The impact of breathing motion versus heterogeneity effects in lung cancer treatment planning

Abstract

The purpose of this study is to investigate the effects of tissue heterogeneity and breathing-induced motion/deformation on conformal treatment planning for pulmonary tumors and to compare the magnitude and the clinical importance of changes induced by these effects. Treatment planning scans were acquired at normal exhale/inhale breathing states for fifteen patients. The internal target volume (ITV) was defined as the union of exhale and inhale gross tumor volumes uniformly expanded by 5 mm. Anterior/posterior opposed beams (AP/PA) and three-dimensional (3D)-conformal plans were designed using the unit-density exhale ("static") dataset. These plans were further used to calculate (a) density-corrected ("heterogeneous") static dose and (b) heterogeneous cumulative dose, including breathing deformations. The DPM Monte Carlo code was used for dose computations. For larger than coin-sized tumors, relative to unit-density plans, tumor and lung doses increased in the heterogeneity-corrected plans. In comparing cumulative and static plans, larger normal tissue complication probability changes were observed for tumors with larger motion amplitudes and uncompensated breathing-induced hot/cold spots in lung. Accounting for tissue heterogeneity resulted in average increases of 9% and 7% in mean lung dose (MLD) for the 6 MV and 15 MV photon beams, respectively. Breathing-induced effects resulted in approximately 1% and 2% average decreases in MLD from the static value, for the 6 and 15 MV photon beams, respectively. The magnitude of these effects was not found to correlate with the treatment plan technique, i.e., AP/PA versus 3D-CRT. Given a properly designed ITV, tissue heterogeneity effects are likely to have a larger clinical significance on tumor and normal lung treatment evaluation metrics than four-dimensional respiratory-induced changes.

Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects

“…The motivation is clear: The less breathing phases are required in the accumulation process, the less images have to be acquired and the less computational time is needed for the 4D dose calculation. In related studies in literature, the number of phases can be found to vary mainly between two (endexpiration and end-inspiration) [10,41] and ten phases (e.g. [42,12]).…”

Design and Testing of a Simulation Framework for Dosimetric Motion Studies Integrating an Anthropomorphic Computational Phantom into Four-dimensional Monte Carlo

“…It is worth stressing that these results were obtained considering constant lung density over the breathing cycle. Recent studies show that such approximation results in minor effects if compared to motion and heterogeneities correction (20, 35). Normal lung DVHs illustrate that tracking has a dosimetric advantage vs. gating, at least in a large motion case.…”

“…Maximum anterior-posterior (AP) expansion of the chest wall applies to motion studies in the lung (20). …”

“…Monte Carlo emerges as the best solution, given the intrinsic accuracy, especially for low density materials and in presence of heterogeneities (19). This specifically applies to motion studies in the lung (20). The use of 4D Monte Carlo dose calculations for time dependent geometries, including variations in patient anatomy due to organ motion, has proven to be highly efficient (21–23).…”

Extension of the NCAT phantom for the investigation of intra-fraction respiratory motion in IMRT using 4D Monte Carlo

“…Many studies have investigated the effect of motion in radiotherapy and shown that it can be a large source of error between planned and delivered dose distributions if it is not accounted for (Bortfeld et al 2002, Court et al 2008, Flampouri et al 2006, Ford et al 2003, Jiang et al 2003, Langen and Jones 2001, Lujan et al 2003, Rosu et al 2007, Schaefer et al 2004, Seco et al 2007, 2008, Shimizu et al 2001, Yu et al 1998). The use of four-dimensional computed tomography (4D-CT) and periodic portal imaging has allowed the nature of the target volume to be seen both over the course of a breathing cycle and throughout a course of radiotherapy, respectively (Keall 2004, Keall et al 2004, Liu et al 2007, Low et al 2003, Maxim et al 2007, Pan et al 2004, Rietzel et al 2005a, 2005b, Rosu et al 2006, Vedam et al 2003).…”