Quantitative assessment of anatomical change using a virtual proton depth radiograph for adaptive head and neck proton therapy.

The aim of this work is to demonstrate the feasibility of using water-equivalent thickness (WET) and virtual proton depth radiographs (PDRs) of intensity corrected cone-beam computed tomography (CBCT) to detect anatomical change and patient setup error to trigger adaptive head and neck proton therapy. The planning CT (pCT) and linear accelerator (linac) equipped CBCTs acquired weekly during treatment of a head and neck patient were used in this study. Deformable image registration (DIR) was used to register each CBCT with the pCT and map Hounsfield units (HUs) from the planning CT (pCT) onto the daily CBCT. The deformed pCT is referred as the corrected CBCT (cCBCT). Two dimensional virtual lateral PDRs were generated using a ray-tracing technique to project the cumulative WET from a virtual source through the cCBCT and the pCT onto a virtual plane. The PDRs were used to identify anatomic regions with large variations in the proton range between the cCBCT and pCT using a threshold of 3 mm relative difference of WET and 3 mm search radius criteria. The relationship between PDR differences and dose distribution is established. Due to weight change and tumor response during treatment, large variations in WETs were observed in the relative PDRs which corresponded spatially with an increase in the number of failing points within the GTV, especially in the pharynx area. Failing points were also evident near the posterior neck due to setup variations. Differences in PDRs correlated spatially to differences in the distal dose distribution in the beam's eye view. Virtual PDRs generated from volumetric data, such as pCTs or CBCTs, are potentially a useful quantitative tool in proton therapy. PDRs and WET analysis may be used to detect anatomical change from baseline during treatment and trigger further analysis in adaptive proton therapy.

Assessment of cumulative external beam and intracavitary brachytherapy organ doses in gynecologic cancers using deformable dose summation.

PURPOSE: Due to inter-fraction variation in applicator position, organ displacement and deformation, doses to targets and normal tissues may not be accurately represented by adding the doses from external beam radiation therapy (EBRT) and intracavitary brachytherapy (ICBT) using rigid image registration. Deformable image registration permits organ and applicators to be spatially matched in 3D, enabling more accurate tracking of the accumulated volumetric dose to the target as well as organs at risk (OAR). This study assesses the dosimetric impact of using deformable image registration to determine the cumulative EBRT and ICBT doses to the rectum and bladder. METHODS AND MATERIALS: Data from 20 patients with stage IB1-IVA cervical cancer were analyzed. Nine of the patients were treated with ICBT and EBRT which included a nodal or parametrium boost while eleven were treated with ICBT and EBRT with no boost. Dose summation was performed in two stages. For the first stage, only the ICBT fractional doses were added using both "parameter adding" and deformable registration techniques. In the second stage, the ICBT and EBRT doses were combined using "parameter adding" in two ways. Partial "parameter adding" considers the cumulative ICBT dose from deformable registration as one parameter while full "parameter adding" uses fractional ICBT parameters. The cumulative minimum doses to 2cc (D2cc) of the rectum and bladder were compared between deformable registration and "parameter adding" techniques. RESULTS: Dose summation of ICBT fractions only using deformable registration yielded D2cc values that were (10.1±9.5)% lower for the rectum and (7.2±6.3)% lower for the bladder compared to "parameter adding". When ICBT and EBRT doses were summed deformably, the group without EBRT boost had D2cc that were (0.0±4.6)% and (-1.2±2.9)% lower for the rectum and bladder respectively compared to partial "parameter adding". With EBRT boost, the differences were (-2.9±4.0)% and (-3.2±3.3)% for the rectum and bladder respectively. For full "parameter adding", the differences from deformable sum were (2.7±5.0)%, (2.6±5.0)% without EBRT boost and (0.6±4.8)%, (-1.5±3.7)% with EBRT boost. CONCLUSION: Comparison of deformable dose summation with the technique of "parameter adding" suggests that "parameter adding" can be used as a good approximation of D2cc when adding ICBT and EBRT doses with or without boost. With EBRT boosts, deformable dose summation may more accurately represent dose to normal critical structures but these differences remain small compared to "parameter adding".

Comparative assessment of liver tumor motion using cine-magnetic resonance imaging versus 4-dimensional computed tomography.

PURPOSE: To compare the extent of tumor motion between 4-dimensional CT (4DCT) and cine-MRI in patients with hepatic tumors treated with radiation therapy. METHODS AND MATERIALS: Patients with liver tumors who underwent 4DCT and 2-dimensional biplanar cine-MRI scans during simulation were retrospectively reviewed to determine the extent of target motion in the superior-inferior, anterior-posterior, and lateral directions. Cine-MRI was performed over 5 minutes. Tumor motion from MRI was determined by tracking the centroid of the gross tumor volume using deformable image registration. Motion estimates from 4DCT were performed by evaluation of the fiducial, residual contrast (or liver contour) positions in each CT phase. RESULTS: Sixteen patients with hepatocellular carcinoma (n=11), cholangiocarcinoma (n=3), and liver metastasis (n=2) were reviewed. Cine-MRI motion was larger than 4DCT for the superior-inferior direction in 50% of patients by a median of 3.0 mm (range, 1.5-7 mm), the anterior-posterior direction in 44% of patients by a median of 2.5 mm (range, 1-5.5 mm), and laterally in 63% of patients by a median of 1.1 mm (range, 0.2-4.5 mm). CONCLUSIONS: Cine-MRI frequently detects larger differences in hepatic intrafraction tumor motion when compared with 4DCT most notably in the superior-inferior direction, and may be useful when assessing the need for or treating without respiratory management, particularly in patients with unreliable 4DCT imaging. Margins wider than the internal target volume as defined by 4DCT were required to encompass nearly all the motion detected by cine-MRI for some of the patients in this study.

The use of proton therapy in the treatment of head and neck cancers.

External beam radiation therapy is a commonly utilized treatment modality in the management of head and neck cancer. Given the close proximity of disease to critical normal tissues and structures, the delivery of external beam radiation therapy can result in severe acute and late toxicities, even when delivered with advanced photon-based techniques, such as intensity-modulated radiation therapy. The unique physical characteristics of protons make it a promising option in the treatment of advanced head and neck cancer, with the potential to improve sparing of normal tissues and/or safely escalate radiation doses. Clinical implementation will require the continued development of advanced techniques such as intensity-modulated proton therapy, using pencil beam scanning, as well as rigorous methods of quality assurance and adaptive techniques to accurately adjust to changes in anatomy due to disease response. Ultimately, the widespread adaptation and implementation of proton therapy for head and neck cancer will require direct, prospective comparisons to standard techniques such as intensity-modulated radiation therapy, with a focus on measures such as toxicity, disease control, and quality of life.