Ytterbium‐169: Calculated physical properties of a new radiation source for brachytherapy
1992; Wiley; Volume: 19; Issue: 3 Linguagem: Inglês
10.1118/1.596813
ISSN2473-4209
AutoresDarcy Mason, Jerry Battista, Rob Barnett, A.T. Porter,
Tópico(s)Advanced X-ray and CT Imaging
ResumoSeeds containing radioactive Ytterbium‐169 ( 169 Yb) have recently been manufactured for possible application to brachytherapy. Ytterbium‐169 emits photons with an average energy of 93 keV (excluding energies less than 10 keV), and decays with a half‐life of 32 days. Analytic and Monte Carlo computations have been used to predict physical quantities useful in treatment planning and radiation protection. Analytic calculations based on the primary photon spectrum of 169 Yb (excluding energies less than 10 keV) yield an air‐kerma rate constant of 0.0427 cGy cm 2 h −1 MBq −1 , and an exposure rate constant of 1.80 R cm 2 mCi −1 h −1 for this radionuclide. Calculated f med factors are 0.922 cGy/R for soft tissue and 2.12 cGy/R for bone. The first half‐value layer in lead is 0.2 mm; the first tenth‐value layer is 1.6 mm. Using Monte Carlo simulations, the relative dose distributions around 169 Yb seeds (Amersham, prototypes 4 and 5) are provided, and are then compared with those around an 125 I seed (3M model 6702). The 169 Yb seeds produce more isotropic dose distributions, and for permanent implants, can deliver it at a greater initial dose rate. A value of 1.19 cm −2 was also calculated for the specific dose constant D 0 , a value which is applicable to both seed types. Radiation protection is not as easily achieved for permanent implants with 169 Yb because of the higher energy emissions (vs 125 I). However, for temporary implants, Ytterbium‐169 may prove to be a useful substitute for 192 Ir or 137 Cs because of its relatively lower energy emissions. It is concluded that 169 Yb merits further investigation, including dosimetry, radiobiological, and clinical studies.
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