Biological effects of passive scattering and spot scanning proton beams at the distal end of the spread-out Bragg peak in single cells and multicell spheroids

Abstract

Purpose

The present study investigated the biological effects of spot scanning and passive scattering proton therapies at the distal end region of the spread-out Bragg peak (SOBP) using single cell and multicell spheroids.

Materials and methods

The Geant4 Monte Carlo simulation was used to calculate linear energy transfer (LET) values in passive scattering and spot scanning beams. The biological doses of the two beam options at various points of the distal end region of SOBP were investigated using EMT6 single cells and 0.6-mm V79 spheroids irradiated with 6 and 15 Gy, respectively, by inserting the fractions surviving these doses onto dose-survival curves and reading the corresponding dose.

Results

LET values in the entrance region of SOBP were similar between the two beam options and increased at the distal end region of SOBP, where the LET value of spot scanning beams was higher than that of passive scattering beams. Increases in biological effects at the distal end region were similarly observed in single cells and spheroids; biological doses at 2–10 mm behind the distal end were 4.5–57% and 5.7–86% higher than physical doses in passive scattering and spot scanning beams, respectively, with the biological doses of spot scanning beams being higher than those of passive scattering beams (p < .05).

Conclusions

In single cells and spheroids, the effects of proton irradiation were stronger than expected from measured physical doses at the distal end of SOBP and were correlated with LET increases.

Keywords:

• Spot scanning
• passive scattering
• spheroids
• distal end of SOBP
• proton therapy

Disclosure statement

The authors declare that they have no conflict of interest.

Additional information

Funding

This work was supported by the JSPS KAKENHI Grant Number 15H05675.

Notes on contributors

Kento Nomura

Kento Nomura, MD, Radiation oncologist, graduate student of Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.

Hiromitsu Iwata

Hiromitsu Iwata, MD, PhD, Radiation oncologist, Assistant manager of Nagoya Proton Therapy Center, Nagoya, Japan.

Toshiyuki Toshito

Toshiyuki Toshito, PhD, Medical physicist, Chief of Department of Proton Therapy Physics, Nagoya Proton Therapy Center, Nagoya, Japan.

Chihiro Omachi

Chihiro Omachi, PhD, Medical physicist, Postdoctoral Researcher of Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.

Junpei Nagayoshi

Junpei Nagayoshi, MS, radiologic technologist, Chief of Department of Radiation Therapy, Nagoya City West Medical Center, Nagoya, Japan.

Koichiro Nakajima

Koichiro Nakajima, MD, PhD, Radiation oncologist, Postdoctoral Researcher of Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.

Hiroyuki Ogino

Hiroyuki Ogino, MD, PhD, Radiation oncologist, Director of Nagoya Proton Therapy Center, Nagoya, Japan.

Yuta Shibamoto

Yuta Shibamoto, MD, PhD, Radiation oncologist, Professor and Chairman Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.