New strategies in radiation therapy: exploiting the full potential of protons

Abstract

Protons provide significant dosimetric advantages compared with photons because of their unique depth-dose distribution characteristics. However, they are more sensitive to the effects of intra- and intertreatment fraction anatomic variations and uncertainties in treatment setup. Furthermore, in the current practice of proton therapy, the biologic effectiveness of protons relative to photons is assumed to have a generic fixed value of 1.1. However, this is a simplification, and it is likely higher in different portions of the proton beam. Current clinical practice and trials have not fully exploited the unique physical and biologic properties of protons. Intensity-modulated proton therapy, with its ability to manipulate energies (in addition to intensities), provides an entirely new dimension, which, with ongoing research, has considerable potential to increase the therapeutic ratio.

Figure 1

The green curve shows the depth dose data of a typical 15 MV photon beam. The red curve shows the depth dose curve of a monoenergetic proton beam. The maximum dose point of this curve is termed the Bragg peak. Scanning thin monoenergetic proton beams are used for intensity-modulated proton therapy. The cyan curve is obtained by electromechanically spreading the monoenergetic proton beam laterally and longitudinally and is used in passively scattered proton therapy. The top flat portion of this curve is called the spread-out Bragg peak. The potential advantages of proton vs. photon dose distributions are clear.

Figure 2

Comparison of IMRT and IMPT dose distributions in a thorax treatment plan. The large “low dose bath” in IMRT (left panels) is considerably reduced in IMPT dose distributions (right panels). (Courtesy Joe Y. Chang, PTCOG 47 presentation)

Figure 3

Optimization of IMPT taking advantage of the physical and biologic characteristics of protons. The panel on the left shows illustrative Bragg curves for variable RBE (orange) and fixed RBE of 1.1 (blue). It shows that, over and above the dose advantage, the use of variable RBE may lead to an additional 40% differential (more in some cases, less in others) between normal tissue and tumor dose. A goal of current research is to selectively place the high RBE portions of the Bragg curve (represented by small translucent red and blue circles, or “spots”, in the right panel) within the gross target volume (GTV). If there is a normal critical structure present adjacent to the distal edge of the target volume along the beam direction, the intensities of spots (blue translucent circles) at the distal edge are set to zero. The underdosing in the target volume thus created at the distal edge is compensated by beamlets of beams from other directions.