Background: Despite improvements in diagnostics and therapy, the majority of lung tumours are diagnosed at advanced stage IV with a poor prognosis. Due to the nature of an incurable disease, patients need to engage in shared decision making on advance care planning. To implement this in clinical practice, effective communication between patients, caregivers and healthcare professionals is essential. The Heidelberg Milestones Communication Approach (MCA) is delivered by a specifically trained interprofessional tandem and consists of four milestone conversations (MCs) at pivotal times in the disease trajectory. MC 1 (Diagnosis): i.e. prognosis; MC 2 (Stable disease): i.e. prognostic awareness; MC 3 (Progression): i.e. reassessment; MC 4 (Best supportive care): i.e. end of treatment. In between MCs, follow-up calls are carried out to sustain communication. This study aimed to assess to what extent the MCA was implemented as planned and consolidated in specialized oncology practice. Methods: A prospective observational process evaluation study was conducted, which focused on the implementation fidelity of the MCA. All MCs during two assessment periods were included. We analysed all written records of the conversations, which are part of the routine documentation during MCs and follow-up calls. Adherence to key aspects of the manual was documented on structured checklists at the beginning of the implementation of the MCA and after 6 months. The analysis was descriptive. Differences between the two assessment periods are analysed with chi-square tests. Results: A total of 133 MCs and 54 follow-up-calls (t1) and of 172 MCs and 92 follow-up calls (t2) were analysed. MC 2 were the most frequently completed conversations (n = 51 and n = 47). Advance care planning was discussed in 26 and 13% of MC 2 in the respective assessment periods; in 31 and 47% of MC 2, prognostic awareness was recorded. The most frequently documented topic in the follow-up calls was the physical condition in patients (82 and 83%). Conclusion: The implementation of a trajectory-specific communication concept was largely successful. Additional studies are needed to understand how fidelity could be further improved. Trial registration: DRKS00013469 / Date of registration: 22.12.2017.