COMS 16 mm

Eye Plaque Description

The dimensions of the COMS 16 mm eye plaque are available here   (given in the TG129 report ^{1, 2}). This plaque has indentations for 13 seeds within the Silastic insert (seed coordinates here) in the Modulay backing, as shown in the above figures (generated using egs_view). The 3D dose distributions for the plaque containing ¹²⁵I (6711), ¹⁰³Pd (200),  and ¹³¹Cs (CS-1 Rev2) (Ref. 3) are provided here (normalized per unit seed air kerma strength): 

  Central axis depth dose curves

Doses along the plaque's central z-axis (CAX) for HOMO and HETERO scenarios are presented here for three radionuclides and compared with published BrachyDose ^{2, 4, 5} and MCNP ^{2, 6} results. Overall, HETERO doses are lower than the HOMO doses, due to the attenuation and scattering in the plaque backing and insert. For instance, HETERO/HOMO dose ratio at 0.5 cm along the CAX is 0.81, 0.88, and 0.89, for ¹⁰³Pd, ¹²⁵I, and ¹³¹Cs, respectively. The mean (range) percentage difference of HOMO doses along the CAX  between egs_brachy and BrachyDose were 0.4% (-0.5%, 0.7%) for ¹⁰³Pd, 0.6% (0.4%, 1%) for ¹²⁵I, and 0.3% (-0.7%, 1.3%) for ¹³¹Cs, respectively. The corresponding values between egs_brachy and MCNP were 0.3% (-3.9%, 2.5%) for ¹⁰³Pd, 0.7% (-0.7%, 2.9%) for ¹²⁵I, and 2.1% (1.2%, 3.1%) for ¹³¹Cs, respectively. The mean (range) percentage difference of HETERO doses along the CAX  between egs_brachy and BrachyDose were -0.02% (-0.11, 0.22%) for ¹⁰³Pd, 0.63% (0.20%, 0.91%) for ¹²⁵I, and 0.10% (-1.4%, 1.3%) for ¹³¹Cs, respectively. The corresponding values between egs_brachy and MCNP were -1.1% (-4.1%,1.2%) for ¹⁰³Pd, -0.40% (-1.7%, 2.7%) for ¹²⁵I, and 1.0% (0.30%, 1.5%) for ¹³¹Cs, respectively. Statistical uncertainties for egs_brachy, BrachyDose ^4, and MCNP ⁶ are less than  0.20%, 1.0%,  and 2.0%, respectively.  

103Pd model 200	125I model 6711	131Cs (CS-1 Rev2)

 Transverse dose profile

Transverse dose profiles (along x and y axes) at depths of z = 0.5 cm and z = 1 cm are shown for three radionuclides. These dose profiles compared to available BrachyDose results from the previous database are in good agreement (< 1%).

103Pd, x view, z =0.5 and 1 cm	103Pd, y view, z =0.5 and 1 cm
125I, x view, z =0.5 and 1 cm	125I, y view, z =0.5 and 1 cm
131Cs, x view, z =0.5 and 1 cm	131Cs, y view, z =0.5 and 1 cm

 Off-axis dose locations

The percentage dose differences (%Δ) along the CAX, and off-axis for some organ at risks (e.g, fovea, optic disk, lens, and lacrimal gland) at 8 different EP positions ³ in the eye between egs_brachy and BrachyDose (eb, BD), and egs_brachy and MCNP (eb, MCNP) for ¹⁰³Pd (200) and ¹²⁵I (6711) seeds are presented here. Overall deviations are lower for HOMO rather than HETERO scenario. The differences were < 2.2% between rgs_brachy and BrachyDose, and < 3.0% between egs_brachy and MCNP; with an exception in the dose to lacrimal gland for HETERO where MCNP differ by 12.5% for ¹⁰³Pd and 17% for ¹²⁵I (below figure).  

Click images for higher res versions

 
 References

Carleton Laboratory for Radiotherapy Physics