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Abstract: Purpose: Accelerated partial breast irradiation (APBI) continues to increase in popularity. Up to 14% of patients treated with the MammoSite (MS) report some degree of chronic pain, which may be related to chest wall toxicity. Reports from several institutions using the multicatheter (MC) technique have not shown associated elevated chest wall toxicity. Additionally, a recent investigation has suggested that increased toxicity may occur with the MS when the dose to the chest wall exceeds 125% of the prescribed dose. This investigation compares the skin and chest wall doses of a cohort of patients treated with the MC technique to a group treated with the MS. Methods and materials: The dosimetric data for 43 patients treated with the MC technique and 83 patients treated with the MS at Virginia Commonwealth University were reviewed. This cohort represents consecutively treated patients from our most recent experience to minimize any learning curve effect on dosimetry. Plans were generated using 3D software (Brachyvision, Varian Medical Systems, Inc., Palo Alto, CA). Multiple dwell positions were used for all MS patients to optimize dose delivery. The minimum distances from the planning target volume to the skin and chest wall were calculated, as well as the maximum doses delivered to the skin and chest wall. Results: The mean skin distances for patients treated with the MC technique and the MS were 0.5 and 0.9cm, respectively (p <0.002). Despite the significantly smaller mean skin distance, the mean skin dose for the MC technique was only 2.3Gy per fraction (67% of prescription dose). The mean skin dose for the MS was 3.2Gy per fraction (94% of prescription dose, p <0.001). The mean chest wall distance was 0.9cm for the MC technique and 1.0cm for the MS (p =0.55). Again, the mean chest wall dose for the MC technique was only 2.3Gy per fraction (67% of prescription dose). The mean skin dose for the MS was 3.6Gy per fraction (105% of prescription dose, p <0.001). The percentage of patients receiving skin doses in excess of 125% for the MC and MS were 0% and 9.6%, respectively. The percentage of patients receiving chest wall doses in excess of 125% for the MC and MS were 0% and 38.6%, respectively. Conclusion: The MC technique results in more conformal dose delivery, with significantly lower mean skin and chest wall doses. Treatment with the MS was associated with significantly more patients receiving doses to the skin or chest wall in excess of 125% of the prescription. Given the limited followup available for the MS, and the significant dose delivered to the chest wall, the use of this device may be associated with a higher incidence of late chest wall toxicity than previously expected. [Copyright &y& Elsevier]