Dosimetric feasibility of real-time MRI-guided proton therapy

Abstract

Purpose: Magnetic resonance imaging (MRI) is a prime candidate for image-guided radiotherapy. This study was designed to assess the feasibility of real-time MRI-guided proton therapy by quantifying the dosimetric effects induced by the magnetic field in patients' plans and identifying the associated clinical consequences.

Methods: Monte Carlo dose calculation was performed for nine patients of various treatment sites (lung, liver, prostate, brain, skull-base, and spine) and tissue homogeneities, in the presence of 0.5 and 1.5 T magnetic fields. Dose volume histogram (DVH) parameters such as D95, D5, and V20 as well as equivalent uniform dose were compared for the target and organs at risk, before and after applying the magnetic field. The authors further assessed whether the plans affected by clinically relevant dose distortions could be corrected independent of the planning system.

Results: By comparing the resulting dose distributions and analyzing the respective DVHs, it was determined that despite the observed lateral beam deflection, for magnetic fields of up to 0.5 T, neither was the target coverage jeopardized nor was the dose to the nearby organs increased in all cases except for prostate. However, for a 1.5 T magnetic field, the dose distortions were more pronounced and of clinical concern in all cases except for spine. In such circumstances, the target was severely underdosed, as indicated by a decrease in D95 of up to 41% of the prescribed dose compared to the nominal situation (no magnetic field). Sites such as liver and spine were less affected due to higher tissue homogeneity, typically smaller beam range, and the choice of beam directions. Simulations revealed that small modifications to certain plan parameters such as beam isocenter (up to 19 mm) and gantry angle (up to 10°) are sufficient to compensate for the magnetic field-induced dose disturbances. The authors' observations indicate that the degree of required corrections strongly depends on the beam range and direction relative to the magnetic field. This method was also applicable to more heterogeneous scenarios such as skull-base tumors.

Conclusions: This study confirmed the dosimetric feasibility of real-time MRI-guided proton therapy and delivering a clinically acceptable dose to patients with various tumor locations within magnetic fields of up to 1.5 T. This work could serve as a guide and encouragement for further efforts toward clinical implementation of hybrid MRI-proton gantry systems.

FIG. 1.

The skull-base case (patient 8) is a representative patient with CTV located within highly heterogeneous tissue.

FIG. 2.

(a) Dose distribution and (b) pristine peak profile in the presence of 0.5 T (solid line) and 1.5 T (dashed line) magnetic fields for (left) 90 MeV and (right) 200 MeV proton pencil beams.

FIG. 3.

Dose distribution for all patients, for nominal (B = 0), 0.5 T and 1.5 T magnetic field settings. Dashed and solid arrows indicate the regions of target underdose and nearby tissue overdose, respectively.

FIG. 4.

Dose difference maps for the spine case (patient 9), show no differences inside CTV and CTV + 0.5 cm structures for (a) 0.5 T and (b) 1.5 T magnetic fields compared to the nominal case (0 T).

FIG. 5.

Dose distributions for (a) patient 2 and (b) patient 5 at 1.5 T field (top left). The directions of beam isocenter shifts are shown by arrows. The dose distributions after beam repositioning are compared (top right). The dose differences (nominal vs 1.5 T) without (bottom left) and with corrections (bottom right) show improved target conformity after implementing the plan corrections.

FIG. 6.

Dose volume histograms showing the tumors and the representative OARs for nominal, 0.5 T, 0.5 T (corrected) (for prostate only), 1.5 T, and 1.5 T (corrected) for all patients.

FIG. 7.

Difference between nominal EUD (in percent of the prescribed dose) and ones with magnetic fields, illustrating the impact of the magnetic fields on the organ doses and the effectiveness of the plan corrections. Negative values indicate decrease and positive values indicate increase of EUD compared to nominal.

FIG. 8.

Comparison of percent differences of target ΔD₉₅ and D_mean for 0.5 T, 1.5 T, and 1.5 T (corrected) with respect to nominal (0 T), for passive scattering, SFUD, and IMPT plans.