Development of optimization method for uniform dose distribution on superficial tumor in an accelerator-based boron neutron capture therapy system

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was low and the skin S/B ratios were high. For tumors with relatively

small sizes, such as 40 × 40 mm, with thicknesses equal to 20 mm,

intensity-modulated irradiation could deliver increased doses to the

tumor, while it can minimize the dose to normal tissue, compared with

single irradiation. Therefore, independent of the tumor size, intensitymodulated irradiation can potentially facilitate superior treatment outcomes compared with existing irradiation methods.

In the proposed method, the intensity modulator used for IF-A

was a PE disk that enhanced the thermal neutron flux on the skin’s

surface to treat superficial tumors. The optimal shape of the intensity

modulator for IF-B was then determined with the use of a Monte Carlo

simulation. However, it has been reported that the dose distribution to

tumors deep inside the body can be improved by designing an intensity

modulator inside the collimator [20]. Therefore, it may be possible

to develop an effective intensity-modulated irradiation method for

the treatment of deep-seated tumors by varying and combining the

intensity modulators used for IF-A and IF-B.

It is also known that BNCT has a longer irradiation time than

X-ray therapy. This raises concerns about the effects on dose distribution that are associated with errors in body positioning and patient

body movements during irradiation [21, 22]. The intensity-modulated

irradiation method improves the dose distribution by delivering a uniform thermal neutron flux to the tumor. The thermal neutron flux

intensity at the edges of the tumor is comparable to that at the center.

Therefore, it is expected to reduce the effects of errors owing to position

setting and patient body movements.

Although intensity-modulated irradiation improves the dose distribution, the irradiation time is long. To apply the intensity-modulated

irradiation method, irradiation must be completed within 1 h owing

to the protocol of the boron drug administration. The time required

to change the intensity modulator and patient’s positional settings

must also be considered. Therefore, increasing the neutron intensity

of the accelerator-based neutron source is necessary to implement the

intensity-modulated irradiation method.

In terms of dose distribution, flattening the distribution of thermal

neutron fluence will also homogenize the boron dose in the normal

tissue distribution. Although the proportion of boron dose is not as

large as that of the tumor dose, the normal tissue dose is also more

homogenized than with conventional one-port irradiation, which may

result in more areas with dose values close to the maximum dose value.

In this study, D5 and D50 were also evaluated for brain doses and

increased. When the developed IM technique will be used in the future,

the distribution of normal tissue should be carefully evaluated, and the

treatment protocol may also need to be improved.

Optimization for uniform dose distribution in BNCT • 611

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