Purpose: Conventional proton therapy uses a beam spreading technique called passive scattering in which a small beamlet of particles is spread laterally to create field sizes needed to cover targets in lateral direction and subsequently an SOBP proton energy spectrum is formed using wheel modulators to deliver spread out dose distributions in depth. This beam delivery method however provides desirable dose distributions only in situations where no considerable tissue inhomogeneity is present. Otherwise, the SOBP dose distribution suffers significant modification known as the distal edge degradation effect, thus limiting the clinical use of passive scattering technique. As a result, such treatment site as lung where significant tissue heterogeneity is present cannot benefit from the dosimetric advantages of protons. The main aim of this work is to show that using spot scanning technique with beamlet size determined by the dimension of the tissue inhomogeneity yields conformal dose distribution with low target dose heterogeneity. Methods and Materials: Using Monte Carlo dose calculation with track repeating technique, in‐phantom dose distributions are calculated for different beamlet and density parameters. Conditions for improved dose distribution are presented. Results: It is shown that when the beamlet size is larger than the dimension of tissue inhomogeneity, significant distal dose degradation effect is observed. However, when the beamlet size becomes comparable to that of tissue inhomogeneity, improved target dose heterogeneity is seen. In the limiting case when the beamlet size is much smaller than the characteristic dimension of inhomogeneity, the dose distribution converges to that of homogeneous density phantom irradiated by laterally spread beam. Conclusions: Since the dose degradation effect is due to proton scattering at different tissue interfaces within the same field, reducing the field size of the beam results in significantly improved target dose coverage and limits the deleterious effects of distal edge degradation.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging