Dose Heterogeneity Index Research Articles

The institutional experience of the implementing 4DCT in NSCLC radiotherapy planning.

The study was to evaluate the effectiveness of dose distribution of four-dimensional computed tomography (4DCT) simulation. The gross tumor volume (GTV) and clinical target volume (CTV) were contoured in all 10 respiratory phases of 4DCT in 30 patients with non-small cell lung cancer (NSCLC). Both 3D and 4D treatment plans were made individually for each patient using the planning volume (PTV). The PTV3D was taken from a single CTV plus the recommended margin, and the PTV4D was taken from the 4D internal target volume, including all 10 CTVs plus the setup margins. The mean PTV was 460 ± 179 (69-820) cm3 for 3DCT and 401 ± 167 (127-854) cm3 for 4DCT (p = 0.0018). The dose distribution (DD) of organs at risk, especially the lungs, was lower for the 4DCT simulation. The V5%, V10%, and V20% of the total lung dose for 4DCT were significantly lower for the 3DCT. However, lung V30% the heart, esophagus, and spinal cord were not significantly different. In addition, the conformity index and the dose heterogeneity index of the PTV were not significantly different. The normal tissue complication probability (NTCP) of the lung and heart was significantly lower for 4DCT than for 3DCT. The 4DCT simulation gives better results on the NTCP. The organs at risk, especially the lungs, receive a significantly lower DD compared with the 3DCT. The conformity index (CI), heterogeneity index (HI) and the DD to the heart, spinal cord, and esophagus were not significantly different between the two techniques.

Prediction of plan adaptation in head and neck cancer proton therapy using clinical, radiographic, and dosimetric features

Purpose Because proton head and neck (HN) treatments are sensitive to anatomical changes, plan adaptation (re-plan) during the treatment course is needed for a significant portion of patients. We aim to predict re-plan at plan review stage for HN proton therapy with a neural network (NN) model trained with patients’ dosimetric and clinical features. The model can serve as a valuable tool for planners to assess the probability of needing to revise the current plan. Methods and Materials Mean beam dose heterogeneity index (BHI), defined as the ratio of the maximum beam dose to the prescription dose, plan robustness features (clinical target volume (CTV), V100 changes, and V100 > 95% passing rates in 21 robust evaluation scenarios), as well as clinical features (e.g., age, tumor site, and surgery/chemotherapy status) were gathered from 171 patients treated at our proton center in 2020, with a median age of 64 and stages from I-IVc across 13 HN sites. Statistical analyses of dosimetric parameters and clinical features were conducted between re-plan and no-replan groups. A NN was trained and tested using these features. Receiver operating characteristic (ROC) analysis was conducted to evaluate the performance of the prediction model. A sensitivity analysis was done to determine feature importance. Results Mean BHI in the re-plan group was significantly higher than the no-replan group (p < .01). Tumor site (p < .01), chemotherapy status (p < .01), and surgery status (p < .01) were significantly correlated to re-plan. The model had sensitivities/specificities of 75.0%/77.4%, respectively, and an area under the ROC curve of .855. Conclusion There are several dosimetric and clinical features that correlate to re-plans, and NNs trained with these features can be used to predict HN re-plans, which can be used to reduce re-plan rate by improving plan quality.

Dosimetric analysis of Rapid Arc (VMAT) treatment planning in head and neck cancer for quality assurance treatment

The current investigation aimed to compare the treatment plans of simultaneous integrated boost (SIB) fixed intensity modulated radiation therapy (IMRT) and SIB RapidArc (RA) using a number of dosimetric indices. In this study, 29 patients of nasopharyngeal carcinoma (NPC) were considered for treatment plan evaluation of SIB RA and SIB IMRT. The plans were evaluated using conformity index (CI), target coverage (TC), gradient index (GI), external volume Index (EI), homogeneity index (HI), dose heterogeneity index (DHI), standard deviation (SD), and unified dosimetric index (UDI). The dose of each planned target volume (PTV) and organs at risk (OARs) was determined using their respective mean and median doses. In accordance with the results, there is no noticeable difference in the values of CI, TC, GI, EI, and UDI for SIB RA and SIB IMRT. DHI of PTV 54 is better for SIB IMRT as compared to SIB RA and DHI of PTV 60, PTV 70 is same for both techniques. HI, SD and sparing of OARs results in better values for SIB RA as compared to SIB IMRT. However, PTV 54 and PTV 60′s doses indicate over dosage. The dose of PTV 70 is found to be within the limits of prescribed dose for both SIB RA and SIB IMRT. SIB RA homogeneity, sparing of OAR, and SD are observed to be superior to SIB IMRT. In the case of RA, less time and a monitor unit are used. In conclusion, SIB RA is thought to be better than SIB IMRT for the treatment of NPC.

Dosimetric and Clinical Features Associated with Replan in Proton Head and Neck Therapy

Intensity modulated proton therapy has become increasingly popular in the treatment of head and neck (HN) cancer because of its high normal tissue sparing compared to photon therapy. However, proton plans are very sensitive to anatomy changes such as weight loss or tumor shrinkage that leads to a 25% replan rate at our center. This analysis aims to predict whether a HN patient will need a replan using various dosimetric and clinical features at the onset of treatment.Data were gathered from 176 HN patients treated at our institution. The most common sites were the oropharynx (40 cases), lip/oral cavity (20 cases), and nasal cavity/sinuses (14 cases). Beam dose heterogeneity index (BHI) was defined as the ratio of the D0.1cc of beam dose to half of the prescription dose. BHI was calculated for every beam and the average BHI for each plan was calculated. Robustness scenarios included a combination of ± 3 mm setup shifts on 3 coordinate directions and ± 3.5% range uncertainty. Plan robustness was evaluated with several features gathered from the highest dose level CTV. These include the fraction of all scenarios where V100 > 95% (passing rate), mean V100, mean V100 deviation from the nominal plan, and mean max dose change from the nominal. 61% of patients were treated to bilateral neck with multi-dose level simultaneous integrated boost plans. A five-beam arrangement (two posterior, two anterior oblique beams, and an AP beam) were used to treat most bilateral HN patients and a 3-beam arrangement was used to treat most unilateral HN patients. Any portion of the target is typically covered by at least two beams.Sixty-three patients were replanned at least once in this subpopulation. Plans were normalized at V100 = 98% when possible for the highest dose level. The BHIs from each planner were significantly different from each other, ranging from 1.01 to 1.27 (P = 0.003). Planner re-plan rates (ranging from 11%-42%) significantly correlate with their BHIs, r = 0.747 (P = 0.03). BHI for re-planned patients was higher than non-replan patients:1.11 ± 0.09 vs. 1.06 ± 0.16 (P = 0.017). The only significant robustness feature was the mean max dose change:3.1 ± 1.3% vs. 2.5 ± 1.4% (P = 0.038).We have identified several features that correlate strongly with HN replans. A fine-tuned planning protocol is being developed to reduce the BHI for all HN plans. Our future work is to build a deep-learning model based on our current features and others such as patients' clinical tumor information, planning image, plan conformity, etc., to predict the probability of a replan before treatment.

Institutional experience with SRS VMAT planning for multiple cranial metastases.

Background and PurposeThis study summarizes the cranial stereotactic radiosurgery (SRS) volumetric modulated arc therapy (VMAT) procedure at our institution.Materials and MethodsVolumetric modulated arc therapy plans were generated for 40 patients with 188 lesions (range 2–8, median 5) in Eclipse and treated on a TrueBeam STx. Limitations of the custom beam model outside the central 2.5 mm leaves necessitated more than one isocenter pending the spatial distribution of lesions. Two to nine arcs were used per isocenter. Conformity index (CI), gradient index (GI) and target dose heterogeneity index (HI) were determined for each lesion. Dose to critical structures and treatment times are reported.ResultsLesion size ranged 0.05–17.74 cm3 (median 0.77 cm3), and total tumor volume per case ranged 1.09–26.95 cm3 (median 7.11 cm3). For each lesion, HI ranged 1.2–1.5 (median 1.3), CI ranged 1.0–2.9 (median 1.2), and GI ranged 2.5–8.4 (median 4.4). By correlating GI to PTV volume a predicted GI = 4/PTV0.2 was determined and implemented in a script in Eclipse and used for plan evaluation. Brain volume receiving 7 Gy (V 7 Gy) ranged 10–136 cm3 (median 42 cm3). Total treatment time ranged 24–138 min (median 61 min).ConclusionsVolumetric modulated arc therapy provide plans with steep dose gradients around the targets and low dose to critical structures, and VMAT treatment is delivered in a shorter time than conventional methods using one isocenter per lesion. To further improve VMAT planning for multiple cranial metastases, better tools to shorten planning time are needed. The most significant improvement would come from better dose modeling in Eclipse, possibly by allowing for customizing the dynamic leaf gap (DLG) for a special SRS model and not limit to one DLG per energy per treatment machine and thereby remove the limitation on the Y‐jaw and allow planning with a single isocenter.

Technical Note: Dose effects of 1.5 T transverse magnetic field on tissue interfaces in MRI-guided radiotherapy

The integration of MRI with a linear accelerator (MR-linac) offers great potential for high-precision delivery of radiation therapy (RT). However, the electron deflection resulting from the presence of a transverse magnetic field (TMF) can affect the dose distribution, particularly the electron return effect (ERE) at tissue interfaces. The purpose of the study is to investigate the dose effects of ERE at air-tissue and lung-tissue interfaces during intensity-modulated radiation therapy (IMRT) planning. IMRT and volumetric modulated arc therapy (VMAT) plans for representative pancreas, lung, breast, and head and neck (HN) cases were generated following commonly used clinical dose volume (DV) criteria. In each case, three types of plans were generated: (1) the original plan generated without a TMF; (2) the reconstructed plan generated by recalculating the original plan with the presence of a TMF of 1.5 T (no optimization); and (3) the optimized plan generated by a full optimization with TMF = 1.5 T. These plans were compared using a variety of DV parameters, including V100%, D95%, DHI [dose heterogeneity index: (D20%-D80%)/Dprescription], Dmax, and D1cc in OARs (organs at risk) and tissue interface. All the optimizations and calculations in this work were performed on static data. The dose recalculation under TMF showed the presence of the 1.5 T TMF can slightly reduce V100% and D95% for PTV, with the differences being less than 4% for all but one lung case studied. The TMF results in considerable increases in Dmax and D1cc on the skin in all cases, mostly between 10% and 35%. The changes in Dmax and D1cc on air cavity walls are dependent upon site, geometry, and size, with changes ranging up to 15%. The VMAT plans lead to much smaller dose effects from ERE compared to fixed-beam IMRT in pancreas case. When the TMF is considered in the plan optimization, the dose effects of the TMF at tissue interfaces (e.g., air-cavity wall, lung-tissue interfaces, skin) are significantly reduced in most cases. The doses on tissue interfaces can be significantly changed by the presence of a TMF during MR-guided RT when the magnetic field is not included in plan optimization. These changes can be substantially reduced or even eliminated during VMAT/IMRT optimization that specifically considers the TMF, without deteriorating overall plan quality.

SU-F-T-613: Multi-Lesion Cranial SRS VMAT Plan Quality

Purpose: Cranial SRS VMAT plans must have steep dose gradient around each target to reduce dose to normal brain. This study reports on the correlation between gradient index (GI=V50%/V100%), target size and target dose heterogeneity index (HI=PTV Dmax/prescription dose) for multi-lesion cranial SRS VMAT plans. Methods: VMAT plans for 10 cranial cases with 3 to 6 lesions (total 39 lesions) generated in Varian Eclipse V11.0.47 with a fine-tuned AAA beam model and 0.125 cm dose grid were analyzed. One or two iso centers were used depending on the spatial distribution of lesions. Two to nine coplanar and non-coplanar arcs were used per isocenter. Conformity index (CI= V100%/VPTV), HI, and GI were determined for each lesion. Dose to critical structures were recorded. Results: Lesion size ranged from 0.05–11.00 cm3. HI ranged from 1.2–1.4, CI ranged from 1.0–2.8 and GI from 3.1–8.4. Maximum dose to brainstem, chiasm, lenses, optic nerves and eyes ranged from 120–1946 cGy, 47–463 cGy, 9–121 cGy, 14–512 cGy, and 17–294 cGy, respectively. Brain minus PTV (Brain-PTV) V7Gy was in the range 1.1–6.5%, and Brain-PTV Dmean was in the range 94–324 cGy. Conclusion: This work shows that a GI 0.4cc. For smaller lesions, GI increases rapidly. GI is lower when HI is increased. Based on this study, recommend HI is 1.4, and recommended GI is for volumes 1.0cc GI<4. CI is < 1.3 for all lesions except for targets < 0.1cc. Cranial SRS VMAT plans must be optimized to lower the GI to reduce the dose to normal brain tissue.

SU-G-JeP2-05: Dose Effects of a 1.5T Magnetic Field On Air-Tissue and Lung-Tissue Interfaces in MRI-Guided Radiotherapy

Purpose: The purpose of the study is to investigate the dose effects of electron-return-effect (ERE) at air-tissue and lung-tissue interfaces under a 1.5T transverse-magnetic-field (TMF). Methods: IMRT and VMAT plans for representative pancreas, lung, breast and head & neck (H&N) cases were generated following clinical dose volume (DV) criteria. The air-cavity walls, as well as the lung wall, were delineated to examine the ERE. In each case, the original plan generated without TMF is compared with the reconstructed plan (generated by recalculating the original plan with the presence of TMF) and the optimized plan (generated by a full optimization with TMF), using a variety of DV parameters, including V100%, D95% and dose heterogeneity index for PTV, Dmax, and D1cc for OARs (organs at risk) and tissue interface. Results: The dose recalculation under TMF showed the presence of the 1.5 T TMF can slightly reduce V100% and D95% for PTV, with the differences being less than 4% for all but lung case studied. The TMF results in considerable increases in Dmax and D1cc on the skin in all cases, mostly between 10-35%. The changes in Dmax and D1cc on air cavity walls are dependent upon site, geometry, and size, with changes ranging up to 15%. In general, the VMAT plans lead to much smaller dose effects from ERE compared to fixed-beam IMRT. When the TMF is considered in the plan optimization, the dose effects of the TMF at tissue interfaces are significantly reduced in most cases. Conclusion: The doses on tissue interfaces can be significantly changed by the presence of a 1.5T TMF during MR-guided RT when the TMF is not included in plan optimization. These changes can be substantially reduced or even removed during VMAT/IMRT optimization that specifically considers the TMF, without deteriorating overall plan quality.

Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: a tomotherapy investigation.

Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy(®) Treatment System plans under varying degrees of rotational beamlet symmetry. The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code geant4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes. The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case. This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

Dosimetric Evaluation of Helical Tomotherapy Low-Dose Total Body Irradiation (TBI) for Mini Allogeneic Transplants

Reduced intensity conditioning (RIC) regimens use less toxic dosages of chemotherapy and/or radiation to prepare the body for allogeneic transplant. When total body irradiation (TBI) is used it is frequently given with a small single dose of 1 - 2 Gy. We speculated that helical tomotherapy could offer simplified treatment planning and that the dose homogeneity and coverage should be excellent with IMRT use. In addition patient comfort and treatment simplicity may be improved because patients can be treated supine on a standard treatment couch without requiring any beam modifying devices. A pre-clinical dosimetry study was performed, utilizing pre-existing full-body CT simulation scans from three patients. Patients had been simulated in full body vacuum immobilization bags and head masks. Patients were not actually treated with these plans. GPU-based planning software was used to generate upper-body and lower-body plans. The treatment machine has a maximum field length of 160 cm, so the upper part of the body would typically be treated first from head to thighs, and then the lower body would be treated from feet to thighs. A gap is calculated between the two fields to create a uniform dose at the junction. In this study the entire body was used as the target volume with no added margins. No avoidance structures were required due to the low dose. Prescription dose was 2.0 Gy in a single fraction, aiming for 95% PTV coverage. Approximately 60 minutes of dosimetry time was required per patient. The planning parameters included a beam field width of 5 cm, a pitch of 0.3 and a modulation factor of 3.0 - 3.6. Average beam-on times were 35.3 minutes upper body and 19.3 minutes lower body, for a total of 54.6 minutes. The upper body received a mean and median dose of 2.11 Gy (SD 0.07), with a D80 of 2.07 Gy and a D20 of 2.15 Gy. The dose heterogeneity index (D20 - D80) / (Rx Dose) was 4.13%. The hottest 5% of the volume (D5) received over 2.24 Gy. The uniformity index (D5 / D95) was 1.115. The proportion of body that received at least 95% of the prescribed dose (V95) was 98.8%. Specific organs were contoured as reference structures, and the median dosages to these organs were: skin 2.11 Gy (SD 0.14), lenses 2.17 Gy, lungs 2.11 Gy (SD 0.04), and kidneys 2.13 Gy. Low dose TBI planning for helical tomotherapy was dosimetrically simple and elegant. Dose coverage and dose homogeneity were excellent. No external shields, bolus, spoilers, or compensating devices were required in the planning process due to IMRT’s ability to achieve homogenous target coverage. Beam-on times were reasonable and patient treatments should be comfortable because patients can lie supine on a standard treatment couch. Helical tomotherapy appears to be a promising technique for delivering low dose TBI in reduced intensity conditioning regimens before mini transplant.

Increasing dose gradient and uniformity in small fields using modulation: theory and prototypes for cone-based stereotactic radiosurgery.

To investigate the theoretical limits to the tradeoff between dose gradient and uniformity when modulation is used in the context of cone based SRS, and to design a prototype collimation device that allows for steeper dose gradients and/or higher target uniformity as compared to a standard circular collimator. An inverse planning optimization is performed in the context of idealized phantom geometry to determine the ideal fluence pattern that best approximates a "rect function" dose distribution. Ideal fluence patterns were approximated in a prototype device and radiochromic film dosimetry was utilized to compare the prototype device to a standard circular collimator. For choices of prescription isodose lines above approximately 50%, utilizing modulation allows for an improved tradeoff between dose gradient index and dose heterogeneity index. Compensators placed within the circular collimator can achieve the necessary modulation. Using modulation with features on a submillimeter distance scale, it is possible to increase the dose gradient and/or uniformity in small fields.

Measurement and Comparison of Surface Dose outside Treatment Area for Different Radiation Treatment Modalities in Breast Cancer Patients Using Thermoluminescence Dosimeters

Purpose: We propose a new intra-uterine tandem design that are capable of creating non-isotropic 192-Ir dose distributions and give unprecedented dose conformality for treatment of cervical cancer. Materials and Methods: The MRI-compatible DMBT tandem design has 6 peripheral holes of 1mm diameter, on a non-magnetic tungsten-alloy rod (18.0 g/cc), enclosed in a polyoxymethylene tube (1.41 g/cc), with a total diameter of 6mm. The tungsten-alloy shield has a diameter of 5mm and the grooves are evenly distributed (60 ) on the surface of tungsten-alloy shield. A VariSource iX HDR source was used for delivering radiation dose. MCNPX Monte Carlo was used to simulate the resulting nonisotropic dose distributions. An in-house developed HDR brachytherapy planning platform, with intensity modulated planning capability using Simulated Annealing and Constrained-Gradient Optimization algorithms, was used to re-plan 75 plans (5 fractions (fx) of 15 patient cases) treated with a conventional tandem & ovoids (T&O) applicator. For the proposed tandem designs, the plans were optimized with the same ovoids in place, as the conventional T&O plans. All DMBT plans were normalized to match the HRCTV V100 coverage of the original T&O plans. Results: Generally, the plan qualities were markedly better using DMBT. Mean 2cc doses to the bladder were 4.0 0.2Gy and 3.37 0.19Gy, for T&O and DMBT, respectively. For the rectum, they were 2.30 0.17Gy and 1.82 0.1Gy. For the sigmoid, they were 1.58 0.19Gy and 1.48 0.17Gy. The best single plan reduction was in the bladder dose, 0.32Gy (40.8%), and this was due to the horseshoe-like wrapping of the bladder around the CTV, thus benefitting the most with the dose conformation enhancements achieved by the proposed designs. The best sigmoid improvement, 0.13Gy (27.5%), was shown for endophytic growth type cervical cancer. The HRCTV dose heterogeneity index (DHI) was 2.56 0.24 and 2.36 0.22 for T&O and DMBT, respectively. Conclusions: We have shown the new tandem designs that advance the conformality of image-guided cervix HDR, in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio.

HDR brachytherapy of rectal cancer using a novel grooved‐shielding applicator design

The aim of this work was to design a novel high-dose rate (HDR) ((192)Ir) brachytherapy applicator for treatment of rectal carcinomas that uses tungsten shielding for possibly improved dosimetric results over commercial brachytherapy applicator(s). A set of 15 single-depth applicators and one dual-depth applicator were designed and simulated using Monte Carlo (MCNPX). All applicators simulated were high-density tungsten alloy cylinders, 16-mm in diameter, and 60-mm long, with longitudinal grooves within which an (192)Ir source can be placed. The single-depth designs varied regarding the number and depth of these grooves, ranging from 8 to 16 and 1-mm to 3-mm, respectively. The dual-depth design had ten channels, each of which had two depths at which the source could be placed. Optimized treatment plans were generated for each design on data from 13 treated patients (36 fractions) with asymmetrical clinical target volumes (CTVs). All results were compared against the clinically treated plans which used intracavitary mold applicator (ICMA), as well as a recently designed, highly automated, and collimated intensity modulation device named dynamic modulated brachytherapy (DMBT) device. All applicator designs outperformed the ICMA in every calculated dosimetric criteria, except the total dwell times (∼30% increase). There were clear, but relative, tradeoffs regarding both the number of channels and the depth of each channel. Overall, the 12-channel, 1-mm depth, and 14-channel 2-mm depth designs had the best results of the simpler designs, sparing the healthy rectal tissues the most while achieving comparable CTV coverage with the dose heterogeneity index and lateral spill doses improving by over 10% and the contralateral healthy rectum dose dropping over 30% compared to ICMA. The ten-channel dual-depth design outperformed each single-depth design, yielding the best coverage and sparing. New grooved tungsten HDR-brachytherapy devices have been designed and simulated. The results of this work attest to the capability of these new, highly anisotropic, intelligently shielded applicators to limit dose to healthy tissues while maintaining a conformal prescription dose to the CTV.

SU-E-T-485: Comparison of the Oncogenic Potential for Radiation-Associated, Second Malignant Neoplasms for Several Prostate Radiotherapy Modalities as a Function of Relative OAR &amp; PTV Volumes

Purpose: To compare plan quality for a comprehensive set of OAR & PTV geometries considering relative risk estimates for the induction of secondary bladder and rectal cancers due to prostate radiotherapy using several modern delivery techniques. Methods: IMRT / 3D-conformal, volumetric-arc (VMAT), and Tomotherapy (TT) plans were generated using PlanUNC, Pinnacle3 ver. 9.2, and TomoTherapy treatment planning systems, respectively, for 12 prostate cases using, RTOG 0815-derived planning goals. Selected cases were representative of relative (i.e. small (Svol), medium (Mvol), large (Lvol)) PTV, rectal & bladder structure volumes encountered clinically. PlanUNC was used to calculate the elevated relative-risk (ERR) of secondary cancer induction in normal tissues, the bladder and rectal ERR, PTV conformity index (CI), dose heterogeneity index (DHI), and bladder and rectal V5, V15, V35, V50 (Bvx / Rvx) for all cases. All metrics were averaged for each relative tissue / target volume, and normalized to corresponding 3D-conformal cases for comparison. Results: Considering relative rectal volumes, all metrics fell within the standard error for each modality excluding bladder ERR (TT 13%ave, IMRT −9% ave & VMAT −18% ave) and BV15 (VMAT −24% ave) for Lvol rectum cases. Considering relative bladder volumes, bladder ERR (TT 4%, IMRT −10%, VMAT −18%) for Lvol and (VMAT −20%) for Mvol; rectal ERR (TT −40%) for Mvol were observed to be significant in addition to bladder ERR (TT 13%) for Svol PTV cases. Generally, all modalities performed better when compared to 3D conformal delivery excluding nominal (<5%) increases in normal tissue dose and increased bladder ERR for several TT cases. Conclusion: We compared three prostate EBRT modalities considering OAR ERR as well as traditional plan quality metrics for a class of cases defined by relative structure size. Preliminary results indicate comparable improvements in PTV conformity & OAR dosing for each modality but differences in OAR ERR.

WE‐A‐108‐06: Dynamic Modulated Brachytherapy for Cervical Cancer

Purpose: We propose two new intra‐uterine tandem designs that are capable of creating non‐isotropic 192‐Ir dose distributions and give unprecedented dose conformality for treatment of cervical cancer. Methods: The first tandem design, MC6, has 6 peripheral holes of 1‐mm diameter, and the in‐between‐space filled with tungsten (18.0 g/cc), wrapped in delrin (1.41 g/cc). The second design, DMBT, has 19 holes of 1‐mm diameter, each occupied by gold wires (19.3 g/cc) that can slide in and out (with motors), and the in‐between‐space filled with silicon (1.14 g/cc). MCNPX Monte Carlo was used to simulate the resulting non‐isotropic dose distributions. An in‐house developed HDR brachytherapy planning platform, with intensity modulated planning capability using Simulated Annealing and Constrained‐Gradient Optimization algorithms, was used to plan 36 patient cases and compare with the clinically treated, conventional T&amp;O applicator‐based plans. For the proposed tandem designs, the plans were optimized with the same ovoids in place, as the conventional T&amp;O plans. Results: All 36 plans were prescribed 6Gy and normalized to D90=6Gy and V100=95%. Generally, the plan qualities were markedly better using MC6 and DMBT, with DMBT usually performing the best overall. Mean 2cc/0.1cc doses to the bladder were 9.14±0.78/9.32± 0.80Gy, 6.87±0.57/7.05±0.59Gy, and 6.61±0.51/6.74±0.52Gy, for T&amp;O, MC6, and DMBT, respectively. For the rectum, they were 5.78± 0.69/5.92±0.70Gy, 5.17±0.52/5.30±0.52Gy, and 4.92±0.50/5.04± 0.50Gy. For the sigmoid, they were 5.23±0.58/5.34±0.59Gy, 5.04± 0.56/5.21±0.57Gy, and 4.86±0.51/5.03±0.52Gy. The most improvement was in the bladder dose and this was due to the horseshoe‐like wrapping of the bladder around the CTV, thus benefitting the most with the dose conformation enhancements achieved by the proposed designs. The CTV dose heterogeneity index (DHI) was 2.43±0.01, 2.30±0.01, and 2.26±0.01, respectively. Conclusions: We have shown two tandem designs that advance the conformality of image‐guided cervix HDR, in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio.

Dynamic modulated brachytherapy (DMBT) for rectal cancer

All forms of past and current high-dose-rate brachytherapy utilize immobile applicators during treatment delivery. The only moving part is the source itself. This paradigm misses an important degree of freedom that, if explored, can in some instances produce previously unachievable dose conformality; that is, the dynamic motion of the applicator itself during treatment delivery. Monte Carlo and treatment planning simulations were used to illustrate the potential benefits of moving applicators for rectal cancer applications in particular. This concept is termed dynamic modulated brachytherapy (DMBT). The DMBT system uses a high-density, 18.0 g∕cm(3), 45 mm long tungsten alloy shield, cylindrical in shape, with a small window on one side to encapsulate a (192)Ir source, to create collimation that results in a highly directional beam profile. This shield can be dynamically translated and rotated, using an attached robotic arm, during treatment to create a volumetric modulated arc therapy-type delivery, but from inside the rectal cavity. Monte Carlo simulations and planning optimization algorithms were developed inhouse to evaluate the effectiveness of this new approach using 36 clinical treatment plans comprised of 13 patients each treated using the intracavitary mold applicator (ICMA, Nucletron, The Netherlands) to quantify the potential clinical benefit. The prescription dose was 10 Gy∕fx and the group had an average clinical target volume of 9.0 ± 3.5 cm(3). Ideal phantom geometries were used to evaluate the impact of various shield dimensions and designs on the resulting plan quality. Simulations of ideal phantom geometries found that shields as small as 10 mm in diameter can produce high quality plans. For the clinical patient cases, compared to the ICMA, for equal prescription tumor coverage, the DMBT plans provided >30% decrease in D(5) (high dose volume) resulting in a ∼40% decrease in dose heterogeneity index. In addition, mean dose and D(98) showed a reduction (typically 40%-60%) on all critical structures evaluated. However, for a 10 Gy prescribed dose there was an increase in total treatment time on average from 7.6 to 20.8 min for a source with an air-kerma strength of 40.25 kU (10 Ci). Dosimetric properties of a novel DMBT system have been described and evaluated. Comparison with the ICMA commercial applicator has shown it to be a prospective step forward in high-dose-rate brachytherapy (192)Ir technology. Dynamic motion of an applicator during treatment, for any applicator and site in general, can provide additional degrees of freedom that, if properly considered, can potentially increase the plan quality significantly.

Stereotactic Body Radiotherapy (SBRT) for Low Risk Prostate Cancer: Plan comparison With Real Time Tracking By Beacons and Helical Tomotherapy

Hypofractionated treatment of low risk prostate cancer by external beam are planned and delivered using rotational arc (RA) treatment planning on a conventional linac. Treatment plans are generated retrospectively on helical Tomotherapy (HT) for comparison. Eight low risk prostate patients were implanted each with three commercial beacons. RA plans were generated on an Eclipse planning system and delivered using CBCT image guidance for localization and real time tracking of beacons. Dose fractionation was 7.25 Gy/fx for 5 fractions to a total dose of 36.25 Gy. Planning goals were set for 95% of the planning target volume (PTV) to receive at least the prescription dose. Treatment plans were also generated on a HT planning system using the same fractionation and dose-volume constraints. Margins for the PTV were set at 5 mm in all directions except 3 mm posteriorly. Treatment plans were evaluated based on the target coverage measured by minimum, mean and maximum dose to the target volumes in the dose-volume-histogram (DVH) analysis and the conformity index (CI).The heterogeneity of the dose distribution in the PTV was measured by a dose heterogeneity index as DHI. The volumes covering 5, 10, 20, 50 and 70% of the prescription dose (V5, V10, V20, V50 and V70) for two OARs were determined. The mean age of the patients was 61 y (range 53-79) and the mean prostate volume was 49.1 cm3 (range 30.3-77.4 cm3). The mean PSA value was 7.0 ng/mL (range 3.8-8.91). All patients were staged at T1cN0M0 except for one at T1bN0M0. For the RA plans the average values of monitor units (MU), actual beam delivery time, CI and DHI are 3193, 328.6 s, 1.194 and 2.88, respectively. The equivalent biological dose at 2 Gy/fx (EQD2) for the SBRT treatment was 90.63 Gy with an a/b ratio of 1.5 for prostate. For HT plans, the average values of the monitor units, treatment beam-on time, CI and DHI are 14330, 984.9 s, 1.307 and 3.22, respectively. Analysis of the DVH data shows that, on average, the minimum dose coverage of the PTV for RA plans is 4% higher than the HT plans. The averages of the maximum and mean dose coverage of the PTV for the two planning systems are almost same. The average monitor unit and beam-on time for HT plans are 4.5 and 3 times, respectively, higher than the RA plans. Thus the RA treatment is more efficient. The CI of the RA plans is slightly better than the HT plans – the average being about 10% lower for the RA plans. Also, the dose heterogeneity of the PTV in the RA plans is around 12% lower than the HT plans. Comparison of the averages of the minimum, maximum and mean doses for the rectum and bladder shows no statistical difference between the two planning systems.

Whole breast irradiation for small-sized breasts after conserving surgery: is the field-in-field technique optimal?

To determine the optimal whole breast irradiation technique in patients with small-sized breasts, tangential and field-in-field IMRT (FIF) techniques were compared. Sixteen patients with ≤3 cm breast height and ≤350 cc volume were included. Seven patients had 4D CTs performed. The planning target volumes (PTV), editing 5 and 2 mm from the surface on the whole breast, were delineated and called PTV(5) and PTV(2), respectively. Dose-volume histograms of tangential techniques with open beam (OT) and wedge filter (WT), conventional FIF (cFIF), and modified FIF (mFIF) blocking out the lung were produced. Various dose-volume parameters, the dose heterogeneity index (DHtrI), dose homogeneity index (DHmI), and PTV dose improvement (PDI) were calculated. OT compared with WT showed a significantly favorable V 90 of the heart and lung, and PTV(5)-dose distribution. Comparing OT and cFIF, OT showed significant improvement in the V 95 of PTV(2), whereas cFIF showed significant improvement in the V 95, DHtrI, DHmI, and PDI of the PTV(5). In comparing cFIF and mFIF, mFIF showed improved dose distributions of the heart and lung, while cFIF presented the better V 95, DHtrI, DHmI, and PDI of the PTV(5). Respiratory influences on the absolute dose were mostly within 1 %. The ratio of free breathing and each respiratory phase was similar among OT, cFIF, and mFIF. cFIF has favorable dose conformity and is suggested to be an optimal method for small-sized breasts. However, OT for dose coverage close to the skin and mFIF for normal tissue may also be potential alternatives. Respiratory effects are minimal.

Dosimetric comparison between intensity-modulated radiotherapy and conformal radiotherapy for upper thoracic esophageal carcinoma

Treatment planning for radiotherapy of upper thoracic esophageal carcinoma is challenging due to the anatomical features. The difficulty may be resolved by intensity-modulated radiotherapy (IMRT). This study was to compare the dosimetric advantages of IMRT to that of conformal radiotherapy (CRT) for upper thoracic esophageal carcinoma, and to explore the clinical application of IMRT. Eleven patients with upper thoracic esophageal carcinoma were enrolled. In addition to the actually used CRT plan, a five-field IMRT plan was generated for each case. The parameters of dose volume histogram for targets and organs at risk were compared between two techniques. For the planning target volume (PTV) of tumor and para-tumor tissues, the mean dose, maximal dose, doses covering 99% and 95% volume were similar in IMRT and CRT plans (P>0.05). However, IMRT plan had a higher conformity index than CRT plan (0.68+/-0.04 vs. 0.46+/-0.11, P<0.01). For the PTV of supraclavicular region, IMRT plan showed a better dose heterogeneity index than CRT plan (1.17+/-0.05 vs. 1.33+/-0.15, P=0.01). IMRT plan had lower maximal dose to the planning risk volume of the spinal cord (44.4 Gy vs. 52.5 Gy, P<0.05) and lower lung volume received dose of 10 Gy or higher [(32+/-6)% vs. (35+/-9)%, P<0.05] than CRT plan. For the upper thoracic esophageal carcinoma, IMRT has more conformal distribution of dose and better spinal cord sparing than CRT, and can reduce the volume of lung that received dose of 10 Gy or higher.

Stereotactic radiotherapy for lung cancer using a flattening filter free Clinac

The objective of this study was to assess the feasibility of stereotactic radiotherapy for early stage lung cancer using photon beams from a Varian Clinac accelerator operated without a flattening filter. Treatment plans were generated for 10 lung cancer patients with isolated lesions less than 3 cm in diameter. For each patient, two plans were generated, one with and one without the flattening filter. Plans were generated with Eclipse 8.0 (Varian Medical Systems) commissioned with beam data measured on a Clinac 21EX (Varian Medical Systems) operated with and without the flattening filter. Removal of the flattening filter increased the dose rate. The median beam‐on time per field was reduced from 25 sec (with the filter) to 11 sec (without the filter), increasing the feasibility of breath‐hold treatments and the efficiency of gated treatments. Differences in a dose heterogeneity index for the planning target volume between plans with flattened and unflattened beams were statistically insignificant. Differences in mean doses to organs at risk were small, typically about 10 cGy over the entire treatment. The study concludes that radiotherapy with unflattened beams is feasible and requires substantially less beam‐on time, facilitating breath‐hold and gating techniques.PACS numbers: 87.56.bd, 87.53.Ly