Should the Lymphatic System Be Treated as an Organ at Risk? Reframing Lymphedema as a Preventable Radiotherapy-Planning Toxicity

Lymphedema is usually approached as a survivorship complication; yet, many of the injuries that produce lymphatic failure begin during cancer treatment planning. Regional nodal irradiation, chemoradiation, lymph-node surgery, systemic therapy, obesity, infection history, and baseline lymphatic reserve can converge to produce chronic swelling, fibrosis, cellulitis risk, functional limitation, and an impaired quality of life. Despite this, lymphatic drainage pathways are rarely contoured, constrained, or prospectively monitored as organs at risk in radiotherapy practice. This commentary argues that the lymphatic system should enter radiotherapy-planning discussions as a candidate toxicity structure, while cautioning against premature universal dose constraints. Evidence signals are clinically meaningful but not yet protocol-defining: in the MA.20 breast cancer trial, regional nodal irradiation increased lymphedema from 4.5% to 8.4%; in nasopharyngeal carcinoma, mean doses of approximately 58.7 Gy to level IV and 58.6 Gy to levels I–VII were proposed as thresholds associated with moderate/severe facial lymphedema; and gynecological cancer studies report wide lower-limb lymphedema incidence ranges, with radiotherapy, lymphadenectomy, number of nodes removed, and body mass index repeatedly implicated as risk factors. The immediate priority is not mandatory lymphatic sparing, but lymphatic-aware planning: a baseline risk assessment, reproducible candidate contours, dose–volume reporting, selective sparing where oncologically safe, and prospective toxicity monitoring. The author proposed a framework to reflect this argument. Making lymphatic toxicity visible, measurable, and modelled may help shift lymphedema from an accepted late effect to a potentially preventable planning endpoint.