Patients will receive a single subcutaneous administration of the immunotherapy sasanlimab in a fixed dose of 300 mg in a neo-adjuvant setting, followed by curative-intended surgery.

Patients will also receive a radiolabeled imaging tracer [89Zr]Zr-crefmirlimab berdoxam that entails two intravenous administrations of a fixed dose of 1.5 mg protein dose labelled with activity dose 37 MBq Zirconium-89; one at baseline and one at 2 weeks after sasanlimab injection.

Patients will receive a single subcutaneous administration of the immunotherapy sasanlimab in a fixed dose of 300 mg in a neo-adjuvant setting, and 3 days of non-ablative dose radiation therapy starting with sasanlimab injection. This is followed by curative-intended surgery.

Patients will also receive a radiolabeled imaging tracer [89Zr]Zr-crefmirlimab berdoxam that entails two intravenous administrations of a fixed dose of 1.5 mg protein dose labelled with activity dose 37 MBq Zirconium-89; one at baseline and one at 2 weeks after sasanlimab injection.

6 mL of the study drug will be administered subcutaneous injection in the abdominal fat fold. If SC injections in the abdominal location are not possible, SC injections can be administered in a distributed manner in the thighs. SC injections in the upper extremities (eg, deltoid, upper and lower arm) are not permitted. Any observed abnormality at the injection site (e.g. erythema, induration, ecchymosis, injection site pain, injection site pruritus) will be monitored and judged by the investigator to determine whether a corresponding AE should be reported.

Feasibility will be determined on all patients who have entered the treatment phase of the study, i.e. received at least one course of neo-adjuvant sasanlimab. Feasibility is defined as successful completion of curative surgery within 42 days after start of neo-adjuvant treatment (= day 1). No delays will be allowed. Results will be reported in a descriptive fashion, including percentages, mean and standard deviation, median and range for the time-related measures.

Safety will be defined as the number of CTC grade ≥3 toxicity related to neo-adjuvant sasanlimab. Results will be reported in a descriptive fashion.

Efficacy will be defined as the detection of treatment induced immune related responses in >15% of the patients. Results will be reported in a descriptive fashion, for both cohorts of each n=10 patients (sasanlimab with and without radiotherapy).

Determine changes from baseline in immune profiles, including maturation stages, activation markers, chemokine receptors and functional markers on different lymphocyte (sub-)populations assessed by multipanel flowcytometry on samples obtained at 3 weeks and expressed in percentages and absolute numbers per mL.