Orthovoltage X-rays
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Orthovoltage X-rays have an energy in the range of 200-500 kV. When used to treat patients, radiation oncologists find that they penetrate to a useful depth of about 4-6cm. That makes them good for ribs, but not for lungs.
Long ago, orthovoltage X-rays were called "deep" X-rays or "DXR". Historically, as radiation machines were improved in the 1930-40s, the beam energy (and so its penetration) were increased. So as the new 200-500kV beams became available, they were considered 'deep'. Now, with modern linear accelerators being able to produce X-rays of much higher energy, the penetration of orthovoltage X-rays is considered quite shallow.