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
Highlights
Motion of the internal organs may create problems in radiation therapy, which relies on a static target to plan a dose delivery
Breathing can result in lung tumor motion with amplitudes above 5 mm and periods close to 4 s, mainly in the superior–inferior (SI) direction.[1,2,3] breathing-induced motion is not limited to the lungs; the liver and kidney may move by 1.5–2 cm in the SI direction during normal respiration.[4]. Pancreatic tumors may move by almost 1 cm in the SI direction and half that in the anterior–posterior (AP) direction.[5]. This ‘intrafraction’ motion can cause unwanted effects, such as the blurring of the intended dose distribution along the direction of motion
These artifacts have been identified as dose rounding, dose rippling, and the leaf opening asynchronization effect.[6]. Some authors have suggested that either the interplay effect itself does not produce significant error beyond dose rounding for sinusoidal breathing patterns,(7-10) or that the error is reduced over several fractions.[7,11,12,13] Other studies have found that there are significant effects, even with sinusoidal motion[6] or with only real patient breathing patterns.[9,14]
Summary
Motion of the internal organs may create problems in radiation therapy, which relies on a static target to plan a dose delivery. With thoracic tumors that move due to breathing, the interplay between all of these motions may lead to artifacts in the dose distribution. The standard method for dealing with uncertainties such as tumor motion is to expand the clinical target volume (CTV) by a certain margin to create a planning target volume (PTV). Because the required size of the margin is uncertain, physicians must overestimate the necessary PTV size in order to ensure coverage. This leads to an increase in the level of radiation given to healthy tissue
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