Treatment Plan Quality for Deep Inspiration Breath-Hold Breast Cancer Radiation Therapy - Comparison between Conventional Technique and Modulated Approaches

Abstract

Introduction: Breast cancer remains a significant health concern globally, impacting millions of individuals each year [1]. Radiation therapy, an adjuvant to surgery and chemotherapy, aims to eliminate residual cancer cells and inhibit tumor growth [2]. The most common breast cancer radiation therapy technique in Sweden is 3D Conformal Radiation Therapy (3DCRT) with static photon beams. 3DCRT is a widely accessible and robust treatment technique concerning positioning uncertainties, respiratory movements, and breast swelling. However, the dose is not well-conformed to the target volume, resulting in greater exposure to nearby healthy tissues such as heart and lungs. [3]. Thus, exploring new techniques is crucial, especially for advanced stages. Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) offer precise radiation delivery and high conform dose to the target region while minimizing damage to nearby healthy tissues, however, they also increase low-dose exposure to nearby healthy tissues[3]. Additionally, there has been concern about the robustness of these modulated techniques, which has been a significant barrier to their widespread adoption. Therefore, hybrid techniques combining 3DCRT with IMRT (Hybrid-IMRT) or VMAT (Hybrid-VMAT) have been developed to improve dose conformity and reduce off-target absorbed dose exposure. The aim of this study is to compare the quality of treatment plans for left-sided breast cancer across four techniques: 3DCRT, Hybrid-IMRT, Hybrid VMAT, and VMAT. The assessment will include homogeneity and conformity indices, dose distribution to target volumes, and nearby organs at risk (OARs) as planned dose distribution. Robustness against positioning uncertainties and breast swelling as delivered dose distribution. Additionally, the feasibility of Deep inspiration breath hold (DIBH) for each technique will be analyzed. Material and Methods: Computed tomography (CT) data from nine female patients under 40 years with left-sided breast cancer previously treated at Sahlgrenska University Hospital were analyzed. The prescribed absorbed dose was 40.05 Gy to the whole breast and 48 Gy to the lumpectomy cavity. All CT data was acquired during DIBH. Treatment planning was conducted for a Varian TrueBeam linear accelerator in the Varian Eclipse (Varian Medical Systems, Siemens Healthineers) treatment planning system (TPS). Four treatment techniques were evaluated: 3DCRT, Hybrid-IMRT, Hybrid-VMAT, and VMAT. The data is a subset from the study by Svensson et al. [4]. Dose distributions to target volumes were evaluated according to Sweden national guidelines [2], with OARs including the heart, lungs, left anterior descending artery (LAD), and contralateral breast [2]. Homogeneity- and Conformity-index were calculated using Dose Volume Histogram (DVH) data. Robustness was assessed by shifting the treatment plan isocenter, 3 mm and 5 mm, following in six directions (±x, ±y, ±z), and variations in D98% for CTV and CTV-T, and mean doses for OARs were examined. For breast swelling, a surface contour of 5 mm thickness, called CTVsurface, was delineated along the edge of the CTV. The isocenter was shifted 1.1 cm medially and dorsally within the CTVsurface. Changes in D98% and V93% were studied for CTVsurface. The number of DIBH and mean DIBH time were analyzed for each technique. Results and Discussion: All four treatment techniques adhered to national guidelines for the target volumes. 3DCRT and Hybrid-IMRT presented significantly lower mean doses to OARs compared to Hybrid-VMAT and VMAT, with VMAT resulting in higher doses to the LAD due to increased low-dose spillage [3]. The Hybrid techniques, especially in Hybrid-IMRT, provided superior dose homogeneity and conformity compared to 3DCRT and VMAT. The analysis of robustness to positioning uncertainties showed no significant differences in dose distribution for CTV and OARs among the four techniques. For CTV-T, Hybrid-IMRT was not as robust as 3DCRT, Hybrid-VMAT, and VMAT. This can be explained by the absence of a margin outside the breast that accounts for breathing motion and breast swelling, leading to loss of dose fluence in Hybrid-IMRT.[5]. VMAT and Hybrid-VMAT presented equal robustness to breast swelling as 3DCRT, while a large under dosage for CTVsurface was observed for Hybrid-IMRT. 3DCRT had shorter breath-hold time which is beneficial for patients with breath-holding challenges. In conclusion, the quality of dose distribution was evaluated for four different radiotherapy techniques. The quality of planned dose distribution was highest for Hybrid-IMRT, both for target volumes and organs at risk. The most robust dose distributions, considering positioning uncertainty and breast swelling, were observed with 3DCRT, VMAT, and Hybrid-VMAT.