Monitor Unit Calculations Part 1 George Starkschall, Ph.D. Department of Radiation Physics U.T. M.D. Anderson Cancer Center Return to our first patient 62 yr old woman with Stage IIIB (T1N3M0) NSCLC rt lower lobe Dose prescription 6 MV x-rays x 1.8 Gy/fraction x 35 fractions IMRT How long do we leave the linac on? Purpose To provide a methodology for the calculation of machine settings Rather than specifying a particular method for calculating machine settings, we will provide a consistent methodology that allows selection of a particular set of measurement conditions to ensure consistency in the machine setting calculations 1
A caveat Different institutions have different methodologies for calculating machine settings All methodologies may be correct, provided they are consistent Goal of calculation Calculate dose delivered to small mass of tissue at a specified point within patient, given a set of physical beam parameters Goal of calculation Beam parameters include: 1. Machine radiation type and energy 2. Machine output 3. Field size 4. Depth 5. Beam modifiers 2
Achieving this goal Brute force method: Simulate beam configuration on phantom and measure dose per monitor unit Likely to be very time-consuming But this is what we presently do for some cases: IMRT Protons Some clinics do this for electrons, as well Achieving this goal Can we calculate the dose per monitor unit instead? Yes, at least for photons and electrons Goal is to be able to do this for protons as well Achieving this goal Relate dose to small mass of tissue in patient to reference dose Measure reference dose Calculate desired dose 3
Calculation of machine setting reference dose machine setting =, reference dose output reference dose output = calibration dose output correction for collimator setting correction for distance from source. correction for beam modifiers correction for attenuation and scatter Calculation of machine setting All these corrections have previously been measured in a water phantom when the treatment machine was commissioned All these corrections are readily available in tables and can be looked up Important to have access to these tables Hard copy Computer files Reference dose Dose prescriptions must be expressed as point dose Dose prescribed to be delivered to isocenter Dose prescribed as percentage of isocenter dose Example: Prescription: 180 cgy to isocenter Prescription: 200 cgy to 95% of isocenter Convert this to isocenter dose: 200/0.95=211 cgy to isocenter 4
A common blunder 200 cgy to the 95% line 95% of what? Must specify isocenter, or maximum dose, or reference point dose MDACC prescription methodology We often prescribe an implied minimum target volume dose 180 cgy means the entire target volume receives 180 cgy Really a more descriptive prescription than a point dose prescription This is converted by the treatment planning system into a point dose prescription Reference dose For isocentric treatments Dose prescribed to be delivered to isocenter Example: Prescription 180 cgy from 3 equally weighted fields Reference dose 60 cgy for each field 5
Reference dose For isocentric treatments Dose prescribed to be delivered to isocenter Example: Prescription 180 cgy from 2 fields weighted 2:1 Reference dose 120 cgy for the heavier- weighted field, 60 cgy for second field Reference dose For fixed SSD treatments Given dose for treatment field Given dose can be found by dividing prescription dose by percent depth dose (%dd) Percent depth dose defined later Not used as much any more for photon treatments Will address electrons and protons later Reference dose For treatments where dose distribution extracted from treatment planning system Reference dose is some other dose Reference dose may not even be actual point dose, rather dose to defined point from hypothetical field 6
Reference dose Reference dose must be identified as dose to a specific point Prescription based on mean dose or dose coverage must be converted to reference dose by treatment planning system Definition of reference dose may be treatment planning system specific Calibration dose output Dose output of treatment machine For linac calibrated to deliver 1.0 cgy/mu for 10 cm 10 cm field at d max Because output is pulsed, a transmission ion chamber is placed in beam path Monitor units are arbitrary units recorded by the ion chamber For cobalt machine calibration dose output is measured (expressed in cgy/min) Output is constant Calibration dose output Calibration dose output may be specified in air or in tissue (or tissue-equivalent equivalent phantom) For cobalt and low-energy linac photons may be specified in air or phantom For high-energy photons specified in phantom 7
Calibration dose output Necessary to know at what depth value is defined Typically at d max Could be at other reference depth, e.g., TG-51 depth specification TG-51 mandates that calibration measurement be done at fixed depth (5 cm or 10 cm), then converted back to d max Calibration dose output Necessary to know at what distance from source value is defined May be equal to source to isocenter distance (SAD) May be equal to SAD + d max, i.e., phantom surface is placed at source to isocenter distance and calibration point at depth d max Calibration dose output At MDACC, 1 cgy/mu specified as follows: 10 cm 10 cm field In phantom at depth d max At distance from source SAD + d max, i.e., phantom surface placed at SAD At other institutions, 1 cgy/mu may be specified differently 8