THERApy De-escalation for TESTicular Cancer

Inclusion Criteria: Each patient must meet all of the following inclusion criteria to be enrolled in the study: 1. Willing and able to provide written informed consent. 2. Male. 3. Age ≥ 16 years. 4. Histologically confirmed seminoma (biopsy/orchidectomy) 5. Clinical stage II (standard of care cross-sectional imaging). 6. Ability to comply with the protocol, including but not limited to, completion of the patient-reported outcome questionnaires. rRPLND cohort specific inclusion criteria Participants must meet the following additional inclusion criteria to register for the rRPLND cohort: 1. Stage IIA and \<3cm IIB with unifocal ipsilateral lymph node within rRPLND template. 2. Negative or mildly elevated serum tumour markers, defined as: 1. AFP (alpha-fetoprotein) \<10ng/ml and non-rising on serial testing 2. BhCG (human chorionic gonadotropin) \<50mg/ml 3. LDH (lactate dehydrogenase) \<1.5x upper limit normal 3. Fit for surgery, defined as meeting all of the following criteria: 1. Body mass index (BMI) \<34 2. Charlson comorbidity index ≤3 3. ECOG Performance status 0-1 4. No significant cardio-pulmonary disease, or other uncontrolled intercurrent illness that would limit fitness for surgery in the opinion of the investigator 5. No previous open intra-abdominal surgery Carboplatin AUC10 cohort specific inclusion criteria Participants must meet the following additional inclusion criteria to register for the Carboplatin AUC10 group: 1. Serum tumour markers, defined by IGCCCG "good risk" criteria: 1. AFP \<10ng/ml 2. any BhCG 3. LDH \<2.5x ULN 2. Glomerular filtration rate by EDTA clearance over 25 ml/min (a measured creatinine clearance using Cockcroft and Gault would be allowed if unable to perform EDTA clearance). 3. ECOG Performance status 0-2. 4. Patients must be sterile or agree to use adequate contraception during the period of therapy. Exclusion Criteria: 1. Raised AFP \> 10ng/ml that does not fall to \<10ng/ml following orchidectomy 2. Previous chemotherapy or radiotherapy for the disease under study. 3. Previous or concurrent malignancy other than testicular cancer, unless treated with curative intent and with no known active disease present for ≥2 years before enrolment and felt to be at low risk for recurrence by the treating physician (for example: non-melanoma skin cancer or lentigo maligna; breast ductal carcinoma in situ; prostatic intraepithelial neoplasia; urothelial papillary non-invasive carcinoma or urothelial carcinoma in situ). 4. Any condition that, in the opinion of the investigator, would interfere with evaluation of study intervention or interpretation of patient safety or study results such as medical comorbidities impacting on QoL or medical conditions or other disorders that would affect adherence to study requirements