Dose calculations for preclinical radiobiology experiments conducted with single‐field cabinet irradiators

Abstract

Purpose To provide percentage depth dose (PDD) data along the central axis for dosimetry calculations in small‐animal radiation biology experiments performed in cabinet irradiators. The PDDs are provided as a function of source‐to‐surface distance (SSD), field size, and animal size. Methods The X‐ray tube designs for four biological cabinet irradiators, the RS2000, RT250, MultiRad350, and XRAD320, were simulated using the BEAMnrc Monte Carlo code to generate 160, 200, 250, and 320 kVp photon beams, respectively. The 320 kVp beam was simulated with two filtrations: a soft F1 aluminium filter and a hard F2 thoraeus filter made of aluminium, tin, and copper. Beams were collimated into circular fields with diameters of 0.5–10 cm at SSDs of 10–60 cm. Monte Carlo dose calculations in 1–5‐cm diameter homogeneous (soft tissue) small‐animal phantoms as well as in heterogeneous phantoms with 3‐mm diameter cylindrical lung and bone inserts (rib and cortical bone) were performed using DOSXYZnrc. The calculated depth doses in three test‐cases were estimated by applying SSD, field size, and animal size correction factors to a reference case (40‐cm SSD, 1‐cm field, and 5‐cm animal size), and these results were compared with the specifically simulated (i.e., expected) doses to assess the accuracy of this method. Dosimetry for two test‐case scenarios of 160 and 250 kVp beams (representative of end‐user beam qualities) was also performed, whereby the simulated PDDs at two different depths were compared with the results based on the interpolation from reference data. Results The depth doses for three test‐cases calculated at 200, 320 kVp F1, and 320 kVp F2 with half value layers (HVLs) ranging from ∼0.6 to 3.6 mm Cu, agreed well with the expected doses, yielding dose differences of 1.2%, 0.1%, and 1.0%, respectively. The two end‐user test‐cases for 160 and 250 kVp beams with respective HVLs of ∼0.8 and 1.8 mm Cu yielded dose differences of 1.4% and 3.2% between the simulated and the interpolated PDDs. The dose increase at the bone‐tissue proximal interface ranged from 1.2 to 2.5 times the dose in soft tissue for rib and 1.3 to 3.7 times for cortical bone. The dose drop‐off at 1‐cm depth beyond the bone ranged from 1.3% to 6.0% for rib and 3.2% to 11.7% for cortical bone. No drastic dose perturbations occurred in the presence of lung, with lung‐tissue interface dose of >99% of soft tissue dose and <3% dose increase at 1‐cm depth beyond lung. Conclusions The developed dose estimation method can be used to translate the measured dose at a point to dose at any depth in small‐animal phantoms, making it feasible for preclinical calculation of dose distributions in animals irradiated with cabinet‐style irradiators. The dosimetric impact of bone must be accurately quantified as dramatic dose perturbations at and beyond the bone interfaces can occur due to the relative importance of the photoelectric effect at kilovoltage energies. These results will help improve dosimetric accuracy in preclinical experiments.

An evaluation of solid state detectors for the relative dosimetry of kilovoltage X‐ray beams

“…Kilovoltage (kV) X-ray beams in the energy range from 30 to 300 kVp are an important and widely used modality in radiotherapy and radiobiological research. [1][2][3][4][5] Due to its property of depositing maximum dose to the surface, 6,7 it has commonly been utilized to treat skin cancers and tumors close to the surface. 8,9 For any radiotherapy treatment, it is crucial to have accurate relative dosimetric measurements such as percentage depth doses (PDDs), lateral profiles, relative output factors (ROFs) etc., which are required for the treatment plan, to understand the dose variations from the reference conditions with field sizes, depths, lateral distances etc.…”

“…Kilovoltage (kV) X‐ray beams in the energy range from 30 to 300 kVp are an important and widely used modality in radiotherapy and radiobiological research 1–5 . Due to its property of depositing maximum dose to the surface, 6,7 it has commonly been utilized to treat skin cancers and tumors close to the surface 8,9 .…”

A review of small animal dosimetry techniques: image-guided and spatially fractionated therapy

“…demonstrated that segmenting at least 39 tissues is required to calculate dose with a 2% accuracy for a 250 kVp beam [116]. Muhavava et al presented a MC study that can be used to convert measured dose at a point to dose at a different depth in single-field irradiators [167]. In addition, it is crucial that appropriate interaction cross-sections and particle energy or distance cutoffs are used and therefore an appropriate application of MC dose calculations requires a good knowledge of the MC code as well as the irradiation system.…”