Comparative planning of flattening-filter-free and flat beam IMRT for hypopharynx cancer as a function of beam and segment number.

Although highly conformal dose distributions can be achieved by IMRT planning, this often requires a large number of segments or beams, resulting in increased treatment times. While flattening-filter-free beams offer a higher dose rate, even more segments may be required to create homogeneous target coverage. Therefore, it is worthwhile to systematically investigate the dependence of plan quality on gantry angles and number of segments for flat vs. FFF beams in IMRT planning. For the practical example of hypopharynx cancer, we present a planning study of flat vs. FFF beams using three different configurations of gantry angles and different segment numbers. The two beams are very similar in physical properties, and are hence well-suited for comparative planning. Starting with a set of plans of equal quality for flat and FFF beams, we assess how far the number of segments can be reduced before the plan quality is markedly compromised, and compare monitor units and treatment times for the resulting plans. As long as a sufficiently large number of segments is permitted, all planning scenarios give good results, independently of gantry angles and flat or FFF beams. For smaller numbers of segments, plan quality decreases both for flat and FFF energies; this effect is stronger for fewer gantry angles and for FFF beams. For low segment numbers, FFF plans are generally worse than the corresponding flat beam plans, but they are less sensitive to a decrease in segment number if many gantry angles are used (18 beams); in this case the quality of flat and FFF plans remains comparable even for few segments

Left: Monitor units and predicted treatment times for different plan scenarios, with standard deviations.

<p>Right: linear fit of calculated treatment time as a function of segment number of each scenario.</p

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

Quality measures and doses to organs at risk for all plan scenarios, plotted with standard deviations.

<p>CI: conformity index, HI: homogeneity index, GI: gradient index. Within each plan scenario, segment number decreases from left to right (shown for 6 MV, 7 beams).</p

Dose-volume histogram of plans shown in Figure 3.

<p>Solid line: 6 MV, dashed line: FFF 7 MV. The right parotid fell inside the PTV, so it received considerable doses as compared with the left parotid, which was spared as much as possible.</p

Final optimized inversion constraints.

<p>*exluding parotis where only edge of parotis reaches into PTV.</p

Plan characteristics and IMRT inversion parameters for three different beam arrangements.

<p>Plan characteristics and IMRT inversion parameters for three different beam arrangements.</p

Example PTV with isocenter, transverse (left), sagittal (center) and coronal views (right).

<p>Example PTV with isocenter, transverse (left), sagittal (center) and coronal views (right).</p