Evaluation of fixed-jaw IMRT and tangential partial-VMAT radiotherapy plans for synchronous bilateral breast cancer irradiation based on a dosimetric study

Abstract

Purpose: To investigate the fixed-jaw intensity-modulated radiotherapy (F-IMRT) and tangential partial volumetric modulated arc therapy (tP-VMAT) treatment plans for synchronous bilateral breast cancer (SBBC).

Materials and method: Twelve SBBC patients with pTis-2N0M0 stages who underwent whole-breast irradiation after breast-conserving surgery were planned with F-IMRT and tP-VMAT techniques prescribing 42.56 Gy (2.66 Gy*16f) to the breast. The F-IMRT used 8-12 jaw-fixed tangential fields with single (sF-IMRT) or two (F-IMRT) isocenters located under the sternum or in the center of the left and right planning target volumes (PTVs), and tP-VMAT used 4 tangential partial arcs with two isocenters located in the center of the left and right PTVs. Plan evaluation was based on dose-volume histogram (DVH) analysis. Dosimetric parameters were calculated to evaluate plan quality; total monitor units (MUs), and the gamma analysis for patient-specific quality assurance (QA) were also evaluated.

Results: For PTVs, the three plans had similar D_mean and conformity index (CI) values. F-IMRT showed a slightly better target coverage according to the V_100% values and demonstrated an obvious reduction in V_105% and D_max compared with the values observed for sF-IMRT and tP-VMAT. Compared with tP-VMAT, sF-IMRT was slightly better in terms of V_100% , V_105% and D_max . In addition, F-IMRT achieved the best homogeneity index (HI) values for PTVs. Concerning healthy tissue, tP-VMAT had an advantage in minimizing the high dose volume. The MUs of the tP-VMAT plan were decreased approximately 1.45 and 1 times compared with the sF-IMRT and F-IMRT plans, respectively, and all plans passed QA. For the lungs, heart and liver, F-IMRT achieved the smallest values in terms of D_mean and showed a significant difference compared with tP-VMAT. Simultaneously, sF-IMRT was also superior to tP-VMAT. For the coronary artery, tP-VMAT achieved the lowest D_mean , while the value for F-IMRT was 2.24% lower compared with sF-IMRT. For all organs at risk (OARs), tP-VMAT was superior at the high dose level. In contrast, sF-IMRT and F-IMRT were obviously superior at the low dose level. The sF-IMRT and F-IMRT plans showed consistent trends.

Conclusion: All treatment plans for the provided techniques were of high quality and feasible for SBBC patients. However, we recommend F-IMRT with a single isocenter as a priority technique because of the tremendous advantage of local hot spot control in PTVs and the reduced dose to OARs at low dose levels. When the irradiated dose to the lungs and heart exceed the clinical restriction, two isocenter F-IMRT can be used to maximize OAR sparing. Additionally, tP-VMAT can be adopted for improving cold spots in PTVs or high-dose exposure to normal tissue when the interval between PTVs is narrow.

Keywords: dosimetric comparison; fixed-jaw intensity-modulated radiotherapy (F-IMRT); synchronous bilateral breast cancer (SBBC); tangential partial volumetric modulated arc therapy (tP-VMAT).

Figure 1

Beam arrangements and isocenter positions for (a) single fixed‐jaw intensity modulated radiotherapy (sF‐IMRT), (b) F‐IMRT, and (c) tangential partial volumetric modulated arc therapy. For F‐IMRT, six fixed‐jaw fields aimed to geometrically cover the left breast (red lines) and the other six (yellow lines) focused on the right breast. The interval angle for the ipsilateral fields was 6–10 degrees. For tangential partial VMAT, two partial arcs, rotating in opposite directions, aimed to geometrically cover primarily either the left (red line) or right (yellow line) breast. Dashed sectors were avoidance areas where the accelerator beam was off while the gantry was rotating.

Figure 2

Example of fixed jaw use to reduce exposure of (a) right lung and (b) heart.

Figure 3

Example of fixed X2 jaw to reduce lung exposure. The X2 jaw can only exceed the central axis by 2 cm, and the white dotted line is where the X2 jaw is expected to be located.

Figure 4

Case of isodose distribution on transverse, coronal, and sagittal views of (a) single fixed‐jaw intensity modulated radiotherapy (sF‐IMRT), (b) F‐IMRT, and (c) tangential partial volumetric modulated arc therapy for synchronous bilateral breast cancer.

Figure 5

Mean dose‐volume histograms (averaged over the 12 patients) of the left (a) and right (b) lungs, heart (c), coronary artery area (d), and liver (e).