Abstract
Helical TomoTherapy has unique challenges in handling intrafractional motion compared to a conventional LINAC. In this study, we analyzed the impact of intrafractional motion on cumulative dosimetry using actual patient motion data from clinically treated patients and investigated real time jaw and multileaf collimator (MLC) compensation approaches to minimize the motion-induced dose discrepancy in clinically acceptable TomoTherapy lung SBRT treatments. Intrafractional motion traces from eight fiducial tracking CyberKnife lung tumor treatment cases were used in this study. These cases were re-planned on TomoTherapy for SBRT, with 18 Gy × 3 fractions to a planning target volume (PTV) defined on the breath-hold CT without ITV expansion. Each case was planned with four different jaw settings: 1 cm static, 2.5 cm static, 2.5 cm dynamic and 5 cm dynamic. In-house 4D dose accumulation software was used to compute the dose distributions with tumor motion and then compensate for that motion by modifying the original jaw and MLC positions to track the trajectory of the tumor. The impact of motion and effectiveness of compensation on the PTV coverage depends on the motion type and plan settings. On average, the PTV V100% (the percent volume of the PTV receiving the prescription dose) accumulated from three fractions changed from 96.6% (motion-free) to 83.1% (motion-included), 97.5% to 93.0%, 97.7% to 92.1%, and 98.1% to 93.7% for the 1 cm static jaw, 2.5 cm static jaw, 2.5 cm dynamic jaw and 5 cm dynamic jaw setting, respectively. When the jaw and MLC compensation algorithm was engaged, the PTV V100% was restored back to 92.2%, 95.9%, 96.6% and 96.4%, for the four jaw settings mentioned above respectively. TomoTherapy lung tumor SBRT treatments using a field width of 2.5 cm or larger are less sensitive to motion than treatments using a 1 cm field width. For 1 cm field width plans, PTV coverage can be greatly compromised, even over three fractions. Once the motion pattern is known, the jaw and MLC compensation algorithm can largely minimize the loss of PTV coverage induced by the motion.
Highlights
Helical TomoTherapy® (Accuray Inc., Sunnyvale, CA) is a method of radiation therapy that utilizes a helical delivery of dose to a patient in a slice-by-slice fashion similar to that of a helical computed tomography (CT) scanner [1, 2]
We aim to investigate the realistic consequences of lung tumor motion and the effectiveness of compensation by using real patient motion, clinical plans optimized on actual patient anatomy and four typical jaw settings used in clinical
Dose calculation with motion The dose calculation software used in this research was implemented as a fast GPU calculation algorithm that accurately predicts the effects of motion in TomoTherapy dose delivery [22]
Summary
Helical TomoTherapy® (Accuray Inc., Sunnyvale, CA) is a method of radiation therapy that utilizes a helical delivery of dose to a patient in a slice-by-slice fashion similar to that of a helical computed tomography (CT) scanner [1, 2]. A compact 6 MV S-band linear accelerator is attached to the gantry which delivers a fan beam that is collimated by longitudinal jaws and a binary multileaf collimator [1, 3]. The fan beam can laterally extend 40 cm at isocenter and has a maximum longitudinal field size of 5 cm at isocenter. Dynamic jaws (TomoEDGETM) have been introduced that allow for the motion of jaws in the beginning and end of treatment to conform to the target [4] This techniques allows for full field size delivery specified by the plan within the target to speed up treatment delivery, while dynamically closing the jaws to 1 cm at the superiorinferior borders of the target for sharper dose fall off [5]
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