Study on the effects of the changes in respiratory status on gated stereotactic radiotherapy using an in-house developed motion phantom / 中华放射医学与防护杂志

ABSTRACT

Objective:To evaluate the effects of the changes in respiratory status on gated stereotactic radiotherapy under free breathing guided by real-time position management (RPM).Methods:This study simulated the baseline shift, change in respiratory rate, end-expiratory delay, end-inspiratory delay, and irregular breathing using an in-house developed motion phantom. Moreover, this study analyzed the correlation between the changes in the above states of three plans, three-dimensional conformal radiotherapy(3D-CRT), intensity modulated radiotherapy(IMRT), and volumetric modulated arc therapy(VMAT) and the position of the ball at the center of the motion phantom (L) and the exposed dose of the phantom in the ionization chamber (the dose).Results:The in-house developed phantom presented high setup repeatability and measurement stability. There was a positive correlation between L and the baseline shift ( r = 0.99, P < 0.01). The change in the dose was less than 4% when the baseline shift was less than the setup error, while the dose declined rapidly and was negatively correlated with the baseline shift otherwise ( r= -0.95, P < 0.01). Moreover, there was statistically significant difference in dose when the baseline shift exceeded the setup error or not ( Z = -3.06, P < 0.01). There was no significant difference in the rate of the dose affected by baseline shift in the three plans ( P > 0.05). The changes in respiratory rate had little effect on L and the dose. Both end-inspiratory delay and end-expiratory delay reduced the planned dose of the three plans, with a maximum decrease of up to -1.74%. Furthermore, the end-inspiratory delay has greater effects on the planned dose than the end-expiratory delay( Z = -2.67, P< 0.01). However, there was no significant correlation between the dose and the delay duration ( P > 0.05), and no significant difference in the rate of the planned dose of the three plans affected by respiratory waveform change ( P > 0.05). Irregular breathing had greater effects on the dose. Specifically, the dose from six repeated measurements of 3D-CR, IMRT, VMAT was (709.68±180.00), (751.40±127.16), and (750.00±185.60) cGy, respectively, all less than the prescribed dose with a poor consistency. Conclusions:The changes in the patients′ respiratory status will reduce the dose, especially when the baseline shift exceeds the setup error threshold or large respiratory waveform variation corresponding to irregular breathing occurs. Moreover, there is no correlation between the decrease in the dose and the radiotherapy technology.