How a carefully selected arsenal of isotopes, based on the latest clinical findings, paved the way to complete remission.

This case of peritoneal carcinomatosis following robot-assisted Da Vinci prostatectomy illustrates the excellent results that can be achieved with personalized theranostics.

Peritoneal carcinomatosis after robot-assisted surgery is not rare and is partly related to the surgical technique itself. It results from the dissemination of tumor cells within the abdominal cavity. Pneumoperitoneum^[1] and continuous irrigation during surgery facilitate the spread of cancer cells even into the most remote peritoneal recesses.

Because prostate cancer typically grows very slowly, such peritoneal metastases may remain clinically undetectable for years. Even PSMA PET/CT with gallium-68 or fluorine-18 can reach its diagnostic limits here – despite PSA levels already well above 1 ng/ml. The lesions are simply too small, PSMA expression too low, and the physiologic background activity of the normal tissue too high.

After mostly unsuccessful local radiotherapy, many patients are started on long-term antihormonal treatment. This approach can mask peritoneal carcinomatosis until the disease eventually progresses to the stage of castration resistance.

In the present case, however, the patient firmly declined empiric hormone therapy and insisted on identifying the cause of the rising PSA. This then led to a ⁸⁹Zr-PSMA PET/CT investigation. The scan not only detected the tiny peritoneal lesions but also confirmed their PSMA expression.

The greater precision of zirconium-89 lies in its much longer half-life of approximately 78 hours (compared with 68 minutes for Ga-68). This allows imaging at later time points, up to several days after tracer injection. During this time, the diagnostic radiopharmaceutical continues to accumulate even in very small or less PSMA-avid lesions while washing out from normal tissues. The result is reduced background activity and sharper contrast.

This finding enabled a highly modern theranostic treatment concept. Based on two recent studies – PSMAddition and VIOLET – the patient received:

• Short antihormonal pretreatment with relugolix to increase PSMA expression (Unlike classical GnRH agonists such as leuprorelin, the GnRH antagonist relugolix avoids the initial testosterone surge (flare effect) and lowers testosterone immediately. After discontinuation, testosterone levels recover quickly, limiting side effects such as fatigue or hot flashes to a short period.)
• PSMA radioligand therapy using a tailored radionuclide combination: one treatment with terbium-161 and two with lutetium-177. Tb-161 is particularly well suited for the treatment of microscopic disease such as the micro-metastases seen in peritoneal carcinomatosis.

The current outcome is excellent, with the patient maintaining maximal quality of life:

• PSA (end of January 2026) – 0.4 ng/ml with normal testosterone levels
• ⁸⁹Zr-PSMA PET/CT – complete metabolic remission.

[1] The targeted insufflation of gas into the abdominal cavity during laparoscopic or robot-assisted surgery to lift the abdominal wall from the intestines and internal organs, thus creating a working space for the camera and instruments.