Treatment planning for neutron radiation therapy

Im. J. Radrannn Onrology iJ,ol Phys.. 1977, Volume 3. pp. 177.183.

Pqmon

Press

Pnnred ,n the U.S.A.

??Neutrons-l

TREATMENT

PLANNING

FOR NEUTRON

RADIATION

THERAPY?

P. WOOTTON, K. WEAVER and J. EENMAA Medical Radiation

Physics,

University

of Washington,

Seattle, WA 98195, U.S.A.

Treatment-planning techniques developed for photon therapy have been freely borrowed for fast-neutron beam therapy, including methods of storing the measured data and subsequent manfpulation to correct for patient contour, inhomogeneities, oblique incidence of the beam, and the presence of beam-modffyii devices. These corrections account for disconthtuities in density in the path of the ray from source to volume of interest, but not for their location along that ray, discontinuYi in adjacent rays, or the finite size of the patients. Given appropriate choice of effective density and attenuation for the dintbutity, resultfng errors are minimal for the photon energfes fn common use and in most clinicalfy realistic geometries. In fast-neutron beam therapy, however, experhnent shows that for accurate prediction of dose in the frradiated volume, correctfons must be made for the finite size and shape of the irradiated volume and the location of dfscontfnuftks along the ray from the source to the vohune of interest. Corrections must afso be made for dfscontfnuities present afong adjacent rays, though absent afong the ray through the volume of interest. Corre&ms are fnffuenced by both density and hydrogen content, being greatest for lung, then bone, and least for fat. Methods wfll be dfscussed. The problem of incorporating the biological data to provide biologically effective dose wfll be consfdered. Neutron radfation

therapy,

Treatment

planning,

Dose’dfstrfbution.

INTRODUCTION Treatment plans for neutron radiotherapy are cur-

tissue inhomogeneities can have significant effects on the dose. This presentation will deal with observations of the magnitude of these effects, and with methods of accounting for them in dose calculations.

rently calculated with the same techniques that are used for photon beams: Dose distributions are calculated from depth-dose and off-center ratio data measured in an effectively infinite phantom. Simple corrections are made for oblique beam incidence, details of patient contour, inhomogeneities in the beam, and the presence of field shaping devices. Effectively, exponential functions are used to correct for altered attenuation of the primary beam by external inhomogeneities and of the total beam by internal inhomogeneities. In principle such functions can account for changes in the primary beam but are not so satisfactory in accounting for resultant changes in the scattered component of total dose: even so their use in supervoltage photon therapy leads to an error of less than 5%. However, neutron beams have a large component of scattered radiation; for example the backscatter factor for a 10 x 10 cm neutron field is about four times larger than for 6oCo. Patient geometry is therefore more important in neutron therapy than in photon therapy. In particular the finite size of the irradiated volume and the position of Bupported

by Grant No. POI-CA-12441 from the National

EXPERIMENTAL RESULTS NRL Experiments At the Naval Research Laboratory (NRL), McGinley and McLaren measured the neutron and photon dose in a simulated lung medium.” Measurements were made in a phantom filled entirely with TE fluid, as well as with simulated lungs in place. In both cases the dose decrease with depth was approximately exponential for depths greater than 5 cm. The dose in the first 6 cm of lung medium was smaller than in the homogeneous phantom by as much as 5% due to the decrease in scattered radiation. At greater depths the dose in the lung exceeded that in the homogeneous phantom by as much as 28% due to increased transmission. The photon dose in the lung was larger than in the TE fluid at all depths; at the back edge of the lung, it was larger by a factor of three. McGinley and McLaren also measured the dose Cancer Institute, 177

National

Institutes

of Health.

17X

Radiation Oncology

0

Biology 0

Physics

changes caused by thicknesses of bone-equivalent material.J The phantom was a tank with a hollow tube through the center into which bone-equivalent samples were inserted. Doses were measured at different depths in TE material with and without bone material present. In all cases, the dose variation with depth for depths >6cm was approximately exponential. The dose behind the bone material was found to be smaller than that in the TE material; for example, the 4.8 cm thick bone sample caused a dose decrease of 15% at 8 cm depth. The photon dose was unchanged by the presence of the bone material. Measurements

with the 7’AMVEC

neutron beam

At Texas A & M University, Smith measured the dose perturbation due to the presence of lung tissue in irradiated volumes.” Silicon diode dosimeters were inserted into the esophagus and bronchi of anesthetized rhesus monkeys. The measured doses D,,, were compared to the doses D,, predicted by isodose distributions obtained from homogeneous TE phantom measurements. The measured doses exceeded the predicted doses in all cases, and the ratio D,,,/Dp increased with increasing thickness dL of lung and was found to depend on the thickness dr of tissue overlying the lung as well; a dependence of D,,,lDp on the ratio dLldr was demonstrated. U W Measurements At the University of Washington the effects on dose of various inhomogeneities and of phantom geometrical configurations were measured. Benelex (pressed wood), fiberboard and paraffin wax were used as inhomogeneities; these materials have compositions and densities which make them suitable substitute materials for bone, lung and fat, respectively (see Table 1). The irradiation configurations investigated are shown in Fig. 1. Results of measurements for configuration 1 are shown in Fig. 2. The dose correction factors in all cases depend on the thickness t of the inhomogeneity and on the distance d, of the point of interest from the inhomogeneity. For lung the correction factor increases slowly as the distance d, increases, and may approach a constant value for large distances. The correction is independent of the amount do of normal tissue overlying the inhomogeneity. This is consistent with results of @Co measurements,‘.6 but appears to disagree with the in vice measurements reported by Smith.’ For bone, with w = 11 cm and t = 5. I cm the correction factor at d = 12 cm is 1.007. Thus bone contributes only a small amount of scattered radiation over that of tissue. For configuration 2, with w = t = II cm of fiberboard and d = 12 cm, the correction factor is 0.91.

Table

I.

pMcm’)

h(cm)

1.43 0.29 0.93 I .07

7.1 34.5 7.4 7.9

Bsnelex Fiberboard Paraffin wax TE fluid

Confiq. One

/Inhomoqeneity

_-----_---____ --

--_ ---._

30-cm Phantom T.E. Fluid

Confiq. Two

___FZjZJ#-TI_ __----_---

A___

u

I*C* _-_/_

-----Tk/-___[ ‘:

t

d

Fig. I. Irradiation configurations for measurements of tissue-inhomogeneity effects. Phantom size and shape effects on dose rate and depth dose were investigated because the dimensions of anatomical structures are often sufficiently different from “standard” phantoms to affect the tumor doses. (1) Dose rates in a 20-cm cubic phantom were about 1% lower, and in a 15-cm phantom about 2% lower, than the dose rate in the standard calibration 30-cm phantom. (2) For a 10 x IOcm field size, depth-dose distributions (Fig. 3) at shallow depths are not measurably affected by phantom size. Near the back surface of a small phantom they deviate from those for a large phantom (approximately 5% for a IS-cm phantom at 14-cm depth).

Treatment

planning

for neutron

radiation

therapy

0

P. WOOTTON er (I/.

179

nificantly from those measured in the standard 30-cm cubical phantom. For the IO-cm field size. the dose rates at 2-cm depth. for the 20-cm, 15cm and IO-cm dia. cylinders respectively, are 1%. 2% and 4% below that for the 30-cm cubical phantom. The distributions are altered because of “missing” scattering material. For the IO-cm cylinder in the IO-cm field, the depth dose varies from 4% to 13% below that in the 30-cm phantom from 2-cm to 9-cm depth. The effects are even more pronounced when the field size (20-cm) exceeds the phantom size. Here. the lack of the scattering medium results in very significant departures from the idealized phantom data. METHODS OF CORRECTING DOSES FOR THE EFFECTS OF INHOMOGENEITIES -

:,

: 34 -

i

&3X

SC -

I_

s-

_>

~_._~_

: ;7

-

--~

34. ~~~-

4 g. 4 I

2

4

DISTANCE

8

6 FROM

c

,i

INHOMOGENEITY

4 d

6

8

km)

Fig. 2. Experimental results of measurements of correction factors for tissue inhomogeneities. The smooth curves are calculated correction factors based on the TAR method.

I I---~-”

describes the variation of dose D with depth y for y > 5 cm. In the lung-equivalent medium the expression D = 95.5 e-“.03wy

I

! AELATIIVE ---.--

45cm

1 DEeTH

DOSE

1

Phantom

I -_A

.90

Fig. 3. Relative

1

0

5

!

IO

percentage depth dose in lkm. normalized to the depth

(2)

describes the dose variation, again for y > 5 cm. To describe the dose variation for points beyond the lung, where y > yl and y, is the depth of the back

I

/

and McLaren

(1)

Cylindrical phantoms were 30-cm high and 10, 15 and 20-cm in diameter. The experimental configurations are shown in Fig. 4, and results of the measurements are shown in Fig. 5. The central axis depth dose distributions in cylindrical phantoms deviate sig1.05

used by McGinley

For the lung- and bone-equivalent configurations described above. McGinley and McLaren found that the dose variations at sufficiently great depths could be described adequately by appropriate exponential functions. For the lung phantom with only muscleequivalent fluid, the expression D = 12,L e-o.o711?

9.4

.

: 96

C9E

Methods

___--

I

15

20

DEPTH

(cm)

I

25

30

I

20-cm and 30-cm cubical phantoms. dose in a 4Scm cubical phantom.

35

The data are shown

180

Radiation

Oncology

??Physics

Biology

0

r-------l

l--------l I I

~-

_A

I I-

I I

I ;

IO-cm diamter

--~

k.J 10 cm x 10 cm flcld size

I 1

I I

I IO-cm diamater

__-A___ I

--p 20 cmx 20 an field size

------(30 x 30 x 30)

' l-

-----_

-J

(30 x 30 x 30)

----r-1 15cm dlamater l/c\ V

-_--

I

10 an x 10 cm field size

I

r\ I-

--I-20 cm X 20 cm field size

L___--__I (30 x 30 x 30)

_----(30 x 30 x 30)

------

r 20-011

1

diaater

4

I

I

10 an X 10 cm

1

I

1

field size

L ------_ (a

Fig. 4. Schematic

x 30 x 30)

J

diagrams of the experimental

surface of the lung, McGinley and McLaren the expression

20 cm X 20 cm field size

configurations phantoms.

propose

for depth-dose

equivalent

L

-----_

1

(33 x 30 x 30)

measurements

in cylindrical

phantom was given by D, = 113 e-“.cn4v.

D = 95.5

e-o.o3%Y

e-o.cnlI(r-Y,).

This expression arises from the assumption that the dose at depth y > yl will be the dose at ye attenuated by the appropriate amount in the muscle equivalent fluid. Expression (3) correctly predicts the additional measured attenuation of 2% at a point 3 mm beyond the lung. The constant term in equations (2) and (3) presumably depends on the phantom geometry, and may be different for different lung size or thickness of overlying tissue. McGinley and McLaren also found an exponential decrease in dose for measurements with bone material. With 4.8 cm of bone-equivalent material in the beam, the dose at depths greater than 6 cm was given by expression

The correction fore given by

while the dose at the same depth

(4) y in a muscle-

factor,

defined as I&/D,,,, was there-

CF = 0.903 e-“~oo6’Y.

(6)

This factor can be multiplied by doses measured in TE phantoms to correct for the presence of bone material. However the constant term and the attenuation coefficient in equation (6) may both change for different thicknesses of bone or the presence of overlying tissue. Furthermore, equation (6) was derived from “good geometry” measurements, and the presence of off-axis bone material could cause additional changes. Correction

DB = 102 e-0.0806’,

(5)

(3)

method used by Smith

Smith’s measurements of the effect of lung material on neutron doses demonstrates a relationship between the dose correction factor C, defined as the

Treatment plannmg for neutron radiation therapy 0 P. WOOTTON er a/.

too

I

I

I

I

I

CENTRAL

I

AXIS

% DEPTH

DOSE

IO X IO

F.S.

150cm

SSD

zo-

\

I IO, 0

I

I

4

8

Fig. 5. Experimental

I

I 12

16

DEPTH

(cm)

I 20

results of central-axis

I

I 24

28

IOL--0

4

’

8

I

I

I

I

12

16

20

24

DEPTH

depth dose measurements

in cylindrical

1 28

lcml

phantoms.

The data are

compared to that measured in a 30-cm cubical phantom. measured dose D,,, divided by the calculated dose Q, and the ratio drld,, where the dL is the lung thickness and d, is the thickness of overlying tissue: C = (O.Orl)(drldr) + 1.0,

(dLldt) < 6.

(7)

Smith’s data can be fit even simple expressions

more closely

by the

(dLld,) ( 1,

(84

C = O.l2(ddd,)

+ 1.0,

and c = 1.12,

1 < (ddd,) < 6.

(8b)

Equation

(8b) suggests that the dependence of C on for values of (dJd,) < 1 may be small. However, either equation (7) or (8) provides a correction factor accurate to about 2 4%. (dL/d,)

TAR method

The method of Batho’ employs tissue/air ratios (TAR’s) and relative electron densities to account for effects of tissue inhomogeneities in ?Zo photon therapy. The method is based on the configuration described in Fig. 6. In the presence of the inhomogeneity, the dose at P in soft tissue is altered by a factor CF given by CF = TAR(A. dz PC-’ TAR(A, d, I

(9)

where pp denotes the relative electron densities of the

Fig. 6. Schematic diagram showing irradiation of inhomogeneous tissue and the parameters for the TAR method of correction for inhomogeneity data.

two media and TAR(A, d) denotes the tissue/air ratio for area A and depth d. The correction is functionally independent of the thickness do of tissue overlying the inhomogeneity. The method was applied to the UW neutron data. The ratio of electron densities was replaced by the ratio of the neutron beam half value layer of tissue

Radiation

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relative results

Oncology

to that of the inhomogeneity.

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Biology

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Physics

The calculated

are shown in Fig. 2. For lung material.

calculated

correction

agrees with experimental

to better than 4% for the three thicknesses For bone and fat material. small. about 4-5%

the corrections

the

results

indicated. are quite

for 5.1 cm of bone and ‘-3%

4.9cm of fat. The corrections agree to better l-2% with experimental results.

for than

Monte Carlo methods The correction methods discussed so far are all designed to deal with dose perturbations caused by inhomogeneities between the source and the point of interest. However. they cannot correct for perturbations caused by inhomogeneities lying adjacent to the path of direct radiation. Since such artifacts can alter the dose in a neutron field by as much as 10%. methods of dealing with the problem should be developed. One possible method of predicting scattering changes is through the use of Monte Carlo calculations. Using such techniques, BGhm et al. have made depth-dose calculations for 14 MeV neutrons incident on a simulated torso containing lungs, bone. etc.’ However, use of this procedure for routine therapy calculations is probably not practical for two reasons. First, the computer time required for such calculations is usually large and the cost is correspondingly high. Second, the neutron beams currently used for therapy have energy spectra extending well above I5 MeV. in regions where the interaction probabilities are largely unknown. Techniques

using measured

scatter doses

We have developed an approximation technique for calculating dose changes due to off-axis scattering. We assume that the dose at a point can be resolved into a direct component Dd and a scatter component 0,. We further assume that inhomogeneities along a ray between the source and the point of interest cause changes in Dd that can be corrected by exponential factors. To calculate changes in 0, due to inhomogeneities adjacent to the direct ray, we utilize measured scatter

doses. Measurements were made of the scatter from polyethylene samples of various sizes. positioned at a range of angles and distances from the dosimeter. The scatter doses F were normalized to a constant neutron fluence. The scatter from an inhomogeneity of size Z. at a distance S and displacement angle 8, is assumed to be the same fraction of F(& S, 2.) as the ratio Ntinhumoj/N(poly)r where iV is the number of nuclei per volume for the inhomogeneity and for polyethylene: this assumption was suggested by the similarity of the neutron cross sections for H, C and 0 at 8 MeV, our most probable neutron energy. Combining the various assumptions discussed above. we derive an expression relating the modified dose D’(p) at depth p in a medium with scattering and attenuating material present to the dose D(p) at the same depth in a homogeneous TE medium:

D’(p) = D(p)[l -K, -K,I

(10)

where K=~(&~(l-exp[zti(F)]) K s

(11)

F c-Id+s)l% (1/l + d)‘( 1 - N;E,;“).

D(P)

(12)

In the above equations, B = P = 1= ti =

backscatter factor for the appropriate field size, fractional depth dose source-to-surface distance, thickness of the ith material in the direct beam, ho = mean free path for neutrons in TE medium, A, = mean free path for neutrons in the ith material, and d = depth to the surface of the inhomogeneity. Equation (11) gives the factor that corrects for materal in the direct beam, while equation (12) gives the factor that corrects for scatter changes. Measurements were made in a phantom with various inhomogeneities present. and the results, shown in Table 2, were compared to the homogeneous-phantom results corrected according to equation (10). In all cases the calculated values agreed with the measurements to within 3%.

Table 2.

Type of inhomogeneity in beam

Thickness of material in beam (cm)

Type of inhomogeneity adjacent to beam

None None Lung Lung Lung

0 0 6 I1 II

Bone Void None None Void

Calculated

Measured

D’(P)

D’(P)

D(P)

D(P)

0.999 0.98 1.20 1.49 1.48

1.0 0.99 1.21 IS3 1.52

Treatment

planning

for neutron

radiation

As a further test of equation (IO) we calculated the dose reduction at 12 cm depth in phantom caused by replacing 1I cm of off-axis TE material by lung material, while leaving the TE material on axis (see Fig. lb). Reduced scatter from the lung material caused the measured dose to decrease by 9%, compared to a calculated value of 12%. CONCLUSION For many patient configurations the dose calculation techniques used in photon therapy can be applied with good accuracy in neutron therapy.

therapy

0

P. WOOITON et al.

183

However, if bone. fat, or lung tissue are present in the neutron beam. special corrections may be necessary. For bone and fat, a simple exponential correction factor usually predicts the dose attenuation with sufficient accuracy, especially if the inhomogeneity is small. For lung or voids in the beam, the TAR correction technique provides good results if the scatter dose is not greatly altered. If extensive volumes of lung tissue or voids lie adjacent to the beam axis. Monte Carlo techniques or correction methods using measured scatter doses may be required for accurate dose calculation.

REFERENCES 1. Batho, H.: J. Can Assoc. Radiolog. 15: 79. 1964. 2. Biihm, J.. Hehn, G., Kramer. H., Pfister. G., Prillinger, G., Stiller, P.: Proc. 2nd Symp. on Neutron Llosimetry in Biology and Medicine, Munich, 1974, EUR-5273 d-e-f, p. 733. 3. McGinley,

P.. McLaren.

J.: Med. Phy. 1: 219. 1974.

4. McGinley,

P., McLaren, J.: Med. Phy. 3: 181, 1976. 5. Smith, A.R.: In vivo measurements of lung corrections for fast neutron therapy. Med. Phys. 3(6): 39l-3%. 1976. M.E.J.. Gaylord. J.D.: Br. J. Radio/. 43: 349. 6. Young, 1970.