Radiation oncology physics ABR Questions With Complete Solutions

TG-51 def: %dd(10)x correct answer: the photon component of the photon beam percentage depth-dose at 10 cm depth in a 10x10 cm2 field on the surface of a water phantom at an SSD of 100 cm TG-51 def: %dd(10) correct answer: the measured photon beam percentage depth dose at 10 cm depth in a 10x10 cm2 field on the surface of a water phantom at an SSD of 100 cm (includes electron contamination). TG-51 def: %dd(10)Pb correct answer: same as %dd(10) except that a 1 mm lead foil is in place below the accelerator at about 50 cm from the phantom surface ( 30 cm if 50 cm clearance is not available) TG-51 def: clinical reference dosimetry correct answer: determination of absorbed dose to water per MU under reference conditions in the clinic. TG-51 def: DQw correct answer: the absorbed dose to water for a given number of monitor units ~or minutes for 60Co) from a radiation beam of quality Q. Unit: gray, Gy. TG-51 def: dmax correct answer: the depth at which the absorbed dose to water (not ionization) is a maximum for a given beam. In photon beams it may include effects of electron contamination in the incident beam. Unit: cm. TG-51 def + formula: dref in electron beams correct answer: the reference depth for electron beams given as dref=0.6R50-0.1, where R50 is in cm. Unit: cm. TG-51 def: I50 correct answer: the depth in an electron beam at which the gradient-corrected ionization curve falls to 50% of its maximum. Unit: cm. TG-51 def: kQ correct answer: the quality conversion factor, which accounts for the change in the absorbed-dose to water calibration factor between the beam quality of interest, Q, and the beam quality for which the absorbed-dose alibration factor applies (usually 60Co). kQ is a function of the beam quality Q [specified by %dd(10)x or R50] and is chamber dependent. For 60Co beams kQ=1.000 by definition. TG-51 def: kR50 correct answer: chamber specific component of kQ in an electron beam which is independent of the ionization gradient at the point of measurement (i.e., kQ=kR50PQgr). kR50 is a function of the electron beam quality specified by R50. TG-51 def: k'R50 correct answer: the electron quality conversion factor. Needed to convert NQecalD,w into NQD,w for any beam quality Q. Is a function of the electron beam quality specified by R50. Note that kR50=k'R50kecal TG-51 def: kecal correct answer: photon-electron conversion factor. Needed to convert N60CoD,w into an electron beam absorbed-dose calibration factor NQecalD,w for a selected beam quality Qecal. Note that: kR50=k'R50kecal TG-51 def: Mraw(d) correct answer: uncorrected ion chamber reading with the point of measurement of the ion chamber at a depth d in water, for a given number of monitor units (or minutes for 60Co). If no sign is indicated, the measurement is made collecting the same charge as during calibration. If a sign is indicated (+ or -), it is the sign of the charge collected. Unit: C or rdg (meter reading). TG-51 def: M correct answer: fully corrected ion chamber reading corrected to the standard environmental conditions of temperature and pressure for which the ion chamber calibration factor applies; and also corrected for polarity effects, lack of complete ion collection efficiency, and electrometer accuracy. Unit: C or rdg. TG-51 def: MU correct answer: number of monitor units (or minutes for 60Co) for which a given irradiation is performed TG-51 def: ND,w correct answer: the absorbed-dose to water calibration factor for an ion chamber located under reference conditions in a radiation beam. The absorbed dose measured is that at the chamber's point of measurement in the absence of the chamber. For a vented ion chamber the calibration factors from US and Canadian calibration laboratories apply for standard environmental conditions of temperature, pressure, and relative humidity. Calibration factors apply assuming the chamber reading corresponds to 100% charge collection efficiency. In contrast, calibration factors are usually for a stated polarity and corrections are needed if there is a significant polarity effect in the calibration beam. Unit: Gy/C or Gy/rdg. TG-51 def: NQD,w correct answer: the value of ND,w in a photon or electron beam of quality specified by Q TG-51 def: P (pressure) correct answer: air pressure inside ion chamber. In a vented chamber it is assumed to be the same as the local air pressure. Not corrected to sea level, including temperature and latitude corrections for a mercury barometer. Unit kPa, reference 1013.25