Implementation of apertures in a proton pencil-beam dose algorithm.

The use of field-specific apertures, routine in scattered or uniform-scanned proton fields, are still a necessity in pencil-beam scanned (PBS) fields to sharpen the penumbral edge at low energies and in high fraction dose application beyond that achievable with small spot size. We describe a model implemented in our clinical pencil-beam algorithm that models the insertion of a shaped aperture, including shapes adapted per energy layer such as may be achieved with a multi-leaf collimator. The model decomposes the spot transport into discrete steps. The first step transport a uniform intensity field of high-resolution sub-pencil-beams at the layer energy through the medium. This transport only considers primary scattering in both the patient and an optional range-shifter. The second step models the aperture areas and edge penumbral transition as a modulation of the uniform intensity. The third step convolves individual steps over the uniform-transported field including the aperture-modified intensities. We also introduce an efficient model based on a Clarkson sector integration for nuclear scattered halo protons. This avoids the explicit modeling of long range halo protons to the detriment of computational efficiency in calculation and optimization. We demonstrate that the aperture effect is primarily due to in-patient and shifter scattering with a small contribution from the apparent beam source position. The model provides insight into the primary physics contributions to the penumbra and the nuclear halo. The model allowed us to fully deploy our PBS capacity at our two-gantry center without which PBS treatments would have been inferior compared to scattered fields with apertures. Finally, Monte Carlo calculations have (nearly) replaced phenomenological pencil-beam models for collimated fields. Phenomenological models do, however, allow exposition of underlying clinical phenomena and closer connection to representative clinical observables.

The reliability of proton-nuclear interaction cross-section data to predict proton-induced PET images in proton therapy.

In vivo PET range verification relies on the comparison of measured and simulated activity distributions. The accuracy of the simulated distribution depends on the accuracy of the Monte Carlo code, which is in turn dependent on the accuracy of the available cross-section data for ß(+) isotope production. We have explored different cross-section data available in the literature for the main reaction channels ((16)O(p,pn)(15)O, (12)C(p,pn)(11)C and (16)O(p,3p3n)(11)C) contributing to the production of ß(+) isotopes by proton beams in patients. Available experimental and theoretical values were implemented in the simulation and compared with measured PET images obtained with a high-resolution PET scanner. Each reaction channel was studied independently. A phantom with three different materials was built, two of them with high carbon or oxygen concentration and a third one with average soft tissue composition. Monoenergetic and SOBP field irradiations of the phantom were accomplished and measured PET images were compared with simulation results. Different cross-section values for the tissue-equivalent material lead to range differences below 1 mm when a 5 min scan time was employed and close to 5 mm differences for a 30 min scan time with 15 min delay between irradiation and scan (a typical off-line protocol). The results presented here emphasize the need of more accurate measurement of the cross-section values of the reaction channels contributing to the production of PET isotopes by proton beams before this in vivo range verification method can achieve mm accuracy.

Field size dependence of the output factor in passively scattered proton therapy: influence of range, modulation, air gap, and machine settings.

At the Francis H. Burr Proton Therapy Center field specific output factors (i.e., dose per monitor unit) for patient treatments were modeled for all beamlines (two gantries, fixed stereotactic, and fixed eye beamline). The authors evaluated the accuracy of dose calculation and output model for small fields. Measurements in a water phantom were performed in three of our beamlines quantifying the dependency of the output factor on the field size for a variety of proton ranges. The influence of snout size, air gap, modulation, and second scatterer was investigated. The impact of field size on output depends strongly on the depth of interest. The air gap has a notable influence on small field outputs. A field size specific correction factor to the output is necessary if the latter was modeled or measured without the custom hardware in place. The output was shown to be field size dependent even for large fields, indicating an effect beyond charged particle disequilibrium caused by lateral scatter.

Characterization of a mini-multileaf collimator in a proton beamline.

A mini-multileaf collimator (MMLC) was mounted as a field shaping collimator in a proton beamline at the Massachusetts General Hospital. The purpose is to evaluate the device's dosimetric and mechanical properties for the use in a proton beamline. For this evaluation, the authors compared MMLC and brass aperture shaped dose distributions with regard to lateral and depth dose properties. The lateral fall off is generally broader with the MMLC, with difference varying with proton range from 0.2 to 1.2 mm. Central axis depth dose curves did not show a difference in peak-to-entrance ratio, peak width, distal fall off, or range. Two-dimensional dose distributions to investigate the conformity of MMLC shaped doses show that the physical leaf width of approximately 2.5 mm does not have a significant impact. All differences seen in dose distribution shaped by the MMLC versus brass apertures were shown to be clinically insignificant. Measured neutron doses of 0.03-0.13 mSv/Gy for a closed brass beam block (depending on range) are very low compared to the previously published data. Irradiation of the tungsten MMLC, however, produced 1.5-1.8 times more neutrons than brass apertures. Exposure of the staff resulting from activation of the device is below regulatory limits. The measurements established an equivalency between aperture and MMLC shaped dose distributions.