Proton terapiyasi nima?

4.2 Treatment Planning and Plan Evaluation for Scanning Beams and IMPT

Compared to the scattering and range modulation of proton beams for PSPT, the use of magnetic scanning of beamlets of protons of a sequence of incident energies offers considerable additional flexibility to achieve optimal dose distributions. In particular, it allows the delivery of intensity-modulated proton therapy (IMPT) in which scanning beamlets of protons are used to “paint” radiation dose on the target.[19–23] The flexibility of IMPT can be exploited to safely bend beams around complex critical structures, allowing improved sparing of these structures without compromising target coverage. The energy of the beamlets is varied to paint the target layer-by-layer. The intensities of beamlets comprising multiple scanning beams, aimed at the tumor from different directions, are determined using computer-aided mathematical methods to optimally balance the tumor dose versus the limits of normal tissue exposures. Because of its ability to control proton energies and intensities, the process produces dose distributions that are, in general, vastly superior not only to the corresponding photon-based techniques (i.e., intensity-modulated [photon] radiotherapy or IMRT) but also to PSPT (Figures 10 and ​and11).11). While considerable improvement in IMPT technology is possible with ongoing research, thousands of patients have already been treated with it, mainly at Paul Scherrer Institute in Switzerland and MD Anderson Cancer Center in Houston, including prostate, head and neck, CNS, spine, retroperitoneal, pleural and lung. It is expected that IMPT will become the dominant mode of treatments in the near future.


Figure 11
(a) IMRT vs. IMPT dose distributions. Large dose bath outside the target for IMRT is apparent. (b) DVHs for IMRT (squares) and IMPT (triangles) plans are shown.[18]
The relative dosimetric advantages of lower entrance dose and no dose beyond the range of protons over photons in PSPT are also true of IMPT over IMRT. In numerous treatment planning studies, it has been shown that protons deliver two to three times less energy (i.e., integral dose) outside the target volume than high-energy x-rays.[24] This dose advantage can be used either to increase the tumor dose and, hence, the probability of tumor control, or to reduce morbidity, or an intermediate combination of the two.
In IMPT, we often aim to achieve homogeneous dose to the target but not necessarily deliver it using SOBPs from each beam direction. The achievement of homogeneous target dose distribution with minimum and optimally balanced normal tissue doses would generally lead to inhomogeneous per-field target dose distributions as illustrated in Figure 12.


Figure 12
Inhomogeneous individual field IMPT target dose distributions (F1, F2, F3, F4) and a homogenous combined dose distribution for a head and neck case. (Adapted from a figure provided by A. Lomax, PSI, private communication.)
Such highly complex per-beam dose distributions, when combined, fit somewhat like a jigsaw puzzle to create the desired pattern of homogeneous dose distribution in the target and sparing of normal tissues as illustrated in Figure 12. However, in the face of uncertainties (e.g., in range), the fit may be lost, creating hot and/or cold regions. Thus, in general, IMPT dose distributions are more sensitive to (i.e., less robust in the face of) uncertainties in positioning and motion than PSPT dose distributions. For the latter, the dose distribution due to each incident beam is designed independently of other beams to cover the target adequately. It is relatively uniform (in water) and is terminated beyond the distal edge of the target plus a margin. Therefore, the composite dose distribution due to all beams is relatively less sensitive to perturbations. It should also be noted that smearing implicitly improves the robustness of PSPT dose distributions.
To reduce such sensitivity of IMPT to uncertainties, “robust optimization” techniques are being actively investigated (see Section 6.5).