ant J. Radiation Oncology Bid Phys.. Vol. II, PP. 627-630 Printed in the U.S.A. All rights reserved.

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??Technical Innovations and Notes

A BASIS FOR BLOCKING THE SPINAL CORD IN OPPOSED FIELD IRRADIATION

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

‘Northwest Medical Physics Center, P.O. Box 21185, Seattle, WA 9811 I: ‘Radiation Oncology, Mason Clinic, P.O. Box 1930. Seattle, WA 9811 I

The concept of partial tolerance is applied to the equivalent dose formula for spinal cord tolerance to allow for the change in dose when open and blocked fields are used in a course of radiotherapy. The field arrangement considered is parallel opposed irradiation. Certaill assumptions are made regarding the contribution of dose to the spinal cord from open and blocked fields, which allows for the development of an equation to calculate the number of fractions that require spinal cord shielding in the posterior field. The use of a form to facilitate these calculations is described.

Spinal cord tolerance, Blocking.

A safe dose of irradiation to the spinal cord has been assessed to be 4500 cGy at 180 cGy per day, 900 cGy per week. When the daily dose rate rises to 200-250 cGy, the tolerance of the cord to the total dose changes considerably. A common practice has been to introduce into the posterior field a spinal cord block after 4500 cGy. There is, however, still a significant dose delivered to the spinal cord from this field. The purpose of this report is to draw attention to the fraction size dependence of spinal cord tolerance and offer an explicit solution, based on customary manipulation of a time-dose for- mula, to the question of when the posterior block should be inserted.

the exponent for T is small, the error introduced by the variation in this ratio is negligible. We assume that the dose is delivered in equal increments and the total dose D is replaced by the product of the daily dose, d, and the number of fractions. Thus equation 1 can be rewritten as:

ED = N X d X N-o.38 X (N X 1.28)-‘.06

which may be rearranged to:

N= ED X 1.015 ‘.79

d 1 There are a wide variety of time-dose formulae de-

scribing cord tolerance, but all are characterized by a relatively large value of the exponent for the number of fractions compared to the overall time. We have chosen equivalent dose (ED) formula described by Wara et ~1.~ and have taken the liberty of rounding the values of the exponents to two decimal places:

ED = D X N-0.38 X T-0.06 (1)

Thus, if a total dose and fractionation is established that is considered to be spinal cord tolerance, then the equivalent dose can be calculated from equation 1. Given an alternative daily dose, the number of fractions to achieve the same tolerance level can be readily calculated using equation 2. For example, if a daily dose of 200 cGy given in 23 fractions is considered spinal cord tolerance, the equivalent dose is 1139 and a course of 11 fractions at 300 cGy per fraction is an equivalent course.

When daily treatments are given, five fractions per When the spinal cord is shielded during a course of week, the ratio of the overall time (T) to the number of radiotherapy, the lower daily dose results in an increase fractions (N) varies from 1.1 to 1.35 for courses of 10 in the number of fractions required to achieve tolerance. to 30 fractions. The average value in this range is 1.28. Consider a course consisting of a total of N fractions in Thus, we substitute 1.28 X N for T in equation 1. Since which the spinal cord is blocked for B fractions. If Nd

Reprint requests to: Douglas Jones, B.Sc. Accepted for publication 3 October 1984.

627

628 Radiation Oncology 0 Biology e Physics March 1985. Volume il. Number 3

PATIENT NAME: CORD DOSE LIMIT: ED = \\ 35 CLINIC NO.: 4&00 @ aoqJ#y

DATE: 2 \ -DGC- 83

POSTERIOR CORD BLOCKING PARALLEL OPPOSED FIELDS NMPC 12/83

INDICATE F;LD < CENTRAL AXIS LOCATION & THICKNESS SEPCn =

& MINIMUM THICKNESS IN FIELD SEP

TREATMENT PRESCRIPTION:

32 (N) FRACTIONS, MIDPLANE DAILY DOSE MI = 230

ALLOW FOR CORD NOT LOCATED AT MIDPLANE

SEPMIN

Energy <16 16 cm 18 20 22 24 26 28 30 32 31

Co-60 4 MV

d5 1.01 1.02 1.03 1.05 1.08 1.11 1.15 1.19 1.24 1.29 .O 1.01 1.01 1.02 1.03 1.05 1.07 1.10 1.13 1.16 1.20 /_ 0

6 MV 1.0 1.0 1.01 1.01 1.02 1.04 1.06 1.08 1.10 1.13 1.16 10 MV 1.0 1.0 1.01 1.01 1.02 1.03 1.04 1.06 1.09 1.11 1.13

ALLOW FOR THICKNESS VARIATION

SEPMIN /2 = 7.3 SEPCA /2 =

(TAR) SEPMIN ,2 -gx 1.m

ITAR) SEPCA ,‘2 =.83;L =

[I 1*786 = 7.5 MAXIMUM DAILY CORD DOSE d, = 132

B 2 1.67 N - 1.72 y BLOCK CORD FOR FRACTIONS (B)

TOTAL MAXIMUM CORD DOSE = da [N-OAf3]=4342

Calcufated by: Approved by: w-r

Fig. 1. (A) (B) A form used to calculate the number of posterior blocked fractions completed for a typical patient.

Oppcxed field i~diation ??D. JONES el al. 629

is the number of &actions required to achieve a tolerance Ievei in open fields when the daily dose is d and Nb is the number of fractions required to achieve a tolerance

level in blocked fields where the daily dose is b, then the concept of partial tolerance, which has been shown to apply to the spinal cord,’ allows the following:

630 Radiation Oncology 0 Biology 0 Physics March 1985, Volume II, Number 3

N-B

Nd +B<I

Nb (3)

Expanding this relationship using equation 2:

-1.79 (N - W

ED X 1.015 d 1

ED X 1.015 -‘.79 +B

b 1 I 1 (4)

This is somewhat unwieldly and in routine clinical situations some simplifying assumptions can be made to determine the number of fractions that require pos- terior cord blocking.

Blocks used to shield the spinal cord typically project a width of 1.5 to 2 cm at the level of cord and are 5 half-value-layers thick. When a block is included in the posterior field of a parallel opposed air, the dose to the spinal cord will depend on many factors such as cord depth, block width, patient thickness, beam energy and field size. However, a reasonable assumption is that the dose prescribed to midplane from the anterior field is delivered to the cord and 20% of the dose prescribed to midplane is delivered to the cord from the posterior

field. For example, when 180 cGy is prescribed, the cord dose with a posterior block is 108 cGy.

Given the foregoing assumptions the daily dose to the cord in the blocked field is 60% of the prescription dose, i.e., b = 0.6d which is substituted in equation 4 to yield:

B 2 1.67N - 1.72 [ I

ED “79 d (5)

In practice we estimate the maximum dose (d,) to the cord allowing for patient thickness variation, and Figure 1 is a reproduction of the form that has been developed for this purpose. The figure used is adapted from a paper by Brinkly and Masters.* The form is completed with a typical example.

The tolerance level chosen depends upon the particular clinical situation. We believe it is common practice to deliver 4500 cGy in 180 cGy fractions. However, in so doing, patient thickness variations are frequently ne- glected. Thus, the maximum cord dose is on the order of 7% greater than that prescribed. Thus, when the maximum dose is estimated, as is done in our form, it is reasonable to consider a higher tolerance level. The “risky” level is taken from Table 4 of the paper by Cohen and Creditor.3

REFERENCES

1.

2.

Ang, K.K., Van de Kogel, A.J., Van der Schurer, E.: The effect of small radiation doses on the rat spinal cord: The concept of partial tolerance. Int. J. Radiat. Oncol. Biol. Phys. 10: 1487-1491, 1983. Brinkley, D., Masters, H.E.: The depth of the spinal cord below the skin. Br. J. Radiol. 40: 66-68, 1967.

3.

4.

Cohen, L , Creditor, M.: An iso-effect table for radiation tolerance of the human spinal cord. ht. J. Radiat. Oncol. Biol. Phys. 7: 96 l-966, I98 1. Wara, W.M., Phillips, T.L., Sheline, G.E., Schwade, J.G.: Radiation tolerance of the spinal cord. Cancer 35: 1158- 1562, 1975.