Rx = 3000 cGy in 10 fractions, whole brain Rt and Lt laterals, KD Siemens 6x. The collimator is set for 22 X 16 cm2 portal films at 92.5 cm SSD. Calculate the treatment port MU to isocenter. Calibration performed std. SAD; treatment performed SSD (assume SSDs at the calc point = 92.5 cm).
To calculate the treatment port Monitor Units (MU) to isocenter, you'll need to follow these steps:
Step 1: Calculate the Source to Axis Distance (SAD) correction factor.
The SAD correction factor corrects for the difference between the treatment Source to Axis Distance (SSD) and the calibrated Source to Axis Distance (SAD). In this case, since the calibration was performed at SAD and the treatment was performed at SSD, we will need to calculate this correction factor.
SAD correction factor = (SSD^2) / (SAD^2)
SSD = 92.5 cm (given)
SAD = ?
To find the SAD, we need more information. Please provide the calibrated Source to Axis Distance (SAD).
Step 2: Calculate the Field Size Factor (FSF).
The Field Size Factor corrects for the difference in energy dose delivered to the patient between different field sizes. It depends on the collimator size used for treatment.
Field area = 22 cm x 16 cm = 352 cm^2
To find the FSF, you will need to refer to a reference table or use machine-specific data that provides the FSF values for different field sizes. Please consult the equipment documentation or a medical physicist to determine the correct FSF value for a field size of 22 cm x 16 cm.
Step 3: Calculate the Monitor Units per centiGray (MU/cGy) factor.
The MU/cGy factor specifies the number of Monitor Units needed to deliver 1 centiGray of radiation at the reference point. It depends on the machine being used.
Machine factor = ?
Similarly, the machine factor depends on the specific machine used. You will need either machine-specific data or consult a medical physicist to determine the correct machine factor.
Step 4: Calculate the treatment port Monitor Units (MU) to isocenter.
The treatment port Monitor Units (MU) to isocenter can be found using the following formula:
MU = Rx (cGy) * (MU/cGy factor) * (FSF) * (SAD correction factor)
Given:
Rx = 3000 cGy (prescription dose)
FSF = ? (Field Size Factor)
MU/cGy factor = ? (Machine Factor)
SAD correction factor = ? (To be calculated)
Once you have all the required values, simply substitute them into the formula to calculate the treatment port Monitor Units (MU) to isocenter.
Note: Since some of the required values are missing, it is not possible to provide the exact calculation at this time.