<prompt> Maximum damage to skin is caused bySuper voltage X-ray therapyMega voltage X-rayOrthovoltage X-rayCobalt-60  <response>: Ans. c Orthovoltage X-ray Ref. Harrisons medicine 17th ed. 484 External Beam therapy unit includea. X-ray machine orb. Teletherapy unit i.e. Linear Acclerator or Cobalt-60 unit Beams used for external radiotherapy are X-ray beams:a. Conventionalb. Superficial: 40 to 120 kVc. Orthovoltage: 250 to 400 kVd. Megavoltage: 1Mv - 2, 4, 6, 12 35 MeV Gamma ray beams:a. Cobalt -60 beamb. Cesium -137 beam Particle beams:a. Electronsb. Protonsc. NeutronsThus routine radio-therapy treatment is by X-ray beams in the super voltage or mega voltage range. Orthovoltage or deep X-ray therapy machine produce X-rays with a voltage of 200-300 kV. For many years they were the most powerful machines in radiotherapy and were used to irradiate most sites, however now been replaced by megavoltage equipments because of following advantages of megavoltage therapy 2 to 8 million volts; Skin sparing effects: With orthovoltage X-rays maximum dose is received by skin surface whereas with high voltagesmaximum dose is at some distance beneath skin, the skin is thus spared with high dose. Greater penetration of beam Differential absorption in tissues Improved collimation 