Tissue Heterogeneity Corrections Research Articles

Feasibility and Safety of Single-Isocenter/Multi-Lesion (SIML) HyperArc Brain SRT: Clinical Implementation and Early Outcomes

Patients with multiple brain metastases may not tolerate relatively longer treatment times for traditional stereotactic radiation therapy (SRT) with individual isocenter plans for each lesion due to discomfort or co-morbidities. SRT using a single-isocenter/multi-lesion (SIML) HyperArc volumetric modulated arc therapy (VMAT) plan with flattening filter free (FFF) beam could significantly shorten overall treatment time, and improve patient comfort, compliance, and clinic efficiency. We report early clinical results of treating multiple brain metastases with SIML HyperArc SRT. Twenty-three patients with multiple brain metastatic tumors (range, 2-9 lesions; total treated lesions, n = 96) were simulated using Encompass support and Q-fix mask, and treated with a highly-conformal SIML VMAT SRT plans via non-coplanar HyperArc geometry. Mean tumor distance to isocenter was 5.3 cm, maximum up to 7 cm. Common prescriptions were 25-30 Gy/5 fractions, 24-27 Gy/3 fractions, and 20 Gy/1 fraction prescribed to each planning target volume (PTV) using 2 mm margin around standard gross tumor volume (GTV) delineated on contrasted enhanced MP-RAGE MRI fusion. Acuros dose calculation for 6MV-FFF beam was used for tissue heterogeneity corrections. Alliance A071801 criteria was used for dose constraints to organs at risk (OAR) and target conformality. Treatment was delivered every other day with CBCT-guidance, adjustments made with 6DOF couch corrections on a medical linear accelerator, and treatment delivery time within 15 minutes. Local control rates were reported, and toxicity profile rated based on CTCAE v5.0 for brain radionecrosis, optic neuropathy, and brainstem dysfunction. All plans met Alliance A071801 requirements for each tumor coverage, dose to OAR including optic apparatus, brainstem, and spinal cord. Mean GTV and PTV volume were 9.4 cc (range, 0.3-54.8 cc) and 16.13 cc (range, 1.0-80.2 cc). Patient-specific quality assurance results were 98.3% for gamma passing criteria of 2%/2mm. Independent in-house Monte Carlo physics second check agreed with HyperArc plans by ±3.0%. Mean follow up was 6 months (range, 0.0-18.6 months). Of the 23 patients treated, 17 (74%) had post-treatment MRI imaging to assess local control and toxicity. Local control was achieved in 69/73 (95%) of treated and followed lesions. CTCAE grade 2 radionecrosis occurred in 2 patients and were managed with dexamethasone. No CTCAE grade 3+ events of radionecrosis, optic pathway dysfunction, or brainstem toxicity were observed. SIML HyperArc Brain SRT for multiple brain metastases has excellent local control and low toxicity profile in our patients. It can significantly reduce treatment delivery time as compared to traditional multiple-isocenter brain SRT or chronologically separate treatment courses and thus, help to improve patient comfort, compliance, ease of care, and clinic workflow. Longer median follow up of SIML brain SRT on larger patient cohort is warranted.

Clinical Outcomes of Synchronous Lung Stereotactic Body Radiation Therapy Lesions via Single-Isocenter/Multi-Lesion Treatments

Lung cancer patients with synchronous primary or oligometastatic (< 5 lesions) with associated co-morbidities may not retain their treatment position for the extended stereotactic body radiotherapy (SBRT) treatment times with individual isocenter plans for each lesion due to discomfort or shortness of breath. SBRT using a single-isocenter/multi-lesion (SIML) volumetric arc therapy (VMAT) plan with flattening filter free (FFF) beam could significantly shorten overall treatment time, improve patient comfort and compliance and clinic efficiency. We report clinical results of treating multiple lung metastases or synchronous primary lung cancers with SIML lung SBRT. Eighty patients with synchronous primary lung cancers or oligometastatic lung tumors (two, n = 61; three, n = 10; four, n = 4; five n = 5; total treated lesions, n = 193) were simulated with abdominal compression and/or 4D-CT MIP images and treated with a highly-conformal SIML VMAT lung SBRT plans via 3-4 non-coplanar arcs. Common prescriptions were 50-55 Gy/5 fractions and 54 Gy/3 fractions prescribed 70-80% isodose line to the each PTV. Acuros dose calculation for 6MV-FFF beam was used for tissue heterogeneity corrections. RTOG-0618/0813 criteria were used to dose constraints to organs at risk (OAR) and target conformality. Treatment was delivered every other day or twice weekly with CBCT-guidance, adjustments made with 6DOF couch corrections on a medical linear accelerator, and treatment delivery time within 15 minutes. Local control rates and toxicity profile was evaluated using CTCAE v. 5.0 grading for pneumonitis, rib fracture and chest wall pain. All plans met RTOG-0618/0813 requirements for each tumor coverage, dose to OAR including normal lung and ribs. Mean follow up after last fraction of treatment was 16.9 months (range, 1.0-54.2 months). PTV volume ranged from 2.17 to 167.8 cc with mean volume of 16.1 cc. Of the 80 patients treated, 71 had adequate post-treatment thoracic CT imaging to assess local control. Local control was achieved in 167/175 (95.4%) of treated and followed lesions. CTCAE grade 1 asymptomatic pneumonitis was noted on thoracic CT scans in 42/71 (59.2%) of patients and occurred, on average 4.8 months after SBRT. Symptomatic pneumonitis and rib fracture did not occur in any patient. CTCAE grade 2 chest wall pain occurred in 4/80 (5.0%) treated patients and was managed conservatively with over-the-counter NSAIDS or acetaminophen. SIML lung SBRT for synchronous primary lung cancers or multiple lung metastases can be used as a variant to traditional multiple isocenter SBRT or chronologically separate treatment courses, and has excellent local control rates and low toxicity profile in our patient population. It can help improve comfort and compliance of the patients who have difficulty lying still for an extended treatment course, and significantly reduces treatment time via isocenter shifts/repeated CBCTs for image guidance, thus improving clinic efficiency.

Heterogeneous Versus Homogeneous Radiation Dose Calculations of Twice-Daily Fractionation in Small Cell Lung Carcinoma.

PurposeThe standard radiotherapy regimen for small cell lung cancer (SCLC) was determined using dose calculations without corrections for tissue heterogeneity, while modern treatments are planned using algorithms accounting for tissue heterogeneity. We assessed differences in dose delivered using heterogeneous and homogeneous dose calculations in a cohort of patients treated for limited-stage small cell lung cancer (LS-SCLC).MethodsThis is a retrospective analysis of 35 patients (three-dimensional conformal radiation therapy (3D-CRT), n = 22; intensity-modulated radiation therapy (IMRT), n = 13) with LS-SCLC treated with chemoradiotherapy from 2011 to 2017. Treatment plans were developed in the Eclipse Treatment Planning System (TPS) version 13.6 using the Analytical Anisotropic Algorithm (AAA). Two plans were generated for each patient with one using the unit relative electron density and the other maintaining the same monitor units (MUs) with tissue density corrections. The prescription was 45 Gy in 30 fractions of 1.5 Gy delivered twice daily. Individuals who underwent replanning within the same treatment course were evaluated using a separate corrected and uncorrected plan sum. Variations greater than 5% in dose to the tumor or organs at risk were considered clinically relevant. A two-sided paired t-test was used to evaluate the statistical significance of the dosimetric differences.ResultsThe percent dose difference between plans without tissue heterogeneity corrections to those with corrections resulted in an overall median difference of -3% (range: -15.1% to 9.6%; p < 0.01) for the dose covering 95% of the planning target volume (PTV D95) and was -5.6% (range: -17.3% to 5.4%; p < 0.01) for lung volume receiving ≥20 Gy (lung V20). For 3D-CRT, the median difference for the PTV D95 was -0.1% (range: -4.7% to 9.6%; p = 0.62) and the lung V20 was -4.2% (range: -9.4 to 5.4; p < 0.01). For IMRT, the median difference for the PTV D95 was -10.0% (range: -15.1% to -5.3%; p < 0.01) and the lung V20 was -8.9% (range: -17.3 to -3.5; p < 0.01).ConclusionTraditional planning without tissue heterogeneity corrections results in an overall decrease in the dose delivered to the target compared with those that incorporate tissue heterogeneity corrections. These differences are modest for 3D treatment plans but may result in clinically relevant differences for the IMRT cohort (>5% deviation).

Impact of Tissue Heterogeneity Correction on Stereotactic Radiosurgery of Acoustic Neuromas

Stereotactic radiosurgery (SRS) for acoustic neuromas (ANs) yields control rates ≥ 90%. Among the available treatment planning systems, the commonly used algorithms for SRS include TMR10, which assumes tissue homogeneity equivalent to water, and collapsed-cone convolutional (CCC) algorithms, which corrects for tissue inhomogeneity. The dosimetric differences between these approaches for patients receiving SRS is not well understood. Given both bone/brain and air/brain interfaces exist adjacent to the internal auditory canal, ANs pose a clinical scenario in which dose inhomogeneity may be relevant. The aim of this study was to quantify the dosimetric differences between TMR10 and CCC planning algorithms for ANs treated with SRS. We performed a retrospective review of 50 AN treated with SRS at a large academic center from 2015-2018. All patients underwent MRI and CT imaging during their initial treatment and were treated using TMR10 generated plans. Electron density curves from CT images were then used to generate a second plan using the CCC planning algorithm. Shot locations and size were kept constant to allow for comparison of the two planning algorithms. Dosimetric outcomes of interest included differences in Dmax, D50%, Cochlea Dmax, and mean cochlea dose. In effort to identify characteristics associated with significant dosimetric differences, patient dosimetry was also analyzed based on laterality (>5mm from central axis) and size (PTV <1cc). 50 patients were treated from 2015-2018 with median planning target volume (PTV) of 1.5cc (0.3cc - 2.8cc) and median dose prescription of 12Gy prescribed to the 50% isodose line. Compared to CCC algorithms, the TMR10 algorithm overestimated the actual delivered dose. Dmax was overestimated by an average 6.2% (3.4%-10.1%, p<.001), Dmin was overestimated by an average 3.1% (1.1%-7.3%, p<.032). The average D50% difference between plans was 11.3% (4.7%-16.1%, p<0.001). The average max Cochlea dose difference between plans was 20.1% (12.9%-24.0%, p<0.001). Differences in Dmax and D50% were greatest in AN that were lateralized, >5mm from central axis (mean Dmax difference: 8.5% vs 1.5%, p<0.001, D50% mean difference: 13.2% vs 6.1%, p<0.001) and small in size, <1cc (mean Dmax difference: 8.9% vs 3.9%, p<0.003, D50% mean difference: 12.1% vs 7.2% p<0.001). We found statistically significant dosimetric differences among ANs planned using TMR10 and CCC algorithms. Plans generated using the TMR10 algorithm are likely overestimating delivered dose, particularly in small and lateralized ANs. For more accurate dosimetry in AN treatment, institutions should consider heterogeneity corrected algorithms. As the use of heterogeneity correction algorithms increases, clinicians should be cognizant of these dosimetric differences when comparing historical studies and outcomes.

Dosimetric Study of Tissue Heterogeneity Correction for Breast Conformal Radiotherapy

Introduction: Heterogeneity correction is an important parameter in dose calculation for cancer patients where it may be cause inaccuracy in dose calculation as a result of different densities of patients. This study studied the impact of dose calculation of breast cancer patients with and without heterogeneity correction. Material and Methods: Twenty breast cancer patients were treated with Three-Dimensional Conformal Radiotherapy(3DCRT). Dose calculations were performed using two modes: Fast Photon mode for homogeneity and Fast Photon Effective Path length for heterogeneity with two photon energies. Monitor Units(MU), Modulation Factor, Dose Volume Histograms(DVH) and quality indices were used to evaluate the effect of heterogeneity correction on dose calculation and investigate the mechanism of this effect in the low and high energies. Results: Heterogeneity correction compared to without it showed significant reduction in MU and modulation factor at 6MVand 10MV (p <0.05). Dosimetric parameters derived from DVH were significantly lower for Planning Target Volume (PTV) with homogeneity versus heterogeneity (p <0.05) as D95% (95.1%vs93.7%) and V95%(95.3%vs89%) for 6MV while max Dose and D2 increased. Also the dose for organs at risk exhibited an increase with heterogeneity correction. Quality indices were be worst with heterogeneity correction with a significant difference (p <0.05). The differences between the dose with heterogeneity correction and without it in 6MV and 10MV were as follows: ΔD95% (4.4%vs3.4%;P=0.001) and ΔV95%(4.76%vs4.5%;P=0.001). Conclusion: non-use of the heterogeneity correction can be cause to deliver under or overdose dose to the target volume. Tissue heterogeneity correction had an impact on dose calculation for breast cancer patients and this impact was more effective for the low energy.

Local control rates in stereotactic body radiotherapy (SBRT) of lung metastases associated with the biologically effective dose.

To evaluate dose differences in lung metastases treated with stereotactic body radiotherapy (SBRT), and the correlation with local control, regarding the dose algorithm, target volume and tissue density. Several studies showed excellent local control rates in SBRT for lung metastases, with different fractionation schemes depending on the tumour location or size. These results depend on the dose distributions received by the lesions in terms of the tissue heterogeneity corrections performed by the dose algorithms. Forty-seven lung metastases treated with SBRT, using intrafraction control and respiratory gating with internal fiducial markers as surrogates (ExacTrac, BrainLAB AG), were calculated using Pencil Beam (PB) and Monte Carlo (MC) (iPlan, BrainLAB AG).Dose differences between both algorithms were obtained for the dose received by 99% (D 99%) and 50% (D 50%) of the planning treatment volume (PTV). The biologically effective dose delivered to 99% (BED99%) and 50% (BED50%) of the PTV were estimated from the MC results. Local control was evaluated after 24 months of median follow-up (range: 3-52 months). The greatest variations (40.0% in ΔD 99% and 38.4% in ΔD 50%) were found for the lower volume and density cases. The BED99% and BED50% were strongly correlated with observed local control rates: 100% and 61.5% for BED99%>85Gy and <85Gy (p<0.0001), respectively, and 100% and 58.3% for BED50%>100Gy and <100Gy (p<0.0001), respectively. Lung metastases treated with SBRT, with delivered BED99%>85Gy and BED50%>100Gy, present better local control rates than those treated with lower BED values (p=0.001).

Precision IORT – Image guided intraoperative radiation therapy (igIORT) using online treatment planning including tissue heterogeneity correction

BackgroundTo the present date, IORT has been eye and hand guided without treatment planning and tissue heterogeneity correction. This limits the precision of the application and the precise documentation of the location and the deposited dose in the tissue. Here we present a set-up where we use image guidance by intraoperative cone beam computed tomography (CBCT) for precise online Monte Carlo treatment planning including tissue heterogeneity correction. Materials and methodsAn IORT was performed during balloon kyphoplasty using a dedicated Needle Applicator. An intraoperative CBCT was registered with a pre-op CT. Treatment planning was performed in Radiance using a hybrid Monte Carlo algorithm simulating dose in homogeneous (MCwater) and heterogeneous medium (MChet). Dose distributions on CBCT and pre-op CT were compared with each other. Spinal cord and the metastasis doses were evaluated. ResultsThe MCwater calculations showed a spherical dose distribution as expected. The minimum target dose for the MChet simulations on pre-op CT was increased by 40% while the maximum spinal cord dose was decreased by 35%. Due to the artefacts on the CBCT the comparison between MChet simulations on CBCT and pre-op CT showed differences up to 50% in dose. ConclusionsigIORT and online treatment planning improves the accuracy of IORT. However, the current set-up is limited by CT artefacts. Fusing an intraoperative CBCT with a pre-op CT allows the combination of an accurate dose calculation with the knowledge of the correct source/applicator position. This method can be also used for pre-operative treatment planning followed by image guided surgery.

A dosimetric comparison of whole-lung treatment techniques in the pediatric population

To demonstrate the dosimetric advantages and disadvantages of standard anteroposterior-posteroanterior (S-AP/PAAAA), inverse-planned AP/PA (IP-AP/PA) and volumetry-modulated arc (VMAT) radiotherapies in the treatment of children undergoing whole-lung irradiation. Each technique was evaluated by means of target coverage and normal tissue sparing, including data regarding low doses. A historical approach with and without tissue heterogeneity corrections is also demonstrated. Computed tomography (CT) scans of 10 children scanned from the neck to the reproductive organs were used. For each scan, 6 plans were created: (1) S-AP/PAAAA using the anisotropic analytical algorithm (AAA), (2) IP-AP/PA, (3) VMAT, (4) S-AP/PANONE without heterogeneity corrections, (5) S-AP/PAPB using the Pencil-Beam algorithm and enforcing monitor units from technique 4, and (6) S-AP/PAAAA[FM] using AAA and forcing fixed monitor units. The first 3 plans compare modern methods and were evaluated based on target coverage and normal tissue sparing. Body maximum and lower body doses (50% and 30%) were also analyzed. Plans 4 to 6 provide a historic view on the progression of heterogeneity algorithms and elucidate what was actually delivered in the past. Averages of each comparison parameter were calculated for all techniques. The S-AP/PAAAA technique resulted in superior target coverage but had the highest maximum dose to every normal tissue structure. The IP-AP/PA technique provided the lowest dose to the esophagus, stomach, and lower body doses. VMAT excelled at body maximum dose and maximum doses to the heart, spine, and spleen, but resulted in the highest dose in the 30% body range. It was, however, superior to the S-AP/PAAAA approach in the 50% range. Each approach has strengths and weaknesses thus associated. Techniques may be selected on a case-by-case basis and by physician preference of target coverage vs normal tissue sparing.

Technical Note: Dosimetric evaluation of Monte Carlo algorithm in iPlan for stereotactic ablative body radiotherapy (SABR) for lung cancer patients using RTOG 0813 parameters.

For stereotactic ablative body radiotherapy (SABR) in lung cancer patients, Radiation Therapy Oncology Group (RTOG) protocols currently require radiation dose to be calculated using tissue heterogeneity corrections. Dosimetric criteria of RTOG 0813 were established based on the results obtained from non‐Monte Carlo (MC) algorithms, such as superposition/convolutions. Clinically, MC‐based algorithms are now routinely used for lung SABR dose calculations. It is essential to confirm that MC calculations in lung SABR meet RTOG guidelines. This report evaluates iPlan MC plans for SABR in lung cancer patients using dose‐volume histogram normalization per current RTOG 0813 compliance criteria. Eighteen Stage I‐II non‐small cell lung cancer (NSCLC) patients with centrally located tumors, who underwent MC‐based lung SABR with heterogeneity correction using X‐ray Voxel Monte Carlo (XVMC) algorithm (BrainLAB iPlan version 4.1.2), were analyzed. Total dose of 60 Gy in 5 fractions was delivered to planning target volume (PTV) with at least V100%=95%. Internal target volumes (ITVs) were delineated on maximum intensity projection (MIP) images of 4D CT scans. PTV (ITV+5 mm margin) volumes ranged from 10.0 to 99.9 cc (mean=36.8±20.7 cc). Organs at risk (OARs) were delineated on average images of 4D CT scans. Optimal clinical MC SABR plans were generated using a combination of non‐coplanar conformal arcs and beams for the Novalis‐TX consisting of high definition multileaf collimators (MLCs) and 6 MV‐SRS (1000MU/min) mode. All plans were evaluated using the RTOG 0813 high and intermediate dose spillage criteria: conformity index (R100%), ratio of 50% isodose volume to the PTV (R50%), maximum dose 2 cm away from PTV in any direction (D2cm), and percent of normal lung receiving 20 Gy (V20) or more. Other organs‐at‐risk (OARs) doses were tabulated, including the volume of normal lung receiving 5 Gy (V5), maximum cord dose, dose to <15 cc of heart, and dose to <5 cc of esophagus. Only six out of 18 patients met all RTOG 0813 compliance criteria. Eight of 18 patients had minor deviations in R100%, four in R50%, and nine in D2cm. However, only one patient had minor deviation in V20. All other OARs doses, such as maximum cord dose, dose to <15 cc of heart, and dose to <5 cc of esophagus, were satisfactory for RTOG criteria, except for one patient, for whom the dose to <15 cc of heart was higher than RTOG guidelines. The preliminary results for our limited iPlan XVMC dose calculations indicate that the majority (i.e., 2/3) of our patients had minor deviations in the dosimetric guidelines set by RTOG 0813 protocol in one way or another. When using an exclusive highly sophisticated XVMC algorithm, the RTOG 0813 dosimetric compliance criteria such as R100% and D2cm may need to be revisited. Based on our limited number of patient datasets, in general, about 6% for R100% and 9% for D2cm corrections could be applied to pass the RTOG 0813 compliance criteria in most of those patients. More patient plans need to be evaluated to make recommendation for R50%. No adjustment is necessary for OAR dose tolerances, including normal lung V20. In order to establish new MC specific dose parameters, further investigation with a large cohort of patients including central, as well as peripheral lung tumors, is anticipated and strongly recommended.PACS number: 8087

Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans : review

Photon dose calculation algorithms in treatment planning system could affect the accuracy of dose delivery when tissue heterogeneity is involved along the beam path. Treatment planning for lung cancer is challenging, especially in the case of treatment plan involving small fields. The combination of low-density (air) medium and small fields cause charge particle disequilibrium nears the air/tissue interface. Beam modeling within the dose calculation algorithms must also employ an accurate method of accounting tissue heterogeneity corrections in order to avoid dose overestimation or underestimation. Analytical anisotropic algorithm (AAA) is one of the widely tested and validated dose calculation algorithms in external beam photon radiation therapy. Recently, Acuros XB (AXB) was made available for photon dose calculations, and several studies have demonstrated better dose prediction accuracy of the AXB over AAA. This article reviews the results from the treatment planning studies, which have investigated the clinical dosimetric impact of the AXB and AAA on real lung cancer treatment plans. -------------------------------------- Cite this article as: Rana S. Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans: review. Int J Cancer Ther Oncol 2014; 2 (1):02019. DOI : http://dx.doi.org/10.14319/ijcto.0201.9

The CT number accuracy of a novel commercial metal artifact reduction algorithm for large orthopedic implants

Philips Healthcare released a novel metal artifact reduction algorithm for large orthopedic implants (O‐MAR). Little information was available about its CT number accuracy. Since CT numbers are used for tissue heterogeneity corrections in external beam radiotherapy treatment planning, we performed a phantom study to assess the CT number accuracy of O‐MAR. Two situations were simulated: a patient with a unilateral metallic hip prosthesis and a patient with bilateral metallic hip prostheses. We compared the CT numbers in the O‐MAR reconstructions of the simulations to those in the nonO‐MAR reconstruction and to those in a metal‐free baseline reconstruction. In both simulations, the CT number accuracy of the O‐MAR reconstruction was better than the CT number accuracy of the nonO‐MAR reconstruction. In the O‐MAR reconstruction of the unilateral simulation, all CT numbers were accurate within ±5HU (AAPM criterion). In the O‐MAR reconstruction of the bilateral simulation, CT numbers were found that differed more than ±5HU from the metal‐free baseline values. However, none of these differences were clinically relevant.PACS numbers: 87.57.Q‐, 87.57.cp

Evaluation of Acuros XB algorithm based on RTOG 0813 dosimetric criteria for SBRT lung treatment with RapidArc

The Radiation Therapy Oncology Group (RTOG) 0813 protocol requires the use of dose calculation algorithms with tissue heterogeneity corrections to compute dose on stereotactic body radiation therapy (SBRT) non‐small cell lung cancer (NSCLC) plans. A new photon dose calculation algorithm called Acuros XB (AXB) has recently been implemented in the Eclipse treatment planning system (TPS). The main purpose of this study was to compare the dosimetric results of AXB with that of anisotropic analytical algorithm (AAA) for RTOG 0813 parameters. Additionally, phantom study was done to evaluate the dose prediction accuracy of AXB and AAA beyond low‐density medium of different thicknesses by comparing the calculated results with the measurements. For the RTOG dosimetric study, 14 clinically approved SBRT NSCLC cases were included. The planning target volume (PTV) ranged from 3.2‐43.0 cc. RapidArc treatment plans were generated in the Eclipse TPS following RTOG 0813 dosimetric criteria, and treatment plans were calculated using AAA with heterogeneity correction (AAA plans). All the AAA plans were then recalculated using AXB with heterogeneity correction (AXB plans) for identical beam parameters and same number of monitor units. The AAA and AXB plans were compared for following RTOG 0813 parameters: ratio of prescription isodose volume to PTV (R100%), ratio of 50% prescription isodose volume to PTV (R50%), maximal dose 2 cm from the PTV in any direction as a percentage of prescription dose (D2cm), and the percentage of ipsilateral lung receiving dose equal to or larger than 20 Gy (V20). The phantom study showed that the results of AXB had better agreement with the measurements, and the difference ranged from −1.7% to 2.8%. The AAA results showed larger disagreement with the measurements, with differences from 4.1% to 12.5% for field size 5×5 cm2 and from 1.4% to 6.8% for field size 10×10 cm2. The results from the RTOG SBRT lung cases showed that, on average, the AXB plans produced lower values for R100%, R50%, and D2cm by 4.96%, 1.15%, and 1.60%, respectively, but higher V20 of ipsilateral lung by 1.09% when compared with AAA plans. In the set of AAA plans, minor deviation was seen for R100% (six cases), R50% (nine cases), D2cm (four cases), and V20 (one case). Similarly, the AXB plans also showed minor deviation for R100% (one case), R50% (eight cases), D2cm (three cases), and V20 (one case). The dosimetric results presented in the current study show that both the AXB and AAA can meet the RTOG 0813 dosimetric criteria.PACS number: 87.55.D‐, 87.55.dk, 87.55.kd, 87.55.km

A multicenter phase II study of cetuximab in combination with chest radiotherapy and consolidation chemotherapy in patients with stage III non-small cell lung cancer

BackgroundCetuximab has demonstrated improved efficacy in combination with chemotherapy and radiotherapy. We evaluated the integration of cetuximab in the combined modality treatment of stage III non-small cell lung cancer (NSCLC). MethodsPatients with surgically unresectable stage IIIA or IIIB NSCLC were treated with chest radiotherapy, 73.5Gy (with lung and tissue heterogeneity corrections) in 35 fractions/7 weeks, once daily (63Gy without heterogeneity corrections). Cetuximab was given weekly during radiotherapy and continued during consolidation therapy with carboplatin and paclitaxel up to a maximum of 26 weekly doses. The primary endpoint was overall survival. Baseline tumor tissue was analyzed for EGFR by fluorescence in situ hybridization (FISH). ResultsForty patients were enrolled in this phase II study. The median overall survival was 19.4 months and the median progression-free survival 9.3 months. The best overall response rate in 31 evaluable patients was 67%. No grade 3 or 4 esophagitis was observed. Three patients experienced grade 3 rash; 16 patients (69%) developed grade 3/4 neutropenia during consolidation therapy. One patient died of pneumonitis, possibly related to cetuximab. EGFR gene copy number on baseline tumor tissues, analyzed by FISH, was not predictive of efficacy outcomes. ConclusionsThe addition of cetuximab to chest radiotherapy and consolidation chemotherapy was tolerated well and had modest efficacy in stage III NSCLC. Taken together with the lower incidence of esophagitis, our results support evaluation of targeted agents instead of chemotherapy with concurrent radiotherapy in this setting.

SU‐E‐T‐469: Dosimetric Impact of Whole Breast Treatment Planning with Tissue Heterogeneity Correction

Purpose: Dose to normal organs, such as lung and heart, are more and more considered in the design of whole breast radiation treatments. In this study, the dosimetric impacts of heterogeneity correction on doses of such cases are investigated. Methods: Six clinical whole breast + SCV treatment cases were used in the study. These cases were first planned using static 6 MV photon beams with multileaf collimators and wedges where necessary. The plans were calculated using AAA without heterogeneity correction and the plans met the dose constraints set for the breast and critical organs. The plans were then recalculated with heterogeneity correction using the same MUs. We compared doses for breast, ipsilateral lung and heart. Results: With tissue heterogeneity correction, the breast mean and maximum doses increased, on average, by 0.8 % from 98.9 % and 0.5 % from 110.2 % of the prescribed doses, respectively. The breast volume receiving 95 % of the prescribed dose (V95%) increased by 1.5 % from 83.6 %. The ipsilateral lung mean and maximum doses increased by 2.3 % from 25.3 % and −0.2 % from 100.6 % of the prescribed dose, respectively. The ipsilateral lung V40% and V20% increased by 1.1 % from 25.1 % and 5.4 % from 29.2 %, respectively. The mean and maximum doses for heart increased by 0.1 % from 6.9 % and 0.8 % from 87.1 % of the prescribed dose, respectively, and the heart V20% increased by 0.2 % from 1.4 %. Conclusion: Calculated doses to breast and critical structures with tissue heterogeneity correction tend to be a little higher than those without the correction Dose constraints derived from experience without the correction may need to be adjusted accordingly if heterogeneity correction is used.

Radiobiological evaluation of dose calculation algorithms in RapidArc planning of esophageal cancer treatment plans

Normal 0 false false false EN-US X-NONE X-NONE Purpose: The purpose of current study is to investigate the impact of tissue heterogeneity corrections in Acuros XB algorithm (AXB) and Anisotropic Analytical Algorithm (AAA) on RapidArc esophageal cancer treatment plans using equivalent uniform dose (EUD) calculations and tumor control probability (TCP). Methods: Ten esophageal cancer cases were selected for the current study. All cases were planned using RapidArc technique in Eclipse treatment planning system (10.0.28). The treatment plans were inversely optimized using progressive resolution optimizer and each optimized plan was calculated using AAA with tissue heterogeneity correction. The AAA plans were then normalized such that prescription dose covered 95% of the planning target volume (PTV). The AXB plans were generated by recalculating the normalized AAA plans using AXB with tissue heterogeneity correction for identical monitor units and beam parameters as in the corresponding AAA plans. The EUD and TCP calculations were then performed on the AAA and AXB plans. Results: For PTV, the EUD values in AXB plans were always lower than in AAA plans with an average difference of 1.3%. Similarly, AXB calculations produced smaller magnitude of EUD for the organs at risk (OARs) compared to AAA calculations. On average, in AXB plans, the lung EUD was lower by 3.5%, the liver EUD was lower by 2.3%, the heart EUD was lower by 2.8%, and the spinal cord EUD was lower by 1.8%. The TCP of AXB plans was lower by average difference of 7.1% compared to that of AAA plans. Conclusion: In comparison to AAA calculations, the preliminary results presented in the current study showed that AXB calculations produced lower EUD and TCP for the PTV as well as lower EUD for OARs in the esophageal cancer treatment plans created by RapidArc planning technique.

Dosimetric Accuracy of Ray-tracing Algorithm for Treatment of Thoracic Spine Lesions Using Robotic Radiosurgery

Dosimetric Accuracy of Ray-tracing Algorithm for Treatment of Thoracic Spine Lesions Using Robotic Radiosurgery

SU-E-T-569: A Dosimetric Comparison of Helical Tomotherapy versus Intensity Modulated Proton for Lung Cancer.

To evaluate the dosimetric difference between helical tomotherapy (HT) and intensity modulated proton therapy (IMPT) treatment for lung cancer patients. Five patients treated by HT at University of Wisconsin Carbone Cancer Center were selected. HT plans were generated on TomoTherapy treatment planning station (TomoTherapy Inc., USA). The field widths were set to 2.5 cm for all patients in this study. The IMPT plans were generated using the same planning CT and contours with our in-house treatment planning system. Three to five field spot scanning IMPT were used to deliver uniform doses to the targets while minimizing the irradiated lung volume. The proton spots used has a Gaussian sigma of 6mm and are placed on a rectangular grid. The dose distribution of each proton spot is calculated using a pencil beam algorithm with tissue heterogeneity corrections. All the dosimetric analyses are performed using normalized total dose. Alpha/beta ratios were set to 3 for normal tissues and 10 for tumors. IMPT plans showed improvement of critical structure avoidance and target dose uniformity for all patients. Reductions in mean lung doses of between 81% to 27% were observed in the IMPT plans relative to the HT. The equivalent uniform dose of the target improved from 49.2 Gy in HT plan to 60.04 Gy in IMPT for patient #2, and equivalent for other cases. The maximum doses to cord were reduced by 20.5 Gy on average using IMPT. In two patient cases, the normal tissue complication probabilities were reduced by 53% and 14% with IMPT. IMPT provides improved dose homogeneity on the target and normal structure sparing compared with HT in the treatment of non-small cell carcinoma in lung. Significant reduction of mean lung dose was demonstrated, as well as toxicity to organs at risk adjacent to the target.

SU-E-T-286: Verification of Treatment Delivery Monitor Unit (MU) Calculation and Dose Estimation of Bilateral Total Body Irradiation (TBI).

TBI treatment delivery MU and patient dose estimation are calculated manually at our institution. This study was to verify the accuracyof MU calculation and dose estimation of bilateral TBI by application of tissue heterogeneity correction. Twelve TBI patients were simulated from neck to thigh in bilateral TBI position. CT images were imported into the treatment planning system (Philips, Pinnacle3). Treatment dose was prescribed to the midpoint at the level of the umbilicus. Treatment distance was 519 cm. Both 6MV and 23 MV opposite lateral beams delivered 200 cGy to the dose prescription point with a 40 ×40 cm2 field size and 45o collimator angle. A 1 cm thick spoiler was placed about 15 cm from patient skin. Adaptive convolution superposition with and without heterogeneity correction was used for calculation of MUs and doses at the midpoints of the shoulder, chest, abdomen, and pelvis. Monitor units calculated with heterogeneity correction were 1.1% and 0.9% smaller on average than those without heterogeneity correction for 6MV and 23MV beams respectively. The maximum deviations of MU were 3.8% and 2.8% smaller. Average percentage differences of point doses with and without heterogeneity corrections were -0.2%, 17.0%, -0.3%, and -2.7% at the levels of shoulder, chest, abdomen, and pelvis for 6MV beam and 0.4%, 11.0%, 0.2%, and -1.7% for 23MV beam. Discrepancy of doses to the points at the shoulder level varied from -6.8% to 8.9% for 6MV beam and from -1.6% to 5.1% for 23MV beam. Bilateral TBI MU calculation errors caused by ignoring tissue inhomogeneity would be less than 4% and 3% for 6MV and 23 MV beam. Dose estimation is less accurate using 6MV beam and the inaccuracy could be more than 8% for shoulder midpoint and 4% for pelvis midpoint.

Evaluation of the AAA Treatment Planning Algorithm for SBRT Lung Treatment: Comparison with Monte Carlo and Homogeneous Pencil Beam Dose Calculations

PurposeTo evaluate the dose calculation accuracy of the Varian Eclipse anisotropic analytical algorithm (AAA) for stereotactic body radiation therapy (SBRT), and to investigate the dosimetric consequences of not applying tissue heterogeneity correction on complex SBRT lung plans. Materials and MethodsNine cases of non–small-cell lung cancer (NSCLC) that were previously treated with SBRT at our center were selected for this study. Following Radiation Therapy Oncology Group 0236, the original plans were calculated using pencil beam without heterogeneity correction (PBNC). For this study, these plans were recalculated by applying tissue heterogeneity correction with the AAA algorithm and with the Monte Carlo (MC) method, keeping the number of monitor units the same as the original plans. Two kinds of plan comparison were made. First, the AAA calculations were compared with MC. Second, the treatment plans that were calculated with AAA were compared with the original PBNC calculations. The following dose-volume parameters were used for the comparison: V100%; V90%; the maximum, the minimum, and the mean planning target volume (PTV) doses (Dmax, Dmin, and Dmean, respectively); V20Gy, V15Gy, V10Gy, V5Gy; Dmean for the lung; and Dmax for the critical organs. ResultsComparable results were obtained for AAA and MC calculations: except for Dmax, Dmin, and Dmean, the differences in the patient-average values of all of the PTV dose parameters were less than 2%. The largest average difference was observed for Dmin (3.8 ± 5.4%). Average differences in all the lung dose parameters were under 0.2%, and average differences in normal tissue Dmax were under 0.3 Gy, except for the skin dose. There were appreciable differences in the PTV and normal tissue dose-volume parameters when comparing AAA and PBNC calculations. Except for V100% and V90%, PBNC calculations on average underestimated the dose to the PTV. The largest discrepancy was in the PTV maximum dose, with a patient-averaged difference of 11.1 ± 4.6%. ConclusionsBased on our MC investigation, we conclude that the Eclipse AAA algorithm is sufficiently accurate for dose calculations of lung SBRT plans involving small 6-MV photon fields. Our results also demonstrate that, although dose calculations at the periphery of the PTV showed good agreement when comparing PBNC with both AAA and MC calculations, there is a potential to significantly underestimate the dose inside the PTV and doses to critical structures if tissue heterogeneity correction is not applied to lung SBRT plans.

Maximum PTV Dose Predicts for Radiation Pneumonitis Following Lung Stereotactic Body Radiation Therapy

Materials/Methods: Patients were treated with SBRT for medically inoperable non-small cell lung cancer or metastatic lung lesions at Mayo Clinic from January 11, 2008 to January 12, 2010. Treatment planning was performed with full body immobilization, 4-dimensional CT-based planning and daily cone-beam CT for image guidance. Tissue heterogeneity corrections were used in all cases. Dosimetric data was collected prospectively on all patients. Follow-up care consisted of CT imaging and clinical exam every 3 months. The incidence of radiation pneumonitis (RP) was evaluated and graded according to CTCAE v3.0. The impact of demographic and dosimetric variables on the risk of RP . grade 2 was estimated using Cox proportional hazards regression modeling.