Physics Contribution
Four-Dimensional Lung Treatment Planning in Layer-Stacking Carbon Ion Beam Treatment: Comparison of Layer-Stacking and Conventional Ungated/Gated Irradiation

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Purpose

We compared four-dimensional (4D) layer-stacking and conventional carbon ion beam distribution in the treatment of lung cancer between ungated and gated respiratory strategies using 4DCT data sets.

Methods and Materials

Twenty lung patients underwent 4DCT imaging under free-breathing conditions. Using planning target volumes (PTVs) at respective respiratory phases, two types of compensating bolus were designed, a full single respiratory cycle for the ungated strategy and an approximately 30% duty cycle for the exhalation-gated strategy. Beams were delivered to the PTVs for the ungated and gated strategies, PTV(ungated) and PTV(gated), respectively, which were calculated by combining the respective PTV(Tn)s by layer-stacking and conventional irradiation. Carbon ion beam dose distribution was calculated as a function of respiratory phase by applying a compensating bolus to 4DCT. Accumulated dose distributions were calculated by applying deformable registration.

Results

With the ungated strategy, accumulated dose distributions were satisfactorily provided to the PTV, with D95 values for layer-stacking and conventional irradiation of 94.0% and 96.2%, respectively. V20 for the lung and Dmax for the spinal cord were lower with layer-stacking than with conventional irradiation, whereas Dmax for the skin (14.1 GyE) was significantly lower (21.9 GyE). In addition, dose conformation to the GTV/PTV with layer-stacking irradiation was better with the gated than with the ungated strategy.

Conclusions

Gated layer-stacking irradiation allows the delivery of a carbon ion beam to a moving target without significant degradation of dose conformity or the development of hot spots.

Introduction

Worldwide, more than 28 particle treatment centers were operating in 2008, including three carbon ion beam centers, and the construction of new centers is set to continue. Compared with photon beams, charged particle beams provide superior dose conformation and the minimization of excessive dosing to normal tissues. These strengths result from the characteristic increase in energy deposition of particle beams with penetration depth (proton and carbon ion beams) up to a sharp maximum at the end of the range (Bragg peak) (1). The National Institute of Radiological Sciences (NIRS) has treated more than 3,000 patients using carbon ion passive beams since 1994, and several clinical studies have been reported 2, 3, 4, 5.

Although passive beam irradiation benefits from relatively simple treatment planning requirements, its disadvantage is the significant excessive dose delivered to normal tissues along the entrance of the target. Although beam scanning irradiation using proton beam and carbon ion beams was first introduced by Kanai et al. (6) and Goitein et al. (7), respectively, as long as 25 years ago, no treatment centers are yet using scanning irradiation to provide treatment to a moving target, and investigation continues at the research level 8, 9. Kanai et al. proposed layer-stacking irradiation, which uses passive proton beams to achieve higher dose conformation and thereby provide similar dose conformity to scanning irradiation (10). They were followed by Kanematsu et al., who extended this idea to carbon ion beams and integrated it into the Heavy Ion Medical Accelerator in Chiba (HMAC) at the NIRS (11), and the NIRS has now been upgraded to provide layer-stacking irradiation. This advance represents a relatively easy means of upgrading treatment centers currently using passive beams.

Several centers have reported treatment approaches to a moving target using passive beam irradiation 12, 13, 14, 15. Because scanning irradiation irradiates respective beam spots as a function of time within the same layer and then changes to the next layer, repeating this process until the prescribed dose is delivered to the target, it should be naturally less robust against motion than passive irradiation 8, 9. In contrast, layer-stacking irradiation delivers a small spread-out of Bragg peak (SOBP) in a single layer at a time, and should therefore be more robust against motion than scanning irradiation, but more sensitive than conventional irradiation. These findings highlight the importance of evaluating respiratory-induced dose variation, including deformable registration, in layer-stacking irradiation.

Here, we evaluated 4D layer-stacking carbon ion beam distribution in lung cancer and compared findings between respiratory-ungated and respiratory-gated strategies.

Section snippets

Layer-stacking irradiation method

The conventional particle beam irradiation method presently used at the NIRS is a passive irradiation technique, in which the treatment beam is laterally broadened by a pair of wobbler magnets and a scatterer, and the sharp Bragg peaks are broadened along the beam direction by a ridge filter (16). The SOBP length is fixed within the beam field (Fig. 1a).

To achieve higher dose conformation with passive beams similar to that with scanning irradiation, layer-stacking irradiation basically uses a

Results

Carbon ion dose distribution of a beam with 330-degree beam angle for respiratory-ungated layer-stacking irradiation at respective phases is shown in Fig. 3 (Patient 9). Because the total prescribed dose was not given in a single phase, Figs. 3a–3d are derived from different respiratory cycles and therefore not in respiratory phase order. Because beam weight among all layers is highest in the first layer, the magnitude of overdosage due to replacement of the solid tumor density by the lower

Discussion

We evaluated 4D layer-stacking irradiation carbon ion beam distribution using lung 4DCT data sets and compared results with those for conventional irradiation. The benefit of layer-stacking irradiation is its minimization of excessive doses to normal tissues, particularly the skin, the dose of which was decreased by an average 7 GyE from that with conventional irradiation in both treatment strategies. Although the PTV D95 value in layer-stacking irradiation (94.0% and 95.2% for the ungated and

Conclusion

We quantified the 4D dose distribution of lung treatment, including deformable registration, in carbon-layer–stacking irradiation, and compared results with those for conventional irradiation. Respiratory-gated layer-stacking irradiation allowed the minimization of excessive doses to normal tissues and of hot spots without significant degradation of dose conformation to the moving target, making it clearly the safer of the two methods.

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