Ion-beam mask for cancer patient therapy

Abstract

An ion-beam mask has been provided for spatially distributing fast ions used to irradiate tumors so that the region near the Bragg peak receives a uniform radiation dose while the tissue at the point of entrance of the beam has a web of unirradiated volume to promote healing of the surface tissue following radiotherapeutic treatment of embedded tumors.

Government Interests

The present invention was made during the course of, or under, a contract with the United States Atomic Energy Commission.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to radiotherapy and more specifically to a shielding mask for use in ion-beam radiotherapy which allows distributed irradiation of an embedded tumor while shielding a substantial portion of the surface volume tissue.

A treatment of choice for cancer patients having embedded tumors is irradiation of the tumors with a beam of fast ions such as by a beam of 65 MeV protons produced by the Oak Ridge Isochronous Cyclotron (ORIC). Such a treatment frequently results in destruction of the malignant cells with a consequent remission of the cancer. The dose to be delivered to a specified target tumor region is usually prescribed to be uniform throughout the target volume to well within .+-.5 percent. The desired uniformity of accumulated dose laterally across the target volume is readily attained, as indicated, for example, by R. S. Bender's measurements on the proton beam at ORIC as reported in the Annual Progress Report of the Electronuclear Division of the Oak Ridge National Laboratory, Oak Ridge, Tenn., for 1967, on pages 101-102, by using only the control portion of a wide beam of fast ions, masked to cover the lateral extent of the target volume. The desired uniformity longitudinally through the depth of the target volume is readily obtained by modifying the range of the ions for certain fractions of the total exposure time. Unfortunately, healthly tissues between the outer epidermis and the tumor itself must be exposed to the damaging fast protons as they seek out the target tumor.

SUMMARY OF THE INVENTION

In view of the above, it is an object of this invention to provide an ion beam mask for mitigating surface tissue effects of radiotherapeutic treatment of embedded tumors which will permit a beam of fast ions, such as protons to penetrate to the tumor and deliver with the desired uniformity over the lateral extent of the target volume at each exposure any prescribed dose level but which will, at the same time, leave a reasonable fraction of skin area either unirradiated or with a dose level low enough to enable the skin and dermal regions to more readily recover from the deterimental effects produced by the heavy charged particles.

Other objects and many of the attendant advantages of the present invention will be obvious from the following detailed description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a radiotherapy system employing the ion-beam mask according to the present invention.

FIG. 2 is a planar view of the ion-beam mask shown in FIG. 1.

DETAILED DESCRIPTION

The subject invention is best illustrated by referring to the drawings. FIG. 1 shows a cross section of one example of the subject ion-beam mask positioned over tissue having a tumor embedded therein. The mask is shown generally at 10. A beam of fast ions, such as protons from a source 8 in the range of from 50 to 200 MeV, enters from the direction of arrows 12. Mask 10 is preferably made of carbon (graphitized and purified) and has an array of holes 14 drilled therethrough to permit passage of the ions to the surface of the skin 16 whereupon the ions proceed to the target tumor 18. The masked portions of the surface of skin 16 will receive less of a dose of ionizing radiation with the mask in place than they would if no carbon shield were interposed between the patient and the beam. The purpose, therefore, of breaking the beam into many discrete portions whereby a large area of skin and intervening tissue are protected from ionizing radiation can be achieved by means of the subject design provided that the thickness of the mask 10 exceeds sufficiently the range of the ions in the mask material. Although a carbon mask is preferred, other suitable masking materials, such as dense plastics, aluminum, and denser metals, may be used.

FIG. 2 is a planar view of the device showing that the device consists simply of a shield 10 of a material, such as carbon, having parallel holes 14 drilled therein for collimation of the beam which passes therethrough.

The stated object of this invention can be fulfilled by an appropriate choice of the diameter (d) of the holes through the mask and of the wall thickness (w) between the holes to attain sufficient overlap between beamlets passing through adjacent holes at the depth of the Bragg peak, approximately the ion range (R.sub.o) in the patient's tissue. Optimal overlap between adjacent beamlets and thereby optimal lateral uniformity of dose will occur, for the example of each beamlet having a Gaussian spread whose effective dispersion is .sigma..sub.e at the target depth of interest, when the sum of d and w is made equal to 1.5 .sigma..sub.e. The lateral Gaussian spread of each beamlet may be seen from the expression for the beam intensity I(x,y) at depth y being I(x,y) = R(y) exp (-x.sup.2 /.sigma..sub.e.sup.2) where x is the lateral coordinate (from beamlet center) at which I(x,y) is specified. It should be noted that reasonable uniformity may be attained with factors differing considerably from this ideal Gaussian value of 1.5. For large depths, y, the effective dispersion, .sigma..sub.e, is related to the hole diameter, d, and the dispersion of the Gaussian spread associated with each ion's motion through the tissue, .sigma..sub.o, by the expression from the work of A. Koehler and W. Preston of the Harvard Cyclotron,

.sigma..sub.e = (.sigma..sub.o.sup.2 + d.sup.2 /4).sup. 1/2

where .sigma..sub.o gives the Gaussian spread for each fast ion's lateral position as it moves through toward the end of its range R. Measurements show .sigma..sub.o to be 0.031 R for protons moving through water and 0.045 R for protons moving through aluminum. This .sigma..sub.0 is large enough for ions reaching depths such as 20 cm to permit d and w to be optimized. The hole 14 illustrated in FIG. 2 can be arrayed in a variety of configuration but it has been found that the hexagonal pattern, as illustrated in FIG. 2, provides a close packed array for the best uniformity at target depth near R.

Consider the illustration of planning to deliver a certain dose at depth R = 12 cm. The proton beam energy needed is 130 MeV for which each proton's spread has .sigma..sub.o = 0.031 R = 0.37 cm in soft tissue. For the optimal lateral uniformity, the sum d + w is chosen equal to a factor F times the effective Gaussian dispersion of each beamlet .sigma..sub.e at depth R to have the value of F = 1.5 for Gaussian beams (d + w = F.sigma..sub.e). For round holes the optimum uniformity at the proton range R.sub.o = R is attainable by setting F = 1.5 and specifying d/w by the expression ##EQU1## acceptable uniformity may be attainable with F values in the range of about 1 to 2. The variety of consistent values of d and w include one set of d = 0.60 cm and w = 0.12 cm and another set of d = 0.18 cm hole diameter and w = 0.36 cm wall thickness.

To further illustrate the way in which d and w may be determined from the above expressions, suppose the radiotherapist were to relax his requirement of lateral uniformity from F = 1.5 (the ideal) to F closer to 2, say F = 1.73, then the design above of a suitable carbon mask gives a thickness of at least 8 cm and a choice of hole diameter and corresponding wall thickness including the sets of d = 0.037 cm for w = 0.37 cm and d = 0.76 cm for w = 0.12 cm.

This design procedure makes the subject invention compatible with accurate location control, i.e., the beam can be accurately repositioned during a series of exposures to effectively spare a web of skin and still deliver a uniform dose to a deeply embedded tumor.

In summary, the subject development is shown to break a beam of fast ions given to cancer patients for therapeutic purposes into separate and discrete rays whereby the region near the Bragg peak (the end of the range of ions, in tissue, for example) has a lateral dose uniform to within a few percent and whereby the skin surface area and underlying tissues receive a discontinuous dosage of irradiation thereby enabling the skin to heal more readily than it would if exposed to a massive dose covering the entire area interposed between the radiation source and the tumor.

Claims

What is claimed is:

1. A radiation mask for mitigating the surface tissue effects of a radiotherapeutic treatment of embedded tumors in a patient by means of a beam of fast ions from a beam source, comprising:

a block of radiation shielding material positioned in said beam intermediate the patient and said beam source, said block having a surface area larger than the cross section of said beam and a thickness greater than the range of ions of said beam in said shielding material, said shielding material block having a plurality of circular cross section apertures extending therethrough in a direction parallel to the rays of said beam and arranged in an orderly, evenly spaced array, each of said apertures having a diameter (d) and spaced with a wall thickness (w) therebetween defined by d = w = F.sigma..sub.e, with f being in the range of from 1 to 2 and where .sigma..sub.e is the effective dispersion of the ions passing through said apertures at the depth of said embedded tumor to attain sufficient overlap of discrete beam portions passing through adjacent ones of said apertures at the depth of the Bragg peak for said beam source coincident with the tumor region, thereby providing a lateral uniformity of radiation dose over the irradiated area of said tumor while shielding a substantial are of skin and intervening tissue.

2. The radiation mask as set forth in claim 1 wherein said beam of fast ions is protons in the range of from 50 to 200 MeV and said shielding material is carbon.

3. The radiation mask as set forth in claim 2 wherein the apertures in said shield are arrayed in a hexagonal pattern.