Comparison Of Treatment Plans Research Articles

Assessment of the Air-Electron Streaming Effect for MRI Guided Lung SBRT: A Treatment Planning Study

Electron streams outside the body are inevitably generated in the presence of magnetic field during MRI guided radiotherapy. This study aims to investigate the air-electron streaming effects for lung Stereotactic Body Radiotherapy (SBRT) treated on the 1.5T MRI Linac. Three lung SBRT cases with prescription dose ranging from 30Gy to 60Gy in four to five fractions were investigated in this study. The original curative treatment plan was designed by using step-and-shoot intensity modulated radiotherapy (IMRT) technique and then calculated with Monaco (v5.40.01) Monte Carlo dose engine under the 1.5 T magnetic field. To investigate the effect of air-electron streaming, another comparable treatment plan without 1.5T magnetic field was generated using the same planning parameters. In order to display the in-air dose distribution outside the body in Monaco system, an auxiliary structure was created by extending the external with margin of 8cm in anterior. The two plans with or without magnetic field were then compared in terms of in air dose distribution outside the body and dose to the chin. Electron stream effects outside the body were obviously observed for all 3 lung SBRT cases when in the presence of magnetic field, while which were not observed in the absence of magnetic field. From sagittal view, the electron stream can travel a long distance in air along the magnetic field direction, even can reach to the patient’s chin. But only two of the three cases have CT scan including the chin region. For that two cases, the chin surface skin doses were 1.47Gy (2.45% of 60Gy) and 2.24Gy (7.47% of 30Gy) respectively. While for the plans without magnetic field, the corresponding chin surface skin dose were 0.18Gy (0.3% of 60Gy) and 0.32Gy (1.07% of 30Gy) respectively. For the last case, the streaming electrons were expected to reach at the chin area, but due to which was not scanned in CT image, it’s hard to evaluate quantitatively. Electron streaming effects outside the body were also observed in MRI guided lung SBRT plans and which could lead to a higher chin dose compared to conventional scenario without magnetic field. It is therefore recommended for the MRI guided lung SBRT patients that the chin region should be scanned and included in planning CT image. Other suggestions for example shielding with bolus or keeping the patient’s head tilted to the opposite side should also be considered.

Dosimetric Modeling of Intracavitary versus Combined Intracavitary/Interstitial Applicators During High Dose Rate Brachytherapy for Cervical Cancer

With increased adoption of image guided adaptive brachytherapy (IGABT) for definitive treatment of cervical cancer, combined intracavitary/interstitial (IC/IS) brachytherapy applicators are increasingly utilized as a means of dose escalation while minimizing exposure to organs at risk (OAR) structures compared to traditional intracavitary (IC) applicators. Aside from high risk CTV volume, little data exists to guide clinicians as to relative OAR dose reductions that might be predicted for a given patient with use of combined IC/IS vs IC applicators and tumor/target volume characteristics that correlate with greatest benefit. We performed pair-wise comparisons of IGABT treatment plans modeled with IC and IC/IS applicators to determine relative dose reductions to bladder, rectum, and sigmoid. In addition, we hypothesized that CTV asymmetry would be associated with a relatively greater degree of OAR dose reduction with use of an IC/IS applicator. Treatment planning MRI or CT images for fifteen consecutive patients treated with IC or IC/IS brachytherapy applicators as part of definitive therapy for FIGO 2018 stage IB3 to IIIC cervical cancer at a tertiary care academic medical center were obtained. For each dataset, high dose rate brachytherapy treatment plans were generated for both IC and IC/IS brachytherapy applicators using a treatment planning software. When interstitial needles were not placed at the time of applicator insertion, needle/dwell positions were modeled based on ovoid position and patient anatomy. Treatment plans were generated to minimize dose to the D2 cc volume of OAR contours while reaching an equivalent dose to the D90 of the high-risk CTV. OAR (bladder, rectum, and sigmoid colon), CTV, and GTV volumes, consistent with GEC-ESTRO guidelines, were obtained from previously administered treatment plans and were not modified. As a surrogate for target volume asymmetry, the ratio of maximum antero/posterior to lateral CTV dimension (AP:L ratio), was calculated for each patient. Paired t-tests were used to identify significant differences between IC and IC/IS treatment plans with respect to OAR doses, while Spearman correlation tests were used to assess correlations between OAR sparing and CTV AP:L ratios. Mean volumes for GTV and CTV were 7.4 (1.1 to 19.4) and 29.7 (10.4 to 52.3) cubic centimeters. Mean doses to D2 cc volumes were 15.9% lower for bladder (p<0.01); 15.3% lower for rectum(p<0.01); and 10.0% lower for sigmoid (p<0.01) with IC/IS plans. The CTV AP:L ratio demonstrated a significant inverse correlation with D2cc dose reduction for bladder (p<0.05), rectum (p<0.05), and sigmoid (p<0.05). For an equivalent prescribed radiation dose, IC/IS applicators are associated with significant reductions in bladder, rectal, and sigmoid D2cc doses. CTV asymmetry, as measured by AP:L ratio, is associated with relatively greater dose reductions to bladder, rectum, and sigmoid with use of IC/IS applicators.

Dosimetric Comparison of the Newest Version Tomotherapy, Volumetric-Modulated Arc Therapy, and Intensity-Modulated Proton Therapy Plans for Angiosarcoma of the Scalp

Patients with angiosarcoma of the scalp (AS) have an extremely unfavorable prognosis. As local recurrence is one of the main causes of failure, radiotherapy is established as an integral component of the treatment for AS. Delivering an adequate dose to the target while limiting the dose to the surrounding organs at risk (OARs) such as the brain is challenging. We aimed to compare treatment plans among the newest version of tomotherapy (Tomo), volumetric-modulated arc therapy (VMAT), and intensity-modulated proton therapy (IMPT) for AS patients. We created radiotherapy plans using Tomo, VMAT, and IMPT for 19 AS patients treated at our institution and conducted a pilot study comparing the three methods. The clinical target volume (CTV) 1 included the gross tumor volume plus 3.0 cm and CTV2 was defined as the total scalp. The planning target volume (PTV) 1 and PTV2 were defined as CTV1 plus 0.5 cm and CTV2 plus 0.5 cm other than PTV1, respectively. For IMPT, a setup robustness of ± 0.2 cm was used for the setting of PTV. 70 Gy and 56 Gy were prescribed to D95% (i.e., dose to 95% volume of PTV) of PTV1 and PTV2, respectively, using the simultaneous integrated boost technique. Other constraint goals and acceptable ranges of PTV1, PTV2, and OARs were also set. PTV1 and PTV2 with Tomo, VMAT, and IMPT met the dose constraints of D95% and D98% at least within acceptable ranges for all patients. D95% and D98% of PTV1 and PTV2 were similar among the three methods. D2% of PTV1 and PTV2 were significantly higher in IMPT than in Tomo and VMAT. HI for the IMPT plans was significantly higher than for the other plans; HI was 1.22±0.10 (SD) for the IMPT plans, 1.13±0.04 for the Tomo plans, and 1.14±0.04 for the VMAT plans (p < 0.001). D1% and Dmean of the spinal cord (planning organ at risk volume, PRV), brain, brainstem (PRV), optic pathway (PRV), and both inner ears were significantly lower in IMPT than in Tomo and VMAT. Dmax of the spinal cord (PRV) and optic pathway (PRV) and Dmean of the brain, brainstem (PRV), and optic pathway (PRV) were lower in VMAT than in Tomo. Dmean of the brain was 37.4±6.6 Gy in Tomo, 29.7±3.6 Gy in VMAT, and 17.9±2.0 Gy in IMPT (p < 0.001). The three methods could provide adequate coverage of the PTV while compromising the dose constraint of OARs. Although the dose homogeneity within the PTV was worse in IMPT than in Tomo and VMAT, the three methods could provide a well acceptable homogeneity. Among the three methods, IMPT achieved the best OAR sparing, especially for the brain.

Dosimetric comparison of tomotherapy and volumetric-modulated arc therapy for children with neuroblastoma.

ImportanceIrradiation treatment for pediatric patients with neuroblastoma represents a major challenge due to the pediatric dose limits for critical structures and the necessity of sufficient dose coverage of the clinical target volume for local control.ObjectiveTo investigate dosimetric differences between tomotherapy (TOMO) and volumetric‐modulated arc therapy (VMAT) as retroperitoneal radiotherapy for children with neuroblastoma.MethodsEight patients who received retroperitoneal radiotherapy for neuroblastoma were selected for comparison of TOMO and VMAT treatment plans. The Dmin, Dmax, Dmean, D95, D2, and D98 of planning target volume (PTV), conformity index (CI), heterogeneity index (HI), and organs at risk (OARs) parameters were compared. Delivery machine unit (MU) and image‐guide radiotherapy solution results were also compared.ResultsAll patients received a cumulative dose of 19.5 Gy to the PTV. VMAT showed higher CI (0.93 ± 0.02), compared with TOMO (0.87 ± 0.03, P < 0.001). Notably, the average PTV HI was significantly better using TOMO (1.05 ± 0.01) than VMAT (1.08 ± 0.02, P = 0.003). Compared with VMAT, the Dmin, D95, and D98 all exhibited increases in TOMO; Dmax variation was less than 1% in TOMO. The D0.1cc for the spinal cord and D2cc for the small intestine were better in TOMO in terms of OARs. However, TOMO had more MUs and required a longer delivery time.InterpretationBoth planning techniques are capable of producing high‐ quality treatment plans. TOMO is superior for PTV coverage, but inferior for CI. TOMO requires extra treatment time; its cost is greater than the cost of VMAT.

Dose-Volume Comparison of IMRT and PSPT Treatment Plans for Early-Stage Glottic Cancer.

PurposeTo clarify the dose distribution characteristics for early-stage glottic cancer by comparing the dose distribution between intensity-modulated radiation therapy (IMRT) and passive scattering proton therapy (PSPT) and to examine the usefulness of PSPT for early-stage glottic cancer.Materials and MethodsComputed tomography datasets of 8 patients with T1-2 glottic cancer who had been treated by PSPT were used to create an IMRT plan in Eclipse with 7 fields and a PSPT plan in XiO-M with 2 fields. Organs at risk (OARs) included the carotid arteries, arytenoids, inferior constrictor muscles, strap muscles, thyroid cartilage, cricoid cartilage, and spinal cord. The prescription dose was 66 GyRBE in 33 fractions to the planning target volume (PTV). All plans were optimized such that 95% of the PTV received 90% of the prescription dose considering that the skin was slightly spared.ResultsThe superiority of the PSPT was confirmed in all OARs. In the PSPT, the dose to the contralateral carotid artery and the spinal cord, which is slightly distant from the PTV, was dramatically reduced while maintaining the dose distribution uniformity of the PTV by comparison with IMRT.ConclusionPSPT for early-stage glottic cancer resulted in good target dose homogeneity and significantly spared the OARs as compared with the IMRT. PSPT is expected to be effective in reducing late effects and particularly useful for young people.

The risk for developing a secondary cancer after breast radiation therapy: Comparison of photon and proton techniques

Background and purposeTo compare secondary malignancy risks of modern proton and photon therapy techniques for locally advanced breast cancer. Methods and materialsWe utilized dosimetric data from 34 [10 photon-VMAT, 10 photon-3DCRT, 14 pencil beam scanning proton (PBS)] breast cancer patients who received comprehensive nodal irradiation. Employing a model based on organ equivalent dose to account for both inhomogeneous organ dose distributions and non-linear functional dose relationships, we estimated excess absolute risk, excess relative risk, and lifetime attributable risk (LAR) for secondary malignancies. The model uses dose distribution, number of fractions, age at exposure, attained age, the linear-quadratic dose response relationship for cell survival, repopulation factor, as well as gender specific age dependencies, and initial slopes of dose response curves. ResultsThe LAR for carcinoma at age 70 was estimated to be up to 3.64% for esophagus with an advantage of 3DCRT over PBS and VMAT. For the ipsilateral lung, risks were lowest for PBS (up to 5.56%), followed by 3DCRT (up to 6.54%) and VMAT (up to 7.7%). For the contralateral lung, there is a clear advantage of 3DCRT and PBS techniques (risk <0.86%) over VMAT (up to 4.4%). The risk for the contralateral breast is negligible for 3DCRT and PBS but was estimated as up to 1.2% for VMAT. Risks for the thyroid are overall negligible. Independently performed comparative treatment plans on 10 patients revealed that the risk for the contralateral lung and breast using VMAT can be more than an order of magnitude higher compared to PBS. Sarcoma risks were estimated as well showing similar trends but were overall lower compared to carcinoma. ConclusionConventional (3DCRT) techniques led to the lowest estimated risks of, thyroid and esophageal secondary cancers while PBS demonstrated a benefit for secondary lung and contralateral breast cancer risks, with the highest risks overall associated with VMAT techniques.

The dosimetric impact of replacing the TG-43 algorithm by model based dose calculation for liver brachytherapy

PurposeTo compare treatment plans for interstitial high dose rate (HDR) liver brachytherapy with 192Ir calculated according to current-standard TG-43U1 protocol with model-based dose calculation following TG-186 protocol.MethodsWe retrospectively evaluated dose volume histogram (DVH) parameters for liver, organs at risk (OARs) and clinical target volumes (CTVs) of 20 patient cases diagnosed with hepatocellular carcinoma (HCC) or metastatic colorectal cancer (mCRC). Dose calculations on a homogeneous water geometry (TG-43U1 surrogate) and on a computed tomography (CT) based geometry (TG-186) were performed using Monte Carlo (MC) simulations. The CTs were segmented based on a combination of assigning TG-186 recommended tissues to fixed Hounsfield Unit (HU) ranges and using organ contours delineated by physicians. For the liver, V5Gy and V10Gy were analysed, and for OARs the dose to 1 cubic centimeter (D1cc). Target coverage was assessed by calculating V150, V100, V95 and V90 as well as D95 and D90. For every DVH parameter, median, minimum and maximum values of the deviations of TG-186 from TG-43U1 were analysed.ResultsTG-186-calculated dose was found to be on average lower than dose calculated with TG-43U1. The deviation of highest magnitude for liver parameters was -6.2% of the total liver volume. For OARs, the deviations were all smaller than or equal to -0.5 Gy. Target coverage deviations were as high as -1.5% of the total CTV volume and -3.5% of the prescribed dose.ConclusionsIn this study we found that TG-43U1 overestimates dose to liver tissue compared to TG-186. This finding may be of clinical importance for cases where dose to the whole liver is the limiting factor.

The emerging role of proton therapy for esophagus cancer.

Chemoradiotherapy (CRT) plays an essential role in the treatment of esophageal cancer as either curative or neoadjuvant therapy. When delivered with conventional photon-based techniques, multiple adjacent organs at risk including the heart, lungs, kidneys, liver, stomach, and bowel, receive considerable radiation dose which may contribute to acute and late adverse events (AEs). Proton beam therapy (PBT) offers a reduction in radiation exposure to these organs and potentially an improvement in the therapeutic ratio. Herein we discuss the emerging role of PBT for esophageal cancer, including rationale, treatment planning, early dosimetric and clinical comparisons of PBT with photon-based techniques, ongoing prospective trials, and potential areas of opportunity for the incorporation of PBT with the goal of improving outcomes for patients with esophageal cancer.

The impact of computed radiography and teleradiology on patients' diagnosis and treatment in Mweso, the Democratic Republic of Congo.

IntroductionHigh quality diagnostic imaging can provide increased diagnostic accuracy and help guide medical decision-making and management, however challenges for radiology in resource-limited settings are numerous. Diagnostic imaging and teleradiology have financial and logistical implications, so evidence of impact is crucial. We sought to test the hypothesis that the implementation of computed radiography with teleradiology consultation support will significantly change diagnoses and treatment plans in a resource limited setting.MethodPaired before-after study to determine the therapeutic impact of an add-on diagnostic test. ‘Preliminary Plan’ and ‘Final Plan’ forms allowed direct comparison of diagnosis and treatment plans at initial consultation and following radiography and teleradiology. Consecutive consenting patients were included until the sample size (600) was reached. Changes in both diagnosis and treatment plan were analysed in the whole cohort, with sub-analyses of children aged <5 years, and cases of chest radiography.ResultsFinal analysis included 536 cases. Diagnosis changed following radiography and teleradiology in 62% of cases, and treatment plans changed in 61%. In chest radiography cases, 70% of diagnoses and 62% of treatment plans changed, while in children <5 years 66% of diagnoses and 58% of treatment plans changed.Reduced final treatment plans were most common for exploratory surgery (72% decrease), surgical orthopaedic intervention (62% decrease), and TB treatment (52% decrease), allowing more conservative medical or surgical management in 61 cases. Increased final treatment plans were highest in the orthopaedic and interventional surgery and referral categories. Of 42 cases requiring interventional surgery in the final plan, 26 (62%) were identified only after radiography and teleradiology. 16 additional cases were indicated for orthopaedic surgery, 10 cases required patient transfer, and TB treatment was indicated in 45 cases. A change in the original prescription plan occurred in 41% of 536 cases, with one or more prescriptions stopped in 28% of all cases.ConclusionWe found that computed radiography with teleradiology had significant clinical value in this resource-limited setting, with the potential to affect both patient outcomes and treatment costs through providing improved diagnostics and avoiding unnecessary treatments and medications.

Dosimetric Comparison of Treatment Plans Computed With Finite Size Pencil Beam and Monte Carlo Algorithms Using the InCise™ Multileaf Collimator-Equipped Cyberknife® System.

Purpose:InCise™ multileaf collimator (MLC) was introduced for CyberKnife® (CK) Robotic Radiosurgery System (CK-MLC) in 2015, and finite size pencil beam (FSPB) was the only available dose computation algorithm for treatment plans of CK-MLC system. The more advanced Monte Carlo (MC) dose calculation algorithm of lnCise™ was initially released in 2017 for the CK Precision™ treatment planning system (TPS) (v1.1) with new graphic processing unit (GPU) platform. GPU based TPS of the CK offers more accurate, faster treatment planning time and intuitive user interface with smart three-dimensional editing tools and fully automated autosegmentation tools. The MC algorithm used in CK TPS simulates the energy deposited by each individual photon and secondary particles to calculate more accurate dose. In the present study, the dose disparities between MC and FSPB algorithms for selected Stereotactic Ablative Radiation Therapy (SABR) CK-MLC treatment plans are quantified.Materials and Methods:A total of 80 CK-MLC SABR plans computed with FSPB were retrospectively reviewed and compared with MC computed results, including plans for detached lung cancer (or tumors fully surrounded by lung tissues, n = 21), nondetached lung cancer (or tumor touched the chest wall or mediastinum, n = 23), intracranial (n = 21), and pancreas lesions (n = 15). Dosimetric parameters of each planning target volume and major organs at risk (OAR) are compared in terms of normalized percentage deviations (Ndev).Results:This study revealed an average of 24.4% overestimated D95 values in plans using FSPB over MC for detached lung (n = 21) and 14.9% for nondetached lung (n = 23) lesions. No significant dose differences are found in intracranial (0.3%, n = 21) and pancreatic (0.9%, n = 15) cases. Furthermore, no significant differences were found in Ndev of OARs.Conclusion:In this study, it was found that FSPB overestimates dose to inhomogeneous treatment sites. This indicates, the employment of MC algorithm in CK-MLC-based lung SABR treatment plans is strongly suggested.

Dosimetric differences between cesium-131 and iodine-125 brachytherapy for the treatment of resected brain metastases.

PurposeTo compare treatment plans and evaluate dosimetric characteristics of permanent cesium-131 (131Cs) vs. iodine-125 (125I) implants used in brain brachytherapy.Material and methodsTwenty-four patients with 131Cs implants from a prospective phase I/II trial were re-planned with 125I implants. In order to evaluate the volume of brain tissue exposed to radiation therapy (RT), the dose volume histogram was generated for both radioisotopes. To evaluate the dosimetric differences of the two radioisotopes we compared homogeneity (HI) and conformity indices (CI), and dose covering 100% (D100), 90% (D90), 80% (D80), and 50% (D50) of the clinical target volume (CTV).ResultsAt the 100%, 90%, 80%, and 50% isodose lines, the 131Cs plans exposed less mean volume of brain tissue than the 125I plans (p < 0.001). The D100, D90, D80, and D50 were smaller for 131Cs (p < 0.001). The HI and CI for 131Cs vs. 125I were 19.71 vs. 29.04 and 1.31 vs. 1.92, respectively (p < 0.001).ConclusionsCompared to 125I, 131Cs exposed smaller volumes of brain tissue to equivalent doses of radiation and delivered lower radiation doses to equivalent volumes of the CTV. 131Cs exhibited a higher HI, indicating increased uniformity of doses within the CTV. Lastly, 131Cs presented a CI closer to 1, indicating that the total volume receiving the prescription dose was closer to the desired CTV volume. These results suggest that 131Cs is dosimetrically superior to 125I and may explain the reason for the 0% incidence of radiation necrosis (RN) in our previously published prospective study using 131Cs.

2503. Differences Between Experts and Community Clinicians in Selecting HIV Switch Regimens for Patients With Viral Suppression

BackgroundPatients and clinicians may consider switching suppressive HIV regimens for a variety of reasons, including simplification, improved safety and tolerability, drug interactions, or cost. Because switching treatment is a common clinical dilemma in current HIV care, we developed an online treatment decision support tool to assist providers on selecting a new regimen in a variety of patient scenarios.MethodsIn March 2018, 5 HIV experts provided treatment recommendations for more than 1300 unique HIV switch case scenarios based on a simplified set of variables: CVD, HLA-B*5701, HBV, drug resistance, current ART, and component requiring a switch. We then developed a decision support tool that enabled clinicians to specify a patient scenario using these variables and to select their currently intended approach. The experts’ recommendations for that specific case were then shown, and clinicians were asked if the recommendations changed their planned treatment.ResultsIn the 5 months following release of the tool, healthcare providers (HCPs) entered 932 patient case scenarios. A comparison of HCPs treatment plans vs expert selections in select patient case scenarios is shown in Tables 1 and 2. The data demonstrated several key areas of discordance, including the more frequent selection of the following options by HCPs vs. experts across a wide range of case scenarios: (a) Boosted regimens: 18% to 31% vs. 0% to 4% of cases; (b) TDF-containing regimens: 7% to 25% vs. 0% of cases; and (c) PI-based regimens: 9% to 23% vs. 0% to 4% of cases. In a subset of 88 patient case scenarios where HCPs’ intended treatment differed from experts and HCPs also self-identified the impact of the tool, 48% indicated that their treatment plan would change/agreed with experts after using the tool.ConclusionThis online HIV switch decision support tool shows substantial differences between experts’ and HCPs’ treatment choices for switching therapy in multiple case scenarios. Moreover, consensus expert selections in this online tool resulted in a change to the intended treatment plan for approximately one half of users, suggesting use of the tool can help optimize selection of a new ART regimen for patients switching in the setting of virologic suppression. DisclosuresAll authors: No reported disclosures.

Cost-effectiveness of proton therapy in treating base of skull chordoma.

While proton beam therapy (PBT) can offer increased sparing of healthy tissue, it is associated with large capital costs and as such, has limited availability. Furthermore, it has not been well established whether PBT has significant clinical advantages over conventional volumetric modulated arc therapy (VMAT) for all tumour types. PBT can potentially offer improved clinical outcomes for base of skull chordoma (BOSCh) patients compared with photon (X-ray) therapy, however the cost-effectiveness of these treatments is unclear. In this study, the cost-effectiveness of PBT in the treatment of BOSCh patients is assessed, based on an analysis of comparative radiotherapy treatment plans using a radiobiological Markov model. Seven BOSCh patients had treatment plans for the delivery of intensity modulated proton therapy and VMAT retrospectively analysed. The patient outcome (in terms of tumour local control and normal tissue complications) after receiving each treatment was estimated with a radiobiological Markov model. In addition, the model estimated the cost of both the primary treatment and treating any resultant adverse events. The incremental cost-effectiveness ratio (ICER) was obtained for each patient. PBT was found to be cost-effective for 5 patients and cost-saving for 2. The mean ICER was AUD$1,990 per quality adjusted life year gained. Variation of model parameters resulted in the proton treatments remaining cost-effective for these patients. Based on this cohort, PBT is a cost-effective treatment for patients with BOSCh. This supports the inclusion of PBT for BOSCh in the Medicare Services Advisory Committee 1455 application.

Treatment Outcome and Treatment Plan Comparisons with X-ray for HCC Patients with Small Pretreatment Liver Volume and Treated by Proton Beam

Patients with hepatocellular carcinoma (HCC) often have liver volume (LV) reduction caused by cirrhosis and are frequently ineligible for ablative photon radiotherapy. This study is to report the outcomes in HCC patients with small LV treated with high-dose hypofractionated proton beam therapy (PBT) and to elucidate the dosimetric advantages of protons versus photons. Sixty-two HCC patients with pretreatment LV < 1000 ml undergoing curative-intent PBT (with unirradiated LV to standard LV ratios [ULV/SLV] >30% for Child-Pugh [CP] class A and >40% for CP class B ) were included in this study. The most common dose-fractionation schedules were 72.6 GyE/22 fx. for central tumors (≤1 cm of gastrointestinal tract and porta hepatis [PH], N = 42) and 66 GyE/10 fx. for peripheral tumors (>1 cm from gastrointestinal tract/PH, N = 15). Photon treatment plans was performed in all patients using the same dose schedules as those of PBT, and the dosimetric parameters were compared. Additional photon plans with 36 Gy/3 fx were performed to investigate the eligibility of stereotactic body radiotherapy (SBRT) based on the safety threshold that >700 ml of LV receiving ≤15 Gy. The median pretreatment liver volume was 829 ml (range, 595-995 ml), whereas the median tumor size was 6.0 cm (range, 0.8-22.7 cm). After a median follow-up of 12 months, the 1-year LC, PFS, and OS in patients treated with definitive PBT were 96%, 57%, and 86%, respectively. Radiation-induced liver disease (RILD) was recorded in 3 (4.8%) patients. In the treatment plan comparison using the same hypofractionated dose schedules, proton plans were associated with significantly lower mean doses of liver (16.5 vs. 22.3 Gy, P <0.001), stomach (2.4 vs. 8.4 Gy, P <0.001), duodenum (4.3 vs. 7.2 Gy, P <0.001), lower third esophagus (4.6 vs. 13.9 Gy, P <0.001), heart (3.7 vs. 6.7 Gy, P <0.001), left kidney (2.3 vs. 2.9 Gy, P <0.001), spinal cord (from T9 to L3, 4.5 vs. 7.4, P <0.001), and body (from T9 to L3, 6.4 vs. 9.5, P <0.001), mean D2cc of stomach (9.9 vs. 20.7 Gy, P <0.001), duodenum (16.5 vs. 21.3 Gy, P = 0.002), lower third esophagus (10.8 vs. 22.3 Gy, P <0.001), and mean maximum dose of spinal cord (15.3 vs. 22.7 Gy, P <0.001), compared with the photon plans. As converting the dose-volume histogram (DVH) dose-bins to equivalent doses in 2 Gy (EQD2), 21 (34%) patients had mean liver dose >28 Gy in their photon plans and thereof necessitated target dose de-escalation. In the SBRT plans with 36 Gy in 3 fractions, 50 (81%) patients failed to comply with the constraint that >700 ml of LV receiving ≤15 Gy. The majority of HCC patients with LV <1000 ml were ineligible to SBRT and frequently required dose compromization for hypofractionated photon radiotherapy. Strikingly, PBT permitted these patients to undergo ablative radiation treatment and offered substantial local control benefits with <5% of RILD risk.

Preselection Tool for the Model-Based Selection of Head and Neck Cancer Patients for Proton Therapy

In the Netherlands, head and neck cancer (HNC) patients are selected for intensity modulated proton therapy (IMPT) according to the ‘model-based approach’ (MBA) which is done by integrating the results of individual in silico planning comparison studies (ΔDose of protons versus photons) in NTCP-models to assess ΔNTCP (expected difference in toxicity rates). However, performing planning comparisons in all patients is labor intensive. Therefore, tools to identify patients that are most likely to benefit from protons are needed. The aim of this study was to develop a preselection tool (PST) for a more effective selection of patients for a planning comparison procedure. According to Dutch National Indication Protocol, NTCP-models for grade≥2 dysphagia, xerostomia, and tube feeding dependence are used for patient selection for IMPT. The ΔNTCP-thresholds used to select patients for IMPT are ≥10%, ≥5% and/or ≥15% for grade ≥2, grade ≥3, summed risk reduction (ΣΔNTCP) for grade 2 side effects, respectively. At present, a VMAT plan is created for each patient with optimal sparing of organ at risks (OARs) relevant for the ΔNTCP-profile (model-based optimization). Patients are only selected for a plan comparison if the NTCP-value of one of the 3 endpoints is higher than thresholds (OLD-PST). If that is the case, a robust IMPT-plan is optimized using the same criteria to assess ΔDose and to calculate the ΔNTCP-profile. The NEW-PST assumes a mean dose of 0 Gy in the OAR part that is outside the PTV+5mm. For the remaining part of OAR, the IMPT dose is assumed to be similar to that with VMAT to obtain adequate target coverage. For this study, an IMPT plan was created when patients qualified for protons according to the OLD-PST. Then, we investigated how many unnecessary plan comparisons could be avoided using the NEW-PST. The primary endpoint was: the patient qualifies for protons. 120 patients with HNC who would require a plan comparison according to OLD-PST were included in the study. A VMAT plan was made in 143 patients of which 120 (84%) were selected for plan comparison based on OLD-PST. 55 (46%) of these patients qualified for proton therapy based on plan comparison while for 65 (54%) patients unnecessary plan comparisons were performed. With the NEW-PST, unnecessary plan comparisons were avoided in 18 of these 65 patients. This translates into a reduction of 27% of unnecessary plan comparisons with the NEW-PST. No patient was denied a treatment plan comparison with the NEW-PST, who eventually qualified for protons. The sensitivity and specificity of the NEW PST were 100% and 47%, positive and negative predictive values were 54% and 100%, respectively. The NEW-PST predicts best scenario IMPT dose based on VMAT plan and reduced almost one third of unnecessary plan comparisons compared to current PST without wrongfully denying patients a plan comparison. This contributes to an even more cost-effective selection of patients for protons according to the model-based approach.

Circulating Blood Dose As a Surrogate for Radiation-Induced Lymphocytopenia in Radiation Therapy: Treatment Modality Comparison

Radiation-induced lymphocytopenia is a common occurrence of radiation therapy and is correlated with overall patient survival for a wide range of disease types. We propose to use circulating blood dose as a surrogate for lymphocyte toxicity as well as to use a compartmental model to relate to lymphocyte dose and compare the model results between equivalent treatment using different radiation modalities. Beam dose maps and CT data including MU per beam were exported from treatment planning systems and loaded into in-house software. Anatomical information such as total blood volume, blood flow rate and blood content per tissue type were taken from the literature. Dose to blood “voxels” was calculated as the blood “voxels” flow through the beams, where the blood was assumed to redistribute evenly throughout the body after each beam and dose volume histograms (DVHs) to the circulating blood and lymphocytes were created. A compartmental model with lymphocyte reserves is used to simulate replenishment. Blood DVHs were compared between different modalities: small field vs. large field in prostate cancer, SBRT vs. IMRT in pancreas cancer, FFF vs. non-FFF, EBRT vs. HDR brachytherapy for endo-rectal cancer, 103Pd seed implant vs. EBRT for prostate cancer and 131I vs. EBRT for thyroid cancer. . Comparisons show systematically less blood volume dose from brachytherapy and radiopharmaceutical therapy modalities compared to external beam by factors ranging from 1.4 – 2.0 for cell kill and cell dose metrics. SBRT over IMRT and FFF over non-FFF comparisons showed similar benefits. Blood volume is a likely surrogate for estimating lymphocyte toxicity and should be considered when planning. The proposed model can be used to compare treatment plans and modalities to assist in reducing the lymphocyte toxicity. Use of a larger pre-clinical model may provide better parameterization of compartmental model.

Validation of Dolphin dosimetry in three dimensional patient-specific quality assurance programme.

The aim of this study is to commission and validate Dolphin-Compass dosimetry as a patient-specific Quality Assurance (QA) device. The advancement of radiation therapy in terms of highly conformal delivery techniques demands a novel method of patient-specific QA. Dolphin-Compass system is a dosimetry solution capable of doing different QA in radiation therapy. Dolphin, air-vented ionization detector array mounted on Versa-HD Linear Accelerator (LINAC) was used for measurements. The Compass is a dose computation algorithm which requires modelling of LINAC head similar to other Treatment Planning Systems (TPS). The dosimetry system was commissioned after measuring the required beam data. The validation was performed by comparison of treatment plans generated in Monaco TPS against the measurement data. Different types of simple, complex, static and dynamic radiation fields and highly conformal treatment plans of patients were used in this study. For all field sizes, point doses obtained from Dolphin-Compass dosimetry were in good agreement with the corresponding TPS calculated values in most of the regions, except the penumbra, outside field and at build-up depth. The results of gamma passing rates of measurements by using different Multi-leaf Collimator patterns and Intensity Modulated Radiation Therapy fluence were also found to be in good correlation with the corresponding TPS values. The commissioning and validation of dosimetry was performed with the help of various fields, MLC patterns and complex treatment plans. The present study also evaluated the efficiency of the 3D dosimetry system for the QA of complex treatment plans.

Dosimetric and Radiobiological Comparison of External Beam Radiotherapy Using Simultaneous Integrated Boost Technique for Esophageal Cancer in Different Location.

Objectives: To compare treatment plans of intensity modulated radiotherapy (IMRT), volumetric modulated arc radiotherapy (VMAT), and helical tomotherapy (HT) with simultaneous integrated boost (SIB) technique for esophageal cancer (EC) of different locations using dosimetry and radiobiology.Methods: Forty EC patients were planned for IMRT, VMAT, and HT plans, including 10 cases located in the cervix, upper, middle, and lower thorax, respectively. Dose-volume metrics, conformity index (CI), homogeneity index (HI), tumor control probability (TCP), and normal tissue complication probability (NTCP) were analyzed to evaluate treatment plans.Results: HT showed significant improvement over IMRT and VMAT in terms of CI (p = 0.007), HI (p < 0.001), and TCP (p < 0.001) in cervical EC. IMRT yielded more superior CI, HI and TCP compared with VMAT and HT in upper and middle thoracic EC (all p < 0.05). Additionally, V30 (27.72 ± 8.67%), mean dose (1801.47 ± 989.58cGy), and NTCP (Niemierko model: 0.44 ± 0.55%; Lyman-Kutcher-Burman model: 0.61 ± 0.59%) of heart in IMRT were sharply reduced than VMAT and HT in middle thoracic EC. For lower thoracic EC, the three techniques offered similar CI and HI (all p > 0.05). But VMAT dramatically lowered liver V30 (9.97 ± 2.84%), and reduced NTCP of lungs (Niemierko model: 0.47 ± 0.48%; Lyman-Kutcher-Burman model: 1.41 ± 1.07%) and liver (Niemierko model: 0.10 ± 0.08%; Lyman-Kutcher-Burman model: 0.17 ± 0.17%).Conclusions: HT was a good option for cervical EC with complex target coverage but little lungs and heart involvement as it achieved superior dose conformity and uniformity. Due to potentially improving tumor control and reducing heart dose with acceptable lungs sparing, IMRT was a preferred choice for upper and middle thoracic EC with large lungs involvement. VMAT could ameliorate therapeutic ratio and lower lungs and liver toxicity, which was beneficial for lower thoracic EC with little thoracic involvement but being closer to heart and liver. Individually choosing optimal technique for EC in different location will be warranted.

Frame-based radiosurgery of multiple metastases using single-isocenter volumetric modulated arc therapy technique.

Single‐isocenter volumetric modulated arc therapy (VMAT) technique can provide stereotactic radiosurgery (SRS) treatment with improved delivery efficiency for treating multiple metastases. Nevertheless, planning is time consuming and verification of frame‐based SRS setup, especially at noncoplanar angles, can be challenging. We report on a single‐isocenter VMAT technique with a special focus on improving treatment workflow and delivery verification to exploit the minimized patient motion of the frame‐based SRS. We developed protocols for preplanning and verification for VMAT and evaluated them for ten patient cases. Preplans based on MRI were used to generate comparable treatment plans using CT taken on the day of treatment after frame placement. Target positioning accuracy was evaluated by stereoscopic in‐room kV imaging. Dosimetric accuracy of the noncoplanar plan delivery was validated using measurement‐guided 3D dose reconstruction as well as film‐based end‐to‐end test with a Rando phantom. Average absolute differences of homogeneity indices, conformity indices, and V12Gy between MR preplans and CT‐based plans were within 5%. In‐room imaging positioning accuracy of 0.4 mm was verified to be independent of the distance to the isocenter. For treatment verification, average local and global passing rates of the 3D gamma (1 mm, 3%) were 86% and 99%, respectively. D99 values were matched within 5% for individual target structures (>0.5 cc). Additional film analysis confirmed dosimetric accuracy for small targets that had large verification errors in the 3D dose reconstruction. Our results suggest that the advantages of frame‐based SRS and noncoplanar single‐isocenter VMAT technique can be combined for efficient and accurate treatment of patients with multiple metastases.

Comparison of treatment plans between IMRT with MR-linac and VMAT for lung SABR

BackgroundThe aim of this study was to compare the plan quality of magnetic-resonance image-based intensity modulated radiation therapy (MRI-based-IMRT) with the MRIdian Linac system to that of volumetric modulated arc therapy (VMAT) with the TrueBeam STx system for lung stereotactic ablative radiotherapy (SABR).MethodsA total of 22 patients with tumors located in the lower lobe were retrospectively selected for the study. For each patient, both the MRI-based-IMRT and VMAT plans were generated using an identical CT image set and identical structures with the exception of the planning target volume (PTV). The PTVs of the MRI-based-IMRT were generated by adding an isotropic margin of 3 mm from the gross tumor volume, whereas those of VMAT were generated by adding an isotropic margin of 5 mm from the internal target volume. For both the MRI-based-IMRT and VMAT, the prescription doses to the PTVs were 60 Gy in four fractions.ResultsThe average PTV volume of the MRI-based-IMRT was approximately 4-times smaller than that of VMAT (p < 0.001). The maximum dose to the bronchi for the MRI-based-IMRT was smaller than that for the VMAT (20.4 Gy versus 24.2 Gy, p < 0.001). In addition, V40Gy of the rib for the MRI-based-IMRT was smaller than that for the VMAT (1.8 cm3 versus 7.7 cm3, p = 0.008). However, the maximum doses to the skin and spinal cord for the MRI-based-IMRT (33.0 Gy and 14.5 Gy, respectively) were larger than those for the VMAT (27.8 Gy and 11.0 Gy, respectively) showing p values of less than 0.02. For the ipsilateral lung, the mean dose, V20Gy, V10Gy, and V5Gy for the MRI-based-IMRT were smaller than those for the VMAT (all with p < 0.05). For the contralateral lung, V5Gy, V10Gy, D1500cc, and D1000cc for the MRI-based-IMRT were larger than those for the VMAT (all with p < 0.05). The mean dose and V50% of the whole body for the MRI-based-IMRT were smaller than those for the VMAT (0.9 Gy versus 1.2 Gy, and 78.7 cm3 versus 103.5 cm3, respectively, all at p < 0.001).ConclusionsThe MRI-based-IMRT using the MRIdian Linac system could reduce doses to bronchi, rib, ipsilateral lung, and whole body compared to VMAT for lung SABR when the tumor was located in the lower lobe.