Wednesday Case of the Day Physics Jaydev K. Dave, Ph.D. and Eric L. Gingold, Ph.D. Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, PA History: The schematic below represents a fluoroscopically guided interventional procedure examination set-up. A physicist calculates the entrance skin dose as shown below. Assume no change in position of the patient or of the system throughout the procedure. 𝐸𝑛𝑡𝑟𝑎𝑛𝑐𝑒 𝑠𝑘𝑖𝑛 𝑑𝑜𝑠𝑒=𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑖𝑟 𝑘𝑒𝑟𝑚𝑎 𝑎𝑡 𝑖𝑛𝑡𝑒𝑟𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑝𝑜𝑖𝑛𝑡 ∗𝑖𝑛𝑣𝑒𝑟𝑠𝑒 𝑠𝑞𝑢𝑎𝑟𝑒 𝑙𝑎𝑤 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟∗𝑡𝑎𝑏𝑙𝑒 𝑡𝑜𝑝 𝑎𝑛𝑑 𝑝𝑎𝑑 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 ∗𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑚𝑎𝑠𝑠 𝑎𝑡𝑡𝑒𝑛𝑢𝑎𝑡𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑘𝑖𝑛 𝑡𝑜 𝑎𝑖𝑟 =1000 𝑚𝐺𝑦 ∗ 50 2 75 2 ∗0.85∗1.06 =400.4 𝑚𝐺𝑦 What is the relationship between the calculated entrance skin dose and the true peak skin absorbed dose? (a) Calculated value = true value (b) Calculated value < true value (c) Calculated value > true value

Diagnosis: The correct answer is (b), the calculated value of the entrance skin dose is less than the true value of the peak skin dose.

Discussion: The calculated value of the entrance skin dose is less than the true value of the peak skin dose because the backscatter factor has not been included in the calculations. The backscatter factor is a function of the beam quality (depending on kV, inherent and external filtration, and half value layer), radiation field size at the entrance skin surface and the patient thickness. The backscatter values may range from 1.3 to 1.6 depending on above parameters. Thus, the contribution of backscatter factor should be accounted for in the calculations of the peak skin dose. Calculated peak skin dose would therefore be, 𝑃𝑒𝑎𝑘 𝑠𝑘𝑖𝑛 𝑑𝑜𝑠𝑒=𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑖𝑟 𝑘𝑒𝑟𝑚𝑎 𝑎𝑡 𝑖𝑛𝑡𝑒𝑟𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑝𝑜𝑖𝑛𝑡 ∗𝑖𝑛𝑣𝑒𝑟𝑠𝑒 𝑠𝑞𝑢𝑎𝑟𝑒 𝑙𝑎𝑤 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 ∗𝑡𝑎𝑏𝑙𝑒 𝑡𝑜𝑝 𝑎𝑛𝑑 𝑝𝑎𝑑 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 ∗𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑚𝑎𝑠𝑠 𝑎𝑡𝑡𝑒𝑛𝑢𝑎𝑡𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑘𝑖𝑛 𝑡𝑜 𝑎𝑖𝑟 * backscatter factor =1000 𝑚𝐺𝑦 ∗ 50 2 75 2 ∗0.85∗1.06 * 1.4 =400.4 𝑚𝐺𝑦 ∗1.4 =560.6 𝑚𝐺𝑦 Note, that the contribution of the backscatter factor resulted in a 40 % increase in the calculated value of the peak skin dose.

References/Bibliography: National Council on Radiation Protection and Measurements Report No. 168: Radiation dose management for fluoroscopically guided interventional medical procedures. Published in 2011. Jones, AK and Pasciak AS. Calculating the peak skin dose resulting from fluoroscopically-guided interventional. Part I: Methods. Journal of applied clinical medical physics, vol 12 (4), 2011: 231-244. Jones, AK and Pasciak AS. Calculating the peak skin dose resulting from fluoroscopically-guided interventional. Part II: Case Studies. Journal of applied clinical medical physics, vol. 13 (1), 2012: 174-188.