K Kasamatsu¹*, T Matsuura^2,3, K Yasuda⁴, K Miyazaki^2,3, S Takao^2,3, H Aoyama⁴, (1)Graduate School of Biomedical Science and Engineering, Hokkaido University, Sapporo, JP (2)Faculty of Engineering, Hokkaido University, Sapporo, JP (3)Department of Medical Physics, Hokkaido University Hospital, Sapporo, JP (4)Faculty of Medicine, Hokkaido University, Sapporo, JP

• 
  K Kasamatsu

PO-GePV-T-127 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

ePoster Forums

Purpose: To investigate the influence of variable relative biological effectiveness (RBE) on intensity modulated proton therapy (IMPT) for pharyngeal cancer and to determine the superior hyperfractionated schedule under the variability of RBE by using biologically effective dose (BED).

Methods: IMPT plans were created for three pharyngeal cancer patients. For clinical target volume (CTV) and soft tissue, ΔBED was defined as the difference of the BED between the conventional schedule using constant RBE and the altered fractionation schedule using variable RBE. With the dose and linear energy transfer distributions obtained from the patients, ΔBED was comprehensively calculated using several combinations of the schedule parameters (treatment days, number of fractions, and prescribed dose). RBE was calculated with McNamara model. The superior schedule was determined as the one which commonly gave the higher ΔBED for CTV among the three patients. The resultant schedule was compared using the BED volume histogram.

Results: In conventional schedule, compared with the constant RBE, the variable RBE reduced the BED₅₀ for CTV and soft tissue (acute reaction) by 2.7 Gy and increased the BED₅₀ for soft tissue (late reaction) by 7.4 Gy as the average of the three patients. With the comprehensive calculation, the superior schedule was determined as the schedule of 80.52 Gy (RBE = 1.1) in 66 fractions. Compared with the constant RBE and the conventional schedule, the determined schedule successfully increased the BED₅₀ for CTV by 7.6 Gy however with a 7.8 Gy increase for the soft tissue (acute reaction). The BED₅₀ for the soft tissue (late reaction) was reduced by 2.4 Gy.

Conclusion: The results indicated that the hyperfractionated IMPT can possibly outweigh the effect of variable RBE. Although the biological uncertainty remains, hyperfractionation is expected to be a potential strategy in the IMPT for pharyngeal cancers.

Funding Support, Disclosures, and Conflict of Interest: Koichi Miyazaki is paid by Hitachi, Ltd.

Keywords

Radiobiology, RBE, LET

Taxonomy

TH- External Beam- Particle/high LET therapy: Other particle therapy

Contact Email