Proton Beams to Replace Photon Beams in Radical Dose Treatments

Abstract

With proton beam radiation therapy a smaller volume of normal tissues is irradiated at high dose levels for most anatomic sites than is feasible with any photon technique. This is due to the Laws of Physics, which determine the absorption of energy from photons and protons. In other words, the dose from a photon beam decreases exponentially with depth in the irradiated material. In contrast, protons have a finite range and that range is energy dependent. Accordingly, by appropriate distribution of proton energies, the dose can be uniform across the target and essentially zero deep to the target and the atomic composition of the irradiated material. The dose proximal to the target is lower compared with that in photon techniques, for all except superficial targets This resultant closer approximation of the planning treatment volume (PTV) to the CTV/GTV (grossly evident tumor volume/subclinical tumor extensions) constitutes a clinical gain by definition; i.e. a smaller treatment volume that covers the target three dimensionally for the entirety of each treatment session provides a clinical advantage. Several illustrative clinical dose distributions are presented and the clinical outcome results are reviewed briefly. An important technical advance will be the use of intensity modulated proton radiation therapy, which achieves contouring of the proximal edge of the SOBP (spread out Bragg peak) as well as the distal edge. This technique uses pencil beam scanning. To permit further progressive reductions of the PTV, 4-D treatment planning and delivery is required. The fourth dimension is time, as the position and contours of the tumor and the adjacent critical normal tissues are not constant. A potentially valuable new method for assessing the clinical merits of each of a large number of treatment plans is the evaluation of multidimensional plots of the complication probabilities for each of 'n' critical normal tissues/ structures for a specified tumor control probability. The cost of proton therapy compared with that of very high technology photon therapy is estimated and evaluated. The differential is estimated to be approximately 1.5 provided there were to be no charge for the original facility and that there were sufficient patients for operating on an extended schedule (6-7 days of 14-16 h) with > or = two gantries and one fixed horizontal beam.

Can Photon IMRT be improved by combination with mixed electron and photon techniques?

“…It has also been shown that the dose distributions in target volumes in the head and neck region can be improved by photon IMRT compared with conventional treatment (4 Á/6). Compared with photon IMRT, protons have an even greater capacity to deliver highly conformal dose distributions but with minimal integral doses (29,30). Treatment planning studies have also shown that protons offer an advantage compared with IMRT in some head and neck tumours (31).…”

Protons. A step forward or perhaps only more expensive radiation therapy?

“…I must admit that there are good physical arguments in favour of protons [19,20]. On the other hand, much better results than current results may emerge from IMRT and not the least from better diagnostic procedures disclosing the biological properties of a tumour and selection of areas where appropriate radiation can selectively be deposited as mentioned above.…”

Antiproton radiotherapy: peripheral dose from secondary neutrons

“…For many types of tumors, this has led to unacceptably low tumor control probability (TCP) and to high levels of morbidity. An alternative approach involves the use of protons and other heavier ions [1][2][3][4]. As R. R. Wilson pointed out in 1946 [5], for these charged particles both the amount and rate of energy deposition increase dramatically as the particle nears the end of its range.…”