Commissioning of a novel microCT/RT system for small animal conformal radiotherapy

Abstract

The purpose of this work was to commission a 120 kVp photon beam produced by a micro-computed tomography (microCT) scanner for use in irradiating mice to therapeutic doses. A variable-aperture collimator has been integrated with a microCT scanner to allow the delivery of beams with pseudocircular profiles of arbitrary width between 0.1 and 6.0 cm. The dose rate at the isocenter of the system was measured using ion chamber and gafchromic EBT film as 1.56-2.13 Gy min(-1) at the water surface for field diameters between 0.2 and 6.0 cm. The dose rate decreases approximately 10% per every 5 mm depth in water for field diameters between 0.5 and 1.0 cm. The flatness, symmetry and penumbra of the beam are 3.6%, 1.0% and 0.5 mm, respectively. These parameters are sufficient to accurately conform the radiation dose delivered to target organs on mice. The irradiated field size is affected principally by the divergence of the beam. In general, the beam has appropriate dosimetric characteristics to accurately deliver the dose to organs inside the mice's bodies. Using multiple beams delivered from a variety of angular directions, targets as small as 2 mm may be irradiated while sparing surrounding tissue. This microCT/RT system is a feasible tool to irradiate mice using treatment planning and delivery methods analogous to those applied to humans.

Figure 1

The variable-aperture collimator in the microCT/RT system uses two disks with hexagonal shape that open and close like an iris camera. The disks are rotated 30° one with respect to the other to form a field of dodecagonal shape.

Figure 2

(a) Targets for irradiation are placed at the isocenter of the microCT and treated with beams from different angles. (b) To measure the dose rate, an ion chamber was placed at the isocenter, in air, and irradiated with a photon beam of 120 kVp with a field diameter of 6 cm. (c) To measure the percentage depth dose, gafchromic films were sandwiched in 6 × 6 × 0.3 cm³ solid water slabs and the top surface of the solid water was placed at the scanner isocenter. (d) A single dose fraction was delivered to a gafchromic film sandwiched in solid water, centered at the isocenter and placed parallel to the beam and the gantry rotation plane.

Figure 3

Calibration curve used to associate the dose delivered to the film and its optical density. The curve can be fitted to a third order polynomial. The error bar represents the standard deviation in reading the film and calculating the optical density, STD = 0.0054.

Figure 4

Dose rate as a function of the field diameter. The dose rate decreases as the field diameter decreases due to scatter reduction. The contribution of scatter to the primary beam is significant at the energies at which the microCT operates. The error bars represent the overall uncertainty in the film measurements, 3.6%.

Figure 5

Dose rate as a function of depth for field diameters of 2, 1, 0.5 and 0.2 cm. For field diameters of 1.0 cm and 0.5 cm, the most commonly used field diameter range used in mice irradiation, the dose rate decreases about 10% per every 5 mm in depth. The dose rate is still high enough to irradiate targets inside of mice bodies considering that the average radius of a mouse’s body is about 1.25 cm. The error bar represents the overall uncertainty in the film measurements, 3.6%.

Figure 6

(a) Dose profile at 1 mm depth for field diameters of 0.46, 0.93 and 1.86 cm (120 kVp, 50 mA photon beam). (b) Dose profile at 0.1, 1 and 2 cm depth for a field diameter of 0.93 cm. The FWHM of the beam increases to 0.95 and 1.01 cm at depths of 1 and 2 cm, respectively. Penumbra, flatness and symmetry do not change significantly.

Figure 7

(a) Film irradiated with eight beams with diameters of 0.2 cm. (b) Dose distribution in the film. The difference between the calculated and measured dose is 2.8%. This single dose testing proved that the dosimetry of the system was consistent and that it can be used to deliver the dose to internal organs in mice.