RapidArc Plans Research Articles

SU‐F‐P‐37: Implementation of An End‐To‐End QA Test of the Radiation Therapy Imaging, Planning and Delivery Process to Identify and Correct Possible Sources of Deviation

Purpose:To implement an end‐to‐end QA test of the radiation therapy imaging, planning and delivery process, aimed to assess the dosimetric agreement accuracy between planned and delivered treatment, in order to identify and correct possible sources of deviation. To establish an internal standard for machine commissioning acceptance.Methods:A test involving all steps of the radiation therapy: imaging, planning and delivery process was designed. The test includes analysis of point dose and planar dose distributions agreement between TPS calculated and measured dose. An ad hoc 16 cm diameter PMMA phantom was constructed with one central and four peripheral bores that can accommodate calibrated electron density inserts. Using Varian Eclipse 10.0 and Elekta XiO 4.50 planning systems, IMRT, RapidArc and 3DCRT with hard and dynamic wedges plans were planned on the phantom and tested. An Exradin A1SL chamber is used with a Keithley 35617EBS electrometer for point dose measurements in the phantom. 2D dose distributions were acquired using MapCheck and Varian aS1000 EPID.Gamma analysis was performed for evaluation of 2D dose distribution agreement using MapCheck software and Varian Portal Dosimetry Application.Varian high energy Clinacs Trilogy, 2100C/CD, 2000CR and low energy 6X/EX where tested.TPS‐CT# vs. electron density table were checked for CT‐scanners used.Results:Calculated point doses were accurate to 0.127% SD: 0.93%, 0.507% SD: 0.82%, 0.246% SD: 1.39% and 0.012% SD: 0.01% for LoX‐3DCRT, HiX‐3DCRT, IMRT and RapidArc plans respectively. Planar doses pass gamma 3% 3mm in all cases and 2% 2mm for VMAT plans.Conclusion:Implementation of a simple and reliable quality assurance tool was accomplished. The end‐to‐end proved efficient, showing excellent agreement between planned and delivered dose evidencing strong consistency of the whole process from imaging through planning to delivery. This test can be used as a first step in beam model acceptance for clinical use.

SU‐F‐T‐284: The Effect of Linear Accelerator Output Variation On the Quality of Patient Specific Rapid Arc Verification Plans

Purpose:The aim of the current study is to investigate the effect of machine output variation on the delivery of the RapidArc verification plans.Methods:Three verification plans were generated using Eclipse™ treatment planning system (V11.031) with plan normalization value 100.0%. These plans were delivered on the linear accelerators using ArcCHECK− device, with machine output 1.000 cGy/MU at calibration point. These planned and delivered dose distributions were used as reference plans. Additional plans were created in Eclipse− with normalization values ranging 92.80%–102% to mimic the machine output ranging 1.072cGy/MU‐0.980cGy/MU, at the calibration point. These plans were compared against the reference plans using gamma indices (3%, 3mm) and (2%, 2mm). Calculated gammas were studied for its dependence on machine output. Plans were considered passed if 90% of the points satisfy the defined gamma criteria.Results:The gamma index (3%, 3mm) was insensitive to output fluctuation within the output tolerance level (2% of calibration), and showed failures, when the machine output exceeds ≥3%. Gamma (2%, 2mm) was found to be more sensitive to the output variation compared to the gamma (3%, 3mm), and showed failures, when output exceeds ≥1.7%. The variation of the gamma indices with output variability also showed dependence upon the plan parameters (e.g. MLC movement and gantry rotation). The variation of the percentage points passing gamma criteria with output variation followed a non‐linear decrease beyond the output tolerance level.Conclusion:Data from the limited plans and output conditions showed that gamma (2%, 2mm) is more sensitive to the output fluctuations compared to Gamma (3%,3mm). Work under progress, including detail data from a large number of plans and a wide range of output conditions, may be able to conclude the quantitative dependence of gammas on machine output, and hence the effect on the quality of delivered rapid arc plans.

SU-F-T-571: Objective Assessment of 3D Dosimetry for Flattened and Flattened Filter Free Stereotactic Rotational Delivery Using 729-Array Detector with Octavius 4D Phantom

Purpose: The purpose of this study was to assess the potential of 3D dosimetry for flattened and flattened filter free stereotactic rotational delivery in high definition MLC using 729-detector array with Octavius 4D phantom Methods: Twenty rapid arc plans were assessed for this study. For each patient two plans for 6X and 6FFF photon beams were generated with same prescription and critical organ constraints in Eclipse TPS version 13.0 using high definition MLC. Verification plans were generated in scanned Octavius 4D phantom in TPS. 3D dose measurements were collected from 729-ion chamber detector array in Octavius 4D phantom using verisoft software v 6.0. TPS calculated dose was compared with measured 3D dose in verisoft using the following gamma analysis parameters such as 3D volumetric, 3D planar and 2D global gamma in transverse, sagittal and coronal planes for 3mm/3% and 2mm/2% distance to agreement criteria.Passing rate and arithmetic mean of global gamma were analysed for 2D and 3D global gamma in all planes. Results: The average number of dose points passing rate for 2D global gamma with 3mm/3% criteria in transverse, sagittal and coronal planes was 99.06%±2.89%, 98.8%±0.88% and 99.06%±91%, respectively. For 2mm/2% criteria 97.86%±2.26%, 94.49± 2.64% and 94.34%±2.9% was observed. In 3D planar global gamma with 3mm/3% was 99.53%±0.49%, 98.93%±1.03% and 99.29%±1.29%, for 2mm 2% criteria was 97.50%±2.24%, 94.5%±2.5% and 95.38%±4.5%. The maximum arithmetic mean gamma deviation of 0.505%±0.13% was observed in coronal plane for 2D global gamma with 2mm/2% criteria. The 3D volumetric gamma passing rate was observed as 99.61%±0.433% for 3mm /3% and 95.91%±2.51% for 2mm/2%. Conclusion: The objective assessment of 3D dosimetry have demonstrated that the rotational delivery accuracy for flattened and flattened filter free stereotactic plans can be verified by using Octavius system comprising with 729 ion chamber array and Octavius 4D phantom.

SU-F-T-607: Study of Dose Falloff Slope in RapidArc Planning of Lung SBRT

Purpose: SBRT of lung tumor results in steep dose fall-off outside the PTV to limit the normal tissue dose. RTOG report 0915 recommended metrics for plan quality measure, CI, D2cm, R50% and HDloc were evaluated. This study was aimed at ascertaining moderate to low dose spillage using 5 mm ring structures outside the PTV. Methods: In this retrospective study, 41 clinical RapidArc lung SBRT plans (Eclipse TPS ver 8.9) previously treated at our institution in a TrueBeam STx linac were analyzed. The delivered plans were re-planed using either 2 or 3 complete or partial arcs in coplanar or non-coplanar arrangement with upto ±15 degree couch angle. Seven concentric 5 mm wide rings were created outside the PTV for evaluation of dose fall-off based on mean, maximum doses as a function of distance. Mean and maximum percent dose fall-off in the rings (% dose/mm) were compared between the delivered and research plans. The differences in the mean and maximum percent doses between coplanar and non-coplanar arc plans were studied for any significant differences. Results: Mean dose falloff in the rings outside the PTV followed an exponential curve. For all the patients, interestingly the slope of maximum dose falloff increased at the intersection of 1st and 2nd ring and monotonically fell at outer rings. Altering coplanar to non-coplanar arc arrangements had a minimal impact on the CI as well as D2cm, R50% values (p-value > 0.05). Conclusion: Slope of dose falloff from the PTV surface could serve as an additional metric. Coplanar arc plan had slightly steep dose falloff outside the PTV than non-coplanar arcs, but the differences are statistically insignificant.

SU-F-T-584: Investigating Correction Methods for Ion Recombination Effects in OCTAVIUS 1000 SRS Measurements

Purpose: PTW's Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers (LICs). Collection efficiencies of LICs have been shown to change during IMRT delivery as a function of LINAC pulse frequency and pulse dose, which affects QA results. In this study, two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. Methods: For the first correction, Matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction, MU/min in daily 1000 SRS calibration was chosen to match average MU/min of the VMAT plan. Usefulness of derived corrections were evaluated using 6MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam equipped with an HD- MLC. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. Results: After complex Matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex Matlab corrections. Conclusion: Ion recombination effects can potentially be clinically significant for OCTAVIUS 1000 SRS measurements, especially for higher pulse dose unflattened beams when using tighter gamma tolerances. Matching daily 1000 SRS calibration MU/min to average planned MU/min is a simple correction that greatly reduces ion recombination effects, improving measurements accuracy and gamma pass rates. This work was supported by PTW.

Dosimetric and radiobiological impact of intensity modulated proton therapy and RapidArc planning for high-risk prostate cancer with seminal vesicles.

IntroductionThe purpose of this study was to evaluate the dosimetric and radiobiological impact of intensity modulated proton therapy (IMPT) and RapidArc planning for high‐risk prostate cancer with seminal vesicles.MethodsTen high‐risk prostate cancer cases were included in this retrospective study. For each case, IMPT plans were generated using multiple field optimisation (MFO) technique (two fields) with XiO treatment planning system (TPS), whereas RapidArc plans were generated using double‐arc technique (two full arcs) with Eclipse TPS. IMPT and RapidArc plans were optimised for a total prescription dose of 79.2 Gy (relative biological effectiveness (RBE)) and 79.2 Gy, respectively, using identical dose–volume constraints. IMPT and RapidArc plans were then normalised such that at least 95% of the planning target volume (PTV) received the prescription dose.ResultsThe mean and maximum PTV doses were comparable in IMPT plans (80.1 ± 0.3 Gy (RBE) and 82.6 ± 1.0 Gy (RBE) respectively) and RapidArc plans (80.3 ± 0.3 Gy and 82.8 ± 0.6 Gy respectively) with P = 0.088 and P = 0.499 respectively. The mean doses of the rectum and bladder were found to be significantly lower in IMPT plans (16.9 ± 5.8 Gy (RBE) and 17.5 ± 5.4 Gy (RBE) respectively) when compared to RapidArc plans (41.9 ± 5.7 Gy and 32.5 ± 7.8 Gy respectively) with P < 0.000 and P < 0.000 respectively. For the rectum, IMPT produced lower V30 (21.0 ± 9.6% vs. 68.5 ± 10.0%; P < 0.000), V50 (14.3 ± 5.8% vs. 45.0 ± 10.0%; P < 0.000) and V70 (6.9 ± 3.4% vs. 12.8 ± 3.6%; P < 0.000) compared to RapidArc. For the bladder, IMPT produced lower V30 (23.2 ± 7.0% vs. 50.9 ± 15.6%; P < 0.000) and V50 (16.6 ± 5.4% vs. 25.1 ± 9.6%; P = 0.001), but similar V70 (9.7 ± 3.5% vs. 10.5 ± 4.2%; P = 0.111) compared to RapidArc. RapidArc produced lower mean dose for both the right femoral head (19.5 ± 4.2 Gy vs. 27.4 ± 4.5 Gy (RBE); P < 0.000) and left femoral head (18.0 ± 4.3 Gy vs. 28.0 ± 5.6 Gy (RBE); P < 0.000). Both IMPT and RapidArc produced comparable bladder normal tissue complication probability (NTCP) (0.6 ± 0.2% vs. 0.5 ± 0.2%; P = 0.152). The rectal NTCP was found to be lower using IMPT (0.8 ± 0.7%) than using RapidArc (1.7 ± 0.7%) with P < 0.000.ConclusionBoth IMPT and RapidArc techniques provided comparable mean and maximum PTV doses. For the rectum, IMPT produced better dosimetric results in the low‐, medium‐ and high‐dose regions and lower NTCP compared to RapidArc. For the bladder, the NTCP and dosimetric results in the high‐dose region were comparable in both sets of plans, whereas IMPT produced better dosimetric results in the low‐ and medium‐dose regions.

The dosimetric impact of target volume delineation variation for cervical cancer radiotherapy

BackgroundCervical cancer inter-observer delineation variation has been demonstrated. This article addresses its dosimetric impact. Methods21 centres outlined two INTERLACE trial quality assurance test cases. A gold standard clinical target volume (GSCTV) was created from a consensus and STAPLE outline. RapidArc plans were created for all centres’ planning target volumes (PTVs; PTV1+2). Gold standard PTVs (GSPTVs) were created for each plan by applying each centre’s CTV–PTV margins to GSCTV. DVH parameters including D95% and Dmean for each PTV1+2 and GSPTV were compared, representing planned versus GSPTV delivered dose. PTV1+2 and GSPTV V95% was also calculated. ResultsReviewing all parameters, no plans achieved acceptable GSPTV coverage. GSPTV V95%⩾95% was not achieved for any plan. GSPTV V95%<90% in 15/21 (case 1) and 14/22 (case 2) and <80% in 2 plans from both cases. GSPTV V95% is on average 10–15% lower than planned and GSPTV D95% is 10–20% lower than planned. Most common GSCTV anatomical areas not receiving 95% dose were vagina, obturator and external iliac nodes and, in case 1, the superior nodal aspect. ConclusionCervical cancer CTV delineation variation leads to significant reductions in dose delivered to GSPTV. This highlights the ongoing importance of standardising delineation in the IMRT era.

Investigating ion recombination effects in a liquid-filled ionization chamber array used for IMRT QA measurements.

PTW's Octavius 1000 SRS array performs IMRT quality assurance (QA) measurements with liquid-filled ionization chambers (LICs) to allow closer detector spacing and higher resolution, compared to air-filled QA devices. However, reduced ion mobility in LICs relative to air leads to increased ion recombination effects and reduced collection efficiencies that are dependent on Linac pulse frequency and pulse dose. These pulse parameters are variable during an IMRT delivery, which affects QA results. In this study, (1) 1000 SRS collection efficiencies were measured as a function of pulse frequency and pulse dose, (2) two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. To obtain collection efficiencies, the OCTAVIUS 1000 SRS was used to measure open fields of varying pulse frequency, pulse dose, and beam energy with results normalized to air-filled chamber measurements. Changes in ratios of 1000 SRS to chamber measured dose were attributed to changing collection efficiencies, which were then correlated to pulse parameters using regression analysis. The usefulness of the derived corrections was then evaluated using 6 MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam (Varian Medical Systems) linear accelerator equipped with a high definition multileaf collimator. For the first correction, matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency, and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction the MU/min in the daily 1000 SRS calibration was chosen to match the average MU/min of the volumetric modulated arc therapy plan. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. Collection efficiencies correlated linearly to pulse dose, while correlations with pulse frequency were less defined, generally increasing as pulse frequency decreased. After complex matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6 MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6 MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex matlab corrections. OCTAVIUS 1000 SRS ion recombination effects have little effect on 6 MV measurements. However, the effect could potentially be clinically significant for higher pulse dose unflattened beams when using tighter gamma tolerances, especially when small aperture sizes are used, as is common for SRS/SBRT. In addition, ion recombination effects are strongly correlated to changing MU/min, therefore MU/min used in daily 1000 SRS calibrations should be matched to the expected average MU/min of the IMRT plan.

PO-0820: Volumetric quality assurance of RapidArc plans for multiple intracranial targets using gel dosimetry

PO-0820: Volumetric quality assurance of RapidArc plans for multiple intracranial targets using gel dosimetry

Dosimetric study of RapidArc plans and conventional intensity modulated radiotherapy for prostate cancer involving seminal vesicles and pelvis lymph nodes

Purpose: The main purpose of this study is to (1) identify the continual diversity between conventional fixed field intensity modulation radiotherapy (IMRT) and RapidArc (RA) for high-risk prostate cancer; and (2) determine potential benefits and drawbacks of using for this type of treatment. Methods: A cohort of 20 prostate cases including prostate, seminal vesicles and pelvic lymph nodes was selected for this study. The primary planning target volume (PTV P ) and boost planning target volume (PTV B ) were contoured. The total prescription dose was 75.6 Gy (45 Gy to PTV P and an additional 21.6 Gy to PTV B ). Two plans were generated for each PTV: multiple 7-fields for IMRT and two arcs for RA. Results: A Sigma index (IMRT: 2.75 ± 0.581; RA: 2.8 ± 0.738) for PTV P and (IMRT: 2.0 ± 0.484; RA: 2.1 ± 0.464) for PTV B indicated similar dose homogeneity inside the PTV. Conformity index (IMRT: 0.96 ± 0.047; RA: 0.95 ± 0.059) for PTV P and (IMRT: 0.97 ± 0.015; RA: 0.96 ± 0.014) for PTV B was comparable for both the techniques. IMRT offered lower mean dose to organ at risks (OARs) compared to RA plans. Normal tissue integral dose in IMRT plan resulted 0.87% lower than RA plans. All the plans displayed significant increase (2.50 times for PTV P and 1.72 for PTB B ) in the average number of necessary monitor units (MUs) with IMRT beam. Treatment delivery time of RA was 2 ‒ 6 minutes shorter than IMRT treatment. Conclusion: For PTV including pelvic lymph nodes, seminal vesicles and prostate, IMRT offered a greater degree of OARs sparing. For PTV including seminal vesicles and prostate, RA with two arcs provided comparable plan with IMRT. RA also improved the treatment efficiency due to smaller number of MUs required.

Plan Quality and Treatment Efficiency for Radiosurgery to Multiple Brain Metastases: Non-Coplanar RapidArc vs. Gamma Knife.

This study compares the dosimetry and efficiency of two modern radiosurgery [stereotactic radiosurgery (SRS)] modalities for multiple brain metastases [Gamma Knife (GK) and LINAC-based RapidArc/volumetric modulated arc therapy], with a special focus on the comparison of low-dose spread. Six patients with three or four small brain metastases were used in this study. The size of targets varied from 0.1 to 10.5 cc. SRS doses were prescribed according to the size of lesions. SRS plans were made using both Gamma Knife(®) Perfexion and a single-isocenter, multiple non-coplanar RapidArc(®). Dosimetric parameters analyzed included RTOG conformity index (CI), gradient index (GI), 12 Gy isodose volume (V 12Gy) for each target, and the dose "spread" (Dspread) for each plan. Dspread reflects SRS plan's capability of confining radiation to within the local vicinity of the lesion and to not spread out to the surrounding normal brain tissues. Each plan has a dose (Dspread), such that once dose decreases below Dspread (on total tissue dose-volume histogram), isodose volume starts increasing dramatically. Dspread is defined as that dose when volume increase first exceeds 20 cc/0.1 Gy dose decrease. RapidArc SRS has smaller CI (1.19 ± 0.14 vs. 1.50 ± 0.16, p < 0.001) and larger GI (4.77 ± 1.49 vs. 3.65 ± 0.98, p < 0.01). V 12Gy results were comparable (2.73 ± 1.38 vs. 3.06 ± 2.20 cc, p = 0.58). Moderate to lower dose spread, V6, V4.5, and V3, were also equivalent. GK plans achieved better very low-dose spread (≤3 Gy) and also had slightly smaller Dspread, 1.9 vs. 2.5 Gy. Total treatment time for GK is estimated between 60 and 100 min. GK treatments are between 3 and 5 times longer compared to RapidArc treatment techniques. Dosimetric parameters reflecting prescription dose conformality (CI), dose fall off (GI), radiation necrosis indicator (V 12Gy), and dose spread (Dspread) were compared between GK SRS and RapidArc SRS for multi-mets. RapidArc plans have smaller CI but larger GI. V 12Gy are comparable. GK appears better at reducing only very low-dose spread (<3 Gy). The treatment time of RapidArc SRS is significantly reduced compared to GK SRS.

A.225 - Characteristics and criticalities of Octavius-4D detector system for IMRT and RapidArc plan verification

A.225 - Characteristics and criticalities of Octavius-4D detector system for IMRT and RapidArc plan verification

Dosimetric evaluation of 4 different treatment modalities for curative-intent stereotactic body radiation therapy for isolated thoracic spinal metastases

To investigate the dosimetric characteristics of 4 SBRT-capable dose delivery systems, CyberKnife (CK), Helical TomoTherapy (HT), Volumetric Modulated Arc Therapy (VMAT) by Varian RapidArc (RA), and segmental step-and-shoot intensity-modulated radiation therapy (IMRT) by Elekta, on isolated thoracic spinal lesions. CK, HT, RA, and IMRT planning were performed simultaneously for 10 randomly selected patients with 6 body types and 6 body + pedicle types with isolated thoracic lesions. The prescription was set with curative intent and dose of either 33Gy in 3 fractions (3F) or 40Gy in 5F to cover at least 90% of the planning target volume (PTV), correspondingly. Different dosimetric indices, beam-on time, and monitor units (MUs) were evaluated to compare the advantages/disadvantages of each delivery modality. In ensuring the dose-volume constraints for cord and esophagus of the premise, CK, HT, and RA all achieved a sharp conformity index (CI) and a small penumbra volume compared to IMRT. RA achieved a CI comparable to those from CK, HT, and IMRT. CK had a heterogeneous dose distribution in the target as its radiosurgical nature with less dose uniformity inside the target. CK had the longest beam-on time and the largest MUs, followed by HT and RA. IMRT presented the shortest beam-on time and the least MUs delivery. For the body-type lesions, CK, HT, and RA satisfied the target coverage criterion in 6 cases, but the criterion was satisfied in only 3 (50%) cases with the IMRT technique. For the body + pedicle-type lesions, HT satisfied the criterion of the target coverage of ≥90% in 4 of the 6 cases, and reached a target coverage of 89.0% in another case. However, the criterion of the target coverage of ≥90% was reached in 2 cases by CK and RA, and only in 1 case by IMRT. For curative-intent SBRT of isolated thoracic spinal lesions, RA is the first choice for the body-type lesions owing to its delivery efficiency (time); the second choice is CK or HT; HT is the preferential choice for the body + pedicle-type lesions. This study suggests further clinical investigations with longer follow-up for these studied cases.

The effect of dose escalation on gastric toxicity when treating lower oesophageal tumours: a radiobiological investigation.

PurposeUsing radiobiological modelling to estimate normal tissue toxicity, this study investigates the effects of dose escalation for concurrent chemoradiation therapy (CRT) in lower third oesophageal tumours on the stomach.Methods and materials10 patients with lower third oesophageal cancer were selected from the SCOPE 1 database (ISCRT47718479) with a mean planning target volume (PTV) of 348 cm3. The original 3D conformal plans (50Gy3D) were compared to newly created RapidArc plans of 50GyRA and 60GyRA, the latter using a simultaneous integrated boost (SIB) technique using a boost volume, PTV2. Dose-volume metrics and estimates of normal tissue complication probability (NTCP) were compared.ResultsThere was a significant increase in NTCP of the stomach wall when moving from the 50GyRA to the 60GyRA plans (11–17 %, Wilcoxon signed rank test, p = 0.01). There was a strong correlation between the NTCP values of the stomach wall and the volume of the stomach wall/PTV 1 and stomach wall/PTV2 overlap structures (R = 0.80 and R = 0.82 respectively) for the 60GyRA plans.ConclusionRadiobiological modelling suggests that increasing the prescribed dose to 60Gy may be associated with a significantly increased risk of toxicity to the stomach. It is recommended that stomach toxicity be closely monitored when treating patients with lower third oesophageal tumours with 60Gy.

Dosimetric study of different radiotherapy planning approaches for hippocampal avoidance whole-brain radiation therapy (HA-WBRT) based on fused CT and MRI imaging.

The purpose of this study was to compare the dosimetric characteristics for hippocampal avoidance (HA) between the treatment plans based on fused CT and MRI imaging during whole brain radiotherapy (WBRT) pertaining to: (1) 3-dimensional conformal radiotherapy (3D-CRT), (2) dynamic intensity modulated radiation therapy (dIMRT), and (3) RapidArc for patients with brain metastases. In our study, HA was defined as hippocampus beyond 5 mm, and planning target volume (PTV) was obtained subtracting HA volume from the volume of whole brain. There were 10 selected patients diagnosed with brain metastases receiving WBRT. These patients received plans for 3D-CRT (two fields), dIMRT (seven non-coplanar fields) and RapidArc (dual arc). The prescribed dose 30 Gy in 10 fractions was delivered to the whole-brain clinical target volume of patients. On the premise of meeting the clinical requirements, we compared target dose distribution, target coverage (TC), homogeneity index (HI), dose of organs at risk (OARs), monitor units (MU) and treatment time between the above three radiotherapy plans. V90 %, V95 % and TC of PTV for 3D-CRT plan were lowest of the three plans. V90 %, V95 % and HI of PTV in RapidArc plan were superior to the other two plans. TC of PTV in RapidArc plan was similar with dIMRT plan (P > 0.05). 3D-CRT was the optimal plan in the three plans for hippocampal protection. The median dose (Dmedian) and the maximum doses (Dmax) of hippocampus in 3D-CRT were 4.95, 10.87 Gy, which were lowest among the three planning approaches (P < 0.05). Dmedian and Dmax of hippocampus in dIMRT were 10.68, 14.11 Gy. Dmedian and Dmax of hippocampus in RapidArc were 10.30 gGy, 13.92 Gy. These parameters of the last two plans pertain to no significant difference (P > 0.05). When WBRT (30 Gy,10F) was equivalent to single dose 2 Gy,NTDmean of hippocampus in 3D-CRT, dIMRT and RapidArc were reduced to 3.60, 8.47, 8.20 Gy2, respectively. In addition, compared with dIMRT, MU of RapidArc was reduced and the treatment time was shortened by nearly 25 %. All three radiotherapy planning approaches in our study can meet the clinical requirements of HA. Although TC in 3D-CRT was lowest, hippocampus was protected best by this plan. So many radiation fields and the design of non-coplanar fields lead to the complication of dIMRT. TC and HI in RapidArc were superior to the other two plans with the precise of meeting the clinical requirements. The difference in protection for hippocampus between dIMRT and RapidArc was statistically significant. In addition, RapidArc can remarkably reduce MU and the treatment time.

Cranio Spinal Irradiation of Medulloblastoma Using High Precision Techniques - A Dosimetric Comparison.

Radiotherapy planning, delivery and junction dose verification remain exigent for Cranio Spinal Irradiation (CSI) in medulloblastoma patients. This study aims to evaluate high precision techniques such as Intensity Modulated Radiation Therapy (IMRT), Rapid Arc Therapy (RA) with and without flattening filter (FF) on the basis of dosimetric analysis. Five patients treated with jagged junction Intensity Modulated RadioTherapy (IMRT) using dynamic Multi Leaf Collimators (MLC) were randomly selected for this retrospective study. IMRT, Rapid Arc (RA) plans were simulated in the same CT data set with and without flattening filter. Total dose prescribed was 28.80 Gy in 16 fractions. An evaluation criterion of 98% of PTV receiving 100% of the prescription dose was followed in all plans. Twenty treatment plans with 260 Dose Volume Histograms (DVHs) was created. Dosimetric parameters such as Dmax, Dmin, Dmean, V95%, V107%, CI for PTV and Dmax, Dmean, V80%, V50%, V30%, V10% for Organs At Risk (OAR) were extracted from DVHs. Treatment delivery efficiency was also evaluated for total Beam On Time (BOT). FFF Rapid Arc therapy : 6F_RA) resulted in conformal doses throughout the cranio spinal axis. FF and FFF dynamic IMRT had minimal V107%, 1.23% and 2.88% compared to 49.15 and 66.36 of rapid arc therapy (with and without FF). 6F_IMRT resulted in lesser mean doses to eyes, liver, lungs and kidneys. Heart mean dose was less (3.08 Gy) with 6X_IMRT. Thyroid and esophagus doses could be reduced to about 41.2% and 10% respectively with 6F_RA. The BOT for the treatment techniques were 3.43 min (6X_IMRT), 1.59 min (6F_IMRT), 5min (6X_RA), 4.5 min (6F_RA). Removal of flattening filter in IMRT could improve dose coverage along the caniospinal axis and normal tissue sparing. A reduction of 46.3% BOT could increase treatment efficiency of 6F_IMRT compared to 6X_IMRT. CSI could be simpler since junction doses can be evaded in IMRT and RA techniques.

P0176 Dosimetric comparison of volumetric intensity-modulated arc (RapidArc) therapy, intensity-modulated radiotherapy, and 3D conformal radiotherapy for primary liver tumours

Background To compare the dosimetric outcomes of volumetric arc radiotherapy (RapidArc), intensity-modulated radiotherapy (IMRT), and 3D conformal radiotherapy (3DCRT) plans for primary liver tumours. Methods 7 patients with localised unresectable hepatocellular carcinoma were included in this study. CT simulation was done with voluntary deep inspiratory breath-holding after administering 80–90 mL of intravenous iodinated contrast. The mean tumour size was 5.6 cm (range 2.1–9.8) and two patients had multiple lesions (range 1–3). All patients were planned for partial liver irradiation for a total dose of 50–66 Gy in 2 Gy per fraction with conformal techniques. 3DCRT, IMRT, and RapidArc plans were generated using Eclipse planning system v.13 and dosimetric analysis was done to evaluate the plan quality and efficiency, including CI, HI, MU delivered, PTV D mean and D max . IMRT plans had 3–5 fields and RapidArc plans had 3–5 arcs. V 10 ,V 20 ,V 30 and V 40 of normal liver and D mean of organs at risk (OAR) and D max of spinal cord were also evaluated. Analysis was done using ANOVA and paired t -test with two tailed p Findings All the three techniques had comparable PTV coverage, dose homogeneity, and OAR sparing. IMRT and Rapid Arc had a significantly better conformity index than 3DCRT ( p = 0.03). The high dose areas within the normal liver; V 40 and V 30 were significantly lower in RapidArc and IMRT plans ( p = 0.03 and p = 0.04, respectively), although no significant differences were noted between IMRT and RapidArc. One patient could not attain the normal liver constraint V 33 Interpretation RapidArc and IMRT provide better normal liver sparing and conformity than 3DCRT. However, RapidArc was not better than IMRT for liver protection. Further large trials are required to clearly establish the benefits of IMRT and Rapid Arc techniques to treat primary liver tumours.

Treatment plan comparison of linac step and shoot, tomotherapy, rapidarc, and proton therapy for prostate cancer by using the dosimetrical and the biological indices

The purpose of this study was to use various dosimetrical indices to determine the best intensitymodulated radiation therapy (IMRT) modality - for treating patients with prostate cancer. Ten patients with prostate cancer were included in this study. IMRT plans were designed to include different modalities, including the linac step and shoot, tomotherapy, RapidArc, and proton systems. Various dosimetrical indices, like the prescription isodose to target volume (PITV) ratio, conformity index (CI), homogeneity index (HI), target coverage index (TCI), modified dose homogeneity index (MHI), conformation number (CN), critical organ scoring index (COSI), and quality factor (QF), were determined to compare the different treatment plans. Biological indices, such as the generalized equivalent uniform dose (gEUD) based the tumor control probability (TCP), and the normal tissue complication probability (NTCP), were also calculated and used to compare the treatment plans. The RapidArc plan attained better PTV coverage, as evidenced by its superior PITV, CI, TCI, MHI, and CN values. Regarding organ at risks (OARs), proton therapy exhibited superior dose sparing for the rectum and the bowel in low dose volumes, whereas the tomotherapy and RapidArc plans achieved better dose sparing in high dose volumes. The QF scores showed no significant difference among these plans (p = 0.701). The average TCPs for prostate tumors in the RapidArc, linac and proton plans were higher than the average TCP for Tomotherapy (98.79%, 98.76%, and 98.75% vs. 98.70%, respectively). Regarding the rectum NTCP, RapidArc showed the most favorable result (0.09%) whereas linac resulted in the best bladder NTCP (0.08%).

Direct plan comparison of RapidArc and CyberKnife for spine stereotactic body radiation therapy

We compared the treatment planning performance of RapidArc (RA) vs. CyberKnife (CK) for spinal stereotactic body radiation therapy (SBRT). Ten patients with spinal lesions who had been treated with CK were re-planned with RA, which consisted of two complete arcs. Computed tomography (CT) and volumetric dose data of CK, generated using the Multiplan (Accuray) treatment planning system (TPS) and the Ray-trace algorithm, were imported to Varian Eclipse TPS in Dicom format, and the data were compared with the RA plan by using an analytical anisotropic algorithm (AAA) dose calculation. The optimized dose priorities for both the CK and the RA plans were similar for all patients. The highest priority was to provide enough dose coverage to the planned target volume (PTV) while limiting the maximum dose to the spinal cord. Plan quality was evaluated with respect to PTV coverage, conformity index (CI), high-dose spillage, intermediate-dose spillage (R50% and D2cm), and maximum dose to the spinal cord, which are criteria recommended by the RTOG 0631 spine and 0915 lung SBRT protocols. The mean CI’ SD values of the PTV were 1.11’ 0.03 and 1.17’ 0.10 for RA and CK (p = 0.02), respectively. On average, the maximum dose delivered to the spinal cord in CK plans was approximately 11.6% higher than that in RA plans, and this difference was statistically significant (p < 0.001). High-dose spillages were 0.86% and 2.26% for RA and CK (p = 0.203), respectively. Intermediate-dose spillage characterized by D2cm was lower for RA than for CK; however, R50% was not statistically different. Even though both systems can create highly conformal volumetric dose distributions, the current study shows that RA demonstrates lower high- and intermediate-dose spillages than CK. Therefore, RA plans for spinal SBRT may be superior to CK plans.

Evaluation of the clinical usefulness of modulated arc treatment

The purpose of this study is to evaluate the clinical usefulness of modulated arc (mARC) treatment techniques. The mARC treatment plans for non-small-cell lung cancer (NSCLC) patients were made in order to verify the clinical usefulness of mARC. A pre-study was conducted to find the best plan condition for mARC treatment, and the usefulness of the mARC treatment plan was evaluated by comparing it with other Arc treatment plans such as tomotherapy and RapidArc plans. In the case of mARC, the optimal condition for the mARC plan was determined by comparing the dosimetric performance of the mARC plans developed by using various parameters, which included the photon energy (6 MV, 10 MV), the optimization point angle (6°- 10°intervals), and the total number of segments (36 - 59 segments). The best dosimetric performance of mARC was observed at a 10 MV photon energy, a point angle 6 degrees, and 59 segments. The treatment plans for the three different techniques were compared by using the following parameters: the conformity index (CI), homogeneity index (HI), the target coverage, the dose to the OARs, the number of monitor units (MU), the beam on time, and the normal tissue complication probability (NTCP). As a result, the three different treatment techniques showed similar target coverages. The mARC plan had the lowest V20 (volume of lung receiving > 20 Gy) and MU per fraction compared with both the RapidArc and the tomotherapy plans. The mARC plan reduced the beam on time as well. Therefore, the results of this study provide satisfactory evidence that the mARC technique can be considered as a useful clinical technique for radiation treatment.