for radiation treatment of choroidal melanoma

At our institution, patients diagnosed with choroidal melanoma requiring external beam radiation therapy are treated with two 6 MV volumetric-modulated arcs delivering 50 Gy over 5 daily fractions. The patient is immobilized using an Orfit head and neck mask and is directed to look at a light emitting diode (LED) during CT simulation and treatment to minimize eye movement. Patient positioning is checked with cone beam computed tomography (CBCT) daily. Translational and rotational displacements greater than 1 mm or 1° off the planned isocenter position are corrected using a Hexapod couch. The aim of this study is to verify that the mask system provides adequate immobilization and to verify our 2-mm planning target volume (PTV) margins are sufficient. Residual displacements provided by pretreatment verification and post-treatment CBCT data sets were used to assess the impact of patient mobility during treatment on the reconstructed delivered dose to the target and organs at risk. The PTV margin calculated using van Herk's method¹ was used to assess patient motion plus other factors that affect treatment position, such as kV-MV isocenter coincidence. Patient position variations were small and were shown to not cause significant dose variations between the planned and reconstructed dose to the target and organs at risk. The PTV margin analysis showed patient translational motion alone required a PTV margin of 1 mm. Given other factors that affect treatment delivery accuracy, a 2-mm PTV margin was shown to be sufficient for treatment of 95% of our patients with 100% of dose delivered to the GTV. The mask immobilization with LED focus is robust and we showed a 2-mm PTV margin is adequate with this technique.     

Clinical implementation of radiosurgery using the Helical TomoTherapy unit

The American Society of Radiation Oncology has recently recommended the use of radiosurgery to manage brain metastases. For such a recommendation to be implemented in a widespread manner, radiosurgery must be accessible at community radiation therapy facilities. The work presented here describes our clinical experience in the implementation of radiosurgery using a Helical TomoTherapy unit. Helical TomoTherapy is a unique dose-delivery system designed to perform intensity-modulated radiation therapy (IMRT). The system built on the ring-based gantry has the tight machine tolerances required for radiosurgery. A frameless system consisting of a thermoplastic mask and a noninvasive “stereotactic radiosurgery (SRS)-stereotactic radiotherapy (SRT)” fixation device is used for patient immobilization. Treatment planning is performed using the TomoHD treatment planning system designed for IMRT. The image-guidance system on the Helical TomoTherapy is used for patient localization. Our clinical experience demonstrated that the radiosurgery procedure can be streamlined as we do for IMRT patients. The treatment time of about 10 minutes is comparable with that for IMRT patients. The same patient-specific quality assurance for IMRT is used for radiosurgery. As demonstrated, SRS using Helical TomoTherapy is not a whole-day event, unlike SRS using other dose-delivery systems or SRS performed in the past.     

Casting of electron field defining apertures: casting with the metal mold kits

Cerrobend alloy casts are made to define the desired electron field shapes. These custom casts are fabricated for the selected electron applicator size that has been chosen for the patient. When the cast is placed into that selected electron applicator, it will block out areas that are not to be treated. When an all metal mold assembly was used for the fabrication of these casts, the lip region of the cast which is used to accurately align the cast in the actual treatment applicator, had an irregular edge that prevented an accurate alignment of the cast. To eliminate the irregular edges on the lip region of the cast, the metal mold assembly was heated to approximately 80-85 degrees C before the molten cerrobend alloy was poured into it. The heating of the metal mold assembly helps eliminate the irregular edges on the lip region of the cast. Unfortunately it also created new flaws such as holes, dents, cracks and/or crystallization of the cast as it solidified. These flaws were controlled by cooling the metal mold assembly and the cast immediately after the pouring of the molten cerrobend alloy, evenly with water.     

Evaluation of fiberglass versus plaster of Paris for immobilization of fractures of the arm and leg

We conducted a prospective randomized study comparing the costs, comfort, and effects on activities of daily living of fractures managed in plaster of Paris (POP) vs. fiberglass (FG) for at least 4 weeks. A total of 183 patients with short arm and short leg casts were evaluated. Twenty-seven patients were excluded, leaving 156 patients in the study. The average cost per fracture immobilized in a short arm cast was $12.90 for POP and $15.45 for FG. For short leg casts, the average cost of immobilization was $49.06 for POP and $47.85 for FG. FG had a significantly lower breakdown rate in short leg casts (17% vs. 66%) but not in short arm casts. In 21 measures of activities of daily living and comfort, FG short arm casts were significantly better than POP in only the weight of the cast (p = 0.01). FG short leg casts were superior to POP in getting dressed, odor, sweating under the cast, work/school accomplishments, and overall fewer restrictions and comfort.     

Retinoblastoma: review of 30 years' experience with external beam radiotherapy

A review of the experience at the Peter MacCallum Cancer Centre (Peter Mac), Melbourne, Australia in treating retinoblastoma with external beam radiotherapy was conducted. Outcomes of particular interest were tumour control, vision preservation and treatment late effects. The review was restricted to patients that had intact eyes treated at Peter Mac from 1965 until 1997 with at least 2 years of follow up. Histories were reviewed regarding patient and tumour characteristics and treatment details. Thirty-five patients were identified in whom 47 eyes were treated. Of the tumours, 47% were Reese-Ellsworth stage IV or V and the majority of others were at high risk for vision loss because of tumour location. The radiation treatment technique became increasingly sophisticated during the study period. Radiation dose and fraction size have similarly evolved but most patients received 30-50 Gy. Since 1989, a highly accurate contact lens immobilization technique has been used to deliver 40 Gy in 20 fractions. Thirteen eyes required additional local therapy. Of the treated eyes, 34 (72%) remain intact and 74% of these have useful vision. One patient died from retinoblastoma and three from second malignant neoplasms. With modern radiotherapy, late toxicities other than growth arrest and non-progressive cataract did not occur during the study period. Tumour control was high and a very acceptable rate of organ and vision preservation was achieved in a relatively high-risk population. Modern radiotherapy continues to develop in an attempt to improve treatment accuracy and minimize late radiation toxicity.     

Plastic bronchitis: large, branching, mucoid bronchial casts in children

Expectoration of large, branching, bronchial casts, termed plastic bronchitis, is an uncommon condition in children. Asthma or allergy often is the cause of cast production, but in some instances no etiology is found. Five children produced large, obstructing bronchial casts that either were expectorated or were extracted at bronchoscopy. Four of the children had asthma or allergies and the fifth had congenital tricuspid atresia and chronic pericardial and pleural effusions. In two patients, expectorated casts initially were thought to be aspirated food material. Radiographic findings during periods of cast formation included atelectasis, obstructive emphysema, bronchiectasis, pleural effusion, and pneumomediastinum. Cast formation may be self-limited but generally ceases with appropriate treatment of the causative disorder. Bronchoscopic extraction of casts may benefit some patients.     

Quality of patient positioning during cerebral tomotherapy irradiation using different mask systems

Background and purpose

Patient immobilization during brain tumor radiotherapy is achieved by employing different mask systems. Two innovative mask systems were developed to minimize the problems of claustrophobic patients. Our aim was to evaluate whether the quality of patient immobilization using the new mask systems was equivalent to the standard mask system currently in use.

Material and methods

Thirty-three patients with cerebral target volumes were irradiated using the Hi-Art II tomotherapy system between 2010 and 2012. Each group of 11 patients was fitted with one of the two new mask systems (Crystal® or Open Face® mask, Orfit) or the standard three-point mask (Raycast®-HP, Orfit) and a total of 557 radiotherapy fractions were evaluated. After positioning was checked by MV-CT, the necessary table adjustments were noted. Data were analyzed by comparing the groups, and safety margins were calculated for nonimage-guided irradiation.

Results

The mean values of the table adjustments were: (a) lateral (mm): ??0.22 (mask 1, standard deviation (σ): 2.15); 1.1 (mask 2, σ: 2.4); ??0.64 (mask 3, σ: 2.9); (b) longitudinal (mm): ??1 (mask 1, σ: 2.57); ??0.5 (mask 2, σ: 4.7); ??1.22 (mask 3, σ: 2.52); (c) vertical (mm): 0.62 (mask 1, σ: 0.63); 1.2 (mask 2, σ: 1.0); 0.57 (mask 3, σ: 0.28); (d) roll: 0.35° (mask 1, σ: 0.75); 0° (mask 2, σ: 0.8); 0.02° (mask 3, σ: 1.12). The outcomes suggest necessary safety margins of 5.49–7.38 mm (lateral), 5.4–6.56 mm (longitudinal), 0.82–3.9 mm (vertical), and 1.93–4.5° (roll). There were no significant differences between the groups.

Conclusions

The new mask systems improve patient comfort while providing consistent patient positioning.     

Is one head and neck immobilization system as good as another? One center's experience.

The William Buckland Radiotherapy Center has used 2 different immobilization systems for patients requiring radiotherapy to the head-and-neck region. A polycarbonate mask was manufactured for radical treatments and a thermoplastic mask for palliative treatments. This study evaluated field placement accuracy, staff opinion, and production costs of both systems. The manual matching program of Varian PortalVision Electronic Portal Imaging (EPI) System was used to assess field placement accuracy on a daily basis. Radiation therapists (RTs) were surveyed before and after the study to determine their opinions of each system. Production time and required materials were recorded to assess cost. Nineteen patients from each system had daily EPI results compiled with no statistically significant difference observed in field placement accuracy. The thermoplastic system was found to be more cost efficient due to a combination of the reduced production time and reuseability of the masks. User acceptability of the thermoplastic system has increased so that it is now the preferred system. In conclusion, the thermoplastic system is a viable alternative to the polycarbonate system in terms of treatment accuracy and cost. It is recommended that the thermoplastic system be used for all radical and palliative treatments. In addition, RTs prefer the thermoplastic system.     

Simulator Verification of the Accuracy of Patient Repositioning after Virtual Simulation

PURPOSE: To evaluate the frequency and amount of displacements after repositioning a patient on the physical simulator following virtual simulation. MATERIAL AND METHODS: After laser marking at the CT scanner and virtual simulation, patients were repositioned on the simulator. The isocenter obtained from the calculated table movements was checked by fluoroscopically measuring the distances to standardized anatomic landmarks and comparing them to the treatment plan. RESULTS: In 86% of patients, displacements were < or = 0.5 cm. There was no significant difference between the supine and prone position, diagnosis categories or CT reconstruction indices. The use of immobilization devices and cranial versus body stem localization did make a significant difference. Rates of exact repositioning were high in brain and head and neck patients and comparatively low in abdominal tumors and breast cancer. CONCLUSIONS: Immobilization devices play an important role for the precision of radiotherapy. Whenever precise positioning is possible (e. g. with a head mask), virtual simulation alone might be sufficient. Patients with abdominal and breast tumors, were repositioning precision is often suboptimal, might profit from an additional physical simulation.     

Immobilization in stereotactic radiotherapy: the head and neck localizer frame.

Stereotactic radiotherapy refers to multiple daily fractions of radiation, over days or weeks of treatment, with the patient in a relocatable stereotactic frame. The linear accelerator-based, couch-mounted system from Radionics utilizes the Gill-Thomas-Cosman (GTC) frame and the new Tarbell-Loeffler-Cosman (TLC) pediatric frame for accurate positioning reproducibility. Radionics has now made available the Head and Neck Localizer (HNL) frame to be used with its XPlan treatment planning system and the mini multileaf collimator (MMLC). This will extend the overall capability of stereotactic radiotherapy to the treatment of head and neck cancers. However, with no data available on the HNL frame, a study is being undertaken to assess the accuracy in patient position reproducibility using the frame. This report provides the preliminary findings of comparing depth-helmet readings with radiographic data, together with recommended modifications to the frame.     

Comparison of the effectiveness of different immobilization systems in different body regions using daily megavoltage CT in helical tomotherapy

Objective:

Effective immobilization is crucial for the accurate delivery of radiotherapy. This study aimed to compare the effectiveness of the commonly used immobilization systems for different body regions using megavoltage CT (MVCT).

Methods:

Daily treatment set-up data from 212 patients treated by helical tomotherapy (Accuray, Sunnyvale, CA) in 6 body regions (52 head and neck, 41 chest, 38 abdomen, 36 pelvis, 18 breast and 27 cranium) were obtained. Based on a verification tool using the pre-treatment MVCT, set-up corrections for each patient were recorded. Mean systematic and random errors of lateral, longitudinal, vertical and roll directions and three-dimensional vectors were compared between immobilization systems of each region.

Results:

Smaller set-up deviations were observed in the Orfit system (Orfit Industries NV, Wijnegem, Belgium) of the head and neck region, while the performance of immobilization systems for the chest, abdomen and pelvis regions was similar. Larger differences were noted in the breast group, where the prone BodyFIX® system (Medical Intelligence, Medizintechnik GmbH, Schwabmünchen, Germany) was less stable than the supine VacLok® system (CIVCO Medical Solutions, Orange City, IA).

Conclusion:

Differences were found between the immobilization systems in the head and neck region, in which the Orfit system was relatively more effective, whereas the VacLok and BodyFIX systems performed similarly in the chest, abdomen and pelvis regions. For the breast case, the supine position with VacLok was much more stable than the prone breast technique. The results provided references for the estimation of clinical target volume–planning target volume margins.

Advances in knowledge:

This is the first article on comprehensive comparisons performed in immobilization systems for main body regions that provides some practical recommendations.The goal of radiotherapy is to maximize tumour control and minimize complications in the surrounding normal tissue. The success of radiotherapy depends mostly on the accuracy and the reproducibility of daily treatment delivery.^1,2 Many studies have shown that an effective immobilization system can reduce positioning variations^3–6 and improve the outcome of radiotherapy treatment. With the introduction of many commercially available immobilization accessories, oncology departments have developed their own immobilization systems for specific cancer patients and radiotherapy techniques based on their available resources and treatment protocols. As many choices of immobilization devices are now available in the market, more than one immobilization system may have been developed for the treatment of a specific disease. It is the interest of the clinicians to understand the effectiveness of individual immobilization devices and come up with an optimal system that offers the least set-up deviation during treatment.Because of this, many studies have been performed in the past to investigate the effectiveness of different immobilization systems, most of which used portal imaging as the verification method. Since portal images can only provide two-dimensional (2D) information, it is difficult to detect rotational set-up errors. The recent integration of CT in radiotherapy treatment machines has solved this problem and provides a three-dimensional (3D) verification application. Such systems include the integration of cone beam CT in a linear accelerator and the megavoltage (MV) CT in helical tomotherapy (Accuray, Sunnyvale, CA). Many previous studies, including the one by Li et al,⁷ have already demonstrated that the 3D approach in cone beam CT was superior to the 2D radiographic portal images used in the verification of patients treated at the head and neck region.MVCT is inherent in the helical tomotherapy unit, which is used for the daily set-up and verification of the patient position. Such a system provides more detailed set-up data and is more reliable in assessing positional deviations compared with the portal imaging method. Furthermore, the data collected from the MVCT verification system can be used to generate the systematic and random errors of each treatment, which are useful for the evaluation of set-up accuracy. The systematic error is the average error over all treatment fractions, whereas the random error is the average magnitude of errors that are expected to be distributed as a gaussian function about a mean.The aim of this study was to evaluate the effectiveness of different immobilization systems by assessing the systematic and random errors generated from MVCT data of helical tomotherapy for different body regions. The results would provide reference information for the choice of immobilization devices and establish an optimal system for specific treatment conditions, as it is expected that an optimal immobilization system would lead to more accurate treatment and better treatment outcomes.     

Cancer of the oropharynx developed after radiotherapy and chemotherapy for Hodgkin's disease--a case report.

A case of oropharyngeal squamous cell cancer occurring in the radiation field for Hodgkin's disease is reported. The second cancer was diagnosed six years and one month after the patient received 40 Gy/25 fractions. The patient also received salvage chemotherapy two years and six months after the primary radiotherapy. In a review of the world literature, we found 22 cases of head and neck cancer excluding the thyroid gland occurring after radiotherapy alone or radiotherapy combined with chemotherapy for Hodgkin's disease. Although second cancers in the head and neck area after Hodgkin's disease have rarely been reported, those patients cured of the disease should be followed up carefully for a long period of time.     

Thermoplastic Patient Fixation

BACKGROUND AND PURPOSE: Several methods have been developed to reduce tumor motions and patient movements during radiotherapy of lung cancer. In this study, a multislice CT-based analysis was performed to examine the effect of a thermoplastic patient immobilization system on the chest wall and tumor motions. PATIENTS AND METHODS: Ten patients with stage II-IV lung cancer were enrolled into the study. According to tumor localization, five patients had peripheral, and five patients central lung cancer (T2-T4). In total, six series of measurements were made with a multislice CT scanner, both with and without mask fixation, in normal breathing, at maximal tidal volume inhalation, and at maximal tidal volume exhalation. RESULTS: Movements of chest wall, diaphragm and tumor, with and without mask, under different breathing conditions were registered. With the use of the immobilization system, no significant difference was found in diaphragmatic movements (mean deviation of diaphragm: 41.7-40.5 mm to the right, and 40.5-36.8 mm to the left side) and in tumor motions (mean deviation of tumor: 15.3-12.4 mm in craniocaudal, and 11.5-8.8 mm in posterolateral direction, mean medial deviation: 4.6-4.1 mm, mean lateral deviation: 7.2-5 mm). Significant differences were observed concerning tumor motions in anteroposterior direction (mean: 8.9-6.3 mm) and transverse chest movements in anteroposterior direction. CONCLUSION: Besides the advantage of optimal patient positioning, the movements of the bony chest wall can be considerably reduced by using the immobilization system. However, this fixation system has limitations concerning its suitability for minimizing tumor motions.     

Experience in the use of face masks for radiotherapy of head and neck tumors

By means of positioning and fixation aids, the precision and reproducibility of irradiation fields in radiotherapy of malignant tumors of the head and neck can be considerably improved. Face masks made of different synthetic materials have proved to be a practicable solution of this problem. In our hospital we have developed and tested a simple and not expensive possibility of manufacturing the masks with "Baycast" (producer: Bayer AG Leverkusen). The material is generally well tolerated by the patients, and the head is sufficiently fixed. An increased incidence of radiogenic dermatitides is caused by the overlapping of the depth dose of the Co-60 gamma radiation due to additional secondary electrons emanating from the mask material. This effect can be partly prevented by cutting out the irradiation fields in the masks.     

Where is the light field edge: perception of different operators on different surfaces

On most radiotherapy treatment units, a light field indicates on the patient’s skin where the treatment field will irradiate the patient. It was the aim of the present study to investigate the perception of the light field edge by different operators on different surfaces under different lighting conditions. Ten radiation therapists and physicists were asked to mark the light field edge of an 8 × 10-cm² radiation field from a linear accelerator on prepacked radiographic film. Each operator marked the field 4 times each with the room light turned on and dimmed as usual for patient setup. Two operators marked the field on 5 different surfaces (film envelope, brown solid water^R, clear plastic used for the manufacturing of immobilization shells, black rubber, and Orfit^R patient immobilization material). The interoperator reproducibility (± 0.39 mm, 1SD) was larger than the intraoperator reproducibility (± 0.27 mm). The light field was judged consistently to be 0.6 mm smaller in the light room than under dimmed light conditions and the physicists judged the field to be approximately 0.4 mm smaller compared to the radiation therapists’ judgement. Compared to the yellow film wrapping, the light field on solid water, black rubber, and the clear plastic were judged to be 0.6 mm smaller by both operators. The same observation was made using a slotted block tray, which also gave the worst reproducibility of perceived field edges. While these systematic errors are relatively small and difficult to correct for, it appears to be important to be at least aware of them, in particular if the light field is used to junction radiation fields with steep penumbras, as commonly done in megavoltage treatments of head & neck and breast cancer.