Determination of critical dosimetric parameters for breast radiation using forward-planned segmented fields for intensity modulation

The purpose of this study was to determine factors associated with acute skin toxicity from breast radiation for optimizing forward-planned intensity modulation. Treatment plans in 100 patients who received breast radiation using three-dimensional treatment planning were analyzed. Fifty-two patients were treated with tangent fields using wedges (nonsegmented), and 48 patients were treated with forward-planned fields segmented by a multileaf collimator to modulate intensity. Clinical and dosimetric variables were recorded. Acute skin toxicity was prospectively documented using a standard scale. Body weight, breast target volume, maximum body dose (encompassing 10 mL), and volume of body receiving >50 Gy and 55 Gy (V50Gy, and V55Gy) were associated with acute toxicity. Patients treated with segmented plans had significantly larger breast targets and were treated to lower prescription isodoses, confounding comparison with nonsegmented plans. Consequently, datasets from patients treated with segmented plans were used to design new nonsegmented plans for paired comparison. Segmented plans were superior with respect to dosimetric endpoints predictive of toxicity in this paired comparison. Limitations of 55 Gy for maximum body dose and 1100 mL for V50Gy appeared to be appropriate values to guide forward treatment planning of segmented fields.