• Able and willing to provide written informed consent and to comply with the clinical study protocol (CSP).

• Female of age ≥ 18 years.

• Patients with primary advanced high-grade serous or high-grade endometrioid epithelial ovarian, fallopian tube, or primary peritoneal cancer (FIGO Stage IIIB/C or IV).

• Peritoneal and other metastases eligible for IDS to no residual tumour.

• Adverse events recovered to at least Grade 1 from the effects (excluding alopecia) of any prior medical therapy for malignancy.

• Confirmed HR proficient by Myriad MyChoice CDx testing.

• Completed 3 or 4 cycles of NACT with regress or stable disease on diagnostic imaging and assessed to be operable to R0.

• Eastern Cooperative Oncology Group (ECOG) Performance Status Score of 0 to 2 and patient fit enough to undergo IDS and further treatment according to standard of care.

• Adequate renal function:

• • Calculated creatinine clearance using the Cockcroft-Gault formula ≥ 40 ml/min or measured creatinine clearance ≥ 40 ml/min.

⁃ Adequate hepatic function:

∙ Serum bilirubin \< 1.5 x upper limit of normal (ULN), and

‣ Aspartate transaminase and alanine transaminase ≤ 3 x ULN.

⁃ Adequate bone marrow function:

∙ Absolute neutrophil count ≥ 1.0 x 10\^9/l, and

‣ Platelets ≥ 100 x 10\^9/l, and

‣ Haemoglobin ≥ 9 g/dL.

⁃ For females of childbearing potential, a negative pregnancy test must be documented prior to enrolment.

⁃ For females of childbearing potential agreement to use at least one of the following highly effective (failure rate \< 1%) methods of contraception during the treatment period and for at least 9 months if they receive Radspherin®, unless hysterectomy or oophorectomy is performed during IDS.

∙ Total abstinence (when this is in line with the preferred and usual lifestyle of the patient), periodic abstinence (e.g. calendar, ovulation, symptothermal, post ovulation methods) and withdrawal are not acceptable methods of contraception.

‣ Female sterilisation (have had surgical bilateral oophorectomy with or without hysterectomy), total hysterectomy, or tubal ligation at least 6 weeks before enrolment. In case of oophorectomy alone, only when the reproductive status of the woman has been confirmed by follow up hormone level assessment.

‣ Use of oral (oestrogen and progesterone), injected or implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system, or other forms of hormonal contraception that have comparable efficacy (failure rate \< 1%), for example hormone vaginal ring or transdermal hormone contraception. In case of use of oral contraception women should have been stable on the same pill for a minimum of 3 months before taking study treatment.

• Note: In addition to the use of one highly effective method of contraception as listed above, a condom is required for all male partners during the treatment period and for at least 9 months after the dose of IMP, unless vasectomised at least 6 months prior to enrolment.