ABSTRACT
Respiratory motion is a large source of dosimetric error when treating lung cancer with Intensity Modulated Radiation Therapy (IMRT). The asynchronicity of the tumour motion and the multileaf collimator (MLC) used to modulate the radiation beam intensity, leads to the interplay effect. One method to account for this effect is respiratory gating. Treatment planning optimization for gated IMRT is performed on a subset average 4D-CT which includes the phases surrounding end exhalation. However, this assumes that the beam delivery will be evenly distributed amongst those phases. This study investigates the distribution of beam delivery during gated step-and-shoot IMRT (SS-IMRT) for both early and late stage non-small cell lung cancer (NSCLC). Four Stage I NSCLC patients, prescribed a dose of 54 Gy in 3 fractions, and five Stage III NSCLC patients, prescribed a dose of 60 Gy in 30 fractions, were retrospectively planned with high and low modulation beams-IMRT, and delivered using the QUASAR™ Programmable Respiratory Motion Platform with 15 mm and 20 mm peak-to-peak sinusoidal motion and real patient breathing motion. The percent monitor units delivered at each phase were compared. For Stage I patients, the monitor units delivered were evenly distributed over the gating window due to a high number of monitor units delivered per control point. For Stage III patients, as the complexity of SS-IMRT increases, there were more monitor units delivered in the initial gating phase. This dose discrepancy could potentially lead to geographic miss of the tumour and should be taken into account during treatment planning.
ABSTRACT
Volumetric modulated arc therapy (VMAT) is a time efficient treatment delivery platform capable of producing highly conformal dose distributions with a single 360° arc. However, additional arcs can be used to further improve the conformal dose distribution. For these reasons, VMAT is often used for stereotactic body radiation therapy (SBRT) in which the treatment deliveries are hypofractionated. The dosimetric impact of tumour motion, especially in lung SBRT where tumour motion is most significant and treatments are hypofractionated, has always been a clinical concern. Through the use of 4-dimensional computed tomography (4D-CT), 4D dose distributions can be calculated that account for dosimetric errors due to motion and temporal variation in lung density that are not accounted for in clinical treatment plans. The purpose of this study was to quantify the dosimetric differences that arise due to tumour motion and variations in lung density between single and dual VMAT SBRT treatment plans. Six patients previously treated for stage I/II non-small-cell lung cancer with SBRT were included in this retrospective study. 3D and 4D dose distributions were calculated for both single and dual arc plans for each of the six patients. Dose-volume histogram metrics are reported for the target and critical structures. The results show significant differences (p ≤ 0.05) between the 3D and 4D dose distributions for the ratio of the prescription isodose volume to the primary target volume (PTV). This result was consistent for both single and dual arc VMAT plans.
