A study of longitudinal tumor motion in helical tomotherapy using a cylindrical phantom

Abstract

Tumor motion during radiation treatment on a helical tomotherapy unit may create problems due to interplay with motion of the multileaf collimator, gantry rotation, and patient couch translation through the gantry. This study evaluated this interplay effect for typical clinical parameters using a cylindrical phantom consisting of 1386 diode detectors placed on a respiratory motion platform. All combinations of radiation field widths (1, 2.5, and 5 cm) and gantry rotation periods (16, 30, and 60 s) were considered for sinusoidal motions with a period of 4 s and amplitudes of 5, 6, 7, 8, 9, and 10 mm, as well as real patient breathing pattern. Gamma comparisons with 2% dose difference and 2 mm distance to agreement and dose profiles were used for evaluation. The required motion margins were determined for each set of parameters. The required margin size increased with decreasing field width and increasing tumor motion amplitude, but was not affected by rotation period. The plans with the smallest field width of 1 cm have required motion margins approximately equal to the amplitude of motion (±25%), while those with the largest field width of 5 cm had required motion margins approximately equal to 20% of the motion amplitude (±20%). For tumor motion amplitudes below 6 mm and field widths above 1 cm, the required additional motion margins were very small, at a maximum of 2.5 mm for sinusoidal breathing patterns and 1.2 mm for the real patient breathing pattern.PACS numbers: 87.55.km, 87.55.Qr, 87.56.Fc

Study of Motion-induced Dose Error Caused by Irregular Tumor Motion in Helical Tomotherapy

“…5) As a result of this study, 5 cm of field width exhibited better gamma passing rates than 1 cm and 2.5 cm widths in the gamma comparisons with 2% dose discrepancy and 2 mm distance-to-agreement. In a different study focused on couch motion, 7) the couch motion was varied within a range of values, and the effects on the dose profiles were assessed.…”

“…[1][2][3] The motion-induced dose error presents additional complications in helical tomotherapy, as the tumor motion interacts with the couch motion. 4,5) Owing to the tumor motion, the treatment field does not reach the tumor region constantly, and repeatedly shifts in and out of the tumor area. 6,7) This effect prevents the delivery of the planned dose to the tumor region.…”

Dosimetric Analysis of Microscopic Disease in SBRT for Lung Cancers

“…Several studies have noted that lung respiratory motion was predominant (though not exclusively) in the patient superior–inferior (SI) direction. 8 , 9 Thus, for simplicity, we assumed in this study that (1) the organ motion was uniform along 1-dimensional (SI) direction and (2) the anatomical structures (both tumor and MD) were rigid with no deformation.…”

Outcome and toxicity of stereotactic body radiotherapy with helical tomotherapy for inoperable lung tumor: analysis of Grade 5 radiation pneumonitis

“…For hypofractional treatment such as SBRT for lung tumors, the helical tomotherapy unit has advantages but also causes some concerns: as advantages, conformal delivery by IMRT and precise set-up using the onboard MVCT system [ 4 ]; as concerns, expansion of the low-dose irradiated lung volume caused by the multidirectional and coplanar delivery method, and a discrepancy between calculated and delivered dose distributions due to movements of tumors and mechanical dynamics peculiar to helical tomotherapy: motions of the multileaf collimator, gantry rotation, and couch translation through the gantry. Despite these concerns, several experimental studies have shown that efforts to restrict respiration-associated tumor motion and optimize the planning target volume (PTV) margin lead to an acceptable minimal dose error [ 5 – 7 ], and a number of clinical reports on HT-SBRT have demonstrated its feasibility, with a promising outcome and favorable tolerance [ 4 , 8 – 10 ].…”