Concerning the inferior portion of the spinal radiotherapy field for malignancies that disseminate via the cerebrospinal fluid

Abstract

Purpose: Many radiotherapists widen the inferior aspect of the craniospinal irradiation field to encompass the sacroiliac joints and "cover the sacral nerve roots." This is commonly referred to as a "spade" field. Other therapists, however, reject the notion of widening the bottom of the field--feeling that straight field borders are adequate. We have evaluated this controversy by an anatomical study.

Methods and materials: Twenty-five skeletons were measured to ascertain the distance between the outermost portions of the posterior pedicles of L3, L4, and L5; the distance between the outermost portions of the intervertebral foramina between L3-L4, L4-5, and L5-S1 through which the spinal nerves pass; and the width of the most lateral portions of the posterior and anterior foramina of S1 and S2 through which the sacral spinal nerves would pass. Twenty-two cranial spinal irradiation simulator films of patients with medulloblastoma were used to measure the distance between the outermost portions of the posterior pedicles of L3, L4, and L5 and the foramina of S1 and S2. These measurements were then corrected for film magnification.

Results: Skeleton measurements showed that the mean width between the outer portions of the posterior pedicles of L3 and 4.1 cm, for L4 it was 4.4 cm, and for L5 it was 5.1 cm. Measurements of the mean maximum width of the intervertebral foramen for nerve root exit at the bottom of L3 was 4.1 cm, for L4 4.4 cm, and for L5 4.7 cm. The mean distance between the outermost portions of the intervertebral foramen for nerve root exit at the front of S1 was 5.9 cm while it was 5.0 cm for the back of S1. The mean distance between the outermost portions of the anterior foramen of S2 was 5.7 cm and 4.8 cm for the back. Measurements from the 22 simulation films show that the mean maximum width between the outer portion of the posterior pedicles of L3 was 3.3 cm, for L4 3.5 cm, and for L5 3.8 cm. The mean maximum width of the intervertebral foramen for nerve root exit at the bottom of S1 was 4.4 cm and 4.5 cm for S2.

Conclusions: While the caudal end of the craniospinal field needs to be widened by 1.2 to 1.8 cm to encompass the increase in distance between nerve root exits as one moves inferiorly down the spine, coverage of the sacroiliac joints is not necessary.