3351 - Toxicity and Efficacy of Radiotherapy with or without Pelvic Prophylactic Radiotherapy in Patients with Oligometastatic Prostate Cancer

Purpose/Objective(s):

The value of pelvic prophylactic irradiation in prostate cancer radiotherapy has always been controversial. The application of radiotherapy in oligometastatic prostate cancer is becoming increasingly widespread. Whether further combination of pelvic prophylactic irradiation on the basis of drug therapy and radiotherapy can bring additional benefits is the question to be explored in this study.

Materials/Methods:

In this phase II prospective study, patients with oligometastatic prostate cancer scheduled to receive radiotherapy were enrolled. The inclusion criteria were as follows: 1-3 bone metastases confirmed by PSMA PET, regional lymph node metastases were allowed. Patients with a history of radical prostatectomy were excluded. According to the opinions of the attending physicians, patients were assigned to Group A: pelvic lymph node drainage area radiotherapy + prostate and metastatic lesion radiotherapy group (PRT), or Group B: prostate and metastatic lesion radiotherapy group (NPRT). RT regimens: 70 Gy/25 fx for the prostate area, 65 Gy/25 fx for pelvic metastatic lymph nodes and bone metastases within the pelvis, 47.5 Gy/25 fx for pelvic prophylactic irradiation, and 30 - 37.5 Gy/5 fx for bone metastases outside the pelvis. The drug therapy after radiotherapy was a combination of at least two-year ADT ± ARPI). The primary endpoint was progression-free survival. Key secondary endpoints included acute bone marrow toxicity and late toxicity 6-month after radiotherapy.

Results:

From May 2024 to July 2025, 133 consecutive patients with oligometastatic prostate cancer were enrolled, with 86 cases in Group A and 47 cases in Group B. The median age was 77 years. 72.2% had pelvic metastatic lymph nodes. 81.3% had baseline PSMA PET/CT. 93.2% received first-line ADT combined with ARPI. 8.9% received docetaxel. The median interval from diagnosis to radiotherapy was 6 months. 82.7% were mHSPC and 17.3% were mCRPC. The median follow-up time was 13 months (range: 6 - 20 months). Group A: leukopenia, grade 1 was 22.4%, grade 2 was 17.1%; lymphopenia, grade 1 was 18.4%, grade 2 was 30.2%, grade 3 was 40.8%, and grade 4 was 2.6%. In group B, leukopenia, grade 1 was 16.2%, and grade 2 was 8.1%; lymphopenia, grade 1 was 27.0%, grade 2 was 21.6%, grade 3 was 10.8%. For late bone marrow suppression 6 months after radiotherapy, in Group A, lymphopenia, grade 1 was 25% and grade 2 was 27.5%; in Group B, lymphopenia, grade 1 was 28.6%, and grade 2 was 4.8%. There was no significant difference in 1-year PFS between Group A (92.2%) and Group B (93.6%).

Conclusion:

For patients with oligometastatic prostate cancer, compared with radiotherapy for the primary and all metastatic lesions, adding radiotherapy to pelvic lymph node drainage area significantly reduces lymphocytes, and this inhibitory effect persists for up to 6 months after radiotherapy. However, there was no significant improvement in short-term efficacy. Extensive pelvic prophylactic irradiation should be carried out with caution.