Visualisation of Respiratory Tumour Motion and Co-Moving Isodose Lines in the Context of Respiratory Gating, IMRT and Flattening-Filter-Free Beams

<div><p>Respiratory motion during percutaneous radiotherapy can be considered based on respiration-correlated computed tomography (4DCT). However, most treatment planning systems perform the dose calculation based on a single primary CT data set, even though cine mode displays may allow for a visualisation of the complete breathing cycle. This might create the mistaken impression that the dose distribution were independent of tumour motion. We present a movie visualisation technique with the aim to direct attention to the fact that the dose distribution migrates to some degree with the tumour and discuss consequences for gated treatment, IMRT plans and flattening-filter-free beams. This is a feasibility test for a visualisation of tumour and isodose motion. Ten respiratory phases are distinguished on the CT, and the dose distribution from a stationary IMRT plan is calculated on each phase, to be integrated into a movie of tumour and dose motion during breathing. For one example patient out of the sample of five lesions, the plan is compared with a gated treatment plan with respect to tumour coverage and lung sparing. The interplay-effect for small segments in the IMRT plan is estimated. While the high dose rate, together with the cone-shaped beam profile, makes the use of flattening-filter-free beams more problematic for conformal and IMRT treatment, it can be the option of choice if gated treatment is preferred. The different effects of respiratory motion, dose build-up and beam properties (segments and flatness) for gated vs. un-gated treatment can best be considered if planning is performed on the full 4DCT data set, which may be an incentive for future developments of treatment planning systems.</p></div

PTV and tumour motion characteristics.

<p>PTV and tumour motion characteristics.</p

Example for a field segment with low monitor units (18 MU) located at the edge of the PTV.

<p>The segment is so small that the tumour may move completely outside in the respiratory phase (at the most cranial position). Due to the short irradiation time, this segment might hence miss the tumour. If this effect becomes significant for a number of fields, the dose distribution shown in Movie 2 is no longer valid, since it is based on the assumption that all respiratory phases will experience the same beam/segment configuration. More realistically, the low-MU segments will only be experienced by some respiratory phases. If the “wrong” phases are associated with the segments, the dose associated with these segments will not contribute to the PTV dose, but still increase the dose to organs at risk.</p

Baseline Demographic and Patient Characteristics.

<p><i>Abbreviations:</i> 1) NSCLC = non-small cell lung cancer; 2) thereof one patient with two lung lesions.</p

The Impact of Diffusion-Weighted MRI on the Definition of Gross Tumor Volume in Radiotherapy of Non-Small-Cell Lung Cancer - Fig 3

<p>Scatter plots and Bland-Altman analysis of tumor volume <b>A.</b> Linear regression analysis with fitted line. <i>p</i> = 0.005. <b>B.</b> Bland-Altman analysis showing upper and lower limits of agreement (mean volume difference ± 2 standard deviations) between stGTV (x-axis) and dwGTV (y-axis). The p-value indicates a significant dependence of measured volume differences from the average tumor size, as tested with linear regression analysis. <b>C.</b> Bland-Altman analysis showing upper and lower limits of agreement (mean volume difference ± 2 standard deviations) between stGTV (x-axis) and dwGTV (y-axis) in logarithmic scale. In logarithmic form, no significant dependence of measured volume differences from the average tumor size was observed (tested with linear regression analysis).</p

Patient characteristics, measured tumor volumes for all 16 tumors in 15 patients.

<p>Patient characteristics, measured tumor volumes for all 16 tumors in 15 patients.</p

MRI sequences of a patient with stage T1b NSCLC in the left upper lobe.

<p><b>A</b>. T2 HASTE sequence (non-gated). <b>B.</b> ADC-map. <b>C.</b> DWI (calculated b-value of 1400). <b>D.</b> Fusion of T2 and DWI, color represents diffusion restriction.</p