Dose Falloff Research Articles

Dosimetric Comparison of Various Spot Placement Techniques in Proton Pencil Beam Scanning.

PurposeTo present quantitative dosimetric evaluations of five proton pencil beam spot placement techniques.Materials and MethodsThe spot placement techniques that were investigated include two grid-based (rectilinear grid and hexagonal grid, both commonly available in commercial planning systems) and three boundary-contoured (concentric contours, hybrid, and optimized) techniques. Treatment plans were created for two different target volumes, one spherical and one conical. An optimal set of planning parameters was defined for all treatment plans and the impact of spot placement techniques on the plan quality was evaluated in terms of lateral/distal dose falloff, normal tissue sparing, conformity and homogeneity of dose distributions, as well as total number of spots used.ResultsThe results of this work highlight that for grid-based spot placement techniques, the dose conformity is dependent on target cross-sectional shape perpendicular to beam direction, which changes for each energy layer. This variable conformity problem is mitigated by using boundary contoured spot placement techniques. However, in the case of concentric contours, the conformity is improved but at the cost of decreased homogeneity inside the target. Hybrid and optimized spot placement techniques, which use contoured spots at the boundary and gridlike interior spot patterns, provide more uniform dose distributions inside the target volume while maintaining the improved dose conformity. The optimized spot placement technique improved target coverage, homogeneity of dose, and minimal number of spots. The dependence of these results on spot size is also presented for both target shapes.ConclusionThis work illustrates that boundary-contoured spot placement techniques offer marked improvement in dosimetry metrics when compared to commercially available grid-based techniques for a range of proton scanned beam spot sizes.

High-Dose-Rate Brachytherapy Boost Following External Radiotherapy in a Case of Carcinoma External Ear

Squamous cell carcinoma of the external ear is rare and the combination of external beam radiotherapy and brachytherapy has been beneficial for tumors in this location due to the advantage of organ preservation and thereby cosmesis. The incorporation of brachytherapy allows the delivery of doses large enough for the eradication of tumor and the advantage of rapid dose fall off due to brachytherapy minimizes the radiation damage to the nearby organs at risk. Herein we report a case of elderly male with early stage squamous cell carcinoma of external ear treated with a combination of external beam radiation with brachytherapy. An eighty nine year old male reported to our hospital with the complaints of left sided ear ache the presence of some mass in the left external ear. A diagnostic Contrast enhanced Computed Tomography (CECT) scan of head and neck showed a 1.6*1.5*1.4 cm mass lesion (maximum diameter) involving anterior cartilaginous part of left external auditory canal and preauricular soft tissue, thereby staged as T2N0, stage II. Patient was planned for External Beam Radiotherapy (EBRT) using 3D CRT(Conformal Radiotherapy) technique by 6 MV photon beam to a dose of 50 Gy in 25 fractions (2 Gy/#) and following this, a gap of seven days was given and after that four fractions of HDR brachytherapy using Iridium 192 of 3 Gy/# keeping a gap of six hours between two fractions on a single day, Two flexible plastic tubes were used for radiation delivery. The size of the catheter used for radiation delivery was 6 French. Planning CT scan was taken and target volume defined as PTV and a dose of 3 Gy per fraction to a total of 4 fractions was prescribed to the target volume. The spike of intravenous site was used for fixing the plastic tube in the mould. The tumor responded ver well to radiotherapy. Brachytherapy is an effective modality to boost high risk areas without significant damage to the surrounding area. External Beam Radiotherapy in case of early stage carcinoma external auditory canal combined with brachytherapy boost can be considered as a better modality with less severe side effects and better cosmesis.

Significance of Dosimetric Parameters in Patients Undergoing Gamma Knife Radiosurgery for Vestibular Schwannoma.

Vestibular schwannoma (VS) is a benign, encapsulated, and slow-growing tumor of the myelin-forming cells of the 8th cranial nerve. Gamma Knife radiosurgery (GKRS) has become a widely accepted primary treatment modality for small- to medium-sized VSs. In the case of VS, highly conformal, precisely focused radiation is delivered to the acoustic tumor in a single session under the direct supervision of a radiosurgery teamAim:This study aims to determine the significance of Conformity Index and Gradient Index (GI) in patients undergoing GKRS for VS, retrospectively, and re-assess the plans.Materials and Methods:A dosimetric study of 112 patients of VS (both operated and nonoperated) treated on Gamma Knife Perfexion unit at our hospital, over a 3-year period, was carried out retrospectively. The patients’ mean age at the time of GKRS was 48 years and the mean dose to the tumor margin was 13 Gy. The conformality of the treatment plan was determined by Conformity Index. GI determines fall off dose outside the target.Results:The dosimetric parameters such as Conformity Index and GI were calculated using the dose-volume histograms and the volume analysis tools available in the Leksell Gamma Plan using TMR 10 algorithm. The mean Paddick Conformity Index was found to be around 0.80 ± 0.085 and the mean GI was 2.67 ± 0.22.Conclusions:The dosimetric parameters can be used to evaluate the dose coverage and conformity and dose fall off outside the target.

Optimising tumour coverage and organ at risk sparing for hypofractionated re-irradiation in glioblastoma.

Background and purposeRe-irradiation may be used for recurrent glioblastoma (GBM) patients. In some cases Planning Target Volume (PTV) under-coverage is necessary to meet organ at risk (OAR) constraints. This study aimed to develop a Volumetric Modulated Arc Therapy planning solution for GBM re-irradiation including a means of assessing if target coverage would be achievable and how much PTV ‘cropping’ would be required to meet OAR constraints, based on PTV volume and OAR proximity.Materials and methodsFor 10 PTVs, 360°, 180°, two coplanar 180° and 180° + non-coplanar 45° arc arrangements were compared using 35 Gy in 10 fractions. Using the preferred arrangement, dose fall-off was modelled to determine the separation required between PTV and OAR to ensure OAR dose constraints were met, with data presented graphically. To evaluate the graph as an aid to planning, seven cases with overlap were replanned in two treatment planning systems (TPSs).ResultsThere were no significant dosimetric differences between arc arrangements. 180° was preferred due to shorter treatment times. The graph, which indicated if 95% PTV coverage would be achievable based on PTV volume and OAR proximity, was employed in seven cases to guide planning in two TPSs. Plans were deliverable.ConclusionsRe-irradiation treatment planning can be challenging, especially when PTV under-coverage is necessary. 180° was considered optimal. To assist in the planning process, graphical guidance was produced to inform planners whether PTV under-coverage would be necessary and how much PTV ’cropping’ would be required to meet constraints during optimisation.

Dosimetric comparison of robotic- and LINAC-based treatment of spine stereotactic body radiotherapy.

To determine which treatment technique and modality would offer better dosimetric results and be preferable for spinal stereotactic body therapy (SBRT) depending on the three different regions of the vertebrae. Linear accelerator (LINAC)- and CyberKnife (CK)-based treatment techniques were compared in terms of their dosimetric quality, treatment efficiency, and delivery accuracy. Thirty previously treated patients were included in this study. Intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques were used for LINAC-based treatment, whereas CK-based treatment plans were generated for two different collimator systems: fixed and multileaf collimator (MLC). The plans were compared based on spinal cord sparing, dose homogeneity, conformity index (CI), gradient index (GI), monitor unit (MU), and beam-on time. The percentage volumes of V2Gy, V5Gy (representing volume low of the dose spillage region), V10Gy, and V20Gy (representing the volume of the high-dose spillage region) of the healthy tissue were analyzed. The CI and GI of the VMAT plans were better than those of the IMRT plans. For spinal cord sparing, the VMAT and MLC-based CK (CK-MLC) techniques were superior. The percentage of low-dose spillage regions was the lowest for IMRT and fixed cone-based CK (CK-FIX) plans. The percentage of the high-dose spillage region was the lowest for the VMAT and CK-MLC plans. In terms of treatment efficiency, the VMAT and CK-MLC plans were superior to the IMRT and CK-FIX plans. The VMAT technique lowered the MU and beam-on time values. The plan delivery accuracy of the VMAT and CK-FIX plans was better than that of the IMRT plans. VMAT is the best option for LINAC-based spinal SBRT. For CK-based spinal SBRT, MLC-based plans are preferred. If the clinic has both treatment modalities and the patient can tolerate long treatment times, CK-MLC-based treatment should be chosen because of its superiority in sparing the spinal cord and sharp dose fall-off.

A New Benchmark of the Sharpest Dose Fall-Off for Hypo-Fractionated Radiosurgery of Large Single Target Brain Lesions

Previous studies have characterized dose gradient patterns for various hypo-fractionated brain radiosurgery (hSRS) treatment platforms. The Gamma Knife Icon (GKI) has consistently exhibited either superior or non-inferior peripheral dose fall-off and normal brain sparing characteristics when compared to other Linac-based hSRS treatment platforms. Given the prevalence of Linac-based treatments, identifying planning techniques to optimize treatment parameters is desirable. In this study, we investigated a novel Linac-based treatment approach that aimed to create the sharpest dose fall-off for hSRS of large brain lesions.A cohort of patient cases (n = 10) with single brain lesions (volume 27.6+/-8.1 mL, range 20.-42.1 mL) treated with GKI at our institution were selected as sample test cases for our study. A non-coplanar unconstrained VMAT (NCU-VMAT) treatment planning approach was developed, and its script was implemented on a commercial treatment planning system for clinical Linac treatment equipped with the latest high-definition multi-leaf collimators (MLCs). Treatment plans produced via the NCU-VMAT approach were then compared against further optimized treatment plans from GKI, as well as conventional coplanar and non-coplanar VMAT treatment planning approaches. The comparison was carried out using DVH-derived parameters including target volume coverage, target dose conformity, modified dose fall-off index defined as the volume of 50% prescribed target dose divided by the target volume (PIV50).For each case studied, NCU-VMAT achieved practically identical target coverage (0.98 ± 0.01) and Paddick dose conformity index (0.87 ± 0.02) compared to optimized GKI treatments. The mean PIV50 values were 2.99 ± 0.14, 3.77 ± 0.22, and 3.36 ± 0.19 for optimized GKI, conventional coplanar, and non-coplanar VMAT treatment plans, respectively. These results were in excellent agreement with previously published studies. However, the NCU-VMAT technique yielded a mean PIV50 of 2.41 ± 0.07. This represents an improvement of approximately 40% (P < 0.001, paired two-tailed Student t-test) over all existing approaches including GKI. Further analyses revealed that PIV50 of NCU-VMAT approached the theoretical minimum for all the studied cases.Our results demonstrate that in treatment of large brain tumors, a treatment planning approach using Bremsstrahlung x-ray based Linacs can achieve superior dose fall-off for intracranial hSRS compared to GKI. We dispel the myth that physical characteristics of Cobalt γ-rays are needed to create the sharpest dose fall-off. Given the prevalence of Linac-based treatments compared to GKI, we present a novel optimized planning technique that achieves a new benchmark of sharpest dose fall-off.T. Nano: None. O. Morin: None. B.P. Ziemer: None. D. Raleigh: None. L. Boreta: None. J.L. Nakamura: None. S.E. Fogh: Independent Contractor; Accuray. P.K. Sneed: None. S. Hervey-Jumper: None. P.V. Theodosopoulos: None. S.E. Braunstein: Advisory Board; Radiation Oncology Questions, LLC.L. Ma: Patent/License Fees/Copyright; University of California Regents.

ITV-Negation and Dose Fall-Off Features Result in OAR Dosimetric Superiority Favoring MR-Guided Radiotherapy (MRgRT), and Intensity Modulated Proton Therapy (IMPT) Over Volumetric Modulated Arc Therapy (VMAT) for Treatment of Distal Esophageal Cancer

Purpose/Objective(s)Radiation therapy (RT) for distal esophageal cancer (EC) may deliver significant unintended dose to organs at risk (OARs). Advanced RT modalities and motion management strategies could substantially reduce OAR dose, potentially resulting in clinically meaningful impact on clinical outcomes. The aim of this study is to investigate dosimetric plan quality of intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT), both in free breathing (FB), compared to real-time monitoring breath hold (BH) MR-guided radiotherapy (MRgRT) for distal EC.Materials/MethodsNine EC patients originally treated in mid-inspiration BH on a 0.35T MR-guided LINAC using step-and-shoot IMRT were retrospectively planned in FB using IMPT and VMAT. FB-IMPT and FB-VMAT plans incorporated an internal target volume (ITV) whereas BH-MRgRT plans did not. The prescribed dose was standardized at 50.4 Gy in 28 fractions for all 3 techniques. Both MRgRT and VMAT plans used planning target volumes (PTV) margins for optimization employing Monte Carlo and Acuros dose algorithms, respectively. All IMPT plans used single field optimization (SFO) technique and Monte Carlo dose algorithm (Robust optimization - 5% range and 5mm setup uncertainties) to ITVs. A comparison of plan quality was performed using RTOG-defined metrics for target coverage (TC), high dose conformity (PITV), homogeneity index (HI), and dose fall-off metrics: R50% and D2cm. Additionally, dose metrics to the OARs, (lungs, heart and liver) were evaluated.ResultsThe conformity, target coverage and homogeneity indices among FB-IMPT, BH-MRgRT and FB-VMAT plans were similar; FB-IMPT plans generally had superior dose fall-off (Table 1, top). Achieved lung, liver and heart doses are displayed in Table 1 (bottom).ConclusionBoth FB-IMPT and BH-MRgRT achieved a roughly 2-fold or greater reduction in nearly all assessed heart, lung, and liver dose metrics compared to FB-VMAT. Reducing, or ideally eliminating, respiratory motion should be strongly considered for distal EC management regardless of radiation modality or technique. Radiation therapy (RT) for distal esophageal cancer (EC) may deliver significant unintended dose to organs at risk (OARs). Advanced RT modalities and motion management strategies could substantially reduce OAR dose, potentially resulting in clinically meaningful impact on clinical outcomes. The aim of this study is to investigate dosimetric plan quality of intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT), both in free breathing (FB), compared to real-time monitoring breath hold (BH) MR-guided radiotherapy (MRgRT) for distal EC. Nine EC patients originally treated in mid-inspiration BH on a 0.35T MR-guided LINAC using step-and-shoot IMRT were retrospectively planned in FB using IMPT and VMAT. FB-IMPT and FB-VMAT plans incorporated an internal target volume (ITV) whereas BH-MRgRT plans did not. The prescribed dose was standardized at 50.4 Gy in 28 fractions for all 3 techniques. Both MRgRT and VMAT plans used planning target volumes (PTV) margins for optimization employing Monte Carlo and Acuros dose algorithms, respectively. All IMPT plans used single field optimization (SFO) technique and Monte Carlo dose algorithm (Robust optimization - 5% range and 5mm setup uncertainties) to ITVs. A comparison of plan quality was performed using RTOG-defined metrics for target coverage (TC), high dose conformity (PITV), homogeneity index (HI), and dose fall-off metrics: R50% and D2cm. Additionally, dose metrics to the OARs, (lungs, heart and liver) were evaluated. The conformity, target coverage and homogeneity indices among FB-IMPT, BH-MRgRT and FB-VMAT plans were similar; FB-IMPT plans generally had superior dose fall-off (Table 1, top). Achieved lung, liver and heart doses are displayed in Table 1 (bottom). Both FB-IMPT and BH-MRgRT achieved a roughly 2-fold or greater reduction in nearly all assessed heart, lung, and liver dose metrics compared to FB-VMAT. Reducing, or ideally eliminating, respiratory motion should be strongly considered for distal EC management regardless of radiation modality or technique.

A Phase II Study to Determine the Efficacy of Pre-Operative Stereotactic Radiosurgery Followed by Resection for Brain Metastasis

<h3>Purpose/Objective(s)</h3> Brain metastasis is a frequent cause of morbidity and mortality in stage IV cancer patients. Efforts to treat brain metastasis include surgical resection, whole brain radiation treatment (WBRT), and stereotactic radiosurgery (SRS). In contemporary times, SRS is deployed utilizing conformal, high dose radiation with rapid dose fall off to limit neurocognitive toxicity seen with WBRT. In the combined modality setting wherein surgical resection is required, post-operative SRS is utilized with excellent local control. However, several difficulties may arise from treatment in the post-operative setting. Changes to the tumor bed necessitating increased margin for uncertainty, increased treatment normal brain tissue, treatment in a hypoxic field may portend for less-than-ideal treatment. Therefore, pre-operative SRS is being explored due to potentially better-defined target, less target uncertainty, and reports of less radionecrosis and less leptomeningeal disease. Therefore, our group conducted a phase II clinical trial exploring the efficacy of the preoperative SRS approach in regard to local brain control, distant brain control, and overall survival. <h3>Materials/Methods</h3> A phase II institutional non-randomized clinical trial was initiated at UPMC Hillman Cancer Center enrolling patients with brain metastasis of any malignant histology. Lymphoma, leukemia, multiple myeloma, or germ cell tumors were excluded due to their radiosensitive nature. Patients were to receive SRS followed by surgical resection. Inclusion criteria included ≥18yo, KPS≥50 with a life expectancy of 12 weeks. Patients could have no more than four lesions measuring 1.5-4.0cm in size. Patients that had tumors < 3cm in size were required to be symptomatic to enroll on trial. Patients were subsequently selected to receive 15Gy, 18Gy, or 24Gy, based on tumor size. All patients proceeded to surgical resection within seven days of SRS. The primary endpoint was local control of ≥ 85% when compared to historical control rate of 60%. <h3>Results</h3> A total of 24 patients were enrolled on study. One patient was deemed unresectable due to risk to the peri- rolandic location. Median number of brain metastasis treated was one (range 1-3). The most common treated histology was non-small cell lung carcinoma (NSCLC) (9). Median days from consult to SRS was 3.9 days. Median SRS dose and fraction was 16.5Gy [range: 15-24Gy] in a single fraction. The median number of surveillance MRIs performed was 6 [range 1-17]. Local brain control was 90%, 90%, and 74% at 6, 12, and 24 months. Distant brain control at 6, 12, and 24 months was 66%, 60%, and 54% respectively. Progression free survival (PFS) and overall survival (OS) at 6, 12, and 24 months were as followed [46%, 38%, 29%] and [66%, 58%, 50%] respectively. <h3>Conclusion</h3> Preoperative stereotactic radiosurgery appears to have excellent local control with a 6- and 12-month local control rates of 90%. Further study is warranted in the continued exploration of preoperative radiation therapy.

Acute Toxicity Following Ipsilateral Head and Neck Radiation Therapy: Proton vs. Intensity-Modulated Photon Radiotherapy

Given its unique properties, proton beam radiation therapy has the ability to reduce dose to at-risk organs. Proton's Bragg peak gives sharp distal dose fall off, which results in little or no dose beyond this region. As such, ipsilateral head and neck radiation with protons can spare the contralateral side to a greater extent than photon-based radiation, potentially causing reduced acute toxicity.Eighty-six patients (59 protons and 27 intensity modulated radiation therapy [IMRT]) who received primary or adjuvant ipsilateral head and neck radiation were retrospectively reviewed. Patients received radiation to one side and this must have included at least the parotid region in the treatment field. Primary disease site was major salivary gland in 40, cutaneous in 23, tonsil in 19 and other in 4 patients. Passive scatter proton therapy was delivered with 1-3 fields daily. Median dose delivered was 66 cobalt gray equivalent with protons and 60 gray with IMRT.No grade 4 or greater acute mucositis was present in either group. Median acute mucositis was grade 1 for protons and 2 for IMRT. Grade 0, 1, 2 or 3 acute mucositis toxicity was present in 28 (47%), 11 (19%), 17 (29%) and 3 patients (5%) respectively with proton treatment and present in 7 (26%), 3 (11%), 12 (44%) and 5 patients (19%) respectively with IMRT (P = 0.053). Percutaneous endoscopic gastrostomy (PEG) tube was placed during radiation treatment for 1 patient (2%) with protons and 4 patients (15%) for IMRT (P = 0.032). Two patients in the proton group had a PEG tube placed prior to starting radiation and one patient in the IMRT group refused PEG tube placement despite treating physician recommendation. No grade 4 or greater acute radiation dermatitis was experienced by patients in either group. Median acute radiation dermatitis was grade 2 for protons and 2 for IMRT. Grade 0, 1, 2 or 3 toxicity was present in 0 (0%), 7 (12%), 47 (80%) and 4 patients (7%) respectively with proton treatment and present in 0 (0%), 5 (19%), 20 (74%) and 2 patients (7%) respectively with IMRT (P = 0.718). The majority of patients in both treatment groups completed treatment with ≤ 5 days of missed fractions (86.4% with protons, 85.1% with IMRT). Median number of missed treatment days was 2 in both groups.Ipsilateral radiation therapy is well tolerated with IMRT and proton based radiation therapy. Fewer patients underwent PEG tube placement in the proton cohort, however these results should be further validated with prospective patient-toxicity centered studies.

Characterization of an x-ray tube-based ultrahigh dose-rate system for in vitro irradiations.

To present an x-ray tube system capable of in vitro ultrahigh dose-rate (UHDR) irradiation of small<0.3mm samples and to characterize it by means of a plastic scintillation detector (PSD). A conventional x-ray tube was modified for the delivery of short UHDR irradiations. A beam shutter system with a sample holder was designed and installed in a close proximity of an x-ray tube window to enable<1 s irradiations at UHDR. The dosimetry was performed with a small 0.5-mm long 0.5-mm in diameter PSD irradiated with 80, 100, and 120 kVp beams and beam currents of 1-37.5mA. The PSD signal was recorded at frame rates of 20 and 50 fps for shutter exposure between 100 and 1125ms. Irradiation reproducibility was studied with the PSD. The x-ray tube irradiation setup was modeled with Monte Carlo (MC) and dose on a surface of a phantom was also measured with films. The effect of dose delivery uncertainty to 300-μm spheroids due to positioning and spheroid size was evaluated. MC simulations showed good agreement with PSD measurements acquired at both frame rates of 20 and 50 fps in terms of beam temporal profile. PSD-measured dose exhibited excellent linearity as a function of instantaneous dose rate from 3.1 to 118.0Gy/s as well as shutter exposure time from 100 and 1125ms for all investigated beam energies. PSD absorbed dose for the 80, 100, and 120 kVp beams agreed with MC simulations to within 5%. The total delivered doses ranged from 0.4Gy for a 1-mA, 80 kVp beam, and 100ms shutter exposure to 166.9Gy for a 37.5-mA, 80 kVp beam, and a 1125ms exposure. PSD irradiation reproducibility was<0.5%. Simulated and measured dose fall off agreed and it was steep along the axis of the shutter slit (1%/0.1mm) and with depth (2%/0.1mm at 1-mm depth). Spheroid positioning uncertainty of 300μm resulted in dose difference of<3% for x and y shifts but up to 7% uncertainty for a z-shift parallel to the beam axis. A 16% difference in spheroid size resulted in<5% dose difference in spheroid absorbed dose. We have presented a cost-effective x-ray tube-based system with a beam shutter designed for in vitro UHDR delivery and reaching dose rates of up to 118.0Gy/s. The described shutter system can be easily implemented at other institutions, which might enable new researchers to investigate the radiobiology of UHDR irradiations in vitro.

Influence of target dose heterogeneity on dose sparing of normal tissue in peripheral lung tumor stereotactic body radiation therapy

ObjectiveTo evaluate the influence of target dose heterogeneity on normal tissue dose sparing for peripheral lung tumor stereotactic body radiation therapy (SBRT).MethodsBased on the volumetric-modulated arc therapy (VMAT) technique, three SBRT plans with homogeneous, moderate heterogeneous, and heterogeneous (HO, MHE, and HE) target doses were compared in 30 peripheral lung tumor patients. The prescription dose was 48 Gy in 4 fractions. Ten rings outside the PTV were created to limit normal tissue dosage and evaluate dose falloff.ResultsWhen MHE and HE plans were compared to HO plans, the conformity index of the PTV was increased by approximately 0.08. The median mean lung dose (MLD), V5, V10, V20 of whole lung, D2%, D1cc, D2cc of the rib, V30 of the rib, D2% and the maximum dose (Dmax) of the skin, and D2% and Dmax of most mediastinal organs at risk (OARs) and spinal cord were reduced by up to 4.51 Gy or 2.8%. Analogously, the median Dmax, D2% and mean dose of rings were reduced by 0.71 to 8.46 Gy; and the median R50% and D2cm were reduced by 2.1 to 2.3 and 7.4% to 8.0%, respectively. Between MHE and HE plans there was little to no difference in OARs dose and dose falloff beyond the target. Furthermore, the dose sparing of rib V30 and the mean dose of rings were negatively correlated with the rib and rings distance from tumor, respectively.ConclusionsFor peripheral lung tumor SBRT, target conformity, normal tissue dose, and dose falloff around the target could be improved by loosening or abandoning homogeneity. While there was negligible further dose benefit for the maximum target dose above 125% of the prescription, dose sparing of normal tissue derived from a heterogeneous target decreased as the distance from the tumor increased.

Automated robust SBPT planning using EUD-based prediction of SBRT plan for patients with lung cancer

PurposeTo evaluate the quality of robust stereotactic body proton therapy (RSBPT) plans generated by one-clicking scripting method for patients with lung cancer. Materials and methodsRetrospective analysis was performed on fifty lung cancer patients whose plan with robustly stereotactic body radiation therapy (SBRT). Thirty out of fifty patients were used for training to build a regression model, based on robust SBRT reference doses, to predict EUD values of ROIs for robust SBPT planning. Thereafter, robust SBPT plans with both automated EUD-Based mimicking (Automated Robust Proton ARP) and manual (Manual Robust Proton MRP) methods were evaluated in the remaining 20 patients. Plans were compared in terms of dosimetric parameters and planning time. ResultsA statistically significantly improvement in target dose fall off was observed for ARP plans compare to MRP plans (Dose fall off: 135 for MRP and 88 for ARP, p < 0.01), while no differences in target coverage and conformity. A statistically significantly reduce in normal lung tissue were observed for ARP plans compare to MRP plans (Lung [Dmean cGy (RBE)]: MRP: 478 vs. ARP: 351, p < 0.01; Lung [V5Gy (RBE) (%)]: MRP: 16.1 vs. ARP: 12.1, p < 0.01; Lung [V20Gy (RBE) (%)]: MRP: 8.5 vs. ARP: 6.8, p < 0.01). Planning time was reduced for ARP plans compare to MRP plans (optimization time: 12 min for MRP vs. 8 min for ARP; total plan time: 23 min for MRP vs. 18 min for ARP). ConclusionThe automated robust SBPT plans using EUD-Based mimicking of SBRT reference dose improve target dose fall off, reduced the radiation doses to the lungs, reduce planning time, which might be beneficial for patient with lung cancer in clinical.

Comparison of 2 dynamic conformal arc plans based on high-dose rate flattening filter free beams for peripheral lung cancer.

To compare 2 dynamic conformal arc plans based on the high dose rate flattening filter free (FFF) beams, and to evaluate the dosimetric differences. A total of 20 patients with early peripheral non-small cell lung cancer were selected, and 2 dynamic conformal arc plans were designed in the Eclipse 10.0 treatment planning system (TPS). One of them was based on tumor-center (T-DCA), and the other was based on iso-center (Iso-DCA). Both plans were created by using the Truebeam linear accelerator, based on 6 MV FFF photons with a dose rate at 1 400 monitor unit (MU)/min. All patients received the prescribed dose of 4 800 cGy in 4 fractions (1 200 cGy/fraction). Target coverage and organ at risk limits were planned and designed according to the Radiation Therapy Oncology Group (RTOG) Criteria, and were compared between the T-DCA and the Iso-DCA plans. There was no significant difference in the target coverage between the T-DCA and Iso-DCA plans (P>0.05). Conformal index and homogeneity index had no significant differences (both P>0.05), but the percentage of the maximum dose in any direction 2 cm away from the planned target area (D2 cm) and the ratio of the volume wrapped by the isodose line of 50% prescription dose to the volume of the planned target area (R50%) showed significant differences (both P<0.05). The MU of the Iso-DCA plan was increased by 21% compared with that of the T-DCA plan. Except the maximum dose of spinal cord and esophagus, there was no significant difference in the other dosimetric parameters of the organs at risk between the T-DCA and the Iso-DCA plans (all P>0.05). The dose fall-off of Iso-DCA plan is better than T-DCA plan, but the T-DCA plan is consistently superior in sparing dose to spinal cord and esophagus, and the T-DCA plan has fewer MU.

Efficient optimization of R50% when planning multiple cranial metastases simultaneously in single isocenter SRS/SRT.

Simultaneous optimization of multiple Planning Target Volumes (PTVs) of varying size and location in the cranium is a non‐trivial task. The rate of dose falloff around PTV structures is variable and depends on PTV characteristics such as the volume. The metric R50% is one parameter that can be used to quantify dose falloff achieved in a given treatment plan. An important treatment planning question is how to construct optimization conditions that result in the efficient production of acceptable plan outcomes considering metrics such as R50%. Guidance provided in literature suggests generating multiple shell control structures around each PTV. The constraints applied to these shells can vary significantly depending on PTV volume. Additionally, there is no clear guidance on how to prospectively determine objective constraints for the optimization shells to achieve a specified goal of R50%. Based on physical principles and empirical evidence, we provide clear quantitative guidance on how to translate the desired R50% outcome into appropriately sized optimization structures around PTVs via an equation that depends on a desired goal for R50% and the volume of PTV. Optimization schema are also provided that allow the goal R50% to be approached or achieved for all PTVs individually. We demonstrate the application of the methodology using commercially available treatment planning software and radiotherapy treatment equipment.

PLEN03 Presentation Time: 1:45 PM: HDR Interstitial Spine Brachytherapy Using an Intraoperative CT-Guided Surgical Navigation System Compared with Stereotactic Radiosurgery

Purpose: Stereotactic radiosurgery (SRS) has been used to treat spinal metastases. However, reirradiation to the tumor is limited by spinal cord tolerance due to dose fall off outside the target. In order to escalate dose to the target with high accuracy and precision, we have developed a novel high-dose-rate (HDR) interstitial brachytherapy (IS-BT) technique using an intraoperative mobile 3D CT-guided surgical navigation system and compared dosimetry with SRS.

PHSOR11 Presentation Time: 10:50 AM: Utilizing Tumor Control Probability to Aid in Selection of Multi-Dwell or Single-Dwell Methodology for Leipzig-Style Applicators: A Preliminary Study

Purpose: A recent American Brachytherapy Society working group reports that non-melanoma skin cancer is the most prevalent malignancy in the United States, with over two million cases yearly. 192Ir brachytherapy delivered with a single dwell in a Leipzig-style applicator is a common and convenient treatment option. However, this single-dwell design is inherently limited in clinical target coverage due to significant dose inhomogeneity caused by rapid dose fall-off. At the conventional 3 mm depth, targets are constrained to 1 × 1 cm2 area to remain within the traditional constraint of 150% prescription dose at the surface.

Abstract PO-047: Early T cell responses are enhanced by heterogenous dose delivery in a murine model of head and neck cancer

Abstract Background: Combination treatment with radiotherapy can prime the immune system due and improve responses to PD-1 based immunotherapy. However, the optimal dose and schedule of combination therapy is still unclear. Although hypofractionated radiotherapy has been shown to be superior to traditional fractionation schedules, the immunostimulatory effects of this regimen may be limited by death of radiation sensitive T-cells. Here we examine the effects of heterogenous dose delivery on early T cell responses in a murine model of head and neck cancer. Methods: Dose delivery was validated for the SARRP irradiator using high spatial resolution radiochromic film absolute dosimetry. The nozzles size was selected to deliver either a uniform dose of 12 Gy or SORT using two abutted nozzles to deliver conventional (2Gy) and hypofractionated (12 Gy) dose regions. C3H/HeJ mice bearing SCCVII/SF xenografts (n=8/group) were irradiated with either a single fraction of a uniform dose of 12 Gy using the 1 × 1 cm2 nozzle to cover the entire tumor or SORT (2-12Gy) using 0.5 × 0.5 cm2 nozzle. To assess the effect on activation of T cells present within the tumor at the time of radiation, mice were sacrificed 24 hrs after treatment and the presence of T cell subsets and cytokines was determined by real-time PCR. Statistical differences were determined using a one-way ANOVA followed by a post-hoc Tukey T-Test. Results: Radiochromic film measurements indicated highly uniform dose profiles with sharp dose fall off region for 0.5 × 0.5 cm2 nozzle, allowing butting of high and low dose fields. Quantitative PCR revealed increases in CD8 gene expression in both treatment groups over sham-irradiated mice with a slight but significant increase in 2Gy-12Gy treated mice compared to mice treated with 12Gy (7 fold vs 5 fold). No statistical differences in FoxP3 gene expression were observed between the groups. Significant increases of IFN-γ gene expression were observed in SORT treated mice compared to mice irradiated with 12Gy. Mice treated with a uniform 12Gy dose showed a 13.75-fold increase in IFN-γ over sham irradiated mice. This increase was significantly larger in 2Gy-12Gy treated mice in which a mean 67 fold-difference over sham-irradiated mice was observed. In contrast, tumors treated with 12Gy had a significantly higher level of TGF-beta gene expression than mice treated with 12Gy-2GY compared to sham irradiated mice (10 fold vs 6 fold). Conclusion: Although increases in CD8 gene expression were observed in both treatment groups, this increase was not proportional to the large increase in IFN-γ observed in mice treated with a heterogenous dose, indicating enhanced T-cell survival and/or activation. Further study is needed to determine if early immune activation observed with heterogeneous dosing correlates to a sustained immune response and improved response to immunotherapy. Citation Format: Jennifer Mourtada, Adam Raben, James Keller, Neil Hockstien, Yan Yu, Bo Lu, Firas Mourtada. Early T cell responses are enhanced by heterogenous dose delivery in a murine model of head and neck cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-047.

Characterization of the ZAP-X® Peripheral Dose Fall-Off.

Various small-field radiation dose detectors were systematically compared and their impact on measured beam performance of the ZAP-X® dedicated stereotactic radiosurgery system (ZAP Surgical Systems, Inc., San Carlos, CA, USA) was determined. Three Physikalische Technische Werkstaetten (PTW) diodes, i.e., the microSilicon, the microDiamond, and the Stereotactic Radiosurgery (SRS) diode detectors of (PTW-Freiburg, Freiburg, Germany), as well as Gafchromic™ External Beam Therapy 3 (EBT) film (Ashland, Inc., Wilmington, DE, USA), were used and compared to arrive at a recommended standard for this critical component of small-field beam measurements. Beam profiles, including the dose fall-off region near the edge of the beam, were measured with the PTW diodes and EBT3 film and subsequently contrasted. The impact of detector physical and dosimetric characteristics on the results of the measurements was investigated and compared with film measurements. The beam penumbra was used to quantify the dose fall-off. The measurement acquired with the diodes and film showed the most significant differences in the fall-off region near the field edge. The film-based measurements clearly showed the steepest dose gradient verified by the penumbra value of 1.21 mm, followed by the SRS diode with 1.60 mm, the microSilicon diode with 1.67 mm, and the microDiamond diode with 1.83 mm. A clear correlation of each detector’s sensitive area with the penumbra was found, with the microDiamond detector at 2.2 mm diameter sensitive area having the largest penumbra, followed by the microSilicon and SRS diodes. Beam measurements for the purposes of system characterization or treatment planning system beam data acquisition depend, to a large extent, on detector characteristics. This is especially true for small-field dosimetry performed during stereotactic radiosurgery beam measurements. Careful consideration should be practiced which allows for the measurements to represent true beam characteristics and minimize the impact of the detector on the measurements. We conclude that film should be considered the reference method for such measurements with the ZAP-X due to its smallest physical measurement resolution of 23.1 µm. Potential drawbacks to this methodology are the need to calibrate the film relative to the dose and possible problems with saturation and non-linear film response for very high and very low optical densities.

Dosimetric study between a single isocenter dynamic conformal arc therapy technique and Gamma Knife radiosurgery for multiple brain metastases treatment: impact of target volume geometrical characteristics

PurposeTo compare linac-based mono-isocentric radiosurgery with Brainlab Elements Multiple Brain Mets (MBM) SRS and the Gamma Knife using a specific statistical method and to analyze the dosimetric impact of the target volume geometric characteristics. A dose fall-off analysis allowed to evaluate the Gradient Index relevancy for the dose spillage characterization.Material and methodsTreatments were planned on twenty patients with three to nine brain metastases with MBM 2.0 and GammaPlan 11.0. Ninety-five metastases ranging from 0.02 to 9.61 cc were included. Paddick Index (PI), Gradient Index (GI), dose fall-off, volume of healthy brain receiving more than 12 Gy (V12Gy) and DVH were used for the plan comparison according to target volume, major axis diameter and Sphericity Index (SI). The multivariate regression approach allowed to analyze the impact of each geometric characteristic keeping all the others unchanged. A parallel study was led to evaluate the impact of the isodose line (IDL) prescription on the MBM plan quality.ResultsFor mono-isocentric linac-based radiosurgery, the IDL around 70–75% was the best compromise found. For both techniques, the GI and the dose fall-off decreased with the target volume. In comparison, PI was slightly improved with MBM for targets < 1 cc or SI > 0.78. GI was improved with GP for targets < 2.5 cc. The V12Gy was higher with MBM for lesions > 0.4 cc or SI < 0.84 and exceeded 10 cc for targets > 5 cc against 6.5 cc with GP. The presence of OAR close to the PTV had no impact on the dose fall off values. The dose fall-off was higher for volumes < 3.8 cc with GP which had the sharpest dose fall-off in the infero-superior direction up to 30%/mm. The mean beam-on time was 94 min with GP against 13 min with MBM.ConclusionsThe dose fall-off and the V12Gy were more relevant indicators than the GI for the low dose spillage assessment. Both evaluated techniques have comparable plan qualities with a slightly improved selectivity with MBM for smaller lesions but with a healthy tissues sparing slightly favorable to GP at the expense of a considerably longer irradiation time. However, a higher healthy tissue exposure must be considered for large volumes in MBM plans.

Complications of intracavitary brachytherapy for gynecologic cancers and their management: A comprehensive review

Intracavitary gynecologic brachytherapy in the form of tandem-based brachytherapy and vaginal cylinder–based brachytherapy represents a fundamental component of the treatment of women with cervical or uterine cancer due to the ability to deliver a therapeutic dose of radiation with sharp dose falloff. This results in highly effective treatment in terms of oncologic outcomes with an overall favorable toxicity profile. Still, complications and side effects of brachytherapy do exist. While advances in brachytherapy techniques have led to a significant decrease in the rates of toxicity, a thorough understanding of the potential complications is crucial to ensuring optimal outcomes for women with gynecologic cancer undergoing brachytherapy. Use of equivalent dose at 2 Gy per fraction (EQD2) models has allowed incorporation of external beam radiotherapy dose to the brachytherapy dose leading to development of consolidated dose constraints for organs-at-risk in the modern era. This manuscript offers a comprehensive review of potential complications associated with intracavitary brachytherapy for gynecologic cancer including predictive factors, mitigation tactics, and management strategies.