Post-prostatectomy biochemical recurrence with a rising PSA
A 66-year-old man underwent radical prostatectomy and pelvic lymph node dissection 3 years ago for Gleason 4+3 prostate cancer, pT3a with a positive apical margin. His PSA was undetectable for 2 years, then rose to 0.2, 0.35 and now 0.6 ng/mL over 9 months. PSMA PET shows a single 8 mm avid node in the left obturator fossa, with no other sites of uptake. He is referred for salvage radiotherapy. The radiation oncologist must decide on target volumes, whether to treat the whole pelvic nodal basin, and how to account for the vesicourethral anastomosis and the positive apical margin.
1. Describe the anatomical boundaries of the prostate bed clinical target volume and explain how the positive apical margin modifies your volume.
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The prostate bed CTV is bounded anteriorly by the posterior surface of the pubic symphysis and the retropubic fat, posteriorly by the anterior rectal wall (mesorectal fascia), superiorly by the vesicourethral anastomosis and bladder neck, inferiorly by the urogenital diaphragm and levator ani, and laterally by the levator ani and obturator internus. The vesicourethral anastomosis sits at the apex of the prostate bed and is the most common site of local recurrence, so it must be included with a margin.
A positive apical margin increases the risk of residual disease at the apex and along the urethra. The CTV should extend inferiorly to include the anastomosis and at least 1 cm of proximal urethra into the urogenital diaphragm, and posteriorly to cover the rectourethralis and Denonvilliers' fascia. The superior border should include the bladder neck and the seminal vesicle bed if the vesicles were involved. A 5–7 mm planning margin is typical, reduced posteriorly to 3–5 mm to respect rectal tolerance.
2. The PSMA-avid obturator node lies between the external iliac vein and the obturator nerve. Explain the anatomy of this region and whether whole-pelvis radiotherapy is justified.
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The obturator fossa is bounded superiorly by the external iliac vein, inferiorly by the obturator nerve and vessels as they enter the obturator canal, medially by the bladder and lateral pelvic wall, and laterally by the obturator internus. The obturator nerve arises from L2–L4 and descends medial to the psoas before entering the pelvis, so an involved node here indicates spread along the internal iliac/obturator chain, which drains the prostate via the seminal vesicles and base.
A single PSMA-avid node in this region with a low PSA (0.6) suggests oligometastatic nodal recurrence. Whole-pelvis radiotherapy (levels I–IV and presacral) is justified because the obturator node is part of the pelvic nodal drainage and there is a high probability of microscopic disease in adjacent nodes; the alternative is a focal boost to the node plus prostate bed alone. The node should receive a simultaneous integrated boost to approximately 60–66 Gy while the elective pelvis receives 45–50 Gy.
3. What structures must be contoured as organs at risk, and what dose constraints would you prioritise in this plan?
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Organs at risk include the rectum (from the anal verge to the rectosigmoid junction), the bladder and bladder neck, the bowel (small bowel and sigmoid), the femoral heads and necks, the penile bulb, and the urethra. The rectum is the dominant constraint because the prostate bed lies directly anterior to it; aim for V70 < 15–20%, V60 < 35% and V50 < 50%. The bladder neck and urethra should be constrained to reduce incontinence and stricture risk, and the femoral heads to V45 < 5%.
With whole-pelvis treatment, the bowel constraint becomes important: keep the small bowel V45 < 195 cc and the sigmoid V45 < 150 cc. The penile bulb should be kept below 30 Gy where possible to preserve erectile function. Prioritise rectal and bowel constraints over target coverage only if coverage falls below 95% of the prescription, and document any compromise.