Design, Construction and Dosimetric Considerations of Cerrobend Compensator for High Energy Photon Beam

Abstract

In external beam radiotherapy (EBRT), dose optimization is achieved by conforming the dose distribution to the shape of the intended target whilst minimizing radiation to normal tissues in close proximity to the target. This can be achieved by modulating the intensities of the radiation across the radiation portals forming the irradiation geometry used for the treatment. Compensators may be used to achieve the above effect based on intensity fluence map generated with inverse planning for the irradiated region. For Treatment planning systems (TPSs) without inverse planning capabilities, bolus placed on the surface of the patient at point of beam entrance may be used to provide the required intensity modulation of the beam. During treatment, compensating filters (compensators) placed at distance from the patient's skin are used to mimic the dosimetric effects of the radiation within the irradiated region produced with the bolus to preserve the skin sparing effect of the radiation. The shape of the compensating filters needs to be adjusted for beam divergence and reduction of scatter contribution to the deposition of dose within the patient Procedures for the design and construction of cerrobend compensator had been developed and proposed for a cobalt- 60 beam and Prowess Panther version 4.6 TPS without inverse planning capabilities. Transmission measurements were done with the compensator (in the form of cerrobend slabs) mounted on a tray within the beam for various treatment depths and field sizes in a full scatter water phantom and the same measurements repeated with the tray, but without the compensator such that the thicknesses of the phantom were adjusted to get the same doses as before. These measurements were used to develop and propose algorithm for the conversion of bolus thickness to compensator thickness which would produce the same dosimetric effect as the appropriate bolus thickness. Correction factors were introduced into the algorithm to account for the effects of field size, treatment depth and variation in the thickness of the compensating filter on the dose distribution within the irradiated region. To check the efficacy of the developed and proposed method and also, a number of compensators, some in the form of step wedges were designed and constructed based on the developed and proposed procedure for various treatment depths and field sizes. The constructed compensators on trays were mounted on the accessory holder of Equinox 100 cobalt - 60 teletherapy machine and the dose distribution beneath the compensator measured in a solid water phantom with a Farmer type ionization chamber and Gafchronic films at the appropriate treatment depth. The doses obtained were compared with corresponding dose distributions generated with the TPS for the solid water phantom with the bolus on the surface of phantom. Percentage deviation of the doses ranges from ±1.69% 𝑡𝑜 ± 16.36% with a mean of ±9.51% and standard deviation of ±5.64%. Though most of the deviations were within the tolerance of ± 5% required for external beam radiotherapy, there were significant number of the deviations beyond the tolerance due to inherent errors associated with the manual fabrication of the mould for the cerrobend, which needs to be streamlined.