W Fu

A cone beam CT-guided online plan modification technique to correct interfractional anatomic changes for prostate cancer IMRT treatment

The purpose of this work is to develop an online plan modification technique to compensate for the interfractional anatomic changes for prostate cancer intensity-modulated radiation therapy (IMRT) treatment based on daily cone beam CT (CBCT) images. In this proposed technique, pre-treatment CBCT images are acquired after the patient is set up on the treatment couch using an in-room laser with the guidance of the setup skin marks. Instead of moving the couch to rigidly align the target or re-planning using the CBCT images, we modify the original IMRT plan to account for the interfractional target motion and deformation based on the daily CBCT image feedback. The multileaf collimator (MLC) leaf positions for each subfield are automatically adjusted in the proposed algorithm based on the position and shape changes of target projection in the beams eye view (BEV). Three typical prostate cases were adopted to evaluate the proposed technique, and the results were compared with those obtained with bony-structure-based rigid translation correction, prostate-based correction and CBCT-based re-planning strategies. The study revealed that the proposed modification technique is superior to the bony-structure-based and prostate-based correction techniques, especially when interfractional target deformation exists. Its dosimetric performance is closer to that of the re-planned strategy, but with much higher efficiency, indicating that the introduced online CBCT-guided plan modification technique may be an efficient and practical method to compensate for the interfractional target position and shape changes for prostate IMRT.

Dosimetric effects of patient rotational setup errors on prostate IMRT treatments

The purpose of this work is to determine dose delivery errors that could result from systematic rotational setup errors (DeltaPhi) for prostate cancer patients treated with three-phase sequential boost IMRT. In order to implement this, different rotational setup errors around three Cartesian axes were simulated for five prostate patients and dosimetric indices, such as dose-volume histogram (DVH), tumour control probability (TCP), normal tissue complication probability (NTCP) and equivalent uniform dose (EUD), were employed to evaluate the corresponding dosimetric influences. Rotational setup errors were simulated by adjusting the gantry, collimator and horizontal couch angles of treatment beams and the dosimetric effects were evaluated by recomputing the dose distributions in the treatment planning system. Our results indicated that, for prostate cancer treatment with the three-phase sequential boost IMRT technique, the rotational setup errors do not have significant dosimetric impacts on the cumulative plan. Even in the worst-case scenario with DeltaPhi=3 degrees, the prostate EUD varied within 1.5% and TCP decreased about 1%. For seminal vesicle, slightly larger influences were observed. However, EUD and TCP changes were still within 2%. The influence on sensitive structures, such as rectum and bladder, is also negligible. This study demonstrates that the rotational setup error degrades the dosimetric coverage of target volume in prostate cancer treatment to a certain degree. However, the degradation was not significant for the three-phase sequential boost prostate IMRT technique and for the margin sizes used in our institution.

Dosimetric influences of rotational setup errors on head and neck carcinoma intensity-modulated radiation therapy treatments

The purpose of this work is to investigate the dosimetric influence of the residual rotational setup errors on head and neck carcinoma (HNC) intensity-modulated radiation therapy (IMRT) with routine 3 translational setup corrections and the adequacy of this routine correction. A total of 66 kV cone beam computed tomography (CBCT) image sets were acquired on the first day of treatment and weekly thereafter for 10 patients with HNC and were registered with the corresponding planning CT images, using 2 3-dimensional (3D) rigid registration methods. Method 1 determines the translational setup errors only, and method 2 determines 6-degree (6D) setup errors, i.e., both rotational and translational setup errors. The 6D setup errors determined by method 2 were simulated in the treatment planning system and were then corrected using the corresponding translational data determined by method 1. For each patient, dose distributions for 6 to 7 fractions with various setup uncertainties were generated, and a plan sum was created to determine the total dose distribution through an entire course and was compared with the original treatment plan. The average rotational setup errors were 0.7°± 1.0°, 0.1°±1.9°, and 0.3°±0.7° around left-right (LR), anterior-posterior (AP), and superior-inferior (SI) axes, respectively. With translational corrections determined by method 1 alone, the dose deviation could be large from fraction to fraction. For a certain fraction, the decrease in prescription dose coverage (Vp) and the dose that covers 95% of target volume (D95) could be up to 15.8% and 13.2% for planning target volume (PTV), and the decrease in Vp and the dose that covers 98% of target volume (D98) could be up to 9.8% and 5.5% for the clinical target volume (CTV). However, for the entire treatment course, for PTV, the plan sum showed that the average Vp was decreased by 4.2% and D95 was decreased by 1.2 Gy for the first phase of IMRT with a prescription dose of 50 Gy. For CTV, the plan sum showed that the average Vp was decreased by 0.8% and D98, relative to prescription dose, was not decreased. Among these 10 patients, the plan sum showed that the dose to 1-cm(3) spinal cord (D(1 cm(3))) increased no more than 1 Gy for 7 patients and more than 2 Gy for 2 patients. The average increase in D(1 cm(3)) was 1.2 Gy. The study shows that, with translational setup error correction, the overall CTV Vp has a minor decrease with a 5-mm margin from CTV to PTV. For the spinal cord, a noticeable dose increase was observed for some patients. So to decide whether the routine clinical translational setup error correction is adequate for this HNC IMRT technique, the dosimetric influence of rotational setup errors should be evaluated carefully from case to case when organs at risk are in close proximity to the target.

4D VMAT planning and verification technique for dynamic tracking using a direct aperture deformation (DAD) method

&NA; We developed a four‐dimensional volumetric modulated arc therapy (4D VMAT) planning technique for moving targets using a direct aperture deformation (DAD) method and investigated its feasibility for clinical use. A 3D VMAT plan was generated on a reference phase of a 4D CT dataset. The plan was composed of a set of control points including the beam angle, MLC apertures and weights. To generate the 4D VMAT plan, these control points were assigned to the closest respiratory phases using the temporal information of the gantry angle and respiratory curve. Then, a DAD algorithm was used to deform the beam apertures at each control point to the corresponding phase to compensate for the tumor motion and shape changes. Plans for a phantom and five lung cases were included in this study to evaluate the proposed technique. Dosimetric comparisons were performed between 4D and 3D VMAT plans. Plan verification was implemented by delivering the 4D VMAT plans on a moving QUASAR™ phantom driven with patient‐specific respiratory curves. The phantom study showed that the 4D VMAT plan generated with the DAD method was comparable to the ideal 3D VMAT plan. DVH comparisons indicated that the planning target volume (PTV) coverages and minimum doses were nearly invariant, and no significant difference in lung dosimetry was observed. Patient studies revealed that the GTV coverage was nearly the same; although the PTV coverage dropped from 98.8% to 94.7%, and the mean dose decreased from 64.3 to 63.8 Gy on average. For the verification measurements, the average gamma index pass rate was 98.6% and 96.5% for phantom 3D and 4D VMAT plans with 3%/3 mm criteria. For patient plans, the average gamma pass rate was 96.5% (range 94.5–98.5%) and 95.2% (range 94.1–96.1%) for 3D and 4D VMAT plans. The proposed 4D VMAT planning technique using the DAD method is feasible to incorporate the intra‐fraction organ motion and shape change into a 4D VMAT planning. It has great potential to provide high plan quality and delivery efficiency for moving targets.

SU‐F‐T‐01: Optimization of the Accelerated Partial Breast Brachytherapy Fractionation with Consideration of Physical Doses to Tumor and Organ at Risk

PURPOSE The accelerated partial breast irradiation (APBI) with brachytherapy prescribes 34Gy to be delivered in 10 fractions over 5 consecutive working days without considering the physical dose to the target and organs at risk (OARs) for an individual patient. The purpose of this study is to optimize the fractionation scheme by evaluating the radiation effect on tumor and OARs with a modified linear-quadratic (LQ) model based on dose-volume histograms (DVHs). METHODS Five breast patients treated with multilumen balloon brachytherapy were selected. The minimum skin and rib spacing were ranged from 2.5mm to 14.3mm and from 1.0mm to 25.0mm, respectively. The LQ model parameters were set as: (1) breast: α=0.08, β=0.028, doubling time Tpot=14.4 days, and starting time Tk=21days; (2) skin: acute reaction α=0.101, β=0.009; late reaction α=0.064, β=0.029; (3) rib: α=0.3, β=0.12. Boundary dose Dt was 6 Gy for both target and OARs. The relation between radiation effects on the tumor (ET) and OARs (EOAR) were plotted for fraction number from 1 to 20. RESULTS The value of radiation effect from routine 3.4Gyx10 fractions was used as reference, ETref and EOARref. If set ET=ETref, the fractionation that results in minimum EOAR values correspond to the optimal fractionation. For these patients, the optimal numbers are 10 fractions for skin acute reaction, 18 fractions for skin and rib late reaction while the doses per fraction are 3.4Gy and 2.05-2.10Gy, respectively. If set EOAR=EOARref, the fractionation that results in a maximum ET value corresponds to the optimal fractionation. The optimal fractionation is 3.4Gyx10 fractions for skin acute reaction, and 2.10-2.25Gyx18 fractions for skin late reaction and rib. CONCLUSION For APBI brachytherapy, the routine 3.4Gyx10 fractions is optimal fractionation for skin acute reaction, while 2.05-2.25Gyx18 fractions is optimal fractionation for late reaction of skin and rib.

Dosimetric experience with 2 commercially available multilumen balloon-based brachytherapy to deliver accelerated partial-breast irradiation

The purpose of this work was to report dosimetric experience with 2 kinds of multilumen balloon (MLB), 5-lumen Contura MLB (C-MLB) and 4-lumen MammoSite MLB (MS-MLB), to deliver accelerated partial-breast irradiation, and compare the ability to achieve target coverage and control skin and rib doses between 2 groups of patients treated with C-MLB and MS-MLB brachytherapy. C-MLB has 5 lumens, the 4 equal-spaced peripheral lumens are 5 mm away from the central lumen. MS-MLB has 4 lumens, the 3 equal-spaced peripheral lumens are 3 mm away from the central lumen. In total, 43 patients were treated, 23 with C-MLB, and 20 with MS-MLB. For C-MLB group, 8 patients were treated with a skin spacing < 7 mm and 12 patients with rib spacing < 7 mm. For MS-MLB group, 2 patients were treated with a skin spacing < 7 mm and 5 patients with rib spacing < 7 mm. The dosimetric goals were (1) ≥ 95% of the prescription dose (PD) covering ≥ 95% of the target volume (V(95%) ≥ 95%), (2) maximum skin dose ≤ 125% of the PD, (3) maximum rib dose ≤ 145% of the PD (if possible), and (4) the V(150%) ≤ 50 cm(3) and V(200%) ≤ 10 cm(3). All dosimetric criteria were met concurrently in 82.6% of C-MLB patients, in 80.0% of MS-MLB patients, and in 81.4% of all 43 patients. For each dosimetric parameter, t-test of these 2 groups showed p > 0.05. Although the geometric design of C-MLB is different from that of MS-MLB, both applicators have the ability to shape the dose distribution and to provide good target coverage, while limiting the dose to skin and rib. No significant difference was observed between the 2 patient groups in terms of target dose coverage and dose to organs at risk.

SU‐E‐T‐476: Volumetric Modulated Arc Therapy (VMAT) Plan Validation Using MatriXX Measurements and Monte Carlo Calculation

Purpose: To establish confidence in VMAT plan delivery for recently launched VMAT with Varianˈs RapidArc technique at our clinic, a sophisticated plan validation has been performed by comparison of ion- chamber measurements, planning system calculations, and independent Monte Carlo calculations. Methods: Although MatriXX has been used as a reliable device for validations of intensity modulate radiation therapy (IMRT) treatment plans, it was challenging to use it for VMAT plan validations because of its angle-dependent dose response. A new version of MatriXX Evolution with a gantry angle sensor is able to correct the angle dependency and promises accurate measurements while the gantry rotates. To evaluate the performance of MatriXX for RapidArc plan validations, Monte Carlo calculations using XVMC was used as an independent method to our IMRT validation program. DICOM plans and CT images from Eclipse planning system were translated by an in-house code for XVMC input. Measured dose planes were compared with XVMC dose planes as well as Eclipse dose planes for 12 pelvis plans, 19 prostate plans, and 5 head and neck (H&N) plans. Construction of a scatterplot by taking differences of calculations from measurements displayed a positive correlation between them. Results: Eclipse calculations were off from measurements by 2.64%, 1.89%, and 1.84% in quadratic mean for pelvis, prostate, and H&N plans, respectively. XVMC calculations were off from the measurement by 2.18%, 1.73%, and 2.30% in quadric mean for pelvis, prostate, and HN plans, respectively. Passing percent of Gamma test for Eclipse calculations was over 97.0% and that for XVMC calculations was about 95.7% due to inherent statistical uncertainty of Monte Carlo calculation. Eclipse and XVMC calculations agreed to each other within 4 % Conclusions: The present study shows that Eclipse calculations and measurements using MatriXX evolution can be used for validation of VMAT plans using Rapid Arc delivery technique.

SU‐GG‐T‐11: Dosimetric Impacts of Rotational Setup Errors on Head & Neck IMRT Treatments

Purpose: To investigate the dosimetric impact of the residual rotational setup errors on head&neck (HNC) IMRT with conventional clinical three translational setup corrections, and to evaluate the necessity of the full six degree (6D) setup correction. Methods and Materials: A total of 66 kV CBCTimage sets were acquired on the first day of treatment and weekly thereafter for 10 HNC patients. The CBCTimages were registered with the planning CTimages using two 3D rigid registration methods. Method1 determines the translational errors only and method2 determines 6D errors. To simulate the clinical setup correction, the 6D setup errors were simulated in the treatment planning system and then were corrected by using the corresponding translational data determined by method1. For each patient, dose distributions for 6 to 7 fractions with various setup uncertainties were generated, and plan sum was created to simulate the total dose distribution through an entire course and was compared with the original treatment plan. The first phase plan with prescription about 50 Gy was used in this study. Results: Without rotational corrections and with only translational corrections, for PTV, the plan sum showed that the average Vp (prescription dose coverage) was decreased by 5.9%, D95 (dose that covers 95% of target) was decreased 1.8 Gy. For CTV, though the Vp could be decreased by 8% for treatments on selected days, the plan sum showed that the average Vp was decreased by 1.7% and D98 was decreased 0.1 Gy. The average dose increase to spinal cord was 0.7 Gy. Conclusions: With conventional translational setup correction, the overall CTV prescription dose coverage decreases insignificantly because of the 5mm margin from CTV to PTV. The dose increase to spinal cord is not significant. So for HNC IMRT treatment with target margin, the six degree setup correction is not necessary.

SU‐GG‐T‐114: A Feasibility Study of a 4D Intensity‐Modulated Arc Tracking Technique for Treatment of Moving Targets

Purpose: A major challenge of Intensity‐modulated arc therapy(IMAT) is in dealing with moving targets since the conventional gating technique may not be easily applied. The purpose of this work is to develop a 4D IMAT tracking technique for moving targets and investigate its feasibility for clinical use. Method and Materials: 4DCT images at 10 different breathing phases were acquired. The target was contoured for each phase and a 3D RapidArc plan was generated based on the 50%phase CTimages. Each control point in the RapidArc plan was then correlated to the patients respiration phases with consideration given to changes in gantry speed and dose rate. A novel program was developed to read the RapidArc plan and target contours from all 10 phases. In order to generate a 4D RapidArc plan with the capability of DMLC tracking, the MLC modification algorithm originally developed for on‐line plan modification was used to modify the MLC leaf positions at each control point to correspond to the respiration phases. 4D RapidArc plans for phantom and lung cases were used to evaluate the proposed technique. Experimental measurements for both 4D and 3D plans in a moving phantom were performed and the results were compared with the calculated and measured results for the 3D RapidArc plans in a “static” phantom. Results: Analysis of isodose and gamma distributions indicated that the dose distribution in the moving target delivered using the proposed technique is comparable to that in a static target delivered using a 3D RapidArc plan. The results were much improved using the 4D IMAT delivery compared with the 3D plans for a moving target. Conclusion: A 4D RapidArc tracking technique can be implemented and our results indicated that it is feasible to incorporate the intra‐fraction organ motion into a 4D IMAT planning to track the moving targets.

TH-D-AUD A-06: Study of Translational and Rotational Setup Errors and Their Correction Methods for Head & Neck Patients Using Kilovoltage Cone-Beam Computed Tomography (kV CBCT)

Purpose: To investigate the magnitude of the six degree setup errors in head&neck patients (HNC) and evaluate which correction data, obtained from three degree and six degree 3D/3D registration, is more appropriate for setup correction if the setup errors are corrected by translational shifts. Method and Materials: kV CBCTimages were acquired on the first day of treatment and weekly thereafter for 21 HNC patients treated with IMRT. A total of 145 CBCTimage sets were acquired. The CBCTimages were registered with the corresponding planning CTimages using two different 3D rigid registration approaches. With Approach1 the registrations were conducted with translations alone, with Approach2 all six degrees were taken into account. The setup error with the maximum rotational error was simulated on planning CT for two patients, then the errors were corrected by applying the translational data obtained from Approach1 (Correction1) and Approach2 (Correction2), respectively. Dosimetric indices were compared for the two corrections. Results: For these 21 HNC patients, the average translational errors determined with Approach1 were 1.0±3.5, 0.8±3.5, 1.6±3.8mm and the values determined with Approach2 were 1.1±5.0, 0.4±3.8, 2.2±4.7mm in LR, AP and SI directions respectively. The average rotational errors determined by Approach2 were 0.6°±1.1°, 0.1°±1.9°, 0.3°±0.8° and the average maximum errors were 0.9°±1.6°, 0.5°±3.0°, 0.4°±1.1° around LR, AP and SI axes respectively. The PTV prescription dose coverage was 86.1% and 92.3% for patient1, 92.1% and 92.4% for patent2 with Correction1 and Correction2 respectively. Conclusion: Relatively large rotational errors were observed in HNC patients. Instinctively, it appeared that the Correction1 were more accurate than Correction2 if only translational corrections were involved. The result for patient1 showed that it may not be the case. The dosimetric impact of both corrective approaches has to be further investigated to evaluate which approach should be applied to correct the setup errors.