Investigation of the effects of treatment planning variables in small animal radiotherapy dose distributions

PURPOSE

Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation using a system recently developed for this purpose, the authors characterized dose distributions produced from conformal radiotherapy of small animals in a microCT scanner equipped with a variable-aperture collimator.

METHODS

Dose distributions delivered to a cylindrical solid water phantom were simulated using a Monte Carlo algorithm. Phase-space files for 120 kVp x-ray beams and collimator widths of 1-10 mm at isocenter were generated using BEAMnrc software, and dose distributions for evenly spaced beams numbered from 5 to 80 were generated in DOSXYZnrc for a variety of targets, including centered spherical targets in a range of sizes, spherical targets offset from centered by various distances, and various ellipsoidal targets. Dose distributions were analyzed using dose volume histograms. The dose delivered to a mouse bearing a spontaneous lung tumor was also simulated, and dose volume histograms were generated for the tumor, heart, left lung, right lung, and spinal cord.

RESULTS

Results indicated that for centered, symmetric targets, the number of beams required to achieve a smooth dose volume histogram decreased with increased target size. Dose distributions for noncentered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Irradiation of a mouse lung tumor with multiple beam widths was found to effectively deliver dose to the tumor volume while minimizing dose to other critical structures.

CONCLUSIONS

Overall, this method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapy treatment planning. Future work should address methods to improve conformality in asymmetric targets.

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

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.