A fast analytic dose calculation method for arc treatments for kilovoltage small animal irradiators

Integrating X-ray kV millimetric field dosimetry with a synthetic diamond detector into the treatment planning system commissioning of a preclinical irradiator

PURPOSE

Small animal irradiators are equipped with x-ray beams and cone collimators with millimeter dimensions to be used in preclinical research. The use of small fields in the kV energy range may require the application of energy-dependent, field size-dependent, or depth-dependent correction factors to the dosimetric data acquired for treatment planning system (TPS) commissioning purposes to obtain accurate dose values. Considering that these corrections are also detector dependent, the suitability of a synthetic single-crystal diamond detector for small-field relative dosimetry in a preclinical irradiator (220-kVp) was evaluated to avoid the necessity of applying correction factors during TPS commissioning.

METHODS

The detector response was assessed during the transition for field sizes ranging from 20 × 20 mm² to 3 × 3 mm² , using the small animal radiation research platform (SARRP). The percentage depth dose distributions (PDDs), lateral profiles and output factors (OFs) were measured. The PDDs for the synthetic diamond detector were compared to the distributions acquired using a small-volume microchamber (0.016 cm³ ) and with Monte Carlo calculations using the MC3D in-house software package. The profiles and OFs were compared to the data from a silicon solid-state detector and to radiochromic film data provided by the manufacturer; for the OF determination, measurements made using a microchamber were added for comparison. The performance of several detectors used as references was previously validated for relative dosimetry in preclinical irradiators. A commercial TPS was commissioned for the factor-based algorithm, using the data acquired with the diamond detector, and no additional correction factors were applied. To verify the performance of the TPS and the accuracy of the dosimetric methodology, radiochromic film irradiation in water was conducted, and two-dimensional (2D) dose distributions in the coronal and axial planes were compared under different gamma criteria.

RESULTS

Compared with the microchamber and MC3D distributions, the agreement of the PDDs using the synthetic diamond detector was better than 2%. The profile data exhibited very good agreement compared with the data from the silicon detector, with an average and a maximum difference of 0.31 and 0.39 mm in the penumbras, respectively. Compared with the data from the radiochromic film, the average and maximum differences were equal to 0.77 and 0.89 mm, respectively. Very good agreement, within 1%, was obtained between the OFs measured with the synthetic diamond detector and the radiochromic film, compared only for the cone collimators. The validation of the TPS commissioning using gamma criteria compared to film showed an average passing rate of 100% and 93.2% with a global gamma criterion of 1 mm/3% for the coronal and axial planes, respectively, including the 3 × 3 mm² field size and penumbra regions.

CONCLUSIONS

Synthetic diamond is a suitable detector for the complete relative dosimetry of small x-ray fields. The commissioning of the TPS with its own beam dosimetric data exhibited encouraging results even in a 3 × 3 mm² field and penumbra region. This methodology allows for the prediction of 2D dose distributions with an accuracy in water ranging from 3 to 5% compared to the 2D distribution from film dosimetry.

A kernel-based dose calculation algorithm for kV photon beams with explicit handling of energy and material dependencies

Objective:

Mimicking state-of-the-art patient radiotherapy with high-precision irradiators for small animals is expected to advance the understanding of dose–effect relationships and radiobiology in general. We work on the implementation of intensity-modulated radiotherapy-like irradiation schemes for small animals. As a first step, we present a fast analytical dose calculation algorithm for keV photon beams.

Methods:

We follow a superposition–convolution approach adapted to kV X-rays, based on previous work for microbeam therapy. We assume local energy deposition at the photon interaction point due to the short electron ranges in tissue. This allows us to separate the dose calculation into locally absorbed primary dose and the scatter contribution, calculated in a point kernel approach. We validate our dose model against Geant4 Monte Carlo (MC) simulations and compare the results to Muriplan (XStrahl Ltd, Camberley, UK).

Results:

For field sizes of (1 mm)² to (1 cm)² in water, the depth dose curves show a mean disagreement of 1.7% to MC simulations, with the largest deviations in the entrance region (4%) and at large depths (5% at 7 cm). Larger discrepancies are observed at water-to-bone boundaries, in bone and at the beam edges in slab phantoms and a mouse brain. Calculation times are in the order of 5 s for a single beam.

Conclusion:

The algorithm shows good agreement with MC simulations in an initial validation. It has the potential to become an alternative to full MC dose calculation.

Advances in knowledge:

The presented algorithm demonstrates the potential of kernel-based dose calculation for kV photon beams. It will be valuable in intensity-modulated radiotherapy and inverse treatment planning for high precision small-animal radiotherapy.

Comprehensive quality assurance phantom for the small animal radiation research platform (SARRP)

PURPOSE

To develop and test the suitability and performance of a comprehensive quality assurance (QA) phantom for the Small Animal Radiation Research Platform (SARRP).

METHODS AND MATERIALS

A QA phantom was developed for carrying out daily, monthly and annual QA tasks including: imaging, dosimetry and treatment planning system (TPS) performance evaluation of the SARRP. The QA phantom consists of 15 (60 × 60 × 5 mm(3)) kV-energy tissue equivalent solid water slabs. The phantom can incorporate optically stimulated luminescence dosimeters (OSLD), Mosfet or film. One slab, with inserts and another slab with hole patterns are particularly designed for image QA.

RESULTS

Output constancy measurement results showed daily variations within 3%. Using the Mosfet in phantom as target, results showed that the difference between TPS calculations and measurements was within 5%. Annual QA results for the Percentage depth dose (PDD) curves, lateral beam profiles, beam flatness and beam profile symmetry were found consistent with results obtained at commissioning. PDD curves obtained using film and OSLDs showed good agreement. Image QA was performed monthly, with image-quality parameters assessed in terms of CBCT image geometric accuracy, CT number accuracy, image spatial resolution, noise and image uniformity.

CONCLUSIONS

The results show that the developed QA phantom can be employed as a tool for comprehensive performance evaluation of the SARRP. The study provides a useful reference for development of a comprehensive quality assurance program for the SARRP and other similar small animal irradiators, with proposed tolerances and frequency of required tests.