Should breast/thoracic wall and lymph node volumes have the same margin in hypofractionated radiotherapy?

Purpose.The aim of this study was to investigate if the same planning target volume margins should be used for the breast/thoracic wall and lymph nodes when treated simultaneously with hypofractionated radiotherapy in deep inspiration breath-hold (DIBH) and free breathing (FB).Methods.For 80 patients (40 treated in FB and 40 in DIBH), nine anatomy sites were evaluated. Three were located in relation to the lymph node region and six in relation to the breast/thoracic wall. For three treatment fractions, a cone beam CT was acquired before and after the treatment and compared to the planning CT. Margins were calculated based on the inter- and intra-fractional uncertainties, ensuring 90% of the patients receiving 95% of the prescribed dose to the clinical target volume. Margin calculation was done for the full patient data set as well as for a subgroup of patients with set-up errors below the clinical action level, excluding patients with set-up errors larger than 7 mm.Results.For the full patient data set, margins of up to 14 mm were calculated. However, for patients with set-up errors below the clinical action level, the largest margins calculated for the lymph node region for treatments in DIBH were 8.5 mm, 8.7 mm and 6.6 mm in the vertical, longitudinal, and lateral directions respectively compared to 8.4 mm, 7.9 mm and 5.9 mm for treatments in FB. For the breast/thoracic wall, the largest margins were 5.9 mm, 6.4 mm and 5.5 mm for treatments in DIBH and 5.8 mm, 7.2 mm and 5.8 mm for treatments in FB in the vertical, longitudinal and lateral directions respectively.Conclusion.Treatments in DIBH and FB can use the same margins. However, the lymph node region requires a larger margin than the breast/thoracic wall region, due to greater anatomical mobility.