Treatment Planning for Small Animals

Authored by: Frank Verhaegen

Handbook of Small Animal Imaging

Print publication date:  April  2016
Online publication date:  February  2016

Print ISBN: 9781466555686
eBook ISBN: 9781466555693
Adobe ISBN:

10.1201/b19052-27

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Abstract

In Chapter 18, we discussed why small animal precision irradiation with photons is preferably done at kilovolt (kV) instead of megavolt (MV) energies. This is mostly to avoid extensive dose buildup and dose re-buildup regions near medium interfaces and to avoid large beam penumbras (see Chapter 18). Some animal studies were done in the past with MV beams, often without treatment planning or dosimetry. In many cases, no imaging was available. When using a treatment planning system (TPS) to calculate the complex 3D dose distribution for human radiotherapy, several problems are encountered. These TPSs use calculation models that are not intended or commissioned for small beams (usually not below 3 cm field size). Forcing them to calculate doses for small fields may lead to very wrong absolute machine radiation output. Among the few animal studies using small beams of high-energy photon beams accurately is that of DesRosiers et al. (2003), who irradiated rats with small beams from a GammaKnife device employing multiple 60Co sources (1.25 MeV) with small collimators. They used a dedicated TPS for this device. A second problem arises because radiotherapy TPSs perform calculations in voxelized geometries derived from computed tomography (CT) images. In case where a human CT scanner is used to image a small animal, the voxel resolution may be too coarse (no better than 1 mm) to capture the details of the animal’s anatomy; in case an animal CT scanner is used, the voxel resolution may be much better, but human radiotherapy TPSs are not equipped to handle very small voxels or very large numbers of voxels. Similar problems may arise with positron emission tomography (PET) or magnetic resonance imaging (MRI) when other forms of anatomical and/or functional imaging are required. A third problem is encountered when the animal irradiator employs kV photon beams. There is currently no clinical TPS that handles kV beams accurately. So, using a clinical TPS may lead to large uncertainties in small animal dose calculations.

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