Modality comparison for small animal radiotherapy: a simulation study

Abstract

Purpose: Small animal radiation therapy has advanced significantly in recent years. Whereas in the past dose was delivered using a single beam and a lead shield for sparing of healthy tissue, conformal doses can be now delivered using more complex dedicated small animal radiotherapy systems with image guidance. The goal of this paper is to investigate dose distributions for three small animal radiation treatment modalities.

Methods: This paper presents a comparison of dose distributions generated by the three approaches-a single-field irradiator with a 200 kV beam and no image guidance, a small animal image-guided conformal system based on a modified microCT scanner with a 120 kV beam developed at Stanford University, and a dedicated conformal system, SARRP, using a 220 kV beam developed at Johns Hopkins University. The authors present a comparison of treatment plans for the three modalities using two cases: a mouse with a subcutaneous tumor and a mouse with a spontaneous lung tumor. A 5 Gy target dose was calculated using the EGSnrc Monte Carlo codes.

Results: All treatment modalities generated similar dose distributions for the subcutaneous tumor case, with the highest mean dose to the ipsilateral lung and bones in the single-field plan (0.4 and 0.4 Gy) compared to the microCT (0.1 and 0.2 Gy) and SARRP (0.1 and 0.3 Gy) plans. The lung case demonstrated that due to the nine-beam arrangements in the conformal plans, the mean doses to the ipsilateral lung, spinal cord, and bones were significantly lower in the microCT plan (2.0, 0.4, and 1.9 Gy) and the SARRP plan (1.5, 0.5, and 1.8 Gy) than in single-field irradiator plan (4.5, 3.8, and 3.3 Gy). Similarly, the mean doses to the contralateral lung and the heart were lowest in the microCT plan (1.5 and 2.0 Gy), followed by the SARRP plan (1.7 and 2.2 Gy), and they were highest in the single-field plan (2.5 and 2.4 Gy). For both cases, dose uniformity was greatest in the single-field irradiator plan followed by the SARRP plan due to the sensitivity of the lower energy microCT beam to target heterogeneities and image noise.

Conclusions: The two treatment planning examples demonstrate that modern small animal radiotherapy techniques employing image guidance, variable collimation, and multiple beam angles deliver superior dose distributions to small animal tumors as compared to conventional treatments using a single-field irradiator. For deep-seated mouse tumors, however, higher-energy conformal radiotherapy could result in higher doses to critical organs compared to lower-energy conformal radiotherapy. Treatment planning optimization for small animal radiotherapy should therefore be developed to take full advantage of the novel conformal systems.

Figure 1

Previous preclinical single-field nonconformal radiotherapy approach using a single x-ray beam angle with a shield placed in direct contact with the animal (a) and conformal radiotherapy with a number of collimated isocentric beams (b).

Figure 2

Anatomy of the mouse with subcutaneous tumors (a) and with spontaneous lung tumor (b). Tumor (PTV), right and left lung, spinal cord and heart and control tumor [orange, tumor implanted but receiving no direct radiation, only in (a)] are shown. The PTV in the mouse with lung tumor can be clearly seen in Figs. 3c, 3d.

Figure 3

Beam setup for the subcutaneous tumor-bearing mouse using a single-field irradiator (a) and conformal irradiators (b) and for the lung mouse using a single-field irradiator (c) and conformal irradiators (d).

Figure 4

Depth dose curves for 1-min irradiation for the single-field irradiator 200 kV beam (a), for the microCT 120 kV beam (b), and for the SARRP 220 kV beam (c) for various beam sizes. Percentage depth dose curves for a 10 mm circular beam delivered on all systems are shown in (d). MC simulations are represented by squares, film measurements by triangles, and ionization chamber measurements by circles. For clarity, statistical uncertainties of MC simulations are not shown in (a)–(c) and are below 1%.

Figure 5

Beam profiles for single-field irradiator 200 kV beam (a), for the microCT 120 kV beam (b), and for the SARRP 220 kV beam (c) at various depths d. MC simulations are represented by squares and film measurements by solid lines.

Figure 6

Energy spectra of the single-field (200 kV), microCT (120 kV), and SARRP (220 kV) treatment beams.

Figure 7

Monte Carlo dose distributions for subcutaneous tumor plans delivered on a single-field irradiator with 0.5 mm (a) and 2.0 mm target margin (b), the microCT system (c), and the SARRP system (d). Target is delineated.

Figure 8

Monte Carlo dose distributions for lung tumor plans delivered on a single-field irradiator with 0.5 mm (a) and 2.0 mm target margin (b), the microCT system (c), and the SARRP system (d). Target is delineated.

Figure 9

Dose volume histograms for the subcutaneous (a) and lung (b) case irradiated on a single-field irradiator (dashed lines) with the 0.5 mm (diamonds) and 2.0 mm (squares) target margin and on conformal systems (solid line) with the microCT (triangles) and SARRP (circles) systems. Right lung in (b) is the right lung minus PTV. PTV dose is planned to D₅₀ = 5 Gy.