Objective:

To carry out a dosimetric evaluation of two different advanced radiotherapy modalities prophylactic-whole brain radiotherapy with hippocampal avoidance in the treatment of patients with small cell lung cancer (SCLC).

Method:

Computed tomography with fused magnetic resonance images of patients who were diagnosed with SCLC without brain metastasis were combined and re-planned with volumetric-modulated arc (VMAT) and also helical tomotherapy (n=10). Doses of 25 Gy were prescribed in 10 fractions for the whole brain volume. Planning was performed according to RTOG 0933 criteria for hippocampal avoidance (Dmax ≤16Gy, D100% ≤9Gy). Planning target volume (PTV), homogeneity index (HI), conformity index (CI), minimum dose (Dmin) maximum dose (Dmax) and dose for organs at risk (OARs) were compared.

Results:

In the analysis of PTV data, no significant difference was found between the helical tomotherapy and VMAT for D95 values, the minimum, maximum and mean values of PTV brain doses were higher in the VMAT arm (p=0,000). When comparing PTV HI and CI values, VMAT was significantly superior in CI value (p=0.033), but there was no significant superiority in HI values (p=0.499). When compared statistically, and radiation doses delivered to hippocampus were evaluated it was seen that helical tomotherapy provided significantly better hippocampal protection than VMAT (right: 6.43Gy vs 8.94Gy; p=0.000; left: 6.10Gy vs 8.96Gy; p=0.000), mean (right 7.60Gy vs 11.42Gy; p=0.000; left: 7.56Gy vs 11.70Gy; p =0.000) and maximum (right: 15.83Gy vs 20.8Gy; p=0.003; left: 15.99Gy vs 20.13Gy; p=0.009). When analyzing lens doses, it was observed that the mean and maximum dose values of both right and left lenses were much lower in helical tomotherapy (mean right: 4.52Gy vs 12,34Gy; p=0,000 and left: 4.66Gy vs 11.69Gy; p=.,000; maximum right: 3.41Gy vs 10.54Gy; p=0.00 and left: 3.53Gy vs 10.10Gy; p=0,000).

Conclusion:

Acceptable treatment plans have been developed in both radiotherapy methods. Both hippocampus avoidance region and lens doses were found to be significantly superior in helical tomotherapy when compared with VMAT.

Keywords: dosimetry, hippocampal avoidance, VMAT, helical tomotherapy