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

Abstract

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.

Figure 1

Beam arrangements, isocentre position and targets localization for IMRT and RapidArc. For RapidArc, two arcs, rotating in opposite directions, are delivered in sequence, each arc aiming to geometrically cover primarily either left (red arc) or right targets (blue arc). For IMRT a similar approach was followed. Six fixed gantry field aimed to geometrically cover left targets (red lines showing the central beam axes) and the other 6 (blue lines) the right targets.

Figure 2

Example of dose distributions on axial views for one case. Color wash thresholds were set to 45 or 54 Gy, 95% of the respective dose prescriptions to PTVII and PTVI, and to 10 Gy to represent the total dose bath.

Figure 3

Mean DVHs (averaged over the 10 patients) for the various PTVs.

Figure 4

Mean DVHs (averaged over the 10 patients) of the left and right lungs, heart and healthy tissue (total body volume in the CT set minus the total PTVII).

Figure 5

Example of pre-treatment dosimetric measurements with the GLAaS method and with the PTW-729: a) IMRT, b) RapidArc. In each figure it is shown: i) 2D dose maps calculated by Eclipse and measured at Linac. Planes shown are at 15 mm depth in water (GLAaS) or at isocentre PTW-729); ii) the 2D γ map for GLAaS or the overlay between the calculated 2D dose map and the detector points violating the threshold γ <1; iii) dose profiles along the dashed line shown in the 2D maps, comparing measure and calculation. In the case PTW-729 calculation is the solid line while measurements are the points.