Abstract
One of the most relevant risks in breast intraoperative electron radiotherapy (IOERT) is the incorrect positioning of the shielding disc. If such a setup error occurs, the treatment zone could receive a nonuniform dose delivery, and a considerable part of the electron beam could hit — and irradiate — the patient's healthy tissue. However misalignment and tilt angle of the shielding disc can be evaluated, but it is not possible to measure the corresponding in vivo dose distribution. This led us to develop a simulation using the Geant4 Monte Carlo toolkit to study the effects of disc configuration on dose distribution. Some parameters were investigated: the shielding factor (SF), the radiation back scattering factor (BSF), the volume–dose histogram in the treatment zone, and the maximum leakage dose (MLD) in normal tissue. A lateral shift of the disc (in the plane perpendicular to the beam axis) causes a decrease in SF (from 4% for a misalignment of 5 mm to 40% for a misalignment of 40 mm), but no relevant dose variations were found for a tilt angle until 10°. In the same uncorrected disc positions, the BSF shows no significant change. MLD rises to 3.45 Gy for a 14 mm misalignment and 4.60 Gy for 30° tilt angle when the prescribed dose is 21 Gy. The simulation results are compared with the experimental ones, and allow an a posteriori estimation of the dose distribution in the breast target and underlying healthy tissue. This information could help the surgical team choose a more correct clinical setup, and assist in quantifying the degree of success or failure of an IOERT breast treatment.PACS number: 87.53.Jw, 87.55.dk, 87.55.Gh, 87.55.K‐, 87.55.N‐
Highlights
Intraoperative electron radiotherapy (IOERT) is a radiotherapy technique that delivers a single dose of radiation directly to the tumor bed, or to the exposed tumor, during surgery
It is mainly used as an adjuvant to surgery or as a preliminary boost to be followed later by fractionated conventional external whole-breast radiotherapy (WBRT).(1) The objective is to achieve a higher dose in the target volume, while dose-limiting structures are surgically displaced.[2]
Experimental percentage profile dose (PPD) parameters and percentage depth dose (PDD) reference points (R100, R90, R50, R30 and R10, where Rx is the depth corresponding to x percentage of the maximum dose released) were measured in reference conditions: source-to-phantom surface distance, SSD = 100 cm, 100 mm collimator diameter, and 0° angle.[12]. The experimental data were acquired inside the water phantom using a p-type silicon diode detector
Summary
Intraoperative electron radiotherapy (IOERT) is a radiotherapy technique that delivers a single dose of radiation directly to the tumor bed, or to the exposed tumor, during surgery. It is mainly used as an adjuvant to surgery or as a preliminary boost to be followed later by fractionated conventional external whole-breast radiotherapy (WBRT).(1) The objective is to achieve a higher dose in the target volume, while dose-limiting structures are surgically displaced.[2]. 75 Russo et al.: Breast dose distribution study For this purpose, a new generation of mobile linear accelerators like NOVAC7 (NRT, Aprilia, Italy),(3) Liac (Sordina SpA, Italy),(4) and Mobetron (IntraOp Medical, Inc. Santa Clara, CA)(5) have been designed to deliver radiation therapy in the operating theater. IOERT is very appropriate for the new trends in breast cancer management where mastectomy is substituted by a more conservative treatment, more appropriate for limited-stage breast tumors.[2]
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