Abstract

Kenni Engstrøm¹, Carsten Brink^1,2, Mette Holck Nielsen³, Martin Kjellgren¹, Karina Lindberg Gottlieb⁴, Irene Hazell⁵, Vibeke Nordmark Hansen¹, Ebbe Laugaard Lorenzen⁴

¹Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark; ²Department of Clinical Research, University of Southern Denmark, Odense, Denmark; ³Department of Oncology, Odense University Hospital, Odense, Denmark; ⁴Laboratory of Radiation Physics , Department of Oncology, Odense University Hospital, Odense, Denmark; ⁵Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Denmark, Odense, Denmark

Conventional radiotherapy for breast cancer is tangential forward planed fields with segments. This is often a manual and time-consuming process, which may be more consistent with automatic planning. Treatment techniques such as Volumetric Modulated Arc Therapy (VMAT) can be used to increase the coverage and conformity, but may increase the dose to contralateral organs. In the present study, we evaluated VMAT in a butterfly field configuration using Pinnacle Autoplan module in comparison with forward planned tangential plans (3DCRT) in terms of dose to organs at risk, target coverage, the number of breath holds required for treatment in deep inspiration breath hold and dosimetric accuracy. 

Twenty left-sided breast cancer patients, ten with mastectomy and ten with lumpectomy, were used for the study. Seventeen of the patients were treated in deep inspiration breath-hold. Patients were prescribed 50 Gy to the CTV’s. For all patients, a VMAT and a forward tangential plan were generated. The forward plan used step-and-shoot 6 MV combined with 18 MV fields for larger breasts. The VMAT plan was made using two 6 MV VMAT fields in a butterfly configuration (each arc spanning 40-70 degrees). A skin flash of 1cm was used to ensure robustness for the VMAT plans. Dose planning was done in Pinnacle 16.0 using the Auto-Planning module. Comparison of 3DCRT and VMAT was made by evaluating the difference in dose to OAR and targets in terms of mean dose, metrics provided by DBCG guidelines¹, dose-volume histograms, and required breath-holds for treatment delivery.

Similar dose to OAR was achieved with VMAT compared to 3DCRT (Figure 1). Small differences were observed in the mean dose to the heart and humeral head. The heart dose was slightly reduced with VMAT, whereas the dose to the humeral head was increased (Table 1).  Target coverage was similar, however, with a small statistically significant increase in coverage of supraclavicular- and internal mammary nodes CTV’s (V_45Gy) for VMAT. Compared to 3DCRT, VMAT reduced the number of needed breath-holds during treatment by 56 %. All the VMAT plans were measured with ArcCHECK and clinically acceptable.  Measurements had a mean gamma pass rate (3 mm and 3 %) of 98.4 % with a range from 96.6 % to 99.8 %.

Auto-planning of VMAT within the treatment planning system Pinnacle produced plans in a fast and consistent manner requiring less interaction from the treatment planner, thus potentially reducing inter-planer variations. The dose distribution of the butterfly VMAT technique was similar to that for the conventional tangential irradiation. The VMAT technique improved dose coverage of lymph nodes, without increased dose to contralateral organs nor heart. Furthermore, the number of breath-holds during treatment was reduced by a factor of two for the VMAT treatments, thereby increasing patient comfort and reducing risk of patient displacement. 1 shorturl.at/dsAGR