Negative effect of seroma on breast balloon brachytherapy dosimetry. 
Balloon brachytherapy is commonly used to deliver Accelerated Partial Breast Irradiation (APBI).  Seroma interference is a relatively common phenomenon during APBI.  The negative effect of seroma accumulation on the planning target volume evaluation (PTV_Eval) coverage is not well understood.  This is a dosimetric replanning study on 10 patients with evidence of seroma collection at time of initial computed tomographic simulation around the catheter.  Total dose was 34 Gy given at 3.4 Gy twice a day over 5 treatment days.  A total of 20 plans were generated, 10 plans without accounting for and 10 after subtracting the seroma.  We then compared the changes seen in PTV_Eval between plans as a factor of the seroma volume.  Median age was 62 years (51-83).  Histology was invasive in 7/10 cases and in situ in 3/10.  Median balloon to skin distance was 8.5 mm (3-14).  Median balloon volume was 39 cc (30-104).  Median seroma volume was 3.34 cc (1.13-13.71).  For every 1 cc of accumulated seroma the percentage of PTV_Eval coverage by the 90% isodose line (V90) was found to decrease by 2.45% (P < .0001; confidence interval [CI], 1.87-3.03) and coverage by the 100% isodose line (V100) was decreased by 1.11% (P < .0001; CI, 0.81-1.41).  Fifty percent (5/10) of previously acceptable plans with seroma not accounted for failed to meet the V90 ≥90% requirement after subtracting the seroma.  Accumulation of seroma was associated with a considerable negative impact on PTV_Eval dosimetry with a greater impact on V90 compared with the V100.  Clinicians must be careful in detecting and accounting for such accumulation in treatment plans to prevent underdosing of the at risk target breast tissue.