Resident Physics Lectures XRay Shielding George David Associate

Resident Physics Lectures X-Ray Shielding George David Associate Professor Department of Radiology Medical College of Georgia

Typical Shielding Problem George David

Shielding Considerations • • Whom are we protecting? Workload Type of studies performed Distances Occupancy Primary / secondary Use George David

Whom Are We Protecting? George David

Exposure Limits • Controlled areas à 0. 1 m. Gy / week à 10 mrad / week à 5 m. Gy / year • Uncontrolled areas à. 02 m. Gy / week à 2 mrad / week à 1 m. Gy / year • Film à 0. 1 m. Gy during storage period

Controlled vs. Non. Controlled Areas

Workload • m. A-min beam on at each k. Vp • # patients • # images / patient George David

Barrier Considerations • Is beam directed at this barrier? • Primary or Secondary? • Distance? • What fraction of time? àUse factor • What’s behind the barrier?

Barriers George David

Use Factor • Fraction of time beam aimed at each barrier Typical Primary Barriers Barrier Floor Cross Table Wall Other Walls Chest Bucky Wall Use Factor 0. 89 0. 02 1. 00 George David

Occupancy Factor • Fraction of time the maximally exposed individual is present while beam on George David

Locations Assumed to have Full Occupancy • Offices • Labs • Reception • Reading room • Nurses station • Control Room

Occupancy Factor Location Exam / treatment Corridors, lounges Corridor doors Public toilets, storage, outdoor seating Outdoor, parking lots Occupancy Factor ½ 1/5 1/8 1/20 1/40

Shielding Design VERY Conservative • Attenuation by patient ignored • Perpendicular incidence assumed • Shields not part of wall structure ignored • Tube leakage normally far below assumed maximum allowable value

Shielding Design VERY Conservative • Field sizes often smaller than maximum assumed value àScatter levels considerably less for smaller field • Occupancy factors are conservatively high à 100% for an individual in an office à 20% for an individual in a hallway • Lead comes in specific thicknesses àOrdered in next thicker size • Distance to occupied area assumed to be 1 foot behind barrier

Notes • Shielding designs very conservative • Designs must be approved by state • Integrity must be tested • Adequacy must be tested

Notes • Design only valid for given layout and workload • Changes to equipment style or position or workload require new shielding calculation