Impact of reduced interval from simulation CT to treatment delivery on dosimetric and positioning accuracy for total marrow lymph-node irradiation.

Purpose: Total marrow (lymph-node) irradiation (TMI/TMLI) is designed to minimize toxicities of conventional total body irradiation in hematopoietic stem cell transplant conditioning. Planning typically relies on a computed tomography (CT) scan acquired many days before treatment (e.g. 15 days; CT-15) to allow time for plan optimization. However, anatomical changes during this interval, influenced by patient condition and concurrent therapies, can compromise dosimetric accuracy. This study evaluates the impact of shortening the CT-to-treatment timeframe to 4 days (CT-4) on dosimetric and positioning accuracy in TMI/TMLI.

Methods: Eighteen patients were enrolled in this prospective study (ClinicalTrials.gov: NCT04976205). Treatment plans, optimized with a multi-isocenter volumetric modulated arc therapy on CT-15, were recalculated on CT‑4 to assess changes in planning target volume (PTV) dose coverage (PTV_D98%). Image matching quality between CT-15/CT‑4 and cone-beam CT acquisitions was assessed with a scale of 1 to 5. Wilcoxon signed-rank test with significance set at p < 0.05 was considered.

Results: A significant reduction in median PTV_D98% was found between CT-15 (98.0%, minimum/maximum [98.0, 98.0]%) and CT‑4 (92.2%, [62.9, 98.9]%). Image matching quality improved in 72% of patients using CT‑4 compared to CT-15. In 11% of cases, relevant discrepancies required re-optimization using CT‑4.

Conclusions: These findings underscore the benefits of a shorter CT-to-treatment timeframe for improving dosimetric and positioning accuracy in TMI/TMLI. Automated planning tools may further enhance TMI/TMLI workflows, particularly for patients undergoing intensive conditioning protocols.