Pencil beam scanning treatments in free-breathing lung cancer patients - is 5 mm motion a limit?


Pencil beam scanning treatments in free-breathing lung cancer patients - is 5 mm motion a limit?

Jakobi, A.; Knopf, A.; Perrin, R.; Richter, C.

Purpose
To evaluate the dose degradation when treating lung cancer patients with proton pencil beam scanning during free-breathing. We assess if treatments without rescanning are feasible in order to avoid prolonged treatment time, especially for slow scanning facilities.
Material and Methods
For 40 lung cancer patients, 4DCT imaging was used to generate 4D dynamic dose distributions of 3D treatment plans with 3 pencil beam scanning fields optimised with the single field uniform dose technique. Simulations included the use of random breathing states of the patient at start of irradiation resulting in multiple possible 4D dynamic dose distributions per fraction. Complete treatment was assumed to consist of 33 fractions of probabilistically chosen single fractions. Treatments were assumed to be delivered with an IBA universal nozzle without rescanning (1.5ms between spots, 2s between energy layers, spot sigma 4mm at highest energy). Tumour motion amplitude was the maximum displacement in tumour centre-of-mass assessed by the 4DCT. Evaluation was done by looking at under- and overdosage in the target structure. In addition, changes in the dose distribution due to changes in motion and anatomy during treatment were analysed using a repeated 4DCT for 4D dynamic dose calculation in one patient case.
Results
Almost 50% of the patients had tumour motion amplitudes of less than 5mm. For these patients, the simulated dose degradation per fraction was much smaller than for patients with larger motion amplitudes, with 2% versus 12 % average absolute reduction of the V95 (p<0.01), and an average increase in absolute V107 of 2% vs 9% (p<0,01). In no patient case studied was the minimum dose in the target degraded to below 80% of the prescribed dose, and rarely increased above 120%. Simulating a 33-fraction treatment, the mean reduction of the V95 was below 1% for patients with motion amplitudes below 5mm, while for patients with larger motion, V95 was degraded on average by 4% with worst case scenarios of 4% versus 19% (p<0.01), cf. Fig. 1. V107 had an average increase of about 0% and 1% (n.s.), with worst case values of 5% and 15%. The additional analysis of one patient case with a repeated CT revealed a large increase of tumour motion by about 5mm during treatment, resulting in a large dose degradation and partial miss of the target (V95<70%), cf. Fig. 2.
Conclusion
Motion amplitude is an indicator of dose degradation caused by the interplay effect. Fractionation reduces the dose degradation to such an amount that rescanning might be unnecessary for patients with a small tumour motion less than 5mm. Patients with larger tumour motion should not be treated without any kind of motion mitigation technique (e.g. rescanning , gating or breath hold) to prevent tumour underdosage persisting through to the end of fractionated treatment. Furthermore, the tumour motion needs to be assessed during treatment for all patients to quickly react to possible changes in motion which might require a treatment adaptation.

Keywords: non-small cell lung cancer; proton therapy; pencil beam scanning; tumour motion; interplay; treatment planning study

  • Poster
    ESTRO 36 - Poster, 05.-09.05.2017, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-25438
Publ.-Id: 25438