A scheme for spot scanning using C-11 beams has been developed in order to
form and verify a three-dimensionally conformal irradiation field for cance
r radiotherapy. By selecting the momentum spread of a C-11 beam, we could c
onsiderably decrease the distal falloff of the irradiation field, thus cons
erving the beam quality. To estimate and optimize the dose distribution in
the irradiation field, it is essential to evaluate the precise dose distrib
ution of spot beams. The coupling of the lateral dose and depth-dose distri
butions originating from a wide momentum spread should be taken into accoun
t to calculate the dose distribution of C-11 beams. The reconstructed dose
distribution of the irradiation field was in good agreement with the experi
mental results, i.e., within +/-0.2%. An irradiation field of 35 x 35 x 43
mm(3) was optimized and spot scanning using C-11 beams was carried out. The
flatness was within +/-2.3 % with an error of 1% in the detector resolutio
n.