Background and purpose: The sacrum as radiation target, raises a conceptual question: should the structure be regarded as a single unit or 5 distinct bones. If the entire sacrum must be irradiated there is a higher risk of rectal morbidity.
Materials and methods: Images of 53 patients with sacral metastases were reviewed. The extent of sacral involvement was documented. The location of the rectum was recorded relative to the individual sacral bones.
Results: In 37.7% only S1 and S2 were involved by metastatic disease. In 41.5% there was metastatic involvement of S1-S3. In 1 patient there was involvement of S5 only. In 10 cases the entire sacrum was infested by metastatic disease. The rectum never extended to the height of S1. In 38% the upper pole of the rectum reached the S3 level. In toto, there were 64.2% where the inferior extension of sacral metastatic involvement did not overlap the upper pole of the rectum. Palliation of pain was achieved in 19/20 patients treated with partial sacral irradiation.
Conclusions: The distal part of the sacrum is rarely involved in the metastatic process. Avoidance of radiation therapy to the lower sacrum simultaneously enables effective palliation and sparing of the adjacent rectum.
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