No abstract

Calvarial reconstruction following resection of tumors involving the skull is often followed by stereotactic radiosurgery. Prior studies have addressed the effects of various cranioplasty materials on dose distributions in linac‐based radiosurgery. We aim to determine the effects of titanium mesh implants on Gamma Knife dose. Radiation backscatter and transmission were measured for eight types of titanium mesh using film, ion chamber, and Theratron Co‐60 teletherapy device. A single mesh was selected for Gamma Knife irradiation using a CaSO4 skull filled with ballistics gel. Dose profiles for reconstructed and intact skulls were compared with the planning system prediction at 2.5 and 5.5 cm depth. Titanium contact backscatter and transmission dose perturbations ranged from ‐18% to 23%. Radiation dose measured at 1.5 cm below the calvarial implant increased by 0.5% to 3.3% relative to bone. Measured Gamma Knife dose profile diameters agreed with expected profiles. Maximum dose within the intact phantom was 3% less than planned due to skull attenuation. Maximum dose within the reconstructed phantom was between the intact phantom and planned doses. Titanium mesh implants and hydroxyapatite cranioplasty result in minimal alteration (<3%) in the delivered Gamma Knife dose.PACS number: 87.00

This study evaluates respiratory excursions and their impact on patient setup and dosimetric coverage with and without the use of cone-beam computed tomography (CBCT) for localization in stereotactic ablative radiotherapy. The datasets of 150 non-small cell lung cancer patients were assessed. Four groups of patients were evaluated based on their tumor location: upper lobe (UL)-peripheral (N=39), UL-chest-wall seated (CWS, N=37), lower lobe (LL)-peripheral (N=48), and LL-CWS (N=26). Tumor excursion and setup error were quantified and correlated. Treatment planning margins were derived based on the van Herk formalism. A dosimetric study investigated the dose coverage with and without the use of CBCT for localization. The percentage of patients showing >5 mm tumor respiratory excursion for UL-peripheral/LL-peripheral was 10.0/42.9%, and was 4.2/ 46.7% for UL-CWS/LL-CWS. Planning margin magnitudes averaged over all patients were M residual (2.7, 3.2, 3.7) mm and M interfxn (9.0, 14.0, 10.0) mm, respectively with and without the use of CBCT for image guidance. Comparatively, the planning margins for the patients with LL tumors exhibiting the largest motion and setup errors were M residualLL (2.8, 3.6, 4.4). Pearson correlation coefficients (for LL-peripheral tumors in the S/I direction) between tumor excursion and, respectively, inter-fraction and residual setup errors were 0.62 and 0.32. Based on skin tattoo setup, the overall average difference in D 95 dose to the planning target volume (PTV) between the delivered and planned doses was 14.1±9.2%. The use of CBCT for localization reduced the overall average ΔD 95 to less than 2%. This analysis is suggestive that: (a) patients with LL tumors undergo the largest respiratory-induced motion, and experience larger setup errors relative to UL tumors; (b) the use of CBCT-based image guidance significantly reduces residual setup errors; planning margins of the order of 5 mm appear to be adequate for proper PTV dose coverage; (c) CBCT image guidance reduces the correlation between respiratory-induced motion and setup errors, implying that there is much less variation in the setup uncertainty between tumors undergoing respiratory motion of varying magnitudes, relative to the variation without CBCT.

Purpose: To evaluate the potential advantages of jaw tracking for intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) in spine radiosurgery. Methods: VMAT and IMRT plans were retrospectively generated for ten patients. Six plans for each patient were created in the Eclipse treatment planning system for a Varian Truebeam equipped with a Millennium 120 MLC. Plans were created to study IMRT and VMAT plans with and without jaw tracking, as well as IMRT plans of different flattening filter free (FFF) energies. Plans were prescribed to the 90% isodose line to 16 or 18 Gy in one fraction to cover 95% of the target. Planning target volume (PTV) coverage, conformity index (CI), dose to spinal cord, distance to fall off from the 90% to 50% isodose line (DTF), as well as delivery time were evaluated. Ion chamber and film measurements were performed to verify calculated and measured dose distributions. Results: Jaw tracking decreased the spinal cord dose for both IMRT and VMAT plans, but a larger decrease was seen with the IMRT plans (p=0.004 vs p=0.04). The average D10% for the spinal cord was least for the 6MV FFF IMRT plan with jaw tracking and was greatest for the 10MV FFF plan without jaw tracking. Treatment times between IMRT and VMAT plans with or without jaw tracking were not significantly different. Measured plans showed greater than 98.5% agreement for planar dose gamma analysis (3%/2 mm) and less than 2.5% for point dose analysis compared to calculated plans. Conclusion: Jaw tracking can be used to help decrease spinal cord dose without any change in treatment delivery or calculation accuracy. Lower dose to the spinal cord was achieved using 6 MV beams compared to 10 MV beams, though 10 MV may be justified in some cases to decrease skin dose.