Elsevier

Radiotherapy and Oncology

Volume 102, Issue 2, February 2012, Pages 268-273
Radiotherapy and Oncology

Lung cancer
Comparison of anisotropic aperture based intensity modulated radiotherapy with 3D-conformal radiotherapy for the treatment of large lung tumors

https://doi.org/10.1016/j.radonc.2011.10.006Get rights and content

Abstract

Purpose/objective(s)

IMRT allows dose escalation for large lung tumors, but respiratory motion may compromise delivery. A treatment plan that modulates fluence predominantly in the transversal direction and leaves the fluence identical in the direction of the breathing motion may reduce this problem.

Materials/methods

Planning-CT-datasets of 20 patients with Stage I–IV non small cell lung cancer (NSCLC) formed the basis of this study. A total of two IMRT plans and one 3D plan were created for each patient. Prescription dose was 60 Gy to the CTV and 70 Gy to the GTV. For the 3D plans an energy of 18 MV photons was used. IMRT plans were calculated for 6 MV photons with 13 coplanar and with 17 noncoplanar beams. Robustness of the used method of anisotropic modulation toward breathing motion was tested in a 13-field IMRT plan.

Results

As a consequence of identical prescription doses, mean target doses were similar for 3D and IMRT. Differences between 3D and 13- and 17-field IMRT were significant for CTV Dmin (43 Gy vs. 49.1 Gy vs. 48.6 Gy; p < 0.001) and CTV D95 (53.2 Gy vs. 55.0 Gy vs. 55.4 Gy; p = 0.001). The Dmean of the contralateral lung was significantly lower in the 17-field plans (17-field IMRT vs. 13- vs. 3D: 12.5 Gy vs. 14.8 Gy vs. 15.8 Gy: p < 0.05). The spinal cord dose limit of 50 Gy was always respected in IMRT plans and only in 17 of 20 3D-plans. Heart Dmax was only marginally reduced with IMRT (3D vs. 13- vs. 17-field IMRT: 38.2 Gy vs. 36.8 Gy vs. 37.8 Gy). Simulated breathing motion caused only minor changes in the IMRT dose distribution (∼0.5–1 Gy).

Conclusions

Anisotropic modulation of IMRT improves dose delivery over 3D-RT and renders IMRT plans robust toward breathing induced organ motion, effectively preventing interplay effects.

Section snippets

Methods and materials

CT-datasets of 20 patients with Stage I–IV non small cell lung cancer (NSCLC) formed the basis of this study. Specific patient characteristics are presented in Table 1.

The basis for 3D and IMRT plans were thin slice computer tomography (CT) scans acquired on a dedicated 8 slice CT simulator (Somatom Plus 4 Volume Zoom, Siemens®, 120 kV, 200 mAs/section, 10-mm thickness, 10-mm increment. 3D and IMRT plans were generated on PrecisePLAN© 2.03 for a Synergy© linac equipped with a multileaf collimator

Results

As a consequence of identical prescription doses and an emphasis on target coverage, mean target doses were similar for 3D and IMRT (Table 2). Wilcoxon signed rank tests showed that the differences between 3D and 13- and 17-field IMRT, were significant for CTV Dmin (43 Gy vs. 49.1 Gy vs. 48.6 Gy; p < 0.001) and CTV D95 (53.2 Gy vs. 55.0 Gy vs. 55.4 Gy; p = 0.001). GTV- and CTV DVH-metrics were therefore anchored at very similar values. Plan characteristics are therefore mainly based on OAR DVH-metrics.

Discussion

Many dose-escalating trials have been introduced over the past decades in an attempt to improve treatment outcome for patients with advanced NSCLC. Already in the early 70’s, Fletcher’s group reported that dose escalation to between 80 Gy and 100 Gy is necessary to increase local control for lung cancer patients [8], which was recently confirmed in the context of hyperfractionated radiotherapy for stage I and II NSCLC [1], [9]. The following RTOG 83-11 study did not result in improved survival

Conclusion

IMRT reduces lung exposure in patients with large targets compared to 3D-conformal radiotherapy. Anisotropic modulation renders IMRT plans robust toward breathing induced organ motion, effectively preventing interplay effects while not significantly prolonging treatment time.

Conflicts of interest

None.

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