Irregular breathing in proton therapy

Abstract

Pencil beam scanning is becoming a more common treatment modality. However, its ability to deal with moving targets is known to be limited, as beam motion and target motion can reinforce each other, deteriorating the planned dose distribution in what is called the interplay effect. Literature concerning breathing motion usually investigates regular patterns. This work aims to investigate the magnitude of the interplay effect when considering irregular breathing signals. In silico calculations of dose distributions were made in the treatment planning system RayStation (version 7.99), using an XCAT phantom with 50 CT phases to model moving patient anatomy. An interplay calculator was included in RayStation, allowing calculation of disturbed doses based on a treatment plan and an irradiation time model for a proton therapy accelerator. The target investigated was a spherical liver tumour with 5cm diameter, irradiated with two beams delivering a prescription dose of 63 Gy. Plans without and with 5x layered repainting were created. Clinically realistic regular breathing patterns were generated to establish a baseline, after which irregularities were introduced. The basic form for all patterns was a sin^4 signal, with regular signal amplitudes ranging from 6 to 18 mm, period ranging from 3 to 4 s and phase between 0 and 2π rad. Considered irregularities were baseline shifts up to 34 mm, changing amplitudes between 6 and 18 mm, changing periods between 1.6 and 5.2 s and combinations. Evaluation was done by looking at dose homogeneity HI_5 and the fraction of the CTV volume that received a dose outside of the clinical limits of 95% and 107%, V_107/95. For the regular patterns, both a systematic and a randomised analysis were carried out. For irregular patterns, only a systematic analysis was carried out. The mean HI_5 was found to be 31% for regular patterns; the means of all irregular patterns stay below this, even though the size of the irregularities for some breathing patterns was very large. The mean V_107/95 was found to be 0.7 for regular patterns. Irregularities did not cause further deterioration. Five times layered repainting causes a statistically significant decrease in magnitude of the interplay effect across all breathing patterns by 50-80%, but is approximately 50% less effective against baseline shift than against other types of breathing. Interplay effect size correlates strongly with amplitude, but this correlation can be obscured because period and phase introduce very large variance. The interplay effect in general is large for investigated target size, prescription dose, beam configuration and machine performance. It can cause up to 100% of the CTV to receive a clinically unacceptable dose and lead to large inhomogeneities. Irregular breathing was not found to be notably worse. Repainting is effective, even against irregular breathing, but baseline shifts can undermine its effectiveness. Separately considering breathing irregularities for tumours similar to that investigated here is deemed of low importance; it is more important to properly model the magnitude of the interplay effect using an accurate, individualised breathing pattern.