Recently, energy- and intensity-modulated electron radiotherapy (MERT) has garnered a growing interest for the treatment of superficial targets. In this work. we carried out a comparative dosimetry study to evaluate MERT, photon beam intensity-modulated radiation therapy (IMRT) and conventional tangential photon beams for the treatment of breast cancer. A Monte Carlo based treatment planning system has been investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We have compared breast treatment plans generated using this home-grown treatment optimization and dose calculation software forthese treatment techniques. The MERT plans were planned with up to two gantry angles and four nominal energies (6, 9, 12 and 16 MeV). The tangential photon treatment plans were planned with 6 MV wedged photon beams. The IMRT plans were planned using both multiple-gantry 6 MV photon beams or two 6 MV tangential beams. Our results show that tangential IMRT can reduce the dose to the lung, heart and contralateral breast compared to conventional tangential wedged beams (up to 50% reduction in high dose volume or 5 Gy in the maximum dose). MERT can reduce the maximum dose to the lung by up to 20 Gy and to the heart by up to 35 Gy compared to conventional tangential wedged beams. Multiple beam angle IMRT can significantly reduce the maximum dose to the lung and heart (up to 20 Gy) but it induces low and medium doses to a large volume of normal tissues including lung, heart and contralateral breast. It is concluded that MERT has superior capabilities to achieve dose conformity both laterally and in the depth direction, which will be well suited for treating superficial targets such as breast cancer.

In this paper we present Monte Carlo studies of intensity modulated radiation therapy using laser-accelerated proton beams. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. Through the introduction of a spectrometer-like particle selection system that delivers small pencil beams of protons with desired energy spectra it is feasible to use laser-accelerated protons for intensity modulated radiotherapy. The method presented in this paper is a three-dimensional modulation in which the proton energy spectrum and intensity of each individual beamlet are modulated to yield a homogeneous dose in both the longitudinal and lateral directions. As an evaluation of the efficacy of this method, it has been applied to two prostate cases using a variety of beam arrangements. We have performed a comparison study between intensity modulated photon plans and those for laser-accelerated protons. For identical beam arrangements and the same optimization parameters, proton plans exhibit superior coverage of the target and sparing of neighbouring critical structures. Dose-volume histogram analysis of the resulting dose distributions shows up to 50% reduction of dose to the critical structures. As the number of fields is decreased, the proton modality exhibits a better preservation of the optimization requirements on the target and critical structures. It is shown that for a two-beam arrangement (parallel-opposed) it is possible to achieve both superior target coverage with 5% dose inhomogeneity within the target and excellent sparing of surrounding tissue.

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Purpose: Beam flatness check has been proposed for beam energy check for photon beams with flattering filters. In this work, beam flatness change with beam energy was investigated quantitatively using the Monte Carlo method and its significance was compared with depth dose curve change. Methods: Monte Carlo simulations for a linear accelerator with flattering filter were performed with different initial electron energies for photon beams of 6MV and 10MV. Dose calculations in a water phantom were then perform with the phase space files obtained from the simulations. The beam flatness was calculated based on the dose profile at 10 cm depth for all the beams with different initial electron energies. The percentage depth dose (PDD) curves were also analyzed. The dose at 10cm depth (D10) and the ratio of the dose at 10cm and 20cm depth (D10/D20) and their change with the beam energy were calculated and compared with the beam flatness variation. Results: It was found that the beam flatness variation with beam energy change was more significant than the change of D10 and the ratio between D10 and D20 for both 6MV and 10MV beams. Half MeV difference on the initial electron beam energy brought in at least 20% variation on the beam flatness but only half percent change on the ratio of D10 and D20. The change of D10 or D20 alone is even less significant. Conclusion: The beam energy impact on PDD is less significant than that on the beam flatness. If the PDD is used for checking the beam energy, uncertainties of the measurement could possibly disguise its change. Beam flatness changes more significantly with beam energy and therefore it can be used for monitoring the energy change for photon beams with flattering filters. However, other factors which may affect the beam flatness should be watched as well.

Purpose: To investigate the beam characteristics of a micro‐MLC developed for a robotic stereotactic radiosurgery/radiotherapy (SRS/SRT) system and to provide insight for the MLC design and the feasibility to replace traditional cones. Methods: A Monte Carlo simulation toolbox was developed in our in‐house Monte Carlo system to investigate the details of a micro‐MLC design and their dosimetric effects based on the beam properties of the robotic SRS/SRT system. The micro‐MLC is designed with a piecewise leaf end, no tongue‐and‐groove structure but a tilt angle to reduce the interleaf leakage. The leaf width is 2.5mm projected at 80cm source‐to‐ surface distance (SSD). The MLC leakage for various leaf thickness and tilt angle combinations was studied to derive the optimum leaf thickness and tilt angle. Penumbral widths (80%–20%) at different depths for different field sizes at 80cm SSD were investigated. The dose distributions were compared with those collimated by the cone collimators. Results: With the standard 60mm cone as a reference, the MLC leakage for 8cm tungsten leaves is about 1.9% under the leaves and 21.8% under the leaf ends when leaves are closed. When the leaf thickness is increased to 10cm, the leaf leakage dropped to 0.2% and 9.2%, respectively. When the leaf tilt angle was changed from 1 degree to 0.5 degree, the leakage does not change significantly. The penumbral widths at 1.5cm depth for MLC‐shaped circular field with a diameter of 60mm are 4.7mm and 6.2mm for the leaf‐ ends and lead‐sides. They are comparable to that of the standard 60mm cone, which is 6mm. At the 10cm depth, the penumbral widths become 5.8mm and 7.4mm, which are also comparable to that of the cone collimator (6.9mm). Conclusions: The Monte Carlo simulation results show that the dosimetric parameters of our micro‐MLC design are suitable for SRS/SRT with the robotic system.