Purpose: To introduce double‐ends quasi‐breath‐hold (DE‐QBH) technique and evaluate its feasibility. Methods: It was previously demonstrated QBH, a hybrid technique combining free‐breathing‐gating (FBG) and breath‐hold, could provide less motion uncertainty and shorter treatment time than conventional FBG. While QBH uses only one phase, either end‐of‐exhalation (EOE) or end‐of‐inhalation (EOI), DE‐QBH utilizes both phases to further improve delivery efficiency. DE‐QBH is realized using an audiovisual biofeedback system (AVBFS) and a respiratory motion management program (RMMP). The in‐house developed AVBFS, consisting of two infrared stereo cameras and a head mounted display, provides dynamic feedback to patients. The RMMP establishes a personalized DE‐QBH model by adding a short quasi‐breath‐hold period to both EOI and EOE phase. Then the patient is guided to follow the model. A simulation study for 6 different maneuvers (2−2, 3−3, 4−4, 5−5, 7−5, and 9−5 sec for EOI‐EOE combination) was performed with 3 volunteers. External motion signals were analyzed to obtain mean absolute error (MAE) between the measured and guided. Duty cycle was also estimated. Results: MAEs were smaller than 1 mm for all maneuvers except 9−5 combination [0.9(+/−)1.3, 0.7(+/−)1.2, 0.6(+/−)1.1, 0.7(+/−)1.1, 0.6(+/−)1.0, and 1.1(+/−)0.8 mm for 2−2, 3−3, 4−4, 5−5, 7−5, and 9−5 combination, respectively]. Estimated duty cycles were 43, 52, 57, 62, 64, and 71% for 2−2, 3−3, 4−4, 5−5, 7−5, and 9v5 combination, respectively. Conclusion: The proposed DE‐QBH technique was feasible for respiratory motion management. It showed excellent feasibility with minimal motion uncertainty and significantly improved delivery efficiency, reaching up to 70% duty cycle.
Purpose: A 90Sr/Y applicator has been used as a ‐source for postoperative irradiation after pterygium excision. As an alternative to 90Sr/Y irradiation, we proposed treatments with 32P. This study aims to provide the dosimetry for this new applicator. Method and Materials: In order to optimize the design and materials of 32P ophthalmic applicators, Monte Carlo simulations were performed. The absorbed dose at the surface of a sealed beta source is often measured by using an extrapolation ionization chamber. Radiochromic film (RCF) was used to measure depth dose distributions and dose profiles at various depths. A micro‐MOSFET detector was used for depth dose measurements. Results: The absorbed dose rates to the reference point were 0.238 ± 0.012 cGy/s for an extrapolation ionization chamber, 0.280 ± 0.001 cGy/s for radiochromic films, and 0.257 ± 0.020 cGy/s for MOSFET. The axial depth dose rate was reduced into approximately 1/10 as 32P betas penetrate every 2 mm depth. Measured data sets in depths of 1 mm to 3.5 mm agreed with Monte Carlo data. Due to non‐uniform absorption of 32P into an absorbent disk, the dose at the center of transaxial plane were 2%–4% less than the peak dose around the periphery. We confirmed no leakage of 32P activities and negligible exposure rate around the hand grip of the applicator. Conclusions: The 32P applicator can deliver uniform therapeutic doses to the surface of the conjunctiva, while sparing the lens better than 90Sr/Y applicators. The doses at any points from the 32P applicator can be calculated by using these measured data sets. The safety of 32P applicator was confirmed. However, prior to the clinical application of every new applicator, safety, dose uniformity, and absorbed dose rate at the reference point should be carefully evaluated by the method developed in this study.
Purpose: To present modulation indices (MIs) for volumetric modulated arc therapy (VMAT). Methods: A total of 40 VMAT plans were retrospectively selected. To investigate the delivery accuracy of each VMAT plan, gamma passing rates, differences in modulating parameters between plans and log files, and differences between the original plans and the plans reconstructed with the log files were acquired. A modulation index (MIt) was designed by multiplications of the weighted quantifications of MLC speeds, MLC accelerations, gantry accelerations and dose‐rate variations. Textural features including angular second moment, inverse difference moment, contrast, variance, correlation and entropy were calculated from the fluences of each VMAT plan. To test the performance of suggested MIs, Spearman's rank correlation coefficients (r) with the plan delivery accuracy were calculated. Conventional modulation indices for VMAT including the modulation complexity score for VMAT (MCSv), leaf travel modulation complexity score (LTMCS) and MI by Li & Xing were calculated, and their correlations were also analyzed in the same way. Results: The r values of contrast (particular displacement distance, d = 1), variance (d = 1), MIt, MCSv, LTMCS and MI by Li&Xing to the local gamma passing rates with 2%/2 mm were 0.547 (p < 0.001), 0.519 (p < 0.001), −0.658 (p < 0.001), 0.186 (p = 0.251), 0.312 (p = 0.05) and −0.455 (p = 0.003), respectively. The r values of those to the MLC errors were −0.863, −0.828, 0.917, −0.635, − 0.857 and 0.795, respectively (p < 0.001). For dose‐volumetric parameters, MIt showed higher statistically significant correlations than did the conventional modulation indices. Conclusion: The MIt, contrast (d = 1) and variance (d = 1) showed good performance to predict the VMAT delivery accuracy showing higher correlations to the results of various types of verification methods for VMAT. This work was in part supported by the National Research Foundation of Korea (NRF) grant (no. 490‐20140029 and no. 490‐20130047) funded by the Korea government.
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