Robustness of three external beam treatment techniques against inter-fractional positional variations of the metal port in breast tissue expanders.

INTRODUCTION: Temporary breast tissue expanders contain a metal port that varies in position throughout the course of radiation treatments. The purpose of this study was to quantify the robustness of the three most common external beam treatment techniques (tangential three-dimensional conformal radiation therapy [3DCRT], volumetric modulated arc therapy [VMAT], and helical tomotherapy) against our measured inter-fractional positional variations of the port. METHODS: For eight breast cases, a clinical plan was created for each of the three techniques. The dosimetric effect of our previously measured inter-fractional port errors was evaluated for two classes of error: internal port errors (IPEs) and patient registration errors (PREs). For both classes of error, daily variable and systematic errors were modeled, and their cumulative effects were compared against the originally planned doses. RESULTS: For systematic IPE, the 1%-99% range in point dose differences inside a 5-mm target abutting the implant was the highest for tangential 3DCRT, and it was within 6% and 9% when calculated with Monte Carlo and collapsed cone calculation engines, respectively. Daily variable PRE resulted in mean changes of -3.0% and -3.5% to V100%Rx of the target for VMAT and tomotherapy, respectively. For nearby organs, daily variable PRE resulted in changes to V20Gy of the ipsilateral lung of less than 2% in all three techniques, while V5Gy of the heart increased by as much as 6% in VMAT and 10% in tomotherapy. CONCLUSIONS: When IPEs were modeled, dose variability was the largest in tangential 3DCRT, leading to areas of underdosage in the shadow of the port. When PREs were modeled, the target coverage and nearby organs were affected the most in VMAT and helical tomotherapy. In reality, port positional errors result from a combination of IPE and PRE, suggesting that VMAT and tomotherapy are more robust when patient registration errors are minimized, despite the presence of IPE.

Dose-sparing effect of deep inspiration breath hold technique on coronary artery and left ventricle segments in treatment of breast cancer.

BACKGROUND AND PURPOSE: The risk of radiation-induced cardiac injury remains a challenging problem in the treatment of breast cancer. Certain cardiac structures receive higher doses than others, which results in variable frequencies of radiation-induced injuries across these structures. Radiation dose can be reduced using the deep inspiration breath hold (DIBH) technique. We aimed to investigate the dose reductions from DIBH in individual cardiac segments. MATERIALS AND METHODS: A dosimetric analysis was performed on left-sided breast cancer patients who underwent breast-conserving surgery and whole breast irradiation. Radiation doses to the cardiac structures were compared between the DIBH and free-breathing (FB) techniques and the dose reductions with DIBH were correlated to the lung expansion. RESULTS: For the 75 patients included in our study, DIBH effectively reduced doses to the heart, left lung, left anterior descending coronary artery (LAD) and left ventricle (LV), but the degree of dose reductions was variable across different structures. The absolute dose reductions were greatest in the distal LAD (14.4 Gy) and apical LV (12.1 Gy) segments, compared with the other LAD (middle 9.7 Gy, proximal 1.6 Gy) and LV (anterior 5.3 Gy, lateral 2.9 Gy, septal 2.0 Gy, inferior 0.2 Gy) segments. Left lung expansion was significantly correlated with the dose reductions in the LAD (Spearman's rank correlation coefficient, ρ, 0.304) and LV (ρ, 0.420) segments. CONCLUSIONS: Our study demonstrates the dose-sparing effects of DIBH in various cardiac structures, especially the distal LAD and apical LV segments. The large dose reductions seen in the distal LAD and apical LV segments could potentially translate into clinical benefit of reduced cardiac toxicity, as these structures have been previously shown to receive the highest doses and are associated with radiation-induced injury.

Functional cranio-spinal irradiation: A hippocampal and hypothalamic-pituitary axis sparing radiation technique using two IMRT modalities.

Cranio-spinal irradiation (CSI) treatment of embryonal tumors is associated with long-term endocrine and neuro-cognitive sequelae. As an example, the radiation regiment for standard risk medulloblastoma is 23.4 Grays (Gy) CSI followed by a boost of 30.6Gy to the tumor bed. We hypothesize that a novel CSI technique, which we named "Functional" CSI (F-CSI) can reduce the dose to the hypothalamic-pituitary axis (HPA) and hippocampi compared to standard CSI (S-CSI) without sacrificing coverage. In this study, we compared the efficacy of Volumetric Modulated Arc Therapy (VMAT) and Helical Tomotherapy (HT) in delivering this novel CSI technique. Plans were constructed from 10 patients with embryonal tumors previously treated at our institution. Target volumes and organs at risk were delineated as per our local protocol and the ACNS0331 Atlas. The HPA and hippocampi contours were verified by an experienced neuro-radiologist. Primary objective was to achieve a D95% to the prescribed dose of 23.4Gy for CSI and 30.6Gy for the boost. Dmean ≤18Gy was assigned to the HPA and hippocampi. A two-sided t-test was used for comparison. F-CSI in both modalities were able to achieve the D95% target coverage. Hot spots (D2%) were lower with HT for both the CSI component (p = 0.03) and boost component (p < 0.01). VMAT was able to achieve better conformality (p < 0.01). Compared to S-CSI, both F-CSI modalities were able to achieve a significant decrease in dose to the HPA and Hippocampi. The average S-CSI HPA and Hippocampi Dmean were 23.9Gy and 23.8Gy. In contrast, respective F-CSI Dmean were 13.9Gy and 17.2Gy in VMAT and 15Gy and 15.9Gy in HT. The average composite (F-CSI plus boost) Dmean to the HPA and hippocampi often exceeded 18Gy. Compared to S-CSI, F-CSI with VMAT and HT were capable of achieving acceptable coverage while sparing the HPA and hippocampi. However, the addition of the boost component often exceeded the mean dose of 18Gy. This may be overcome with more conformal modalities for the boost phase such as stereotactic radiotherapy or proton therapy.