A Monte Carlo model for organ dose reconstruction of patients in pencil beam scanning (PBS) proton therapy for epidemiologic studies of late effects

Abstract

Significant efforts such as the Pediatric Proton/Photon Consortium Registry (PPCR) involving multiple proton therapy centers have been made to conduct collaborative studies evaluating outcomes following proton therapy. As a groundwork dosimetry effort for the late effect investigation, we developed a Monte Carlo (MC) model of proton pencil beam scanning (PBS) to estimate organ/tissue doses of pediatric patients at the Maryland Proton Treatment Center (MPTC), one of the proton centers involved in the PPCR. The MC beam modeling was performed using the TOPAS (TOol for PArticle Simulation) MC code and commissioned to match measurement data within 1% for range, and 0.3 mm for spot sizes. The established MC model was then tested by calculating organ/tissue doses for sample intracranial and craniospinal irradiations on whole-body pediatric computational human phantoms. The simulated dose distributions were compared with the treatment planning system dose distributions, showing the 3 mm/3% gamma index passing rates of 94%-99%, validating our simulations with the MC model. The calculated organ/tissue doses per prescribed doses for the craniospinal irradiations (1 mGy Gy^-1 to 1 Gy Gy^-1) were generally much higher than those for the intracranial irradiations (2.1 μGy Gy^-1 to 0.1 Gy Gy^-1), which is due to the larger field coverage of the craniospinal irradiations. The largest difference was observed at the adrenal dose, i.e. ∼3000 times. In addition, the calculated organ/tissue doses were compared with those calculated with a simplified MC model, showing that the beam properties (i.e. spot size, spot divergence, mean energy, and energy spread) do not significantly influence dose calculations despite the limited irradiation cases. This implies that the use of the MC model commissioned to the MPTC measurement data might be dosimetrically acceptable for patient dose reconstructions at other proton centers particularly when their measurement data are unavailable. The developed MC model will be used to reconstruct organ/tissue doses for MPTC pediatric patients collected in the PPCR.

Figure 1.

Workflow of the developed in-house MC modeling program to automatically match beam properties to beam measurement data.

Figure 2.

Dose distributions for proton irradiation cases planned by the MPTC TPS: (a) Patient A (intracranial irradiation on 1-year-old phantom with a prescribed dose of 50 Gy); (b) Patient B (intracranial irradiation on 5-year-old phantom with a prescribed dose of 50 Gy); (c) Patient C (craniospinal irradiation on 1-year-old phantom with a prescribed dose of 23 Gy); and (d) Patient D (craniospinal irradiation on 5-year-old phantom with a prescribed dose of 23 Gy).

Figure 3.

The differences of the spot sizes simulated with the MC model from the measurement data at the five locations (0, ± 10, and ± 20 cm) in the air without a range shifter.

Figure 4.

Integral depth doses (IDDs) in water calculated using the MC model established in the present study (circle), along with those of the MPTC measurement data (line).

Figure 5.

Gamma distributions between the simulated dose distributions and the TPS dose distributions: (a) Patient A (intracranial irradiation on 1-year-old phantom); (b) Patient B (intracranial irradiation on 5-year-old phantom); (c) Patient C (craniospinal irradiation on 1-year-old phantom); and (d) Patient D (craniospinal irradiation on 5-year-old phantom). Gamma analysis using a 3%/3mm criterion was performed excluding the doses less than 0.2% of the maximum dose. Gamma values for the excluded doses were assumed to be zero in the distributions.

Figure 6.

Organ/tissue averaged, RBE-weighted absorbed dose per prescribed dose (Gy Gy⁻¹) for 25 organs and tissues calculated by using the MC beam model established in the present study: (a) Patient A and Patient B undergoing intracranial irradiations and (b) Patient C and Patient D undergoing craniospinal irradiations.

Figure 7.

Contribution of neutron dose to organ/tissue doses calculated by using the MC beam model established in the present study: (a) Patient A and Patient B undergoing intracranial irradiations and (b) Patient C and Patient D undergoing craniospinal irradiations.

Figure 8.

Ratio of the organ/tissue doses of Patent A (left) and Patient B (right) to those of Ardenfors et al (2018) calculated with a 6-year-old patient for intracranial irradiations in lateral and vertex fields.

Figure 9.

Differences of the organ/tissue doses calculated using the developed MC beam model (D) from those calculated using the simplified MC beam model (S) [= (S/D - 1) × 100)]: (a) Patient A and Patient B undergoing intracranial irradiations and (b) Patient C and Patient D undergoing craniospinal irradiations. In the simplified MC model, the mean energy was set to the nominal energy and the other beam properties (i.e., spot size, spot divergence, and energy spread) were set to zero.