Abstract
The dosimetric advantage of modulated electron radiotherapy (MERT) has been explored by many investigators and is considered to be an advanced radiation therapy technique in the utilization of electrons. A computer‐controlled electron multileaf collimator (MLC) prototype, newly designed to be added onto a Varian linac to deliver MERT, was investigated both experimentally and by Monte Carlo simulations. Four different electron energies, 6, 9, 12, and 15 MeV, were employed for this investigation. To ensure that this device was capable of delivering the electron beams properly, measurements were performed to examine the electron MLC (eMLC) leaf leakage and to determine the appropriate jaw positioning for an eMLC‐shaped field in order to eliminate a secondary radiation peak that could otherwise appear outside of an intended radiation field in the case of inappropriate jaw positioning due to insufficient radiation blockage from the jaws. Phase space data were obtained by Monte Carlo (MC) simulation and recorded at the plane just above the jaws for each of the energies (6, 9, 12, and 15 MeV). As an input source, phase space data were used in MC dose calculations for various sizes of the eMLC shaped field (10×10 cm2, 3.4×3.4 cm2, and 2×2 cm2) with respect to a water phantom at source‐to‐surface distance (SSD)=94cm, while the jaws, eMLC leaves, and some accessories associated with the eMLC assembly as well were modeled as modifiers in the calculations. The calculated results were then compared with measurements from a water scanning system. The results showed that jaw settings with 5 mm margins beyond the field shaped by the eMLC were appropriate to eliminate the secondary radiation peak while not widening the beam penumbra; the eMLC leaf leakage measurements ranged from 0.3% to 1.8% for different energies based on in‐phantom measurements, which should be quite acceptable for MERT. Comparisons between MC dose calculations and measurements showed agreement within 1%/1mm based on percentage depth doses (PDDs) and off‐axis dose profiles for a range of field sizes for each of the electron energies. Our current work has demonstrated that the eMLC and other relevant components in the linac were correctly modeled and simulated via our in‐house MC codes, and the eMLC is capable of accurately delivering electron beams for various eMLC‐shaped field sizes with appropriate jaw settings. In the next stage, patient‐specific verification with a full MERT plan should be performed.PACS number: 87.55.ne
Highlights
Megavoltage electron beams have represented an important treatment modality in modern radiotherapy, providing a unique option in the treatment of malignancies confined to superficial regions due to a characteristically sharp dose fall-off beyond the tumor
From the figure, when the jaws are set at 20 × 20 cm2, secondary dose peaks can be seen outside the defined radiation field in the direction perpendicular to the electron MLC (eMLC) leaf movement for both energies, but much more pronounced for 15 MeV
This paper presents our work with respect to a motorized eMLC prototype, which is designed to deliver modulated electron radiotherapy (MERT) in a Varian linac machine
Summary
Megavoltage electron beams have represented an important treatment modality in modern radiotherapy, providing a unique option in the treatment of malignancies confined to superficial regions (less than 5 cm deep) due to a characteristically sharp dose fall-off beyond the tumor. MERT is designed to modulate both electron energy and beam intensity to achieve the dose distribution conformal to the tumor target, resulting in greater normal tissue sparing as compared to the conventional approach with an applicator/cutout, usually one static single-energy field.[2] The dosimetric advantage of using MERT over other electron and photon modalities has been investigated by many groups. Ma et al[3] published a comparative dosimetric study on tangential photon beams, intensity-modulated radiation therapy (IMRT), and MERT for breast cancer treatment, showing a reduced maximum dose to the lung and heart, while Xiong et al[4] from Ma’s group later demonstrated that using combined modulated electron and photon therapy for breast cancer may have an advantage over conventional treatment techniques. Additional dosimetric benefits using MERT technology as compared to other modalities, including volumetric-modulated photon arc therapy, were investigated in treating a postmastectomized chest wall, head and neck shallow tumors, and tumor beds of the breast by various groups.[6,7,8,9]
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