Helical tomotherapy for whole-brain irradiation with integrated boost to multiple brain metastases: evaluation of dose distribution characteristics and comparison with alternative techniques

Abstract

Purpose: To quantitatively evaluate dose distribution characteristics achieved with helical tomotherapy (HT) for whole-brain irradiation (WBRT) with integrated boost (IB) to multiple brain metastases in comparison with alternative techniques.

Methods and materials: Dose distributions for 23 patients with 81 metastases treated with WBRT (30 Gy/10 fractions) and IB (50 Gy) were analyzed. The median number of metastases per patient (N(mets)) was 3 (range, 2-8). Mean values of the composite planning target volume of all metastases per patient (PTV(mets)) and of the individual metastasis planning target volume (PTV(ind met)) were 8.7 ± 8.9 cm(3) (range, 1.3-35.5 cm(3)) and 2.5 ± 4.5 cm(3) (range, 0.19-24.7 cm(3)), respectively. Dose distributions in PTV(mets) and PTV(ind met) were evaluated with respect to dose conformity (conformation number [CN], RTOG conformity index [PITV]), target coverage (TC), and homogeneity (homogeneity index [HI], ratio of maximum dose to prescription dose [MDPD]). The dependence of dose conformity on target size and N(mets) was investigated. The dose distribution characteristics were benchmarked against alternative irradiation techniques identified in a systematic literature review.

Results: Mean ± standard deviation of dose distribution characteristics derived for PTV(mets) amounted to CN = 0.790 ± 0.101, PITV = 1.161 ± 0.154, TC = 0.95 ± 0.01, HI = 0.142 ± 0.022, and MDPD = 1.147 ± 0.029, respectively, demonstrating high dose conformity with acceptable homogeneity. Corresponding numbers for PTV(ind met) were CN = 0.708 ± 0.128, PITV = 1.174 ± 0.237, TC = 0.90 ± 0.10, HI = 0.140 ± 0.027, and MDPD = 1.129 ± 0.030, respectively. The target size had a statistically significant influence on dose conformity to PTV(mets) (CN = 0.737 for PTV(mets) ≤4.32 cm(3) vs CN = 0.848 for PTV(mets) >4.32 cm(3), P=.006), in contrast to N(mets). The achieved dose conformity to PTV(mets), assessed by both CN and PITV, was in all investigated volume strata well within the best quartile of the values reported for alternative irradiation techniques.

Conclusions: HT is a well-suited technique to deliver WBRT with IB to multiple brain metastases, yielding high-quality dose distributions. A multi-institutional prospective randomized phase 2 clinical trial to exploit efficacy and safety of the treatment concept is currently under way.