Respiratory motion effects on whole breast helical tomotherapy

Abstract

The effects of intrafraction respiratory motion on nonhelical intensity-modulated radiotherapy have been well addressed in the literature, both theoretically and experimentally. However, the consequences of respiratory motion on helical tomotherapy, for patient-specific treatment plans, are less well known. Parameters specific to this treatment modality such as pitch, gantry speed, and degree of modulation may play prominent roles in radiation delivery with respect to intrafraction respiratory motion. This phantom-based study specifically addressed the effects of intrafraction respiratory motion on whole breast helical tomotherapy. A device capable of driving an acrylic phantom with reproducible, one-dimensional, anterior-posterior motion resembling a sinusoid of 4.6 mm crest-trough amplitude was developed. A plan to irradiate the corner of an acrylic phantom using parameters typical of a whole breast helical tomotherapy technique was developed using the TomoTherapy Hi-Art-II System. The treatment was delivered to the phantom, with Kodak EDR2 film in the axial plane, for each of the following conditions: (i) phantom at 270 degrees initial sinusoidal phase and 12 cycles/min motion, (ii) phantom at 270 degrees initial sinusoidal phase and 18 cycles/min motion, and (iii)-(v) phantom at 18 cycles/min motion with 0 degrees, 90 degrees, and 180 degrees initial sinusoidal phases. A measure of technique reproducibility was also performed for several irradiations with the phantom static at 270 degrees initial sinusoidal phase. Films were processed using a Kodak MIN-R mammography film processor, scanned with a Vidar NXR-16 Dosimetry Pro scanner and analyzed with RIT113 v.4.2 software. Films were compared to a reference film irradiated under the conditions of no motion and 270 degrees sinusoidal phase. For all comparisons, 5% dose difference threshold, 3% dose difference and 2 mm distance-to-agreement gamma analysis, and isodose plots were generated. The results of this study show a small area of greater than 5% decrease in dose at the phantom's anterior surface and a 1.5-3 mm posterior-medial shift of isodose lines in the penumbral and apex regions of the PTV. Frequency and phase effects are apparent within the PTV where dose varies with high spatial frequency. As the reference film was produced by delivering the treatment plan to the phantom static and in the position corresponding to maximum expiration, results are representative of extreme deviations between planned and delivered dose with respect to sinusoidal motion of clinically relevant magnitudes and frequencies.