Application Of The Mean Absolute Dose Deviation To The Dosimetric Gain Analysis Of Left Whole Breast Radiotherapy By Prone-Free Breathing Versus Supine-Deep Inspiration Breath Hold (DiBH)

Abstract

The advantage of prone setup compared with supine for left breast radiotherapy is controversial. We evaluate the dosimetric gain of prone versus supine and aim to identify predictors of gain. Left breast cancer patients who had dual CT planning, prone and supine DiBH, were retrospectively identified. Like the Right breast study in this meeting, radiation doses to heart, lungs, breasts, and tumor bed were evaluated using the mean absolute dose deviation (Mi). The Mi of a structure i in a patient's plan is the dose deviation from 0 for organ at risks, and the deviation from prescribed dose for target volumes. A weighted excess dose deviation (WXD) penalty was computed for each plan, WXD = sum_i (wi Mi), where wi is a penalty weight assigned to the structure i. Constraining the sum of wi to equate 1 allows to read WXD on the same unit scale as the dose prescription. The wi were 0.40 for heart, 0.16 right lung, 0.14 left lung, 0.10 right breast, 0.09 left breast, 0.11 tumor bed. Doses were converted to % of prescription. Mi and WXD prone and supine were compared by paired t-test. Linear regression was used to model the effect of patients' breast and body measures on the WXDs. Cutoffs of statistically significant predictors were computed dividing the model intercept by the coefficient. A total of 118 dual planning prone and supine DiBH were identified. Patients' median age was 58 years (range 36–82), weight 67 kg (41–111), breast volume 487 ml (34–1580), ratio of breast volume/body weight 7.0 mL/kg (0.8–13.3). Median breast depth measures were supine 56 mm (22–106), prone 100 mm (29–173), ratio prone/supine 1.7 (1.2–2.3) and difference Δ prone−supine 44 mm (6–87). The Mi of all structures were lower (i.e. better) with supine, except the left lung for which the ' ' Mi supine was 7.7% of prescription, >4-fold higher than prone (Table 1). The subsequent WXD penalty score was significantly higher with supine, 2.5 vs 2.3 prone, P = 0.048. Overall, 74 of 118 (62.7%) had lower WXD prone, versus 37.3% lower WXD supine, P = 0.006. Among patient's characteristics, the most significant predictors of a gain prone were the breast depth prone/supine ratio, the breast depth difference Δ, the breast depth prone, and the breast volume/body weight ratio, with cutoff values > 1.6, 35 mm, 85 mm, and 6 mL/kg, respectively. Prone was associated with a dosimetric gain in 62.7% left breast cancer patients. Low pendulousness and small breast/body ratio predicted a gain supine, but dual planning prospective validation is needed.Abstract 3777; TableMean absolute dose deviation (Mi) and weighted excess dose deviation (WXD) by setup.MetricSupine Mean (SD)Prone Mean (SD)P valueMi Heart1.9 (0.4)3.4 (2.1)< 0.001Mi Right lung0.5 (0.1)0.7 (0.2)< 0.001Mi Left lung7.7 (2.4)1.8 (1.2)< 0.001Mi Right breast0.8 (0.3)1.7 (1.0)< 0.001Mi Tumor bed1.8 (0.9)2.1 (1.8)0.104Mi Left breast2.4 (0.7)2.8 (1.2)< 0.001WXD Penalty score2.5 (0.6)2.3 (1.1)0.048 Open table in a new tab