ABSTRACT
Objective To study positron yield quantitatively in different phantoms by means of PET imaging after high energy photon irradiation,and to investigate the feasibility of bio-dose verification by PET in high energy photon radiotherapy.Methods The phantoms (hydrogel,HDPE,inhomogeneous phantom) were scanned with PET 1 min later after exposure to 2,4,6,8 Gy dose with/using 25 and 50 MV photon irradiation.The positron account rates were recorded every minute in PET scanning process and then fitted to get the half-lives of yielded nuclides.Positron distribution in each phantom was compared with dose distribution to investigate the relationship between positron activity and dose delivered.Results The half-lives of nuclides yielded in hydrogel and HDPE were supposed to be 2.23 and 19.47 min,respectively by fitting results,which had negligible difference with half-lives of 11C (20.2 min) and 15O (2.08 min).The positron amounts induced by 50 MV photon in hydrogel and HDPE were 3.88 and 3.86 times that of 25 MV photon,and increased in proportion to dose delivered.Except for build-up region and cavity,activity distributions in depth and off-axis were similar to dose distribution.Conclusions The amount and distribution of positron induced by high energy photon are related to dose delivered,and it is feasible to do dose verification with PET imaging in high energy photon radiotherapy.
ABSTRACT
Purpose:The image information and technique of positron emitter nuclei generated by high energy X-ray photonuclear reactions with body tissues from MM50 is studied.It is explored to verify for dose delivery and location monitoring in tumor target after high energy photon radiation therapy with the image information.Materials and Methods:The technique is based on the photonuclear reactio-as in body tissues elemental composition ~(12)C,~(16)O and ~(14)N with high energy X-Ray,energies well above 20 MeV,resulting primarily in ~(11)C and ~(15)O but also ~(13)N.The induced positron activity distributions were scanned off-line in a PET/CT after irradiation.The activity distributions and position may be used to verify for dose delivery and location in tumor target.These radiations are similar to RT in three fields with some different dose delivery from TPS.The phantom began to be scanned off-line in a PET/CT a couple of minutes after irradiation.The scanning time is respectively 20 minutes and(2～5)mniutes for ~(12)C and ~(16)O.The levels of the beam energy are 10MV,25MV,50 MV.The extent of dose is 1.0 Gy～10.0 Gy.Since measured PET images change with time post irradiation,as a result of the different decay tim-es of the radionuclides,the signals from activated ~(12)C,~(16)O within the irradiated volume could be separated from each other.Most informationis obrained from the carbon and oxygen radionuclide's which the most abundant elements are in soft tissue.A brain phantom Rlade oneself was irradiated with high energy X-ray beams from IBA MM50.The 3D radiation treatment planning system,Nucletron-TPP 3.2,was used to calculate the delivered dose distributions.The phantom was directly simulated in the Nucletron-TPP after CT scan.PET/CT is fro-m GE Discovery LS Ⅱ.Resuits:It was confirmed that no activity was detected at 10 Mv X-ray energy,which was far below the energy threshold for photonuclear reactions.Totally 25 MV X-ray beams can produce photonuclear reactions and get to activity distributions images in PET/CT.It need to greater dose in order to good images information.For 50 MV X-ray beams,2 Gy-3 Gy dose,a normal RT dose,can get to activity distributions images.Conclusions:It was concluded that the PET-CT image acquired from the activity of the ~(15)O and ~(11)C positron emitter nuclei might provide the area and dose distribution information of 50 MV X-ray irradiation in a phantom.It can verify the in vivo dose delivery and location in tumor target after high energy X-ray RT.