SP-0630: Patient absorbed dose in nuclear medicine: Diagnostics and therapy

chamber.The concept of CTDI is still the basis for dosimetry at modern CT scanners, although the assessment of CTDI in phantoms has been the subject of much discussion and many revisions were required since its initial definition. A fundamental revision was for example needed when helical CT was introduced and the concept of pitch had to be taken into account. With the introduction of diagnostic wide CT beams and the introduction of cone beam computed tomography (CBCT) with CT scanners that are integrated with a linear accelerator a new problem arose. For these wide CT beams nor the CT dose phantoms nor the 100 mm long pencil ionization chamber were compatible with the at that time prevailing concept of CTDI. The problem was that the beam width of diagnostic wide beam scanners and CBCT scanners exceeds the lenght of the cylindrical CT phantoms (typically150 mm) and length of the pencil CT ionization chamber (100 mm) (Geleijns etal.; Wen et al.). Solution for diagnostics wide cone beam CT scanners As a development to overcome the shortcomings described in the previous section, the proposed IEC 60601-2-44 international standard (Amendment1 of Edition 3) describes a two tiered approach to the definition of CTDI. The first tier is for beam widths ≤ 40 mm and uses the conventional definition of CTDI100. In the second tier for beam widths > 40mm, it is proposed to measure a reference value for CTDI in the standard CT dose phantoms, for anominal beam width of about 20 mm. This value is then scaled up by the ratio of free in air measurements of CTDI for the wide beam condition and the reference condition. This approach is also followed in the IAEA Human Health Report 5 and is supported by the scientific work from Boone. Kilovoltage cone beam CT at the linac’s are a special case since at a large field of view the detector is shifted from the centered position, this may complicate the measurement of CTDI considerably. Assessment of patient dose for cone beam CT scans at the linac’s is also complicated. Monte Carlo calculations or measurements with anthropomorphic phantoms may be performed. A pragmatic approach may be to adhere to the methodology that is often used for diagnostic CTscanners, i.e. to use a body part specific conversion factor for calculating effective dose from dose-length product. References Shope TB, Gagne RM, Johnson GC. A method for describing the doses delivered by transmission x-ray computed tomography. MedPhys. 1981 Jul-Aug;8(4):488-95. Geleijns J, Salvadó Artells M, de Bruin PW,Matter R, Muramatsu Y, McNitt-Gray MF. Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner. Phys Med Biol. 2009 May21;54(10) Wen N, Guan H, Hammoud R, Pradhan D, Nurushev T,Li S, Movsas B. Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer. Phys Med Biol. 2007 Apr 21;52(8):2267-76. IEC-60601-2-44 Amendment 1of Edition 3. International Electrotechnical Commission, Medical Electrical Equipment Part 2-44 Edition3, Amendment 1: Particular requirements for basic safety and essential performance of X-ray equipment for computed tomography,; 62B/804/CD, committee draft, IEC Geneva (2010). IAEA Human Health Report 5. Status of Computed Tomography Dosimetry for Wide Cone Beam Scanners,ISBN:978-92-0-120610-7. 2011 (www-pub.iaea.org/MTCD/Publications/PDF/Pub1528_web.pdf) Boone JM. The trouble with CTD100. Med Phys.2007 Apr;34(4):136471.