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ed stereotactic extracranial radiotherapy. Material and Methods: To form the actual vacuum mattress, the patient is pressed into the mattress with a vacuum foil ...

Strahlentherapie und Onkologie

Original Article

Reproducibility of Patient Positioning for Fractionated Extracranial Stereotactic Radiotherapy Using a DoubleVacuum Technique Meinhard Nevinny-Stickel1, Reinhart A. Sweeney1, Reto J. Bale2, Andrea Posch1, Thomas Auberger1, Peter Lukas1

Background and Purpose: Precise reproducible patient positioning is a prerequisite for conformal fractionated radiotherapy. A fixation system based on double-vacuum technology is presented which can be used for conventional as well as hypofractionated stereotactic extracranial radiotherapy. Material and Methods: To form the actual vacuum mattress, the patient is pressed into the mattress with a vacuum foil which can also be used for daily repositioning and fixation. A stereotactic frame can be positioned over the region of interest on an indexed base plate. Repositioning accuracy was determined by comparing daily, pretreatment, orthogonal portal images to the respective digitally reconstructed radiographs (DRRs) in ten patients with abdominal and pelvic lesions receiving extracranial fractionated (stereotactic) radiotherapy. The three-dimensional (3-D) vectors and 95% confidence intervals (CI) were calculated from the respective deviations in the three axes. Time required for initial mold production and daily repositioning was also determined. Results: The mean 3-D repositioning error (187 fractions) was 2.5 ± 1.1 mm. The largest single deviation (10 mm) was observed in a patient treated in prone position. Mold production took an average of 15 min (10–30 min). Repositioning times are not necessarily longer than using no positioning aid at all. Conclusion: The presented fixation system allows reliable, flexible and efficient patient positioning for extracranial stereotactic radiotherapy. Key Words: Body fixation · Immobilization · Repositioning accuracy · Stereotactic radiotherapy Strahlenther Onkol 2004;180:117–22 DOI 10.1007/s00066-004-1146-0 Reproduzierbarkeit der Patientenpositionierung für eine fraktionierte extrakranielle stereotaktische Strahlentherapie unter Verwendung einer Doppelvakuumtechnik Hintergrund und Ziel: Voraussetzung für eine konformale fraktionierte Strahlentherapie ist eine präzise reproduzierbare Positionierung des Patienten und des Zielvolumens. Vorgestellt wird ein auf dem Doppelvakuumprinzip basierendes Fixationssystem, das sowohl für konventionelle als auch extrakranielle stereotaktische Bestrahlungen eingesetzt werden kann. Material und Methodik: Mittels einer Fixationsfolie, mit der er auch zusätzlich fixiert werden kann, wird der Patient vor der Abformung in die Vakuummatratze hineingepresst. Eine exakt auf einer indexierten Bodenplatte positionierbare Plexiglashaube dient als stereotaktischer Rahmen. Bei zehn Patienten mit Zielvolumina im Abdomen und Becken wurden vor jeder Bestrahlung orthogonale digitale Verifikationsaufnahmen angefertigt. Diese wurden mit den digital rekonstruierten Röntgenbildern (DRRs) des dreidimensionalen (3-D) Planungssystems verglichen. Aus den Abweichungen der drei Raumrichtungen wurden die 3-D-Vektoren als Maß für die Repositionierungsgenauigkeit errechnet. Ergebnisse: Der Mittelwert der für alle Patienten errechneten 3-D-Vektoren (187 Fraktionen) betrug 2,5 ± 1,1 mm. Der mit 10 mm größte 3-D-Vektor wurde bei einem in Bauchlage bestrahlten Patienten beobachtet. Die initiale Abformung dauerte im Durchschnitt 15 min (10–30 min). Der tägliche zeitliche Lagerungsaufwand am Bestrahlungsgerät ist nur unwesentlich länger als ohne Fixationshilfe. Schlussfolgerung: Das vorgestellte Fixierungssystem ermöglicht eine zuverlässige, flexible und effiziente Patientenpositionierung für die stereotaktische Bestrahlung extrakranieller Tumoren. Schlüsselwörter: Körperfixation · Stereotaktische Bestrahlung · Extrakranielle Stereotaxie · Lagerungsgenauigkeit · Stereotaxie

1 2

Department of Radiotherapy-Radiooncology, and Department of Radiology 1, Stereotactic Interventional Planning Laboratory, University Hospital Innsbruck, Austria.

Received: January 13, 2003; accepted: September 29, 2003

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Nevinny-Stickel M, et al. Double-Vacuum Fixation System in Stereotactic Radiotherapy

Introduction In extracranial stereotactic radiotherapy (ESRT), as in threedimensional (3-D) conformal radiotherapy, a precise method of repositioning and fixation is of paramount importance to ensure dose delivery and to keep the irradiated volume as small as possible. The prerequisites for implementing such a system into clinical routine are a high repositioning accuracy as well as a simple and fast initial production and daily repositioning process. It is obvious, that not only patient throughput but also minimal patient discomfort need to be kept in mind, especially when considering lengthy treatment times. Many positioning and fixation aids, mostly based on body masks [1, 11–14] and/or vacuum [6, 10, 21, 22] or foam [5, 16, 18, 19] mattresses are currently available and in use. Some of these are integrated in a stereotactic frame to decrease uncertainties imposed by skin shift [6, 9–11, 21]. The reproducibility of patient positioning is, in general, directly related to the quality and definition of anatomic mold. This study evaluates an extracranial stereotactic positioning system based on a double vacuum previously described, in principle, for angiography applications [3, 4] and extracranial 3-D navigated brachytherapy [20] in terms of its possible application for ESRT. Both repositioning accuracy of bony landmarks, time required for initial production and daily repositioning as well as patient and staff acceptance in routine clinical practice are analyzed. Material and Methods Production Process The Body-Fix® (Medical Intelligence GmbH, Schwabmünchen, Germany) is an extracranial stereotactic positioning and fixation device based on a doublevacuum sandwich technique (Figure 1). An indexed carbon fiber base plate is centered on the table with self-centering clamps which guarantee correct positioning of the base plate with respect to any table, be it that of CT, simulator or treatment room. The base plate itself is sturdy enough to carry a 150-kg person around. The respective vacuum mattress (available in various sizes and forms) is reproducibly placed on this carbon table via the indexing system. Then, the patient is positioned as desired and the vacuum conductor is placed over or beside the patient (Figure 2a). This perforated hose with a cotton cover (for hygienic purposes) serves to spread the vacuum over a larger surface. The hose is led through the vacuum foil before attaching it to the vacuum supply. The foil is then draped over the patient and pressed

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against the double-sided tape which runs over the full length of the mattress to create a lateral seal. The vacuum supply is then turned on thus evacuating the air between patient and fixation sheet, “sandwiching” the patient between foil and mattress. This vacuum can be adjusted between 40–100 mbar. The fixation foil is inherently difficult to tear, so that, by pulling upward on this foil, the underlying vacuum mattress also gets pulled up and around the patient contours creating a deep and defined impression. At this point, the vacuum mattress can still be manually molded as desired, and special attention should be paid to heel and/or shoulder and head definition in the mattress as these are mainly responsible for craniocaudal positioning accuracy. Patient position can also still be slightly modified (under fluoroscopy, etc.) at this time. Only after having ensured that the stereotactic frame can reproducibly be snapped onto the indexing pegs on the base plate over the region of interest without being impeded by the mattress, is the air evacuated from the mattress, creating a rigid mold (Figures 2a and 2b). Planning CT The mold can be created either directly preceding the planning CT or in a separate room. The planning CT protocol was 5 mm slice thickness acquired in spiral mode. Treatment planning was performed on Precise Plan 1,0 (Elekta Oncology Systems Ltd., Crawley, West Sussex, UK). For each isocenter, an orthogonal (0° and 90°/270°) pair of 10  10 cm positioning fields with their respective beams-eye-view DRRs (digitally

Figure 1. Schematic representation of the double-vacuum technique (1: vacuum mattress and its hose, connected to the vacuum pump; 2: the air between patient and fixation foil is evacuated, sandwiching the patient between the foil and mattress). The three spheres on each side of the frame serve to define the stereotactic coordinates from the planning CT. Abbildung 1. Schematische Darstellung der Doppelvakuumtechnik (1: Vakuummatratze und deren Schlauch; 2: Vakuumschlauch zur Absaugung der Luft zwischen Patient und Fixationsfolie, wodurch der Patient fixiert und in seine Matratze gepresst wird). Die drei kreisförmigen Strukturen in den drei Seiten des Rahmen dienen der Koordinatenbestimmung aus dem Planungs-CT.

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Nevinny-Stickel M, et al. Double-Vacuum Fixation System in Stereotactic Radiotherapy

Figure 2a. Patient fixated in the Body-Fix® by the fixation foil. The stereotactic frame is reproducibly clamped onto the base plate over the region of interest. Abbildung 2a. Im Body-Fix® gelagerter Patient mit Fixationsfolie und reproduzierbar auf die indexierte Grundplatte aufgestecktem stereotaktischen Rahmen.

reconstructed radiographs) were created. X (lateral) and Y (anterior-posterior [AP]) isocenter coordinates were calculated by measuring the distance from the beam axis to the centerrod of the stereotactic frame. Z (craniocaudal) coordinates were measured by multiplying the distance of the diverging two rods by 2 (Figure 1). Repositioning The patient’s vacuum mattress is snapped onto the base plate at the respective level. Then, the patient lies down in the mold. In this study, the fixation foil was used during the initial mold production process but not for the ensuing planning CT and daily fixation, but, depending, e.g., on fractionation schedule and necessity of maximum fixation during treatment, the fixation foil could also be reused for repositioning of that specific patient (for planning CT and treatments). In this case, after laying the patient down comfortably in the mold, the same vacuum conduction hose and foil are used to create the aforementioned “sandwich”, pressing the patient into the vacuum mattress with a pressure of up to 100 mb. Since this pressure is evenly distributed over the body, it is not in the least uncomfortable and can be tolerated for prolonged periods. The stereotactic frame is CT/MRI-compatible with arrowshaped hoses embedded in the anterior and lateral localizer plates (Figure 1) allowing the stereotactic isocenter coordinates to be calculated from the planning CT. Target positioning plates with the respective coordinate scales are then used for positioning according to lasers at the linac. Repositioning Accuracy Ten patients were included in our analysis. All received fractionated conformal radiotherapy, eight under stereotactic

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Figure 2b. Evacuated vacuum mattress as used for positioning in this study. Abbildung 2b. Fertig abgeformte Vakuummatratze, wie sie in dieser Studie zur Lagerung verwendet wurde.

conditions, and were positioned in the Body-Fix® as described above. Clinical information is listed in Table 1. Prior to the initiation of radiotherapy, 10  10 cm orthogonal digital portal images (Elekta “IVIEW-GT”, amorphous silica detector) were acquired on a daily basis and stored by the radiation therapists for retrospective evaluation. As with all patients, the orthogonal portals were viewed on a once-weekly basis by a physician. If corrections were made, only the original (uncorrected) images were included in this study. Repositioning accuracy was retrospective-ly determined by comparing these with their respective DRRs using the commercially available PIPSpro Portal Image Processing System software package (Masthead Imaging Corporation, Nanaimo, BC, Canada) which allows the user to compare two images by establishing the transformation necessary to orient them into perfect alignment. Either fiducial points or bony structures were outlined in the reference and treatment images as were the respective field contours. A chamfer transformation was calculated by determining variation of the anatomic structures relative to the field edge, whereby a set of orthogonal images provides the overall deviation of the patient’s setup in three axes. Mean 3-D vectors (3-D vector = [x2+y2+z2] as described by other authors [7, 17]) and standard deviations (SD) as well as the 95% confidence intervals (CI = mean + 2 SD) were calculated for each patient. Time required for the initial mold production and daily repositioning was also routinely noted. Results Tables 1 and 2 list the repositioning accuracies in all three axes as well as the SD and the mean and maximum 3-D vectors. The mean 3-D vectors for all patients were < 5 mm (patient 1

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Table 1. Patient characteristics, mean and maximum threee-dimensional (3-D) vector, and number of fractions analyzed. NHL: non-Hodgkin’s lymphoma. Tabelle 1. Charakteristika der Patienten und der Zielvolumina, mittlerer und maximaler dreidimensionaler (3-D) Vektor und Anzahl der analysierten Bestrahlungssitzungen. NHL: Non-Hodgkin-Lymphom. Patient

Diagnosis/treatment area/position

Age (years)

Karnofsky (%)

Mean 3-D vector Max. 3-D vector (SD) (mm) (mm)

1 2 3 4 5 6 7 8 9 10

Metastasis of a squamous cell carcinoma/paraspinal/supine Fibrosarcoma/paravertebral/prone Prostate carcinoma/pelvis/supine Prostate carcinoma/pelvis/supine Local recurrence colon carcinoma/pelvis/spine Metastatic adenocarcinoma/paraspinal/supine Metastasis of breast carcinoma/liver/supine NHL/intraabdominal bulk/supine NHL/retroperitoneal bulk/supine NHL/paraspinal/supine

53 36 56 74 62 61 63 62 32 65

70 90 100 90 90 50 90 50 100 90

4.9 (0.8) 3.4 (2.0) 3.0 (1.4) 3.2 (1.4) 3.1 (1.6) 2.3 (1.1) 1.4 (0.7) 1.7 (0.6) 1.3 (0.7) 0.5 (0.4)

6.8 10.0 7.6 7.6 7.7 5.3 3.4 3.4 3.0 1.8

10 22 24 21 39 10 12 10 18 21

Mean

56.4

82

2.5 (1.1)

5.7

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showed a systematic AP error, discovered only retrospectively when comparing the DRRs with the orthogonal portals). The analysis of setup errors related to the three dimensions revealed little differences between the axes. The patient with the largest repositioning errors (max. 3-D vector: 10.0, mean 3-D vector: 3.4, SD: 2.0 mm) was treated in prone position for a para-spinal process. The 95% CI for the entire group was 4.7 mm. Mold production could be performed in 10–30 min. All vacuum mattresses remained rigid throughout the entire treatment periods. For these ten patients, the manufacturer’s claims regarding air tightness could be confirmed in that intermittent reevacuation under treatment was not necessary, even over an 8-week treatment period. Compared to positioning without aids, the additional setup time for the base plate and mattress

Fractions

on the linac table is compensated by a clear decrease of required manual patient position adjustments by the staff, allowing similar (15-min) treatment slots as without fixation. Thus, after an initial critical hesitancy in the learning phase, the radiation therapists found the system to be acceptable also for routine therapies. No treatment series had to be discontinued or replanned.

Discussion The presented fixation method, as used in routine daily fractionated therapy, allows quick, simple and reproducible patient positioning. The additional time required compared to positioning by means of skin marks without positioning aid is so minimal that this system can also be used for conventionally fractionated therapy thus increasing confidence in patient setup and possibly allowing reduction of planning treatment volumes. Obviously, Table 2. Mean, standard deviation (SD), and maximal deviations in each direction (mm). these data represent positioning accuraAP: anterior-posterior. cy of bony structures or of tumor volTabelle 2. Mittelwert, Standardabweichung (SD) und maximale Abweichung in jeder Richtung umes in direct connection to bones. For (mm). AP: anterior-posterior. treatment volumes not fixated to bony structures and subject to organ movePatient X (lateral) X (lateral) Y (AP) Y (AP) Z (craniocaudal) Z (craniocaudal) mean (SD) max. mean (SD) max. mean (SD) max. ment, we recommend CT-based verification before initiating high-dose hypo1 1.0 (0.6) 2.3 4.7 (0.8) 6.2 0.7 (0.6) 1.7 fractionated stereotactic treatment as do 2 1.2 (1.1) 3.4 2.5 (1.9) 8.3 1.9 (1.3) 4.4 other groups using similar devices [22]. 3 1.7 (1.2) 4.6 1.5 (1.0) 3.9 1.9 (1.3) 4.7 All patients in this series, however, had 4 1.9 (1.6) 5.0 2.1 (1.3) 4.6 1.5 (0.7) 3.4 appropriate PTV (planning target vol5 1.2 (0.7) 2.5 2.2 (1.7) 6.6 1.8 (1.6) 3.0 ume) margins to account for internal 6 1.7 (1.3) 4.0 0.6 (0.7) 1.7 1.4 (0.9) 3.0 movement if required and received a 7 0.9 (0.5) 1.7 0.7 (0.5) 1.7 0.8 (0.8) 2.4 conventionally fractionated treatment 8 0.5 (0.5) 1.3 0.5 (0.5) 1.3 1.6 (0.7) 2.9 scheme, so orientation using bony land9 1.1 (0.7) 2.3 0.5 (0.5) 1.7 0.4 (0.2) 0.8 marks as seen in the DRRs was accept10 0.5 (0.4) 1.7 0.1 (0.1) 0.4 0.1 (0.2) 0.5 able.

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Figure 3. An individually formed vacuum pad reproducibly clamped onto the base plate via Velcro straps serves as abdominal press to reduce diaphragm excursions. Abbildung 3. Bauchpresse mittels eines individuell abgeformten Vakuumkissens zur Minimierung der Zwerchfellbewegung.

With regard to repositioning accuracy, our data as acquired for this study show that although repositioning results show a 95% CI of 4.7 mm, this is similar to other stereotactic body frames reported in literature. Wulf et al., for instance, demonstrated a repositioning accuracy of 2–2.9 mm in all three axes with a mean 3-D value of 4.7 mm [21]. Using a full-body (cranial and body parts) mask-based system, the Heidelberg group showed repositioning accuracies of 1.8, 2.0, and < 5 mm in the X, Y, and Z coordinates, respectively, whereby the Y axis depended on slice thickness used in the planning CT and a 3-D vector of ≤ 3.6 mm was calculated [8, 11]. However, these groups used repeat CT scans under therapy to assess repositioning accuracy and, thus, a direct comparison must be performed with caution. Obviously, daily CT prior to irradiation is a luxury reserved for radiosurgical or hypofractionated ESRT and its clinical feasibility in conventionally fractionated radiotherapy is questionable. On the other hand, portal images represent the true treatment position, without the uncertainties imposed by the transfer from CT to the linac. The worst repositioning accuracy noted in the patient treated in prone position was possibly attributable to the patient’s slight obesity but also to the fact that the point of fixation/repositioning (vacuum mattress) was remote from the area to be treated (paraspinal). This should be kept in mind for all treatments, since in times of multifield conformal therapy, skin dose and build-up effect are not as pronounced as when treating a supine patient through one dorsal field

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through the table. So we now recommend treating such patients in supine position. The learning curve became apparent by a clear decrease in SD with time (Table 1), similar to a phenomenon observed by other groups analyzing extracranial positioning systems [5, 12]. Obviously, when treating volumes in motion, the repositioning accuracy values attained by such systems may well be smaller than the extent of expected organ movement under treatment [9]. Especially for tumors in the region of the liver and lower lung, it has been shown that organ motion can be significantly reduced by using an abdominal press [8, 15, 22]. None of the patients in this series were candidates for an abdominal press but one was available for the Body-Fix® (Figure 3), and an evaluation is currently in progress at our institution. The Body-Fix® itself allows two possible uses: with and without fixation foil for daily repositioning. The vacuum foil, as described earlier in the vacuum mattress-forming process, can be employed for daily positioning, which itself may somewhat reduce diaphragm excursions, but possibly also internal movements not associated with breathing (intestinal, etc.). Sandwiched between the vacuum mattress and the foil, (in)voluntary movement is virtually impossible, even when fixated for > 1 h as analyzed by repeat CT after 1.5 h on five patients in the scope of stereotactic interventional procedures (unpublished data by one of the authors [RJB]). Rigid fixation in the body region is especially important for transport between CT and treatment table, as transfer-induced changes of treatment position between CT and irradiation have been listed as a possibility of positioning errors in the craniocaudal (Z) axis [21]. Also, rigid fixation is of utmost importance when performing stereotactic brachytherapy, not only in the head [2], but also in the body regions [20]. Repositioning accuracy using this foil, however, is the topic of a current study on hypofractionated stereotactic treatments. When using the fixation foil only during the mold production process but not for daily treatments as in this study, we have found that repositioning times are comparable to those without positioning aid. It can be argued that this method is now comparable to nonstereotactic positioning aids such as foam mattresses [5, 14, 16, 18, 19] and thermoplastic mask materials [12, 14]. Malone et al. compared three types of pelvic immobilization (rubber leg cushion, foam mattress, and thermoplastic mask in prone position) and found the repositioning accuracy (mean 3-D vector ranging from 1.9 to 2.6 mm, SD 1,6 mm) of the thermoplastic mask to be superior to the other systems (2.7–3,4 mm, SD 2.7–3.1) [12]. A similar study was performed by Mitine et al., except that a hemi-body foam mattress was used and the patients in the mask were in supine position. Here, the mattress results were similar to those of the previous group, but the thermoplastic masks fared worse leading the authors to question the clinical feasibility of both systems [14]. Aside from the fact that these systems do not lend themselves to stereotactic procedures, we enjoy the flexibility

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offered by the Body-Fix® with its multitude of different-sized mattresses and the possibility of using it for both conventional 3-D conformal as well as ESRT indications. In addition, the vacuum mattresses can be disinfected and reused, possibly offering long-term financial advantages. Conclusion The results of this study have shown that even when using the foil only for the vacuum mattress-forming process, daily setup time can be similar to using no positioning aid while offering very acceptable repositioning accuracy in most cases of fractionated conformal extracranial radiotherapy. For tumors in direct relation to bony targets, we suggest a 5-mm safety margin to account for the repositioning uncertainty aspect of the PTV. In this case, after exclusion of systematic errors, daily portal imaging is not required in conventionally fractionated radiotherapy when using the Body-Fix®. With the increasing quality of modern portal imaging systems with integrated software for quick and on-line positioning assessment, we believe that once-weekly or even daily orthogonal verifications can be performed without significant time constraints. All patients with tumors in direct relation to bony structures undergoing hypofractionated irradiation at our institution, currently receive daily pretreatment orthogonal portal image verification, and corrections are applied on an individual basis depending on PTV margins. Acknowledgment The authors would like to express their sincere gratitude to the radiation therapists at the Department of Radiation Oncology in Innsbruck, for their patience, constructive criticism, and assistance in the clinical integration of this system.

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Address for Correspondence Meinhard Nevinny-Stickel, MD Department of Radiotherapy-Radiooncology University Hospital Innsbruck Anichstraße 35 6020 Innsbruck Austria Phone (+43/512) 504-2801, Fax -2812 e-mail: [email protected]

Strahlenther Onkol 2004 · No. 2 © Urban & Vogel

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