In this article we present the results of particle in cell (PIC) simulations of laser plasma interaction for proton acceleration for radiation therapy treatments. We show that under optimal interaction conditions protons can be accelerated up to relativistic energies of 300 MeV by a petawatt laser field. The proton acceleration is due to the dragging Coulomb force arising from charge separation induced by the ponderomotive pressure (light pressure) of high-intensity laser. The proton energy and phase space distribution functions obtained from the PIC simulations are used in the calculations of dose distributions using the GEANT Monte Carlo simulation code. Because of the broad energy and angular spectra of the protons, a compact particle selection and beam collimation system will be needed to generate small beams of polyenergetic protons for intensity modulated proton therapy.
A particle track-repeating algorithm has been developed for proton beam dose calculation for radiotherapy. Monoenergetic protons with 250 MeV kinetic energy were simulated in an infinite water phantom using the GEANT3 Monte Carlo code. The changes in location, angle and energy for every transport step and the energy deposition along the track were recorded for the primary protons and all secondary particles. When calculating dose for a patient with a realistic proton beam, the pre-generated particle tracks were repeated in the patient geometry consisting of air, soft tissue and bone. The medium and density for each dose scoring voxel in the patient geometry were derived from patient CT data. The starting point, at which a proton track was repeated, was determined according to the incident proton energy. Thus, any protons with kinetic energy less than 250 MeV can be simulated. Based on the direction of the incident proton, the tracks were first rotated and for the subsequent steps, the scattering angles were simply repeated for air and soft tissue but adjusted properly based on the scattering power for bone. The particle step lengths were adjusted based on the density for air and soft tissue and also on the stopping powers for bone while keeping the energy deposition unchanged in each step. The difference in nuclear interactions and secondary particle generation between water and these materials was ignored. The algorithm has been validated by comparing the dose distributions in uniform water and layered heterogeneous phantoms with those calculated using the GEANT3 code for 120, 150, 180 and 250 MeV proton beams. The differences between them were within 2%. The new algorithm was about 13 times faster than the GEANT3 Monte Carlo code for a uniform phantom geometry and over 700 times faster for a heterogeneous phantom geometry.
A series of experiments were carried out to simulate air cavities in a polystyrene phantom. Dose was measured at air/polystyrene interface and as a function of depth. Results of experiments were compared to calculations done using three treatment planning systems. These systems employ Batho, modified Batho, and the equivalent tissue-air-ratio methods for inhomogeneity corrections. The measured interface dose decreased by 55% for a 5 cm air gap, 5ϫ5 cm 2 field size, and 6 MV photons. This has been attributed to a lack of electronic equilibrium and dispersion of secondary particles transported through the air gap. These results are at variance with predictions of calculations using three treatment planning systems, for which only a 10% decrease was calculated. This is because the calculation algorithms employed do not incorporate electron transport. Further experiments were conducted to study the contribution of scatter from sides of the walls of the cavities. Dose measurements as a function of depth were also performed to investigate the effect of primary fluence attenuation. The Batho algorithm did not show any sensitivity to the position of air gap sidewalls. This points to the need for proper inclusion of disequilibrium effects and shape of inhomogeneity.
During total body irradiation (TBI), customized shielding blocks are positioned in front of the lungs to reduce radiation dose. The difficulty is to accurately position the blocks to cover the entire lungs. A new technique based on Computed Tomography (CT) simulation was developed to determine the exact position of lung blocks prior to treatment in order to decrease overall treatment time and reduce patient discomfort. Material/Methods: Patients were CT simulated and lungs were contoured using a treatment planning system. Anteroposterior/posteroanterior (AP/PA) fields were designed with MLC aperture conforming to lung contours. The fields were used to represent the extent of the lungs, which was subsequently marked on the patient's skin. The lung blocks were positioned with their shadow matching the lungs' marks. Their position was radiographically verified prior to the delivery of each beam. To evaluate the efficiency of this technique, the treatment session time and the number of repeated attempts to correctly position the shielding blocks was recorded for each beam. Exact treatment times for patients treated with the old technique were not available and were hence approximated based on previous experience. Results: We succeeded in positioning the shielding blocks from the first attempt in 10/12 beams. The position of the shielding blocks was adjusted only one time prior to treatment in 2/12 beams. These results are compared to an average of 3 attempts per beam for each patient using the conventional technique of trial and error. The average time of a treatment session was 29 min with a maximum of 41 min versus approximately 60 min in past treatments and a maximum of 120 min. Conclusion: This new technique succeeded in reducing the length of the overall treatment session of the conventional TBI procedure and hence reduced patient discomfort while ensuring accurate shielding of the lungs.
Purpose: The purpose of this study was to evaluate patient-related non-dosimetric predictors of cardiac sparing with the use of deep inspiration breath-hold (DIBH) in patients with left-sided breast cancer undergoing irradiation (RT).Materials and Methods: We retrospectively reviewed charts and treatment plans of one-hundred and three patients with left-sided breast cancer. All patients had both free-breathing (FB) and DIBH (with body surface tracking) plans available. (MHD) and V4 (heart volume receiving at least 4 Gy) were extracted from dose volume histograms. Fisher's exact and Chi-square tests were used to identify predictors of reductions in MHD and V4 after DIBH.Results: One-hundred and three patients were identified and most underwent mastectomy. MHD and V4 decreased significantly in DIBH plans (0.74 ± 0.25 Gy vs. 1.72 ± 0.98 Gy, p < 0.0001 for MHD; 4 ± 4.98 cc vs. 20.79 ± 18.2 cc, p < 0.0001 for V4). Body mass index (BMI), smoking and timing of CT simulation (spring/winter vs. summer/fall) were significant predictors of reduction in MHD whereas BMI, field size, chemotherapy, axillary dissection, and timing of CT simulation predicted reduction in V4. On multivariate analysis, BMI, and timing of CT simulation remained significant predictors of the heart-sparing effect of DIBH.Conclusions: In the setting of limited resources, identifying patients who will benefit the most from DIBH is extremely important. Prior studies have identified multiple dosimetric predictors of cardiac sparing and hereby we identified new non-dosimetric factors such as BMI and timing of treatments.
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