Medical Physicists Medical Physicists Responsibilities - PDF document

Joel E. Gray, Ph.D. Medical Physicists’ Medical Physicists ’ Responsibilities Responsibilities Evaluation and Consulting on Evaluation and Consulting on Patient Dose in Diagnostic Imaging Patient Dose in Diagnostic Imaging Provide dose and risk information Provide dose and risk information Advising medical staff of doses and risks Advising medical staff of doses and risks Joel E. Gray, Ph.D. Joel E. Gray, Ph.D. Consulting with IRBs Consulting with IRBs regarding research regarding research Director of Technology Director of Technology uses of radiation uses of radiation Landauer, Inc. Landauer, Inc. Determining specific organ dose, Determining specific organ dose, e.g., uterine and fetal doses e.g., uterine and fetal doses James M. Kofler, Ph.D. James M. Kofler, Ph.D. Department of Diagnostic Radiology Department of Diagnostic Radiology Consulting with patients and allaying their Consulting with patients and allaying their Mayo Clinic and Foundation Mayo Clinic and Foundation fears fears Required Information “Negotiating Negotiating” ” Research Protocols Research Protocols Required Information “ Information necessary to provide dose and Information necessary to provide dose and Medical physicist plays vital role in research Medical physicist plays vital role in research risk estimates risk estimates— — protocols protocols Patient entrance skin exposure (ESE) Provide information regarding dose Patient entrance skin exposure (ESE) Provide information regarding dose and risk for IRB and risk for IRB Specific organ doses based on ESE Specific organ doses based on ESE “Negotiate Negotiate” ” with research proponent with research proponent “ Effective dose Effective dose relative to best modality (provide relative to best modality (provide Risk estimate Risk estimate best images), minimizing dose, best images), minimizing dose, etc. etc. Relative risks of other activities Relative risks of other activities 1

Joel E. Gray, Ph.D. Patient ESE Patient ESE– – Simple?? Simple?? NEXT Survey Results NEXT Survey Results (mR or R/min) (mR or R/min) Know the kVp, mAs, SSD and calculate Know the kVp, mAs, SSD and calculate the ESE? the ESE? Exam Min Max Max/Min Exam Min Max Max/Min NO!!!!!!!!!!!!!!!!! NO!!!!!!!!!!!!!!!!! PA Chest PA Chest 2.4 2.4 81 81 33.8 33.8 Why not?? Why not?? AP L. Spine 62 2,154 34.7 AP L. Spine 62 2,154 34.7 Variation of x- -ray system output? ray system output? Variation of x GI Exams GI Exams Variation from one tech to another Variation from one tech to another Rate Rate 0.7 0.7 16.2 16.2 23.1 23.1 Variation, variation, variation Variation, variation, variation Spot Film Spot Film 38 38 4,815 126.7 4,815 126.7 How much variation? How much variation? CT Head CT Head 1,600 1,600 14,000 14,000 8.8 8.8 From 10X to 130 X!!! From 10X to 130 X!!! Variation Within a Facility Variation Within a Facility Variation Within a Facility Variation Within a Facility Major East Coast teaching hospital Personal experience— — Major East Coast teaching hospital Personal experience No in No in- -house physicist or equipment QC house physicist or equipment QC Major East Coast teaching hospital Major East Coast teaching hospital program program AP Lumbar spine ESE AP Lumbar spine ESE Consulting physicist visited once per year 16.4 X range in ESE!!!!! Consulting physicist visited once per year 16.4 X range in ESE!!!!! to meet regulatory requirements to meet regulatory requirements 1.10 mSv to 18.0 mSv 1.10 mSv to 18.0 mSv Did NOT NOT measure patient measure patient ESEs ESEs, only , only Did Mixture of CR and screen- -film film Mixture of CR and screen mR/mAs mR/mAs Screen- Screen -film ESE ~3.00 mSv film ESE ~3.00 mSv mR/mAs tells nothing about how the mR/mAs tells nothing about how the CR dose based on location and x- -ray ray CR dose based on location and x equipment is used nor absorbers equipment is used nor absorbers equipment manufacturer equipment manufacturer in the beam after the patient!!! in the beam after the patient!!! 2

Joel E. Gray, Ph.D. First Three Rules of Dose First Three Rules of Dose Which ESE Do You Measure? Which ESE Do You Measure? Determination Determination Ask the tech what technique they used? Ask the tech what technique they used? Measure the ESE!!! Measure the ESE!!! Which tech? Which machine? Which Which tech? Which machine? Which Measure the ESE!!! Measure the ESE!!! day? day? Measure the ESE!!!* Measure the ESE!!!* Measure the ESE for a standard phantom? Measure the ESE for a standard phantom? American approach American approach *It is morally, ethically, and *It is morally, ethically, and Determine the ESE for a group of patients? Determine the ESE for a group of patients? professionally irresponsible NOT NOT to to professionally irresponsible European approach (Va European approach ( Vañ ño o, et al.) , et al.) measure the ESE!!! measure the ESE!!! Effect of Patient Variation on Dose Effect of Patient Variation on Dose Standard (Reference) Man (Woman) Standard (Reference) Man (Woman) Patient size variation Patient size variation— — Neonate to beached whale Neonate to beached whale Defined by ICRP 23 and 89 Defined by ICRP 23 and 89 5 cm thickness to 40 cm thickness 5 cm thickness to 40 cm thickness Height, weight, skin area, chemical Height, weight, skin area, chemical Fetal dose estimates Fetal dose estimates— — you will know patient size, you will know patient size, composition, etc., etc., etc. composition, etc., etc., etc. i.e., these are retrospective estimates i.e., these are retrospective estimates Research protocols are prospective estimates Research protocols are prospective estimates– – you will know only the age range of patients you will know only the age range of patients Reference man– Reference man – 70 kg, 170 cm 70 kg, 170 cm AP lumbar spine range AP lumbar spine range— — Reference woman Reference woman– – 58 kg, 160 cm 58 kg, 160 cm 0.30 mGy to > 60.0 mGy 0.30 mGy to > 60.0 mGy Dose tables are based on “ Dose tables are based on “standard man standard man” ” 3

Joel E. Gray, Ph.D. Standard (Reference) Man (Woman) Standard (Reference) Man (Woman) Dose Estimate Error?? Dose Estimate Error?? What error in estimated dose is Defined by ICRP 23 and 89, What error in estimated dose is Defined by ICRP 23 and 89, acceptable in diagnostic imaging? acceptable in diagnostic imaging? Height, weight, skin area, chemical Height, weight, skin area, chemical < 1% (precise therapy doses) < 1% (precise therapy doses) composition, etc., etc., etc. composition, etc., etc., etc. 2% to 4% or 5% (typical therapy doses) 2% to 4% or 5% (typical therapy doses) Reference man– – 70 kg, 170 cm Reference man 70 kg, 170 cm 10% ?? 10% ?? 154 lb, 5’ 154 lb, 5 ’7 7” ” 20% ?? 20% ?? Reference woman– – 58 kg, 160 cm Reference woman 58 kg, 160 cm 30% ?? (personnel dosimetry) 30% ?? (personnel dosimetry) 128 lb, 5’ 128 lb, 5 ’3 3” ” 50% ?? 50% ?? 100% not not unreasonable!! unreasonable!! 100% Dose Estimate Error?? Dose Estimate Error?? Dose Estimate Error?? Dose Estimate Error?? Depends on Depends on— — Location of sensitive tissue relative to Location of sensitive tissue relative to Patient size Patient size irradiated volume irradiated volume Technique (kVp, mAs, SSD, etc.) Technique (kVp, mAs, SSD, etc.) In direct x- -ray beam ray beam In direct x Location of sensitive tissue from beam, Location of sensitive tissue from beam, Depth is important Depth is important e.g., e.g., distance of fetus from distance of fetus from HVL in centimeters of tissue at HVL in centimeters of tissue at irradiated volume irradiated volume 80 kVp ~3.5 cm 80 kVp ~3.5 cm Room- -to to- -room variation room variation Room Outside of irradiated volume Outside of irradiated volume Room scatter?? Room scatter?? Felmlee data data Felmlee Tube leakage?? Tube leakage?? 4

Joel E. Gray, Ph.D. Distance of Fetus From Irradiated Distance of Fetus From Irradiated Effect of Depth in Tissue of Sensitive Organ Effect of Depth in Tissue of Sensitive Organ Volume* and HVL in Tissue For CT Volume* and HVL in Tissue For CT cm NFDR cm NFDR Depth Depth 0 0 0.2600 0.2600 1.0000 1.0000 Relative Dose (cm) (cm) 1 0.0458 0.1762 1.0000 1 0.0458 0.1762 1.0000 0 1 1.00 0 1 1.00 2 2 0.0338 0.0338 0.1300 0.1300 0.7380 0.7380 7 7 1/4 1/4 0.250 0.250 3 3 0.0265 0.0265 0.1019 0.1019 0.5786 0.5786 HVL in tissue ~3.5 cm 14 14 1/16 1/16 0.063 0.063 4 4 0.0206 0.0206 0.0792 0.0792 0.4498 0.4498 5 5 0.0167 0.0167 0.0642 0.0642 0.3646 0.3646 28 28 1/64 1/64 0.016 0.016 6 6 0.0136 0.0136 0.0523 0.0523 0.2969 0.2969 *Felmlee JP, Gray JE, Leetzow ML, Price JC. Estimated fetal radiation dose from multislice CT studies. AJR 1990; 154:185-190. Dose Estimate Error?? Dose Estimate Error?? Taking into account all of the variables— — Taking into account all of the variables How accurately can YOU YOU estimate the dose estimate the dose How accurately can to a specific sensitive organ? to a specific sensitive organ? How accurate must the dose be for a dose How accurate must the dose be for a dose estimate ? estimate ? What are the medico- What are the medico -legal implications legal implications 5