Kilovoltage X-ray Dosimetry

Kilovoltage X-rays Nomenclature and tube potentials of kilovoltage machines Contact beams - <50 kV Superficial beams – 50 to 150 kV Orthovoltage – 150 to 500 kV Beam quality described in terms of HVL of Al or Cu. 30 kV – 1 mm Al 100 kV – 3 mm Al 160 kV – 0.5 mm Cu 200 kV – 1 mm Cu

Protocols for calibration NCRP Report 69 Uses exposure calibration factor Nx AAPM TG-61 Uses air kerma calibration factor Nk Nk = Nx· (W/e)air / (1-g) (W/e)air = 33.97 J/C (0.876 cGy/R) g ≈ 0 for E < 300 kV

Energy dependence on calibration factor 6% between 40 kV and 300 kV Tubes with same potential may have varying HVLs. Chamber should be calibrated using beam of similar tube potential and HVL.

TG – 61 Methodology(s) Low energy (superficial): < 100 kV Use in-air calibration method Medium energy (orthovoltage): > 100 kV Use in-phantom or in-air calibration method Use in-air method if surface dose is desired

Measurement (M) M = Mraw Ppol Pion PTP Pelec End effect (δt) may be significant and correction needed for irradiation time.

TG – 61 In-air method Should use parallel-plate chamber for potentials < 70 kV. Requires material (Table 1) to provide buildup and eliminate e- contamination. Cylindrical chambers do not require buildup for in-air measurements. Thimble thickness is 50 mg cm-2 or more. Will require inverse-square correction for closed-cone systems.

TG – 61 In-air method Dw,z=0 = M· Nk · Bw · Pstem,air · Pstem,air accounts for change in photon scatter from chamber stem between chamber calibration and beam calibration. Pstem,air = 1.0 if same field size is used for both chamber calibration and measurement. Use guidelines in Sec. V C.7 to establish Pstem,air if same field size is not used

TG – 61 In-air method is the ratio for water-to-air of the mean mass energy-absorption coefficients averaged over the incident photon spectrum. Function of HVL Table IV (Section B.1.1) Backscatter factor Bw Function of HVL, field diameter, and SSD Table V (Section B.1.2) Closed-end cones require additional correction. (Table VI)

TG – 61 In-phantom method Calibration of medium-energy x rays can be performed in water. Center of chamber at 2-cm depth. If water-proofing is needed, use appropriate correction factors (Appendix B.2.3) Psheath values listed in Table IX Use talcum powder-free latex.

TG – 61 In-phantom method Dw,z=0 = M· Nk · PQ,cham · Psheath · PQ,cham accounts for change in chamber response due to change in beam characteristics between calibration and measurement, and the pertubation in photon fluence due to chamber in water. Function of HVL and chamber type Use Table VIII (Section B.2.2)

TG – 61 In-phantom method is the ratio for water-to-air of the mean mass energy-absorption coefficients averaged over the photon spectrum at depth 2 cm and 10x10 field. Function of HVL Table VII (Section B.2.1) If field size differs from 10x10: Use Figure 3 to correct ratio of mass energy absorption coefficients Use Figure 4 to correct for PQ, cham

TG - 61 Conversion to dose in other medium. Table X lists the in-air ratios of mass energy-absorption coefficients of biological tissue to water. Use in conjunction with in-air calibration method. Conversion to dose in bone requres correction factor to Bw listed in Table XI.