Mathur Jn

A new radiotherapy surface dose detector: The MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOS-FET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-microns in diameter and consists of a 0.5-microns Al electrode on top of a 1-microns SiO2 and 300-microns Si substrate. Results for % surface dose were within +/- 2% compared to the Attix chamber and within +/- 3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10 x 10-cm and 40 x 40-cm field size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

Radiochromic film as a radiotherapy surface-dose detector

Radiochromic film is shown to be a useful surface-dose detector for radiotherapy x-ray beams. Central-axis percentage surface-dose results as measured by Gafchromic film for a 6 MVp x-ray beam produced by a Varian 2100C Linac at 100 cm SSD are 16%, 25%, 35%, 41% for 10, 20, 30 and 40 cm square field sizes, respectively. Using a simple, uniform light source and a CCD camera connected to an image analysis system, quantitative 3D surface doses are accurately attainable in real time as either numerical data, a black-and-white image or a colour-enhanced image.

Magnetic repulsion of linear accelerator contaminates.

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20 x 30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20 x 20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.

Treatment planning figures of merit in thermal and epithermal boron neutron capture therapy of brain tumours

The boron neutron capture therapy (BNCT) figures of merit of advantage depth, therapeutic depth, modified advantage depth and maximum therapeutic depth have been studied as functions of 10B tumour to blood ratios and absolute levels. These relationships were examined using the Monte Carlo neutron photon transport code, MCNP, with an ideal 18.4 cm diameter neutron beam incident laterally upon all ellipsoidal neutron photon brain-equivalent model. Mono-energetic beams of 0.025 eV (thermal) and 35 eV (epithermal) were simulated. Increasing the tumour to blood 10B ratio predictably increases all figures of merit. concentration was also shown to have a strong bearing on the figures of merit when low levels were present in the system. This is the result of a non-10B dependent background dose. At higher levels however, the concentration of 10B has a diminishing influence. For boron sulphydryl (BSH), little advantage is gained by extending the blood 10B level beyond 30 ppm, whilst for D,L,-p-boronophenylalanine (BPA) this limit is 10 ppm. To achieve a therapeutic depth of 6 cm (brain mid-line from brain surface) using the thermal beam, a tumour to blood ratio of 25 with 10 ppm 10B in the blood is required for BPA. Similarly, a tumour to blood ratio of 8.5 with 30 ppm blood 10B is required for the maximum therapeutic depth of BSH to reach the brain mid-line. These requirements are five times above current values for these compounds in humans. Applying the epithermal beam under identical conditions, the therapeutic depth reaches the brain mid-line with a tumour to blood 10B ratio of only 5.7 for BPA. For BSH, the maximum therapeutic depth reaches the brain mid-line with a tumour to blood ratio of only 1.9 with 30 ppm in the blood. Human data for these compounds are very close to these requirements.

The influence of heavy water on boron requirements for neutron capture therapy.

Neutron penetration in tissue is a major limitation of thermal NCT, as such much work has centered upon the epithermal neutron beam in an effort to improve this situation. Further gains in neutron flux penetration, and thus therapeutic ratios, are possible if natural water is replaced with heavy water prior to therapy. Applying MCNP to a heterogeneous ellipsoidal skull/brain model, advantage depth and therapeutic depth parameters are studied as a function of heavy water replacement for a range of tumor to blood boron ratios. Both thermal (0.025 eV) and epithermal (2-7 keV) ideal neutron beams are analyzed. Using 10B ratios in the range of documented human uptake, the thermal advantage depth improved by approximately 0.7 cm for 20% D2O replacement, however, the therapeutic depth increased by less than half this value. For the epithermal beam, both the advantage depth and the therapeutic depth increased by over 1 cm. Effects of heavy water replacement on 10B requirements to therapeutically treat the midline of the brain are also evaluated.

Monte Carlo Neutron Photon Treatment Planning Calculations

Routine treatment planning using the Monte Carlo technique is becoming ever more possible with the development of increasingly powerful computational hardware. This is applicable not just within the field of Boron Neutron Capture Therapy, but in radiation therapy in general. In the clinical situation, treatment plans must take into account, amongst other factors, the individual patient geometry. This is typically done by incorporating patient CT or MRI information and using surface reconstruction techniques or with small rectilinear volume elements termed “voxels”. The voxel based method typically suffers from being inherently computationally intensive, however this problem may be overcome by introducing a variable voxel size. Regions of interest may be specified where higher resolution anatomical structure and dose calculations can be achieved. Surrounding regions may be modelled with lesser resolution, thereby reducing the overall computational effort required. Applying the Los Alamos Monte Carlo code MCNP, version 3a, with the use of supercomputing hardware enables calculations to be performed in under 10hrs to a resolution of a few millimetres and statistical accuracy of less than 5%. Results are presented for human head and whole body phantoms with incident neutron spectrum and collimator geometry being that of the JRC/ECN Petten epithermal neutron beam.