Sacral chordoma with S3 involvement
A 47-year-old man presents with a 6-month history of low back pain and perineal numbness. MRI shows a 7 cm sacral chordoma centred at S3, with the superior margin at the S2–S3 disc and the inferior margin at S4, abutting the anterior sacral cortex and displacing the rectum anteriorly. There is no evidence of metastasis. He is referred for definitive radiotherapy after a biopsy. The team must decide on target volumes, whether to include the presacral space and the sacral nerve roots, and how to spare the bowel and bladder.
1. Describe the anatomy of the sacrum and explain how the tumour's position at S3 affects your target volume.
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The sacrum comprises five fused vertebrae, with the sacral canal running posteriorly and the sacral nerve roots exiting through the anterior sacral foramina. The S3 level is where the sacral plexus forms from the S2–S4 roots, and the tumour at S3 abuts the anterior cortex, placing the presacral space and the S2–S4 nerve roots at risk. The superior margin at the S2–S3 disc means the S2 body is at risk of microscopic involvement.
The CTV should include the entire sacrum from S1 to S5, the presacral space anterior to the sacrum, and the sacral nerve roots bilaterally. The superior border should be at the L5–S1 disc to cover the lumbosacral junction, and the inferior border should include the coccyx. The presacral space contains the median sacral vessels and lymphatics, which are at risk of spread.
2. What organs at risk must be contoured, and what dose constraints would you apply to the bowel and bladder?
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Organs at risk include the rectum (from the rectosigmoid junction to the anal canal), the bladder and bladder neck, the small bowel, the sigmoid colon, the sacral nerve roots, and the iliac vessels. The rectum is directly anterior to the sacrum and is the dominant constraint; aim for V60 < 30%, V50 < 50% and Dmax < 70 Gy. The bladder should be constrained to V65 < 25% and V45 < 50% to reduce cystitis and fistula risk.
The small bowel should be kept to V45 < 195 cc, and the sacral nerve roots should be constrained to Dmax < 70 Gy to reduce the risk of neuropathic pain and incontinence. Because the tumour abuts the anterior sacral cortex, achieving these constraints may require IMRT or proton therapy, and a spacer may be considered.
3. The patient develops faecal incontinence and perineal numbness after 50 Gy. What is the differential diagnosis and how does it affect your plan?
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The differential includes tumour progression into the sacral nerve roots, radiation-induced sacral plexopathy, and radiation proctitis. Perineal numbness suggests involvement of the S2–S4 roots or the pudendal nerve, while faecal incontinence suggests sphincter dysfunction from direct tumour invasion or radiation effect. An MRI of the sacrum and pelvis should be performed to distinguish progression from radiation change.
If there is progression, the plan should be reviewed for geographic miss, and the field may need to extend to the sacral nerve roots and presacral space. If it is radiation plexopathy, the dose to the nerve roots should be limited for the remaining fractions, and symptomatic management with analgesia and bowel care should be instituted. Documentation of the constraint compromise is essential.