YB-169 - CALCULATED PHYSICAL-PROPERTIES OF A NEW RADIATION SOURCE FOR BRACHYTHERAPY

Seeds containing radioactive Ytterbium-169 (Yb-169) 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 Yb-169 (excluding energies less than 10 keV) yield an air-kerma rate constant of 0.0427 cGy cm2 h-1 MBq-1, and an exposure rate constant of 1.80 R cm2 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 Yb-169 seeds (Amersham, prototypes 4 and 5) are provided, and are then compared with those around an I-125 seed (3M model 6702). The Yb-169 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 D0, a value which is applicable to both seed types. Radiation protection is not as easily achieved for permanent implants with Yb-169 because of the higher energy emissions (vs I-125). However, for temporary implants, Ytterbium-169 may prove to be a useful substitute for Ir-192 or Cs-137 because of its relatively lower energy emissions. It is concluded that Yb-169 merits further investigation, including dosimetry, radiobiological, and clinical studies.