Simultaneous integrated boost radiotherapy for bilateral breast: a treatment planning and dosimetric comparison for volumetric modulated arc and fixed field intensity modulated therapy

PURPOSE A study was performed comparing dosimetric characteristics of volumetric modulated arcs (RapidArc, RA) and fixed field intensity modulated therapy (IMRT) on patients with bilateral breast carcinoma. MATERIALS AND METHODS Plans for IMRT and RA, were optimised for 10 patients prescribing 50 Gy to the breast (PTVII, 2.0 Gy/fraction) and 60 Gy to the tumour bed (PTVI, 2.4 Gy/fraction). Objectives were: for PTVs V(90%)>95%, D(max)<107%; Mean lung dose MLD<15 Gy, V(20Gy)<22%; heart involvement was to be minimised. The MU and delivery time measured treatment efficiency. Pre-treatment dosimetry was performed using EPID and a 2D-array based methods. RESULTS For PTVII minus PTVI, V(90%) was 97.8 +/- 3.4% for RA and 94.0 +/- 3.5% for IMRT (findings are reported as mean +/- 1 standard deviation); D(5%)-D(95%) (homogeneity) was 7.3 +/- 1.4 Gy (RA) and 11.0 +/- 1.1 Gy (IMRT). Conformity index (V(95%)/V(PTVII)) was 1.10 +/- 0.06 (RA) and 1.14 +/- 0.09 (IMRT). MLD was <9.5 Gy for all cases on each lung, V(20Gy) was 9.7 +/- 1.3% (RA) and 12.8 +/- 2.5% (IMRT) on left lung, similar for right lung. Mean dose to heart was 6.0 +/- 2.7 Gy (RA) and 7.4 +/- 2.5 Gy (IMRT). MU resulted in 796 +/- 121 (RA) and 1398 +/- 301 (IMRT); the average measured treatment time was 3.0 +/- 0.1 minutes (RA) and 11.5 +/- 2.0 (IMRT). From pre-treatment dosimetry, % of field area with gamma <1 resulted 98.8 +/- 1.3% and 99.1 +/- 1.5% for RA and IMRT respectively with EPID and 99.1 +/- 1.8% and 99.5 +/- 1.3% with 2D-array (DeltaD = 3% and DTA = 3 mm). CONCLUSION RapidArc showed dosimetric improvements with respect to IMRT, delivery parameters confirmed its logistical advantages, pre-treatment dosimetry proved its reliability.