Sa Yoganathan

Performance evaluation of respiratory motion‐synchronized dynamic IMRT delivery

The purpose of this study was to evaluate the capabilities of DMLC to deliver the respiratory motion‐synchronized dynamic IMRT (MS‐IMRT) treatments under various dose rates. In order to create MS‐IMRT plans, the DMLC leaf motions in dynamic IMRT plans of eight lung patients were synchronized with the respiratory motion of breathing period 4 sec and amplitude 2 cm (peak to peak) using an in‐house developed leaf position modification program. The MS‐IMRT plans were generated for the dose rates of 100 MU/min, 400 MU/min, and 600 MU/min. All the MS‐IMRT plans were delivered in a medical linear accelerator, and the fluences were measured using a 2D ion chamber array, placed over a moving platform. The accuracy of MS‐IMRT deliveries was evaluated with respect to static deliveries (no compensation for target motion) using gamma test. In addition, the fluences of gated delivery of 30% duty cycle and non‐MS‐IMRT deliveries were also measured and compared with static deliveries. The MS‐IMRT was better in terms of dosimetric accuracy, compared to gated and non‐MS‐IMRT deliveries. The dosimetric accuracy was observed to be significantly better for 100 MU/min MS‐IMRT. However, the use of high‐dose rate in a MS‐IMRT delivery introduced dose‐rate modulation/beam hold‐offs that affected the synchronization between the DMLC leaf motion and target motion. This resulted in more dose deviations in MS‐IMRT deliveries at the dose rate of 600 MU/min. PACS numbers: 87.53.kn, 87.56.N‐

Dosimetric effect of multileaf collimator leaf width in intensity-modulated radiotherapy delivery techniques for small- and large-volume targets

The purpose of this study was to evaluate the dosimetric effect of the leaf width of a multileaf collimator (MLC) in intensity-modulated radiotherapy (IMRT) delivery techniques for small- and large-volume targets. We retrospectively selected previously treated 5 intracranial and 5 head-neck patients for this study to represent small- (range, 18.37-72.75 cc; mean, 42.99 cc) and large-volume (range, 312.31-472.84 cc; mean, 361.14 cc) targets. A 6-MV photon beam data was configured for Brianlab m3 (3 mm), Varian Millennium 120 (5 mm) and Millennium 80 (10 mm) MLCs in the Eclipse treatment-planning system. Sliding window and step-shoot IMRT plans were generated for intracranial patients using all the above-mentioned MLCs; but due to the field size limitation of Brainlab MLC, we used only 5-mm and 10-mm MLCs in the head-and-neck patients. Target conformity, dose to the critical organs and dose to normal tissues were recorded and evaluated. Although the 3-mm MLC resulted in better target conformity (mean difference of 7.7% over 5-mm MLC and 12.7% over 10-mm MLC) over other MLCs for small-volume targets, it increased the total monitor units of the plans. No appreciable differences in terms of target conformity, organ at risk and normal-tissue sparing were observed between the 5-mm and 10-mm MLCs for large-volume targets. The effect of MLC leaf width was not quantifiably different in sliding window and step and shoot techniques. In addition, we observed that there was no additional benefit to the sliding-window (SW) technique when compared to the step-shoot (SS) technique as a result of reduction of MLC leaf width.

Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array.

The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams.

Investigation of Inter and Intra-fractional Uncertainties in Lung IMRT delivery

Changes in the tumour position due to inter and intra-fractional motion introduce uncertainty in IMRT delivery. In this work, we experimentally evaluated the effect of inter and intra-fractional motion uncertainties in the IMRT delivery. A lung patient was planned with five field dynamic IMRT for a total dose of 60Gy in 30 fractions using 6MV photon beam at a dose rate of 400MU/min. The plan was delivered on Varian CL2100CD Linear Accelerator with millennium 120 MLC for a single fraction and measured using IMRT MatriXX placed on Quasar motion platform, aligned with respect to isocentre for planar as well as point dose measurements. This measurement was summated for thirty fractions and taken as reference. Thirty measurements were performed for each inter-fractional, intra-fractional and combined effect. The setup errors (Mean±SD) used to simulate the inter-fractional displacements were RL: -0.10±0.27mm, SI: 0.17±0.45mm, AP: -0.04±0.38mm and Rot: 0.02±0.86 degree. The intra-fractional motion was simulated using the motion platform parallel to the MLC leaf motion, for amplitude of 1cm and period of 4s. The planner fluence of inter, intra-fractional motion and combination of both was analyzed against the reference using gamma criteria of 3%/3mm. Similarly the point dose measurements were also compared. The maximum deviation in point dose during a single fraction was -3.8%, 3.1% and -5.9% for interfractional, intra-fractional and combined respectively. The same resulted in deviation of -0.4%, -0.3% and 0.1% respectively when summated for 30 fractions. The percentage of pixels failing the gamma criteria during a single fraction was 28.0%, 16.4% and 28.5% for inter-fractional, intra-fractional and combined respectively. The same revealed 11.4%, 12.4% and 19.2% respectively when summated. Though the point dose deviations were nullified over thirty fractions, the planner fluence variation was observed to be considerable.

Synchronization of Intra-fractional Motion in Dynamic IMRT Delivery

One of the limitations of breath-hold and gated treatments is the prolongation of treatment time. Hence, it is noteworthy to incorporate the intra-fractional motion into the IMRT delivery without holding or gating the beam. The purpose of this study was to develop a method for synchronization of intra-fractional motion in dynamic IMRT delivery. An in-house program was developed in MATLABTM. A dynamic MLC (DMLC) file was imported into the program from the Eclipse treatment planning system (TPS). The program modifies the DMLC file by incorporating the target motion. In this study, target motion was assumed to have amplitude of 2cm and period of 4s. The modified DMLC file was sent for delivery to the linear accelerator (CL2100CD) equipped with millennium 120 MLC. Dosimetric measurements were carried out using IMRT MatriXX 2D array device which was placed over the QUASAR motion platform. In order to evaluate the accuracy of synchronization of intra-fractional motion in delivery (SIMD), the fluence of TPS DMLC file was measured using static detector (no motion). The IMRT MatriXX was moved in cranio/caudal direction (parallel to MLC leaf motion) for the above assumed target motion. The fluence of SIMD was measured for this moving detector using the modified DMLC file. Further, in order to show the benefit of SIMD, the fluence of non-SIMD was also measured, using the original TPS DMLC file which was delivered to the moving detector. The gamma evaluation criterion of 3% / 3mm was used to compare the SIMD and non-SIMD with static delivery. The percentage of pixels passing the criteria in gamma evaluation was 94.4% for SIMD and 63.17% for non-SIMD. This demonstrates that SIMD was able to compensate the intra-fractional motion and could deliver the fluence similar to static.

Investigating the electronic portal imaging device for small radiation field measurements

Purpose: With the advent of state-of-the-art treatment technologies, the use of small fields has increased, and dosimetry in small fields is highly challenging. In this study, the potential use of Varian electronic portal imaging device (EPID) for small field measurements was explored for 6 and 15 MV photon beams. Materials and Methods: The output factors and profiles were measured for a range of jaw-collimated square field sizes starting from 0.8 cm×0.8 cm to 10 cm×10 cm using EPID. For evaluation purpose, reference data were acquired using Exradin A16 microionization chamber (0.007 cc) for output factors and stereotactic field diode for profile measurements in a radiation field analyzer. Results: The output factors of EPID were in agreement with the reference data for field sizes down to 2 cm×2 cm and for 2 cm×2 cm; the difference in output factors was +2.06% for 6 MV and +1.56% for 15 MV. For the lowest field size studied (0.8 cm×0.8 cm), the differences were maximum; +16% for 6 MV and +23% for 15 MV photon beam. EPID profiles of both energies were closely matching with reference profiles for field sizes down to 2 cm×2 cm; however, penumbra and measured field size of EPID profiles were slightly lower compared to its counterpart. Conclusions: EPID is a viable option for profile and output factor measurements for field sizes down to 2 cm×2 cm in the absence of appropriate small field dosimeters.

Computational Model to Simulate the Interplay Effect in dynamic IMRT delivery

The purpose of this study was to develop and experimentally verify a patient specific model for simulating the interplay effect in a DMLC based IMRT delivery. A computational model was developed using MATLAB program to incorporate the interplay effect in a 2D beams eye view fluence of dynamic IMRT fields. To simulate interplay effect, the model requires two inputs: IMRT field (DMLC file with dose rate and MU) and the patient specific respiratory motion. The interplay between the DMLC leaf motion and target was simulated for three lung patients. The target trajectory data was acquired using RPM system during the treatment simulation. The model was verified experimentally for the same patients using Imatrix 2D array device placed over QUASAR motion platform in CL2100 linac. The simulated fluences and measured fluences were compared with the TPS generated static fluence (no motion) using an in-house developed gamma evaluation program (2%/2mm). The simulated results were well within agreement with the measured. Comparison of the simulated and measured fluences with the TPS static fluence resulted 55.3% & 58.5% pixels passed the gamma criteria. A patient specific model was developed and validated for simulating the interplay effect in the dynamic IMRT delivery. This model can be clinically used to quantify the dosimetric uncertainty due to the interplay effect prior to the treatment delivery.

Magnitude, impact, and management of respiration-induced target motion in radiotherapy treatment: A comprehensive review

Tumors in thoracic and upper abdomen regions such as lungs, liver, pancreas, esophagus, and breast move due to respiration. Respiration-induced motion introduces uncertainties in radiotherapy treatments of these sites and is regarded as a significant bottleneck in achieving highly conformal dose distributions. Recent developments in radiation therapy have resulted in (i) motion-encompassing, (ii) respiratory gating, and (iii) tracking methods for adapting the radiation beam aperture to account for the respiration-induced target motion. The purpose of this review is to discuss the magnitude, impact, and management of respiration-induced tumor motion.

Investigating different computed tomography techniques for internal target volume definition

PURPOSE The aim of this work was to evaluate the various computed tomography (CT) techniques such as fast CT, slow CT, breath-hold (BH) CT, full-fan cone beam CT (FF-CBCT), half-fan CBCT (HF-CBCT), and average CT for delineation of internal target volume (ITV). In addition, these ITVs were compared against four-dimensional CT (4DCT) ITVs. MATERIALS AND METHODS Three-dimensional target motion was simulated using dynamic thorax phantom with target insert of diameter 3 cm for ten respiration data. CT images were acquired using a commercially available multislice CT scanner, and the CBCT images were acquired using On-Board-Imager. Average CT was generated by averaging 10 phases of 4DCT. ITVs were delineated for each CT by contouring the volume of the target ball; 4DCT ITVs were generated by merging all 10 phases target volumes. Incase of BH-CT, ITV was derived by boolean of CT phases 0%, 50%, and fast CT target volumes. RESULTS ITVs determined by all CT and CBCT scans were significantly smaller (P < 0.05) than the 4DCT ITV, whereas there was no significant difference between average CT and 4DCT ITVs (P = 0.17). Fast CT had the maximum deviation (-46.1% ± 20.9%) followed by slow CT (-34.3% ± 11.0%) and FF-CBCT scans (-26.3% ± 8.7%). However, HF-CBCT scans (-12.9% ± 4.4%) and BH-CT scans (-11.1% ± 8.5%) resulted in almost similar deviation. On the contrary, average CT had the least deviation (-4.7% ± 9.8%). CONCLUSIONS When comparing with 4DCT, all the CT techniques underestimated ITV. In the absence of 4DCT, the HF-CBCT target volumes with appropriate margin may be a reasonable approach for defining the ITV.

SU‐E‐T‐522: Evaluation of EDW and Sliding Window IMRT in the Presence of Organ Motion with Gating

Purpose: To investigate the interplay effect of intrafractional motion when using Enhanced Dynamic Wedge (EDW) and Intensity Modulated Radiotherapy(IMRT) with gated delivery. Methods: EDW of angles 30, 45 & 60 (field size 10×10sq.cm) and sliding window IMRT bench‐mark fluences were delivered using 6MV beams from a Varian CL2100CD with millennium 120 MLC at 100, 400 and 600MU/min dose‐rates for static and gated operation using RPM. The IMRT MatriXX system and 0.6cc ionization chamber were placed at 5 cm depth on the Quasar motion platform to measure 2D fluence and point doses. The motion platform was programmed for breathing frequencies 0.125, 0.17 and 0.25Hz with the jaw/MLC movement in the same direction. During the gated mode, duty cycles were 25%, 50%, 75% and 100% (un‐gated). The 2D fluence comparison and variation in point dose of gated operation with static measurement for various breathing frequencies, duty cycles and dose rates was performed and fluence comparisons analyzed using gamma criteria of 3%/3mm. Results: Fluence and point‐dose deviations of EDW and IMRT increased with increasing duty cycle and breathing frequency. The gamma evaluation of the 25% duty cycle was found to be closest to the static measurements and the point dose deviation was <2%, while for larger duty cycles, the deviations were upto 15% which increased with increasing EDW angles, although with no apparent relationship with the dose rate. At lower dose rates, more number of pixels passed the gamma evaluation criteria. The deviations for 100% duty cycle with breathing frequency of 0.25Hz and dose rate of 100MU/min were −5.7%, −8.9% and −15.8% for 30, 45 and 60 EDW angles respectively. Conclusions: The interplay effect demonstrates considerable error in the delivery of dynamic intensity modulated treatments, which is reduced by appropriate gating. Acknowledgement: Department of Science & Technology ‐ Grant No: IR/SO/LS 02/2003, Government of India