Radiotherapeutic treatment options for oligotopic malignant liver lesions

Abstract

Background: Several radiotherapeutic approaches for patients with oligotopic malignant liver lesions unfit for surgical resection exist. The most advanced competitive techniques are high-dose-rate (HDR) brachytherapy, Cyberknife, volume-modulated-arc therapy (VMAT) and Tomotherapy. We evaluated the optimal technique by a planning study for a single ablative dose with different lesion sizes.

Methods: We compared dose distributions of HDR-brachytherapy with stereotactic ablative radiotherapy using the Cyberknife, VMAT or Tomotherapy. Tumor-control-probabilities (TCP), normal-tissue-complication-probabilities (NTCP) were determined in a theoretical framework applying a single dose of 20 Gy (demanding 95% coverage) for intrahepatic lesions of 1-5 cm in size. We evaluated therapeutic ratios by TCP (mean dose in the lesion) relative to high-dose (conformality) or low-dose liver exposition in dependency on the lesion size for each technique. In addition, we considered treatment times and accuracy (clinical target volume vs planning target volume).

Results: HDR-brachtherapy has the highest therapeutic ratios with respect to high-dose as well as low-dose liver exposition even for extended lesions, and the Cyberknife being suited second best. However, for lesions ≥ 3 cm diameter the therapeutic ratios of all ablative techniques are increasingly converging, and better tolerance and shorter treatment times of noninvasive external techniques become more important. On the other hand, mean tumor doses of HDR-brachytherapy of near 60 Gy are unattainable by the other techniques gaining only 22-34 Gy, and the conformality of HDR-brachytherapy is still rather good for lesions ≥ 3 cm diameter.

Conclusions: HDR-brachytherapy is by far the most effective technique to treat intrahepatic lesions by a single fraction, but sparing of the surroundings declines with increasing lesion size and approaches the benchmarks of external beam radiosurgery techniques. External beam radiotherapy has the advantage to use suitable fractionation schedules.

Keywords: Brachytherapy; Colorectal cancer; Liver metastasis; Oligometastasis; Radiation therapy.

Fig. 1

NTCP-curves for the liver according to LKB-model and parameters of Table 1. A large volume dependency is found for n = 0.6, if we compare irradiation of the entire organ (1) with irradiation of only one third (1/3)

Fig. 2

Dose distributions comparing the four radiotherapy techniques, if the standard single dose 20 Gy is given to a 2 cm lesion. The isodoses range from 150% (30 Gy, orange), 100% (20 Gy, white), 75% (15 Gy, yellow), 50% (green, 10 Gy), 25% (5 Gy, light blue), 10% (2 Gy, purple) to 5% (1 Gy, light green) according to the inserted scales. The ROIs are also shown such as liver (brown), stomach (blue) and kidneys (green, red). For such a small lesion HDR-brachytherapy is clearly superior in comparison to the external techniques

Fig. 3

Therapeutic ratios for all the four radiation techniques relating TCP (mean dose in the lesion) either to high-dose exposure (conformality, left) or low-dose exposure (long-term risks, right) of the liver