Relevance of radiation penumbra in high-energy photon beam therapy

In external beam therapy, radiation penumbra is one of the factors which hinder the confinement of the dose to tumour volume and cause unnecessary irradiation of the surrounding normal tissues in the penumbral region. In the present work, the magnitudes of radiation penumbras were evaluated for various high-energy X-ray beams and cobalt-60 beams, from their respective isodose charts, at the depth of maximum ionization (dmax) and at the typical tumour centre depth of 10 cm (d10) and for field sizes of 5 x 5 cm2, 10 x 10 cm2 and 15 x 15 cm2. Although the radiation penumbra for high-energy X-ray beams up to 16 MV was found to be smaller in comparison with cobalt-60 beams, at 20 MV and beyond it tends to increase and becomes comparable to that for cobalt-60 beams. In the light of this comparison and the results of some of the cancer treatments with high-energy beams reported by various authors, the relative importance of radiation penumbra and integral dose in the treatment volume is discussed.