Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

The overall goal of the following experiment is to characterize the effects of ion recombination in a liquid ionization detector used for the dosimetry of a stereotactic radiation therapy system. This is achieved by using the two dose rate method, which consists of performing a series of water tank measurements with increasing source to surface distances to gradually reduce the dose per pulse, increasing the collection efficiency in the detector sensitive volume as a second step. The measurements at different doses per pulse are compared and used to obtain values of the general collection efficiency as a function of the dose per pulse.

Next correction factors are calculated based on these collection efficiencies and are applied to the liquid ionization chamber measurements to correct for the recombination effect. The results show that ion recombination has non-negligible effects on do symmetric measurements when spanning a large range of dose per pulse. The size and tissue equivalence of the detector active volume are two important tissues when measuring small field, which makes this detector interesting to characterize as it present a very small sensitive volume filled with a liquid medium.

An advantage of this technique is the accurate positioning of the treatment head that moves along the vertical axis, which allows us to study recombination without having to change the whole setup as in conventional Linux. To begin, place the water tank under the treatment head as low as possible, fill the tank with water. Next, align the water tank with the LINAC so it's vertical sides are parallel to the vertical sides of the head, and use the laser to ensure it is in the correct orientation.

Then verify the vertical orientation of the LINAC by performing X and Y profile measurements at two different depths. Compute the beam declination and correct it using the head's axes of rotation. Next, replace the collimator with the TER accessory and accurately position the head at 78.5 centimeters SSD, so that the tip of the accessory barely touches the water surface.

Then remove the TER and place the 60 millimeter collimator on the treatment head. Next position the liquid ionization chamber or LIC in a vertical position with the axis of the cylindrical cavity parallel to the beam direction. Use the laser to position the LIC at the center of the beam in the lateral direction.

Then place a 0.125 centimeter cubed, air-filled ionization chamber or a IC next to the LIC to be able to correct for attenuation distance and scatter effects. Place the chamber 1.5 centimeters below the water level. Connect the LIC and the high voltage supply to the electrometer and set the voltage to 800 volts.

Connect the A IC to another electrometer and set the voltage to 400 volts. Then wait an hour for the system to stabilize. Perform profile measurements in both transverse directions and correct the zero of the LIC to ensure the accuracy of the detector lateral positioning.

First, deliver a pre irradiation dose of 3000 monitor units or MU in order to stabilize the LIC response. Then zero the TERs. Next, perform a series of charge acquisitions with the beam off to assess the leakage current and stability.

Then set the remote controller to Cartesian mode and perform a 20 centimeter motion in the Z direction to place the treatment head at 58.5 centimeters.SSD. Next, leave the treatment room, close the door and program and a radiation of 100 mu at the operator console. Start both TERs deliver the dose and to note the charges measured by the LIC and the A IC.Repeat the process 10 times to assess statistical uncertainties.

After 10 measurements, move the treatment head to 68.5 centimeters.SSD. Then repeat the irradiation when the head has moved further away from the tank. The distance between measuring points can be increased as the charge varies following the inverse square distance law for every distance.

D, take the ratio of each measured LIC value with the corresponding a IC value obtained at the same distance. Next, plot the ratios against the dose per pulse and use a linear fit to obtain the extrapolated ratio at zero dose per pulse. R zero, assume that the collection efficiency is equal to one at zero milligrams per pulse and normalize all the ratios calculated to the extrapolated value in order to obtain values of F.Then plot the values of F against the values of the dose per pulse to represent the evolution of the collection efficiency.

The error bars can be calculated by propagating the uncertainties on the LIC and A IC charges evaluated from the repeated measurements at each distance.Collection. Efficiencies can also be obtained using method B calculated from the following relation where the parameters are calculated from the charge ratios at the different SSDs. The collection efficiency F obtained from method A was plotted against the dose per pulse, where a 2%loss in signal can be seen.

The error bars show important uncertainties inherent to method A in which can be greatly reduced with the use of method B.The general collection efficiency plotted against the dose per pulse calculated from method B proves more precise and provides absolute values of F compared to calculations generated with method A.The deviations are small and the loss in signal is lower than with method A.This figure shows the relative depth dose obtained before and after recombination correction. When the curves are normalized at the depth of 240 millimeters. Where recombination effects vanish, they coincide the corrections compensate for recombination effects in the buildup area where the correction factors are the highest, alluding to the accuracy of the calculated correction factors, a validation of the two dose rate method Following this procedure.

Other methods like Montecarlo modeling can be used to complete the characterization of the liquid ionization chamber and investigate other factors such as detector materials or the sensitive volume effects.