Tin shielding thicknesses for electrons.

The transmission of 6, 9, 12, 16, and 20 MeV electrons from a linear accelerator through tin and lead is studied at 0.5 cm depth in polystyrene. These measurements are performed using a 10 x 10 cm2 cone and extend well into bremsstrahlung region. The results show certain advantages of using tin to shield sensitive organs from electron beams. Tin is non-toxic and creates less additional bremsstrahlung even though it must be thicker than the equivalent lead shield.

Dosimetric aspects of small circular fields of 10 MV photon beam.

Dosimetric data obtained from different measurement techniques of 10 MV small circular photon beams of 12.5, 20, 30, and 40 mm diameter were studied. The effects of lack of lateral electronic equilibrium and steep dose gradient were noticed even when measurements were performed with a small volume (0.02 cc) ionization chamber. From an examination of the results, it is concluded that an average value of data obtained from several carefully performed film measurements may be used for routine radiosurgery.

Dosimetric analysis and clinical implementation of 6 MV X-ray radiosurgery beam.

The dosimetric data on tissue maximum ratios (TMR), output factors, off axis ratios and beam profiles are presented for small circular fields of diameters ranging from 12.5 to 40 mm for 6 MV radiosurgery beam. It is noticed that dmax increases as the collimator field size increases. Comparison of our data with the published TMR and output factors of similar small circular fields shows that our values are higher than those data. Similarities in trend are noticed with the published isodose volumes for 1-5 and 10 arcs. Not much variation is seen beyond two arcs for 80% isodose volumes for all the field sizes. The variation is small in 20% isodose volumes beyond three arcs. Variations are noticed in 5% isodose volumes for 12.5 mm diameter collimated beam. Our experience has been exclusively with malignant neoplasms. An ideal target volume is covered by 80% isodose volume with 3-4 arcs and a single isocenter. Sixteen patients have been treated to date at our institution, including one patient with brain metastases, two patients with meningiomas, one patient with lymphoma and 12 patients with astrocytomas. The majority of tumors have been treated with single isocenter but some as large as 7 cm have been treated safely with two isocenters.

Effective atomic numbers of composite materials for total and partial interaction processes for photons, electrons, and protons.

Effective atomic numbers for total and dominant partial interaction processes of photons (1-50 MeV), electrons (1-50 MeV), and protons (1-200 MeV) for the composite materials bone (cortical), muscle (striated), water, polystyrene, Perspex, and Nylon-6 are derived. For photons, the effective atomic number from pair production in the nuclear field is greater than it is from the incoherent scattering. For electrons the effective atomic number from the radiative losses is greater than it is from the collision losses. In both of these cases however, the effective atomic numbers from partial interaction processes remain more or less the same, whereas the number from the total interaction increases with increasing energy. But in the energy regions from 1 to 5 MeV for photons and from 1 to 10 MeV for electrons, the number from the total interaction remains approximately the same for each of these composite materials. For all these materials, in these energy regions the interaction is predominantly with atomic electrons and the contributions from the pair production for photons and radiative losses for electrons are small. In the case of protons the number from total interaction remains more or less the same in the energy region considered. In this energy region collisions with atomic electrons dominate, and the contribution to the total stopping power is mainly from this process only. Hence the derived effective atomic number is basically from the partial process involving the interactions with atomic electrons. Thus, for photons from 1 to 5 MeV for electrons from 1 to 10 MeV and for protons from 1 to 200 MeV, the dosimetric data collected with composite tissue equivalent phantoms, designed on the basis of interaction with atomic electrons for treatment planning, will have less uncertainty.

Lead shielding thickness for dose reduction of 5 MeV electrons.

The relative percent intensity reduction by lead (Pb) of 5 MeV electrons produced by Siemens Mevatron 77/74 for 5 cm diameter, 10 X 10, 15 X 15, and 20 X 20 cm2 cones both with and without buildup is measured. The thickness of lead (Pb) required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.

Effective atomic numbers of biological materials in the energy region 1 to 50 MeV for photons, electrons, and He ions.

A study of effective atomic numbers for biological materials such as bone, muscle, spleen, liver, mucin, and water has been carried out in the energy region 1 to 50 MeV for photons, electrons, and He ions. It is noticed that the effective atomic number for photons and electrons increases with energy, and remains, more or less the same, for He ions.

Investigation on the reduction of electron contamination with a 6-MV x-ray beam.

Experimental investigations have been carried out on the reduction of electron contamination of a 6-MV x-ray beam of Clinac model 1800 for square field sizes 5 X 5 to 30 X 30 cm2 in steps of 5 cm and for rectangular field sizes 19 X 7 and 7 X 19 cm2. The electron contamination of both the open beam and the beam with the tray can be effectively reduced by placing a lead foil filter immediately below the blocking tray. Measurements at 100-cm source-skin distance with filter in place showed a reduction in dose in the buildup region and also a displacement of the location of Dmax to greater depths, even for small field sizes such as 10 X 10 cm2.

Investigations on the near surface dose for three 10-MV x-ray beam accelerators with emphasis on the reduction of electron contamination.

A study of three 10-MV x-ray clinical accelerators with emphasis on the reduction of electron contamination was conducted. This study, which was performed with different types of trays and filters, suggests that, at 100-cm source-surface distance (SSD), Pb can be used as an effective filter material up to 30 X 30 cm2; however, due to its transparency, a Clear-Pb tray is useful for field sizes up to a 20 X 20 cm2. Percent depth doses for a few selected depths and field sizes at this nominal SSD were examined. No significant differences, with the exception of the location of Dmax, amongst the three accelerators were noticed.

Lead shielding thickness for dose reduction of 6-MeV electrons for different square fields.

The relative percent dose reduction by lead (Pb) of 6-MeV electrons produced by Clinac 1800 for 6 X 6, 10 X 10, 15 X 15, 20 X 20, and 25 X 25 cm2 cones both with and without buildup is measured. The thickness of Pb required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.

Interaction of low-energy photons with biological materials and the effective atomic number.

The effective atomic numbers for total photon interaction in bone, muscle, liver, spleen, fat, and water are determined and found to decrease up to 50% as the energy increases from 10 to 200 keV. Muscle, spleen, liver, and water are found to behave in an approximately similar manner in this energy region, as far as photon interaction is concerned.