Motion mitigation for lung cancer patients treated with active scanning proton therapy

Abstract

Purpose: Motion interplay can affect the tumor dose in scanned proton beam therapy. This study assesses the ability of rescanning and gating to mitigate interplay effects during lung treatments.

Methods: The treatments of five lung cancer patients [48 Gy(RBE)/4fx] with varying tumor size (21.1-82.3 cm(3)) and motion amplitude (2.9-30.6 mm) were simulated employing 4D Monte Carlo. The authors investigated two spot sizes (σ ∼ 12 and ∼ 3 mm), three rescanning techniques (layered, volumetric, breath-sampled volumetric) and respiratory gating with a 30% duty cycle.

Results: For 4/5 patients, layered rescanning 6/2 times (for the small/large spot size) maintains equivalent uniform dose within the target >98% for a single fraction. Breath sampling the timing of rescanning is ∼ 2 times more effective than the same number of continuous rescans. Volumetric rescanning is sensitive to synchronization effects, which was observed in 3/5 patients, though not for layered rescanning. For the large spot size, rescanning compared favorably with gating in terms of time requirements, i.e., 2x-rescanning is on average a factor ∼ 2.6 faster than gating for this scenario. For the small spot size however, 6x-rescanning takes on average 65% longer compared to gating. Rescanning has no effect on normal lung V20 and mean lung dose (MLD), though it reduces the maximum lung dose by on average 6.9 ± 2.4/16.7 ± 12.2 Gy(RBE) for the large and small spot sizes, respectively. Gating leads to a similar reduction in maximum dose and additionally reduces V20 and MLD. Breath-sampled rescanning is most successful in reducing the maximum dose to the normal lung.

Conclusions: Both rescanning (2-6 times, depending on the beam size) as well as gating was able to mitigate interplay effects in the target for 4/5 patients studied. Layered rescanning is superior to volumetric rescanning, as the latter suffers from synchronization effects in 3/5 patients studied. Gating minimizes the irradiated volume of normal lung more efficiently, while breath-sampled rescanning is superior in reducing maximum doses to organs at risk.

FIG. 1.

Transverse slice of patient 1 after 4D-simulation of a single fraction for different motion mitigation approaches: (A) no motion mitigation, (B) 2x-layered rescanning, (C) 6x-layered rescanning, (D) 2x breath-sampled rescanning, and gating (E). (F) shows the nominal, i.e., expected, distribution without any motion, simulated on the planning CT. The color bar is in percent of prescribed dose, the contours represent the GTV (orange/inner contour), and the IGTV (magenta/outer contour).

FIG. 2.

Results for the CTV of patient 3: EUD (left), D₉₉ (middle), and D_5–95 (right). Triangles, layered rescanning; squares, volumetric rescanning; circles, breath-sampled rescanning (BS-rescanning); and diamonds, gating. The values for EUD and D₉₉ are relative to the planned dose distribution (=stars). Error bars represent the standard deviation from the four starting phases studied for each rescanning setting. Layered and volumetric symbols have been slightly offset to ease differentiation, lines connect data points to guide the eye.

FIG. 3.

Absolute values of normal lung V₂₀, mean and maximum dose averaged over all patients. Triangles, layered rescanning; squares, volumetric rescanning; circles, breath-sampled rescanning (BS-rescanning); diamonds, gating; and stars, static plan.

FIG. 4.

Example of a synchronization effects in patient 2 as shown by the increase in the interplay effect for 4x volumetric rescanning. Triangles denote layered rescanning, squares volumetric rescanning, circles breath-sampled rescanning (BS-rescanning), and diamonds gating. Values are relative to the planned dose distribution. Error bars represent the standard deviation from the four starting phases studied for each rescanning setting. Layered and volumetric symbols have been slightly offset to ease differentiation.