A Simple and Non-Invasive Vacuum Mouthpiece ... - Springer Link

Strahlentherapie und Onkologie

Original Article

A Simple and Non-Invasive Vacuum Mouthpiece-Based Head Fixation System for High Precision Radiotherapy Reinhart A. Sweeney1, Reto Bale2, Thomas Auberger1, Michael Vogele3, Stephanie Foerster4, Meinhard Nevinny-Stickel2, Peter Lukas1 Purpose: To demonstrate why conventional non-invasive mouthpiece-based fixation has not achieved the expected accuracy and to suggest a solution of the problem. Patients and Methods: The Vogele Bale Hohner (VBH) head holder is a non-invasive vacuum mouthpiece-based head fixation system. Feasibility and repositioning accuracy were evaluated by portal image analysis in 12 patients with cranial tumors intended for stereotactic procedures, fixated with the newest version (VBH HeadFix-ARC®). Results: Portal image analysis (8 patients evaluated in 2-D, 4 patients in 3-D) showed that even in routine external beam radiation therapy, treatment can be applied to within a mean 2-D and 3-D accuracy of under 2 mm (SD 0.92 mm and 1.2 mm, respectively) with cost and repositioning time per patient and patient comfort comparable to that of common thermoplastic masks. Conclusion: These preliminary results show that high repositioning accuracy does not rule out simple and quick application and patient comfort. Paramount, however, is tensionless repositioning via the vacuum mouthpiece. Key Words: Fixation devices · Radiotherapy · Head fixation · Brain tumors Kopffixation über Vakuummundstück erlaubt eine einfache, nicht invasive und hoch präzise Repositionierung Ziel: Fixationssysteme, die auf konventionellen (nicht Vakuum-)Mundstücken basieren, erreichen oftmals nicht die erwartete Genauigkeit. Die vorliegende Arbeit beschäftigt sich mit den möglichen Ursachen und bietet entsprechende Lösungen. Patienten und Methoden: Der Vogele-Bale-Hohner-(VBH-)Head-Holder ist ein nicht invasives, auf einem Vakuummundstück (Abbildung 1) basierendes Kopffixationssystem (Abbildungen 2 und 3). Bei zwölf Patienten mit kraniellen Tumoren wurde mit der neuesten Version (VBH HeadFix-ARC®) die Repositionsgenauigkeit mittels Portal Imaging untersucht. Ergebnisse: Die Portal-Imaging-Auswertung (acht Patienten in 2-D, vier Patienten in 3-D) bestätigte, dass eine Bestrahlung im Kopfbereich auch in der klinischen Routine mit einer mittleren Genauigkeit von unter 2 mm (Standardabweichung 0,92 bzw. 1,2 mm) appliziert werden kann, während Kosten, Repositionsdauer und Akzeptanz der Patienten vergleichbar sind mit denen thermoplastischer Masken. Schlussfolgerung: Diese vorläufigen Ergebnisse zeigen, dass eine hohe Repositionierungsgenauigkeit ein einfaches, schnelles und für den Patienten angenehmes System nicht ausschließt. Ausschlaggebend ist die spannungsfreie Lagerung mittels Vakuummundstück. Schlüsselwörter: Kopffixation · Strahlentherapie · Hirntumoren Strahlenther Onkol 2001;177:43–7 DOI 10.1007/s00066-001-0828-0

Introduction Many procedures in the head region requiring separate imaging, treatment planning and subsequent treatment/diagnostic procedures such as fractionated (conformal) radiotherapy, stereotactic radiotherapy, stereotactic radiosurgery, stereotactic biopsies, brachytherapy, nuclear medicine, etc. would

greatly benefit from high repositioning precision. This especially applies to the head region due to the close anatomical proximity of critical structures. Consistent submillimetric repositioning, to the point of confidence allowing even stereotactic radiosurgery, has so far been attained mainly with invasive stereotactic frames [8, 9,

1

Department of Radiotherapy-Radiooncology, Department of Radiology I Interdisciplinary Stereotactic Interventional Planning Laboratory (SIP-Lab), Innsbruck, 3 Department of ENT, 4 Department of Pediatrics, University Hospital Innsbruck, Austria. 2

Presented at RSNA, Chicago 1999. Submitted: 11 Sep 2000; accepted: 17 Oct 2000.

Strahlenther Onkol 2001 · No. 1 © Urban & Vogel

43

Sweeney RA, et al. Vacuum Mouthpiece-Based Head Fixation

11]. Although invasive repositioning over longer periods is possible [12] it is surely uncomfortable for the patient, resulting not only in time constraints in performing imaging, planning and treatment, but also in the risk of infection at the points of insertion into the skull. Nature itself, however, has given us the possibility to access the skull directly and non-invasively over the upper row of teeth. This fact is nothing new and mouthpiece-based fixation systems have been in use for some decades in a variety of forms [4, 6, 7]. Repositioning accuracy, however, was never as excellent as the above simple logic would have led to expect. Some simple reasons why not are listed in Table 1. The problem therefore with mouthpiece-based systems was mainly the difficulty to establish an objective, identical and reproducible connection of the impression tray to the upper palate. This problem can be overcome by using vacuum to suck the mouthpiece against the upper palate [10] and by the possibility of adjusting the fixation system in craniocaudal position under the patient. Various departments at our University hospital have been using such vacuum mouthpiece-based fixation since 1994 [2, 3]. These experiences have led to significant modifications of the head holder as used for radiotherapy, culminating in the VBH HeadFix (Medical Intelligence, Schwabmünchen, Germany) presented herein.

plastic vacuum spacer is peeled off. A plastic hose, leading to the vacuum pump (Atmolit, Germany) is connected to the underside of the tray. A successful mouthpiece is one which, upon insertion against the patient’s upper palate, allows underpressure (-0.8 atm) to build, holding the mouthpiece snugly against the upper palate. This mouthpiece is connected to the transverse plate via the 2 anterior bars of the mouthpiece (Figure 1). During the first positioning of the supine patient, the mouthpiece is connected to the transverse plate against which it can be adjusted in anterior-posterior, lateral and craniocaudal directions. Once the patient is in the desired position, the 4 screws (Figure 2) are tightened. This is the rigid unit required per patient, quite insensitive to misalignment throughout a fractionated course of treatment. The tensionless connection of this mouthpiece to the therapy table via 2 carbon fiber posts is granted by the possibility of cranio-caudal movement of the head plate on the therapy table. After the patient is repositioned, the head plate is fixated against the therapy table instantly by tightening the 2 self-centering brackets (Figure 2). Alignment according to treatment room lasers occurs using a reproducible localization box which is quickly removed immediately preceding treatment (Figure 3). The same box with radiodense fiducial lines ingrained into the plates can be used for stereotactic purposes.

Patients and Methods VBH HeadFix The vacuum mouthpiece impression remains the core element of the VBH HeadFix and its production is described previously [2]. The impression material (CORRECT VPS Jeneric/Pentron, Wallingford, CT) is loaded into the impression tray which comes in 3 sizes and to which anterior and transverse bars are connected. After the loaded impression tray is inserted into the patient’s mouth and pressed against the upper palate for 4 minutes, the mouthpiece is removed and the Table 1. Possible factors affecting repositioning accuracy of conventional (non-vacuum) bite blocks (MP = mouthpiece). Tabelle 1. Probleme konventioneller Mundstücke (MP) (ohne Vakuum). Problem

Result

• Patient must actively bite

➾ Compliance very important, extent and level of bite varies from repositioning to repositioning = MP uncertainty

• No adaptation to the patients cranio-caudal position on the therapy table

➾ May lead to different position of the head with respect to the mouthpiece which itself is in fixed relation to the table

• Skin markings

➾ Skin distortion leading to inaccuracy possible, stigmatizes patient

44

Figure 1. Individual patient unit: Mouthpiece (MP) with vacuum area (1), anterior arms (2) and transverse plate (3). During first positioning the mouthpiece is still loose and adjustable in anterior-posterior and lateral directions as well as around a cranio-caudal axis. In addition, flexion of the head around a left-right axis is possible up to 20° by inserting the desired wedges. After tightening the 4 screws (4) the position of the mouthpiece with respect to the transverse plate is fixated. Abbildung 1. Individuelles Setup pro Patient: Mundstück (MP) mit Vakuumareal (1), anterioren Armen (2) und transversaler Platte (3). Während der ersten Positionierung ist das Mundstück noch in anterior-posteriorer und lateraler Richtung verstellbar und um eine kraniokaudale Achse drehbar sowie über Keile um eine Rechts-links-Achse bis 20° kippbar. Nach Festziehen der Schrauben (4) ist das Mundstück in Relation zur transversalen Platte fixiert.



2 1 3 2

4

Figure 2. Patient in pre-treatment position: Individual patient unit (1) which is clamped against the 2 carbon fiber posts with 2 quick-release clamps (2), removable localization box (3) and self-centering bracket (4)

Figure 3. Patient during treatment (localization box removed after positioning in room isocenter): skin, eyes etc. are visible. In case of an emergency the patient can free himself by pulling out the quick release clamps (1).

Abbildung 2. Setup vor Bestrahlung: Individuelles Patienten-Setup (1), welches mit zwei Klemmen (2) reproduzierbar auf die Karbonposten gesteckt wird, Lokalisationsbox (3) und selbst zentrierende Tischklemmen.

Abbildung 3. Setup während Bestrahlung (Lokalisationsbox nach Positionierung im Isozentrum entfernt): Haut, Augen etc. sichtbar. Im Notfall kann sich der Patient selbst befreien, indem er die Klemmen (1) herauszieht.

In case of an emergency both the patient or staff can disconnect the transverse plate from atop of the posts by simply pulling the connectors (connecting the transverse plate to the carbon posts, see Figure 2) out of their slot. The vacuum pressure itself is minimal.

Protocol was kept for each patient with regard to time required for mouthpiece production, ease of localization and subsequent daily fixation.

Patients Between June 1998 and October 1999, 12 patients were treated in the VBH HeadFix system (mean age 43 years, minimum 9, maximum 63 years) at our institution. All patients required increased repositioning accuracy (7 patients with orbital endocrinopathy, 2 with astrocytoma Grade II and 1 each with pituitary adenoma, optical meningeoma and a benign retrobulbar tumor). Repositioning accuracy of these patients was determined retrospectively by portal imaging (IVIEW, Philips Electronics UK) analysis using commercial portal imaging processing software (PIPSpro, Masthead Imaging Corp., British Columbia, Canada), giving the actual position of the head with respect to the treatment beam compared to the position at first treatment. All portal images analyzed were the original images made before any correction of patient position. Due to time constraints, only 4 of the 12 patients had a set of orthogonal portal images allowing determination of positioning in 3 dimensions (X, Y, Z). For the rest, 2-D accuracy (X, Y) was determined from lateral images. The euclidean distance of the X, Y (and Z) deviations result in the 2D (√ [X2 + Y2]) / 3D (√ [X2 + Y2 + Z2]) vector of positioning deviation.


Results Fabrication of the VBH mouthpiece takes between 10 and 20 minutes and can be performed at the leisure of staff and patient. Repeat production during the course of treatment was never necessary. With some practice, fixation times with the VBH HeadFix system are comparable to those of a thermoplastic mask (< 1 minute). At insertion, the quite noticeable suction of the mouthpiece against the hard palate (guarantees correct position of the mouthpiece with respect to the head as vacuum will not build if not sealed tightly) tended to give patients and staff an increased sense of security. Repositioning time is comparable to that of thermoplastic masks. All patients tolerated the VBH HeadFix well, no therapy had to be discontinued or interrupted and, after the first fraction, patients generally preferred to insert and remove the mouthpiece (with the attached transverse plate) themselves. In 4 patients, slight painless gingival bleeding occurred which resolved after excess impression material was removed and oral rinse was prescribed. Not once did bleeding occur on the upper palate where the vacuum acts. Portal Image Deviations The mean 2-D repositioning deviation of the 8 patients (n = 164) was 1.65 mm (SD = 0.92 mm, maximum = 3.5 mm).

45


The mean 3-D repositioning deviation of the 4 patients (n = 95) was 1.86 mm (SD = 1.2 mm, maximum = 3.9 mm). Discussion Currently available non-invasive mouthpiece-based systems have reported accuracies of 0.5 [5] to 3.1 mm [14] whereby great care must be taken in comparing repositioning results. Some studies determine repositioning of the fixation system itself, whereby there is again a difference between mechanical and in-vivo accuracy, others determine repositioning accuracy in the clinical setting by comparison of localization images to verification/portal images. These again could be evaluated in 1, 2 and 3 dimensions, the resulting values successively increasing [14]. Of course the values of the clinical comparisons will be larger, as not only systematic (which may have many causes and can be unique to a patient, physician, technologist, an institution or a technique) but also random errors (due to daily setup variations including uncertainty of fixation system, table rotation, technician error, gantry error, etc.) summate. This successive increase in repositioning error as one progresses through the various stages has been extensively analyzed at this institution and is depicted in Table 2, showing the progression of inaccuracy as more factors are included in the evaluation. The maximum deviation determined by the 3-D vector from the orthogonal portal images was 4.3 mm, surely not acceptable for stereotactic radiosurgery but the above numbers are those before correction of patient position. In stereotactic radiosurgery there would be less time constraints than in routine external beam radiotherapy and full attention can be devoted to tensionless repositioning. If care is taken to position completely tensionless, consistent submillimetric repositioning can be attained. Our experiences with the VBH HeadFix-ARC ® have shown it to be slightly easier to handle, less prone to misalignment under rough handling and cheaper per patient than the system previously reported [13] which used 2 individual hydraulic arms per patient instead of the rigid carbon bars. Table 2. Repositioning accuracy decrease in the progression from mouthpiece-only to the clinical setting (MP = mouthpiece). Tabelle 2. Abnahme der Repositionsgenauigkeit zwischen Messungen des Mundstücks (MP) allein bis zum Einschluss aller klinischen Faktoren. MP accuracy Mechanical accuracy Clinical accuracy [8] of the VBH head using portal holder [13] images 2-D 3-D (n = 164) (n = 95) Mean (mm) SD (mm) Range (mm)

46

0.3