Dosimetric Comparison of Robotic and Linear Accelerator Multi-Leaf Collimator-Based Stereotactic Radiosurgery for Arteriovenous Malformation.

PURPOSE: To investigate the dosimetric comparison of different collimators which are used in robotic radiosurgery (cyberknife-CK) and linear accelerator (LINAC) for stereotactic radiosurgery (SRS) in arteriovenous malformation (AVM). MATERIALS AND METHODS: Twenty-five AVM patients were planned in CK using FIXED cone, IRIS collimator, and multi-leaf collimator (MLC) based in LINAC. Dosimetric comparison was performed using Paddick conformity index (CIPaddick) and International Commission on Radiation Units and measurements (ICRU) homogeneity index (HIICRU), gradient score (GS), normal brain dose received by 10cc (D10cc) and critical structure (brain stem, optic chiasma, optic nerves) doses. Paired sample t-test was used for statistical analysis. RESULTS: Mean treatment volume was 3.16cc (standard deviation ± 4.91cc). No significant deviation (P =0.45, 0.237 for FIXED vs. IRIS and FIXED vs. MLC, respectively) was found in target coverage. For CIPaddick, the mean difference (MD) between FIXED- and MLC-based plans was 0.16(P = 0.001); For HIICRU, difference between FIXED and IRIS was insignificant (0.5, P = 0.823); but, when FIXED versus MLC, the deviation was 7.99% (P = 0.002). In FIXED- and MLC-based plans, significant difference was found in GS70 and GS40 (P < 0.041 and 0.005, respectively). MD between FIXED- and MLC-based plans for normal brain for 5Gy, 10Gy, 12Gy, and 20Gy were 36.08cc (P = 0.009), 7.12cc (P = 0.000), 5.84cc (P = 0.000) and 1.56cc (P = 0.000), respectively. AVM volume <0.7cc should be treated with CK FIXED and >0.7cc were treated by using FIXED or IRIS collimators. AVM volume > 1.4cc can be treated by either LINAC MLC-based SRS or CK. CONCLUSION: Our study shows CK collimator (IRIS and FIXED) could be able to treat brain AVMs in any size. Linac MLC-based SRS has some limitations in terms of conformity and low-dose spillage, and advantages like reduced treatment time and MU.

Validation of a Software Upgrade in a Monte Carlo Treatment Planning System by Comparison of Plans in Different Versions.

PURPOSE: Validation of a new software version of a Monte Carlo treatment planning system through comparing plans generated by two software versions in volumetric-modulated arc therapy (VMAT) for lung cancer. MATERIALS AND METHODS: Three patients who were treated with 60 Gy/30 fractions in Elekta Synergy™ linear accelerator by VMAT technique with 2% statistical uncertainty (SU) were chosen for the study. Multiple VMAT plans were generated using two different software versions of Monaco treatment planning system TPS (V5.10.02 and V5.11). By keeping all other parameters constant, originally accepted plans were recalculated for the SUs of 0.5%, 1%, 2%, 3%, 4%, and 5%. For plan evaluation, the metrics compared were conformity Index (CI), homogeneity Index (HI), dose coverage to planning target volume (PTV), organ at risk (OAR) doses to spinal cord, pericardium, bilateral lungs-PTV, esophagus, liver, normal tissue integral dose (NTID), volumes receiving dose >5 and >10 Gy, calculation time (tCT), and gamma pass rates. RESULTS: In both versions, CI and HI improved as the SU increased from 0.5% to 5%. No significant dose difference was observed in Dmean to PTV, bilateral lungs-PTV, pericardium, esophagus, liver, normal tissue volume receiving >5, and >10 Gy and NTID. It was observed that while the tCT and gamma pass rates decreased, the maximum dose to PTV increased as the SU increased. No other significant dose differences were observed between the two MC versions compared. CONCLUSION: For lung VMAT plans, in both versions, SU could be accepted up to 3% per plan with reduced tCT without compromising plan quality and deliverability by accepting variations in point dose and an inhomogeneous dose within the target. The plan quality of Monaco™V5.10.02 was similar to Monaco™TPS-V5.11 except for tCT.

SU-E-T-597: Influence of Smoothing Parameters on Dynamic IMRT Plan Quality and Deliverability.

PURPOSE: To study the impact of different smoothing parameters on IMRT plan quality and deliverabilityMethods: Five previously treated patients of carcinoma cervix were chosen. Planning target volume (PTV) and organ at risk (OAR) i.e. bladder and rectum were contoured. In each case, five different dynamic IMRT plans with 6MV photon beam were created in eclipse TPS for Varian 2300C/D linear accelerator. During optimization, dose volume constraints and priorities were kept constant and smoothing parameters were varied as follows: 10/5, 40/30 (TPS default value), 80/60, 100/80 and 200/150 in x/y direction. Total dose was 5040cGy in 28 fractions and prescribed at 95% isodose. Plan quality was analyzed by means of coverage index (CI=PTV covered by prescription dose/PTV), OAR mean doses and total monitor units (MUs) required to deliver a plan. In each case, deliverability of treatment plans were verified with I'matriXX ion-chamber array and compared with TPS dose-plane using gamma index of 3% dose difference and 3mm distance to agreement criteria. RESULTS: The CI values were 0.9435±0.032, 0.9418±0.034, 0.9380±0.041, 0.9330±0.047 and 0.8681±0.072 for 10/5, 40/30, 80/60, 100/80 and 200/150 in x/y direction. PTV dose maximum decreases with the increase of smoothing parameters and values were 5724.38±106.08 5723.30±131.60, 5708.44±1 16.74, 5697.92±116.82 and 5587.50±189.50cGy. The bladder mean doses were 4027.46±630.40, 3821.62±420.62, 3819.58±427.08, 3813.42±435.02 and 3814.78±438.0cGy. Rectum mean doses were 3839.88±466.02, 3835.52±473.18, 3837.52±472.88, 3839.10±471.20 and 3918.94±469.76cGy. Similarly, Total MUs were 1588±205, 1573±214, 1513±274, 1456±335 and 1219±68. Gamma pass rate increases with the increase of smoothing parameters and values were 99.16±0.21%, 99.07±0.19%, 99.24±0.28%, 99.29±0.29% and 99.75±0.15%. CONCLUSIONS: When smoothing parameters decreased below TPS default value, plan quality increases, but deliverability decreases. If smoothing parameters increased above TPS default value, deliverability increases but plan quality decreases. Total MU decreases with the increase of smoothing parameters. Therefore, it's a trade-off between plan quality and deliverability which needs to be justified clinically.

SU-E-T-587: Whole IMRT, Hybrid IMRT and 3D Conformal Plan a Dosimetric Comparison for Large Target.

PURPOSE: To dosimetrically compare the whole-IMRT, hybrid-IMRT (combination of IMRT and 3D-CRT) and 3D-conformal radiotherapy (3D- CRT) plans for larger targets. METHODS: Five previously treated patients of carcinoma cervix with para-aortic lymph-nodes (target length 33-34cm) were selected. PTV-P (PTV-Primary), PTV-PA (PTV-para-aortic) and organ at risks (OARs) were defined. Three plans were generated using Eclipse TPS for Varian CL2300C/D linear accelerator using 6MV photon beam. Three plans were: (i) Whole-IMRT: IMRT for both PTV-P and PTV-PA (ii) Hybrid-IMRT: IMRT for PTV-P and 3D-CRT for PTV-PA (iii) 3D-CRT: 3D-CRT for both PTV-P and PTV-PA. Prescription dose for PTV-P is 50.4Gy and PTV-PA is 45Gy in 28 fractions. Coverage index (CI=Target volume covered by prescription dose/Target volume), mean doses to bladder, rectum and bowel were used for plan comparison by using DVH. Integral dose (liter-Gray) to normal tissue (i.e., patient volume minus PTV-P and PTV-PA) and total monitor units (MUs) required to deliver a plan was also noted. RESULTS: The CI for PTV-P is 0.98±0.20, 0.96±0.09, and 0.95±0.01 for Whole-IMRT, Hybrid-IMRT and 3D-CRT plan and for PTV- PA is 0.98±0.01, 0.98±0.01, and 0.97±0.20. Maximum doses to PTV-P are 5660.85±90.85cGy, 5640.35±70.35cGy and 5813.80±97.40cGy. Maximum doses to PTV-PA are 5000.60±109.10cGy, 5079.85±20.25cGy and 5092.25±19.75cGy. Mean doses to the bladder are 3810±225.80cGy, 3842.10±182.70cGy and 5204±98.25cGy for Whole-IMRT, Hybrid-IMRT and 3D-CRT plan, respectively. Mean doses to rectum are 3955.35±324.95cGy, 3971.15±354.15cGy and 4741.20±371.60cGy. Mean doses to bowel are 2623.35±320.85cGy, 2855.30±371.05cGy and 3011.7±433.80cGy. Average MUs required to deliver one fraction is 1285±87, 1585±186, 485±46 for Whole-IMRT, Hybrid-IMRT and 3D-CRT plans, respectively. Higher integral doses to normal tissue were observed for whole-IMRT (267.60±76 liter-Gy) followed by hybrid-IMRT (259.20±53 liter-Gy) and 3D-CRT (186.30±33 liter-Gy). CONCLUSIONS: Whole-IMRT is useful for larger targets compared to hybrid-IMRT in terms of dose conformity, lesser MUs and reduced critical organ doses with little compromise on integral dose, where 3D-CRT sacrificed the OAR sparing.

Dosimetric characteristics of a MOSFET dosimeter for clinical electron beams.

The fundamental dosimetric characteristics of commercially available metal oxide semiconductor field effect transistor (MOSFET) detectors were studied for clinical electron beam irradiations. MOSFET showed excellent linearity against doses measured using an ion chamber in the dose range of 20-630cGy. MOSFET reproducibility is better at high doses compared to low doses. The output factors measured with the MOSFET were within +/-3% when compared with those measured with a parallel plate chamber. From 4 to 12MeV, MOSFETs showed a large angular dependence in the tilt directions and less in the axial directions. MOSFETs do not show any dose-rate dependence between 100 and 600MU/min. However, MOSFETs have shown under-response when the dose per pulse of the beam is decreased. No measurable effect in MOSFET response was observed in the temperature range of 23-40 degrees C. The energy dependence of a MOSFET dosimeter was within +/-3.0% for 6-18MeV electron beams and 5.5% for 4MeV ones. This study shows that MOSFET detectors are suitable for dosimetry of electron beams in the energy range of 4-18MeV.