Purpose
Total body irradiation (TBI) is an integral component of conditioning regimens for hematopoietic stem cell transplantation. Despite similar biological objectives, planning and delivery of TBI vary substantially across institutions. The aim of this study was to quantitatively assess inter-institutional and technique-dependent variability in contemporary TBI planning by comparing dose distributions generated under standardized conditions across multiple centers in Germany and Switzerland.

Methods
Twenty-four radiotherapy centers received anonymized total-body computed tomography (CT) datasets of a standardized dummy patient in head-first supine and prone orientations, including predefined points of interest (POI) and organs at risk (OARs). Each center was asked to generate TBI plans for two prescription levels (8 Gy in 4 fractions and 12 Gy in 6 fractions) using its local planning standards, with the entire body defined as the planning target volume (PTV). Submissions included treatment-planning-system (TPS)–based 3D dose distributions and in-house table-based calculations. TPS plans were centrally imported into a TPS for extraction of target coverage and OAR dose metrics, while table-based submissions were evaluated using aggregated POI doses.

Results
In total, 45 TBI treatment plans from 24 centers were analyzed, including 24 plans for 8 Gy and 21 plans for 12 Gy. Both TPS-based and table-based planning approaches were represented across a broad range of clinical techniques, including volumetic modulated arc therapy (VMAT), Tomotherapy, sweeping beam, static field, bilateral, and translation couch setups. Marked inter-institutional variability was observed, particularly for lung, lens, and kidney doses. At the 12 Gy prescription level, mean lung doses differed by more than 3 Gy between centers, corresponding to relative variations of up to approximately 30%. Conformal intensity-modulated techniques (VMAT and Tomotherapy) showed the highest consistency across institutions, whereas non-conformal approaches exhibited substantially larger dose variability. Increased lung sparing at 12 Gy was consistently associated with reduced target coverage compared to 8 Gy.

Conclusion
This study provides the first standardized multicenter planning-based comparison of contemporary TBI approaches in Germany and Switzerland. By quantifying inter-institutional variability in dose distributions under uniform conditions, the results outline the current heterogeneity of clinical practice and the range of achievable dosimetric outcomes. Rather than defining a reference standard, these findings establish a structured benchmark of existing planning strategies and highlight the need for harmonization, thereby providing a foundation for future consensus development and end-to-end validation efforts.