The primary objective of this study is to compare the 3-year overall survival of stage III NSCLC patients during follow-up surveillance with 18F-Fluorodeoxyglucose Positron Emission Tomography/ Computerized Tomography (18F FDG PET/CT) versus follow-up with conventional CT surveillance.

Participants will receive usual care until 3 years of follow-up (control group) with additional whole-body 18F FDG PET/CT scans during follow-up visits at 6 months, 12 months, 18 months, 24 months, and 36 months of follow-up in the intervention group.

Other tasks include:

• filling in quality of life (QOL) questionnaires at every time point;
• participating in an interview evaluating the addition of the 18F FDG PET/CT scans (optional);
• collecting blood at the follow-up time points for our secondary endpoint (optional).

Researchers will compare the usual care control group with the intervention group to see if the additional 18F FDG PET/CT scans are (cost-)effective.

Stage III non-small cell lung cancer (NSCLC) patients are at high risk of developing recurrences (50-78%) during follow-up. With more effective treatments available for patients with oligometastatic disease, early detection of tumor recurrence can prolong survival and health-related quality of life and thereby lower the disease burden. With the use of 18F FDG PET/CT during follow-up, recurrences may be detected earlier at an oligometastatic state when curative-intent treatment is still possible.

Primary objective:

- The primary objective of this study is to compare the 3-year overall survival of stage III NSCLC patients during follow-up surveillance with 18F FDG PET/CT versus follow-up with conventional CT surveillance.

The secondary objectives of this study are:

• To compare the 2-year overall survival of stage III NSCLC patients during follow-up surveillance with 18F FDG PET/CT versus follow-up with conventional CT-based surveillance (interim analysis);
• To compare the number of detected (symptomatic and asymptomatic) recurrences of stage III NSCLC patients during follow-up surveillance with 18F FDG PET/CT versus follow-up with conventional CT-based surveillance;
• To compare the event-free survival of stage III NSCLC patients during follow-up surveillance with 18F FDG PET/CT versus follow-up with conventional CT-based surveillance;
• To determine the cost-effectiveness of 18F FDG PET/CT versus conventional CT-based surveillance during follow-up of stage III NSCLC patients;
• To compare the effect of 18F FDG PET/CT versus conventional CT-based surveillance on health-related quality of life during follow-up of stage III NSCLC patients;
• To assess the beneficial value of ctDNA in the detection of recurrences during follow-up in stage III NSCLC patients;
• To identify patients' experiences with the additional 18F FDG PET/CT scans in the follow-up of stage III NSCLC patients.
• To assess differences in type of treatment following recurrence during follow-up in stage III NSCLC patients.

Primary analyses will be performed on an intention-to-treat basis as well as per protocol. Kaplan-Meier curves with stratified log-rank 2-sided tests will be used to compare the survival between groups. In case of empty strata, strata will be collapsed. The clinical relevance of the difference will be primarily expressed in terms of 3-year survival of the intervention versus the control group.

Eligible for this study are patients with stage III NSCLC (8th edition TNM Classification) who (are about to) start(ed) follow-up care (which may include adjuvant treatment) at a participating hospital. Patients may already be included during their curative intent treatment. Patients enter a screening period that runs until their randomization.

Inclusion Criteria:

To be eligible to participate in this study, a subject must meet all of the following criteria at the timing of randomization:

• Cytological or histologically proven stage III non-small cell lung cancer before start of curative intent treatment
• Treated with curative intent and started follow-up care
• All adjuvant treatments are permitted as co-intervention during follow-up care
• Age 18 years or older
• ECOG Performance Status classification 0-2 at moment of inclusion
• Written and signed informed consent by the patient or patient's representative (with the understanding that consent may be withdrawn by the patient or patient's representative at any time without consequences to future medical care)

Exclusion Criteria:

A potential subject who meets any of the following criteria will be excluded from participation in this study:

• Life expectancy shorter than 6 months at the end of curative intent treatment
• Evidence of recurrence after end of curative intent treatment and before randomization (4 months follow-up)
• Any condition that, in the opinion of the investigator, would interfere with evaluation of the intervention or interpretation of HRQOL or other study results.

Final (definitive) versions of data used for analysis will be made findable and shared for reuse and/or verification. Documentation/codebooks necessary for understanding the data will be made findable and shared for reuse and/or verification. Left-over blood samples will be made findable and can be accessed to share for reuse and/or verification.

Metadata will be published in DANS. Metadata of the blood samples and scans will also be published in BBMRI. Crossreferences between DANS and BBMRI will be included.

A DOI will be assigned to the whole dataset published in DANS Data Station Life, Health and Medical Sciences.

Data will become available after NVALT31- PET study investigators have performed the analyses needed for this study.

All data will be available on request. Access to the data will be guarded by the CMO Radboudumc and the NVALT31-PET data access committee (DAC) and may be requested by other researchers from Radboudumc and Amsterdam UMC. Also, the data can be used in collaboration with other groups (abroad).