Dosimetric Feasibility of Preoperative Partial Breast Irradiation in Prone Position Using a Magnetic Resonance Linear Accelerator International Journal of Radiation Oncology, Biology, Physics POSTER VIEWING ABSTRACT| VOLUME 96, ISSUE 2, SUPPLEMENT , E691-E692, OCTOBER 01, 2016
Date
10/01/2016Abstract
Purpose/Objective(s)
The prone accelerated partial breast irradiation (pAPBI) using external beams is attractive since a higher dose can be delivered to a smaller volume of the breast in a shorter period of time in a position that maximizes normal tissue sparing and limits target motion. Current standard care of postoperative (postop) delivery of radiation therapy (RT) suffers from increased normal breast irradiation and inaccurate targeting. Delivery of RT in the preoperative (preop) setting would resolve these issues, particularly if MRI is used to guide the planning and delivery. This work studies the dosimetric benefits of delivering preop pAPBI with a MR-linear accelerator with a 1.5 T transverse magnetic field (TMF).
Materials/Methods
MRI T2 STIR and CT data acquired both pre- and postoperatively in the prone position similar to the subsequent RT position for 4 patients with early-stage breast cancer using a 1.5 T scanner were used in this study. On each preop image set, the gross tumor volume (GTV) was delineated, along with a clinical tumor volume (CTV: GTV plus 1.5-cm margin, a volume typically removed in lumpectomy and shown to correlate with microscopic extent beyond MR defined tumor), and the PTV (CTV plus 0.5 cm). Four dosimetric plans were generated for each case based on CT and MRI using a research system with a Monte Carlo dose engine to consider the effects of 1.5 T TMF. The 4 MRI- and CT-based plans were generated based on bulk electron density assignment based ICRU report 45. The 4 plans were: (1) single arc VMAT without TMF; (2) single arc VMAT with 1.5 T; (3) tangential VMAT with dual 50-60o degree arcs without TMF; and (4) tangential VMAT with dual 50-60o arcs with 1.5 T. Plans were also generated based on the patient’s postop images used for their actual radiation treatment. Dose-volume constraints from the postoperative APBI NSABP B39/RTOG 0413 were used for all plans (prescription dose of 38.5 Gy). The skin was defined as the first 0.5-mm layer of tissue beneath.
Results
The average preop/postop target (GTV for preop and lumpectomy cavity for postop) volumes were 5.7 (4.9-6.7) and 47.6 (8.9-153.5) mL, respectively. The average breast volumes for preop/postop images were 979.4 (835.8-1082.3) and 960.0 (756-1140.1) mL. The average breast volume receiving 34.65 and 19.25 Gy on T2- and CT-based, dual tangent plans without and with a 1.5 T TMF was found to be 18.52% vs 22.4% and 19.5% vs 20.1%, respectively. The average breast volume receiving 34.65 and 19.25 Gy on T2- and CT-based single arc plans without and with a 1.5T TMF was lower in magnitude and found to be 14.1% vs 18.1% and 14.7% vs 16.9%, respectively. The average skin dose to 1 mL was found to be 35.3 (30.5-38.3) Gy vs 36.2 (32.4-39.7) Gy for plans without and with a TMF.
Conclusion
The MRI-guided preop pAPBI could potentially allow for treatment of a smaller volume of breast tissue, which could translate to lower rates of breast fibrosis and improved cosmetic outcomes. The presence of 1.5T has a small dosimetric effect on pAPBI.
Author List
P.W. Prior Jr., A.D. Currey, T.R. Kelly, J.A. Bovi, C. Bergom, & A. LiAuthor
Adam Currey MD Associate Professor in the Radiation Oncology department at Medical College of WisconsinView Online
