Int. J. Radiamn Onco/ogyE~io/. Phys.. Vol. 13, pp. 129-132 Printed in the U.S.A. All rights reserved.

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??Technical Innovations and Notes

A RADIOLUCENT BITE-BLOCK APPARATUS

DOUGLAS JONES, B.Sc.’ AND MARK D. HAFERMANN, M.D.2

’ Northwest Medical Physics Center; and *Division of Radiation Oncology, Mason Clinic, Seattle, WA

A bite-block, patient immobilization apparatus has been constructed using all plastic parts. The apparatus may be used in C-T scanning without detriment to the images produced. The construction details are given and the procedure used to produce a bite block is described. A method to reduce electron backscattering onto the buccal mucosa from metal fillings is given.

Patient immobilization, C-T scanning.

INTRODUCIION

The design goals in developing the apparatus described in this report were 4-fold:

1.

2.

3.

4.

Use plastics in the construction and thus allow a C-T scan to be performed with the patient immobilized in the same position as for radiotherapy. Minimize the number of scales used, thus reducing errors and improving efficiency in patient set-up. Provide separation of the teeth from the buccal mu- cosa when required. Separate the jaws and depress the tongue when de- sired.

METHODS AND MATERIALS

There is a clear requirement for localization studies to be performed with the patient in the same position as for treatment. This can be readily accomplished if all immo- bilization aids are constructed prior to proceeding to lo- calization. Bite-blocks so far described have included some metal in their construction, see for example, van de Geijn et al.,’ which introduce unacceptable artifacts in C-T images. The majority of the bite-block described here is constructed from PVC (polyvinyl chloride) and no metals are employed in the construction. To attach the bite-block apparatus to the various couches, simula- tor, C-T scanner, radiotherapy machine, a universal clamp was constructed.

The use of this bite-block is most easily described by

Figure 1, showing the steps employed in the fabrication of a bite-block which is narrated below:

A.

B. C., D.

E.

F.

G.

H.

I.

J.

The component parts of the apparatus showing six head supports*, a small water heater used for softening the thermoplastic dental impression material and the bite-block apparatus. The arm support is centered on the couch. The couch clamps are attached to the sides. A rubber surface contacts the vertical sides of the simulator couch or side rails on the treatment couch. Stops are added to both sides which are wedged- shaped and serve to flex the white plastic. The arm support assembly is removed from the couch by flexing the white plastic as shown. The arm support assembly is reinstalled on the couch by flexing the white plastic as shown. The head rest clamp is attached in any of 7 lateral positions. One of the head and neck supports is selected and slides into grooves provided in the head rest clamp. A label with the patient’s name is affixed to the threaded black plastic tube which is then screwed into the white support arm. A mark on the label indexes the rotation of this tube with respect to a notch on the white arm. The couch lateral travel is set to the zero position and the patient’s midline adjusted to the center of the couch which is de- fined by the longitudinal crosshair or a sag&al la-

Reprint requests to: Douglas Jones, Northwest Medical PhysicsCenter,P.O.Box21185,Seattle,WA98111-3185. Acknowledgments-We wish to express our appreciation to the staff of the Radiation Oncology Section at Mason Clinic for useful comments during the development of this apparatus, to

Mr. Tom Macnab and Mr. Andrew Jones for their skill in pro- ducing the apparatus and Mary Rouzer for her careful prepara- tion of this manuscript.

Accepted for publication 13 August 1986. * Timo Industries, Bonita, CA.

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1. J. Radiation Oncology 0 Biology 0 Physics January 1987, Volume 13, Number 1

Fig. 1. The steps employed to produce a bite-block. See text for narration.

K.

ser. A head position for the patient is selected and they are temporarily immobilized using tape. The arm is rotated so that the black plastic tube is close to the patient’s cheek and the whole as- sembly is pulled or pushed up and down the couch until the black plastic tube is aligned with the patient’s mouth as viewed from the side. The head rest is then clamped using any of the four screws shown and its position recorded on the label.

L.

M.

N.

The level of the patient’s lips on the plastic tube are noted and the tube cut to this length. The thermoplastic impression plate is softened in hot (170°F) water and wrapped onto the end of the plastic. The arm is lowered into the hole provided on the support plate and the thermoplastic into the pa- tient’s mouth. A screw clamps the arm and pre- vents rotation about a vertical axis. Dentures should be removed.

A radiolucent bite-block apparatus ??D. JONES AND M. D. HAFERMANN 131

0. The thermoplastic sets up hard in about 5 min- utes after which the tape may be removed.

A separate assembly, with the couch clamp adjusted for the therapy couch, is used in set up for treatment. The only index to a scale that is required is the setting of the head and neck support in its slide, which defines the lon- gitudinal position. The vertical location is defined by the length of the attaching piece, which is fixed, and the number of threads engaged in the screw, which is easily indexed, to within one thread pitch, to a mark on the label. With the couch set at zero lateral travel, the lateral position is determined by the rotation of the arm which is indexed to the patient’s midline by the machine cross- hairs or a sagittal alignment laser in the treatment room. The threaded black plastic tube and thermoplastic cast are stored in a paper cup.

Various authors have noted the enhanced dose in soft tissue adjacent to metal fillings in teeth when these areas are included in photon4 and electron beams.‘,2*3 A solu- tion to this problem is to separate the soft tissue from the metal with a low atomic number material. The thickness of the material depends upon the particular clinical situ- ation but may be readily calculated for electron beam therapy.

Klevenhagen et aL2 show that the electron backscatter factor (EBF) is related to the atomic number of the back- scattering material by:

EBF = A - B exp (-kZ) (I)

where, A, B and k are constants for a particular electron energy (E,) at the interface. Z is the atomic number of the backscattering material.

Suppose a cheek, 1.4 cm thick is to be treated with an electron beam, with an incident energy of 9 MeV. To increase the skin dose, a 1 cm thick slab of tissue equiva- lent bolus is added. It can readily be shown that the aver- age electron energy (E,) after passing through the cheek is 4.0 MeV. A gold filling is in contact with the buccal mucosa. The value of constants for these conditions are A = 1.95, B = 1.03, and k = 0.016, which when entered in Eq. 1 yield a backscatter factor of 1.66.

The penetration of backscattered electrons has been reported by Lambert and Klevenhagen3 to be defined by the following equation:

EBI = Aemkt (2)

where EBI is the backscatter intensity, A and k are con- stants dependent on the electron energy. In the particular example discussed, it is considered desirable to reduce the backscatter intensity to less than 10%. The values of A and k at 4 MeV are 1 and 0.258, respectively, hence Eq. 2 can be solved to yield a thickness of 9 mm. The density of the thermoplastic used in this bite-block is 1.26, thus 7 mm of thermoplastic is required to reduce the backscattered electron dose to the proposed level.

Fig. 2. The steps employed to separate the buccal mucosa from the teeth.

The procedure for producing a bite-block which sepa- rates the buccal mucosa from the teeth or when substan- tial jaw separation is required, is slightly modified from that described in Figure 1. The thermoplastic has the property of adhering to itself so that first a plate is made which wraps around the teeth as shown in Figure 2a. A second piece of the softened thermoplastic, attached to the threaded plastic rod, is introduced into the mouth as shown in Figure 2b to produce the bite-block shown in Figure 2c.

132 I. J. Radiation Oncology 0 Biology 0 Physics January 1987, Volume 13, Number I

DISCUSSION

The design goals defined for the bite-block system de- scribed here have been achieved and the system has been in routine use for over 2 years. While the apparatus does force the patient’s head to a fixed position with respect

to itself, we continue to use triangulation tattoos to make small (less than 5 mm on the skin) adjustments of the patient position which is accommodated by the elasticity of the arm. The extent to which such adjustments are made certainly depends on the cooperation of the patient in the set up process.

REFERENCES

Gagnon, W., Cundiff, J.: Dose enhancement from back- scattered radiation at tissue-metal interfaces irradiated with high energy electrons. Br. J. Radiof. 53: 466-470, 1980.

Klevenhagen, S.C., Lambert, G.D., Arbabi, A.: Backscat- tering in electron beam therapy for energies between 3 and 35 MeV. Phys. Med. Biol. 27: 363-373, 1982.

Lambert, G.D., Klevenhagen, S.D.: Penetration of back-

4.

5.

scattered electrons in polystyrene for energies between 1 and 25 MeV. Phys. Med. Biol. 27: 721-725,198l. Thambi, B., Murthy, AK., Alder, G., Kartha, PK.: Dose perturbations resulting from gold fillings in patients with head and neck cancers. Int. J. Radiat. Oncol. Biol. Phys. 5: 58 l-582, 1979. van de Geijn, J., Harrington, F.S., Lichter, AS., Glatstein, E.: Simplified bite-block immobilization of the head. Ra- diology149: 851, 1983.