In Tomotherapy, Why output is described in terms of absorbed dose per unit time rather than the traditional units of dose per monitor unit? The radiation source of Tomotherapy is extranuclear right?
Dear dr Neil Madrigal,
If you have the possibility to access at the Accelerator Output Machine (AOM) parameters (that you can query on a Tomotherapy Operator Station), you'll find a string named: "TX_1 CM_Beam, MU, TriggerRate" where there is an editable (by the specialist only) string: "muPerChamberCount". Here you'll find two values (one for each monitor chamber) representing the scaling factor used by the system to convert the monitor chamber readings in terms of monitor units per minute (referred in the same AOM tab as: "expectedMonitorUnitsPerMinute". This is also the value that appears in the Tomotherapy TPS tab named: "Plan Settings" at the string: "Beam".
During the Acceptance Test Procedure of a Tomotherapy system, the output measurements are performed in order to assess the monitor chamber readings of the system itself. The procedure used is delivered for 70 seconds: 10 sec. for warm-up the system and 60 sec. of beam-on, using a water-equivalent phantom with two ion chambers.
The average value of the monitor chamber readings collected during the test is multiplied by 60 sec. beam-on and divided by the total procedure duration (i.e. 70sec). Suppose that this value is 894 MU/min, and suppose that the value reported at the AOM string: expectedMonitorUnitsPerMinute" is 861. Hence it needs a correction factor (or scaling factor) that is suitable to match the MU/min measured with the 861 MU/min.
This is done by the Specialist: she/he tunes the "muPerChamberCount" (that is expressed in terms of "dose/time" or "cGy/sec") in order that the system output reaches the values of 861 MU/min.
The "muPerChamberCount" is the aswer to your question.
I hope I answered the question.
Have a great day,
Emilio
Tomo source is a linac for sure. The reason for which dose rate is used instead of MU is just because there may be a significant variation in dose rate according to the linac position (linac rotates all around the patient, 360°). However, as required by the IEC 60601-2-1 standard, Tomo is provided with a redundant monitoring system
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