Volumetric Modulated Arc Radiotherapy Plans Research Articles

Investigating the Dosimetric Leaf Gap Correction Factor of Mobius3D Dose Calculation for Volumetric-modulated Arc Radiotherapy Plans

Aims: The dosimetric leaf gap (DLG) is a parameter for correcting radiation transmission through the round leaf end of multileaf collimators. The purpose of this study was to determine and investigate the optimal DLG correction factor for 6 MV volumetric-modulated arc radiotherapy (VMAT) plan dose calculations in Mobius3D. Materials and Methods: Seventeen VMAT plans were selected for the DLG correction factor optimization process. The optimal DLG correction factor was defined as the minimum difference between the measured dose and the Mobius3D-calculated dose on the Mobius Verification Phantom™ with different DLG correction factors. Subsequently, the optimal DLG correction factor was applied for Mobius3D dose calculation, and accuracy was assessed by comparing the measured and calculated doses. For verification and validation, the 17 previous plans and 10 newly selected plans underwent Mobius3D calculations with the optimal DLG correction factor, and gamma analysis was performed to compare them to the treatment planning system (TPS). Gamma analysis was also performed between the electronic portal imaging device (EPID) and the TPS for cross-comparison between systems. Results: The DLG correction factor was optimized to −1.252, which reduced the average percentage differences between measured and Mobius3D-calculated doses from 2.23% ±1.21% to 0.03% ±1.82%. The cross-comparison between Mobius3D/TPS and EPID/TPS revealed a similar trend in gamma passing rate (>95%) in both the verification and validation plans. Conclusion: The DLG correction factor strongly influences the accuracy of Mobius3D-calculated doses. Applying the optimal DLG correction factor can increase dose agreement and gamma passing rate between calculation and delivered doses of VMAT plans, which emphasizes the importance of optimizing this factor during the commissioning process.

Left Anterior Descending Coronary Artery-Optimized Knowledge-Based Radiotherapy Planning in Patients with Locally Advanced Non-Small Cell Lung Cancer

The left anterior descending (LAD) coronary artery volume receiving 15 Gy (V15 Gy) has been associated with major adverse cardiac events and mortality in patients with locally advanced non-small cell lung cancer (LA-NSCLC) treated with radiotherapy (RT). However, coronary arteries are not routinely contoured as standard-of-care and there is limited data describing coronary-sparing planning techniques. Our objective was to evaluate whether LAD-sparing could be improved compared to real-world plans using a commercial knowledge-based planning (KBP) software without clinically unacceptable tradeoffs to other critical organs at risk (OAR). Retrospective replanning analysis was conducted on 31 RT plans developed for patients with LA-NSCLC. Each case was manually replanned with volumetric modulated arc radiotherapy (VMAT) optimized for LAD V15 Gy sparing with a prescription dose of 60 Gy. The resulting plans were used to create an LAD-optimized KBP model. The model included both KBP-generated objectives and additional point objectives to meet LAD (V15 Gy < 10%), lung (V5 Gy < 60%, V20 < 30%, mean < 17 Gy), heart (mean < 20 Gy), spinal cord (D0.03cc < 45 Gy), esophagus (V60 Gy < 17%, mean < 34 Gy), and planning target volume (PTV; D95% = 100%). The LAD-optimized model was deployed to replan 33 additional VMAT plans for LA-NSCLC with prescription dose 60 Gy, delivered between 2018 and 2022. Dose constraints or objectives were only unmet with KBP when not met in the original plan. Descriptive statistics were used to compare dosimetric parameters between original and LAD-optimized KBP plans. For the 33 plans identified for LAD-optimized KBP replanning, the original plans had a mean LAD V15 Gy of 17.9% (standard error [SE], 3.9%), while the LAD-optimized KBP plans had a significantly lower mean LAD V15 Gy of 5.9% (SE, 1.9%; p = 0.031). There was no significant difference in mean heart dose between the original and LAD-optimized KBP plans (8.4 Gy [SE 6.6 Gy] vs. 7.3 Gy [SE 5.8 Gy]; p = 0.48). Of these 33 KBP plans, 42% (n = 14) had mean reductions in LAD V15 Gy from 40.5% (SE 4.3%) to 10.5% (SE 3.5%; p = 0.0002), with 9/14 now able to meet < 10%, without affecting other critical OAR objectives (p > 0.5 for lungs, esophagus, spinal cord). 45% (n = 15) originally had an LAD V15 Gy < 1%, which were unchanged using KBP (p > 0.3). Four plans (12%) had higher LAD V15 Gy in the KBP vs. original plan (2/4 still meeting < 10%), due to the model prioritizing other OARs to a greater degree. Improvements in LAD-sparing was achievable in more than 40% of plans without apparent clinically significant tradeoffs in other critical OARs and without corresponding change in MHD. This result underscores the importance of standard implementation of LAD contouring and dose constraints, particularly given the discordance between whole heart and LAD dose exposure.

Dosimetric effects of oral contrast in the planning of conventional radiotherapy and IMRT, for rectal cancer treatment

Abstract Introduction: Contrast media are frequently used during radiation therapy simulation. However, there are concerns about dosimetric variations when dose calculation is done on contrast-enhanced computed tomography (CT). This study evaluates the dosimetric effect of oral contrast during three-dimensional conformal radiotherapy (3D-CRT) and volumetric modulated arc radiotherapy (VMAT) planning. Methods: Rectal cancer patients were consecutively enrolled. For each patient, one unenhanced CT and one contrast-enhanced CT were taken using oral and intravenous contrast. Then, a 3D-CRT plan and an Intensity-modulated radiation therapy (IMRT)/VMAT plan were generated in the enhanced CT, and the dose distribution was recalculated in the respective unenhanced CT. The beam intensities were kept the same as for the enhanced CT plans. Finally, the unenhanced and enhanced plans were compared by calculating the gamma index. Results: For 3D-CRT plans, there were statistically significant differences in second phase planning target volume (PTV) D2% (Mean difference (MD) between unenhanced and enhanced CT 0·01 Gy, 95% CI [0·003 to 0·02 Gy]) and in maximum doses to the bladder (MD 0·26 Gy, 95% CI [0·05 to 0·47 Gy]). For IMRT/VMAT plans, there were statistically significant differences in small intestine V45 Gy (MD 3·1 cc, 95% CI [0·81 to 5·4 cc]), bladder V45 Gy (MD 2·9%, 95% CI [1·4 to 4·3%]) and maximum dose to the bladder (MD 0·65 Gy, 95% CI [0·46 Gy to 0·85 Gy]). In addition, for PTV D98% the MD between unenhanced and enhanced CT was 0·22 Gy 95% CI [0·05 to 0·39]. Conclusions: For most of the dose metrics, the differences were not clinically meaningful. The greatest differences were found in VMAT plans, especially in V45 Gy of the small intestine. This difference could lead to an underestimation of dose–volume metrics when the plan is based on an enhanced CT. The use of small bowel oral contrast does not significantly influence dose calculations and may not affect the acceptability of plans when adhering to constraints.

Evaluation of an automated template-based treatment planning system for radiotherapy of anal, rectal and prostate cancer.

The Ethos system has enabled online adaptive radiotherapy (oART) by implementing an automated treatment planning system (aTPS) for both intensity-modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) plan creation. The purpose of this study is to evaluate the quality of aTPS plans in the pelvic region. Sixty patients with anal (n=20), rectal (n=20) or prostate (n=20) cancer were retrospectively re-planned with the aTPS. Three IMRT (7-, 9- and 12-field) and two VMAT (2 and 3 arc) automatically generated plans (APs) were created per patient. The duration of the automated plan generation was registered. The best IMRT-AP and VMAT-AP for each patient were selected based on target coverage and dose to organs at risk (OARs). The AP quality was analyzed and compared to corresponding clinically accepted and manually generated VMAT plans (MPs) using several clinically relevant dose metrics. Calculation-based pre-treatment plan quality assurance (QA) was performed for all plans. The median total duration to generate the five APs with the aTPS was 55min, 39min and 35min for anal, prostate and rectal plans, respectively. The target coverage and the OAR sparing were equivalent for IMRT-APs and VMAT-MPs, while VMAT-Aps.demonstrated lower target dose homogeneity and higher dose to some OARs. Both conformity and homogeneity index were equivalent (rectal) or better (anal and prostate) for IMRT-APs compared to VMAT-MPs. All plans passed the patient-specific QA tolerance limit. The aTPS generates plans comparable to MPs within a short time-frame which is highly relevant for oART treatments.

A customizable anthropomorphic phantom for dosimetric verification of 3D-printed lung, tissue, and bone density materials.

To design and manufacture a customized thoracic phantom slab utilizing the 3D printing process, also known as additive manufacturing, consisting of different tissue density materials. Here, we demonstrate the 3D-printed phantom's clinical feasibility for imaging and dosimetric verification of volumetric modulated arc radiotherapy (VMAT) plans for lung and spine stereotactic ablative body radiotherapy (SABR) through end-to-end dosimetric verification. A customizable anthropomorphic phantom slab was designed using the CT dataset of a commercial phantom (adult female ATOM dosimetry phantom, CIRS Inc.). Material extrusion 3D printing was utilized to manufacture the phantom slab consisting of acrylonitrile butadiene styrene material for the lung and the associated lesion, polylactic acid (PLA) material for soft tissue and spinal cord, and both PLA and iron-reinforced PLA materials for bone. CT images were acquired for both the commercial phantom and 3D-printed phantom for HU comparison. VMAT plans were generated for spine and lung SABR scenarios and were delivered as per departmental SABR protocols using a Varian TrueBeam STx linear accelerator. End-to-end dosimetry was implemented with radiochromic films, analyzed with gamma criteria of 5% dose difference, and a distance-to-agreement of 1mm, at a 10% low-dose threshold by comparing with calculated dose using the Acuros algorithm of the Eclipse treatment planning system (v15.6). 3D-printed phantom inserts were observed to produce HU ranging from -750 to 2100. The 3D-printed phantom slab was observed to achieve a similar range of HU from the commercial phantom including a mean HU of -760 for lung tissue, a mean HU of 50 for soft tissue, and a mean HU of 220 and 630 for low- and high-density bone, respectively. Film dosimetry results show 2D-gamma passing rates for lung SABR (internal and superior) and spine SABR (inferior and superior) over 98% and 90%, respectively. The end-to-end testing of VMAT plans for spine and lung SABR suggests the clinical feasibility of the 3D-printed phantom, consisting of different tissue density materials that emulate lung, soft tissue, and bone in kV imaging and megavoltage photon dosimetry. Further investigation of the proposed 3D printing techniques for manufacturability and reproducibility will enable the development of clinical 3D-printed phantoms in radiotherapy.

Dosimetric Comparison of Volumetric Modulated Arc Therapy with Tomotherapy Based Total Body Irradiation for Patients Undergoing Conditioning Prior to Hematopoietic Stem Cell Transplantation for Acute Lymphocytic Leukemia

Total body irradiation (TBI) as a component of myeloablative condition prior to hematopoietic stem cell transplantation (HSCT) for acute leukemia has been associated with high rates of pulmonary toxicity. The purpose of this study is to compare the dosimetry of volumetric modulated arc radiotherapy (VMAT) with tomotherapy (tomo) for TBI in patients undergoing myeloablative conditioning prior to HSCT for acute lymphocytic leukemia (ALL), and to report acute toxicity associated with these regimens.Sixteen patients with ALL undergoing high dose TBI at our institution were retrospectively identified. Prescription dose was to 13.2 Gy for 15 patients and 12 Gy for one patient, delivered in 8 twice daily fractions, at least 6 hours apart. The planning target volume was defined as the external body contour minus the lung, heart, and kidneys. Six or seven isocenters were utilized with VMAT plans with two arcs delivered at each isocenter. Immobilization was achieved with a vacuum cradle for tomo patients and with a stereotactic body radiotherapy (SBRT) board with rigid thermoplastic immobilization for VMAT patients. Daily cone beam CT was acquired at each isocenter for VMAT patients. Mean dose to organs at risk (OARs) were compared with T-tests. Results are presented with 95% confidence intervals (CI).Sixteen patients with ALL planned for high dose TBI were identified for analysis. Ten patients were planned for VMAT-based TBI while six patients were planned for tomo-based treatment. There was no significant difference between mean heart dose for VMAT plans (5.71 Gy, 95% CI 5.16-6.27) vs tomo plans (6.26 Gy, 95% CI 5.51-7.01), P = 0.18. Mean lung dose was lower for VMAT (7.20 Gy, 95% CI 7.01-7.40) compared to tomo (7.75 Gy, 95% CI 7.24-8.26), P = 0.04. The mean kidney dose was lower for tomo-based plans (5.55 Gy, 95% CI 4.91-6.19) vs VMAT (6.31 Gy, 95% CI 5.87-6.75), P = 0.03. The median follow-up from completion of TBI was 198.5 days and the median time to HSCT after completion of TBI was four days. No acute or late pulmonary toxicity was observed. Seven of the 14 patients (50%, 95% CI 23-77) who completed TBI developed acute grade 3 or grade 4 mucositis, including 2 of the 5 tomo patients (40%, 95% CI 5-85) and 5 of the 9 VMAT patients (56%, 95% CI 21-86). One patient treated with VMAT developed acute grade 3 diarrhea while one patient treated with Tomo experienced acute grade 3 enteritis and dermatitis. One patient treated with tomo developed late grade 2 xerophthalmia, and there were no other late radiotherapy related toxicities observed. Four patients expired during the study follow-up period at a median of 21 days following TBI. One patient had relapsed disease approximately 8 months following completion of TBI.High dose TBI planned with VMAT and delivered with SBRT board and thermoplastic immobilization is feasible and decreases mean lung dose compared to tomotherapy based plans. While mucositis was a common adverse event, no pulmonary toxicity was observed.

The Effect of Contrast Agents on Dose Calculations of Volumetric Modulated Arc Radiotherapy Plans for Critical Structures

Radiotherapy dose calculation requires accurate Computed Tomography (CT) imaging while tissue delineation may necessitate the use of contrast agents (CA). Acquiring these two sets is a common practice in radiotherapy. This study aims to evaluate the effect of CA on the dose calculations. Two hundred and twenty-six volumetric modulated arc therapy (VMAT) patients that had planning CT with contrast (CCT) and non-contrast CT (NCCT) of different cancer sites (e.g., brain, head, and neck (H&amp;N), chest, abdomen, and pelvis) were evaluated. Treatment plans were recalculated using CCT, then compared to NCCT. The variation in Hounsfield units (HU) and dose distributions for critical structures and target volumes were analyzed using mean HU, mean and maximum relative dose values, D2%, D98%, and 3D gamma analysis. HU variations were statistically significant for most structures. However, this was not clinically significant as the difference in mean HU values was within 30 HU for soft tissue and 50 HU for lungs. Variation in target volumes’ D2% and D98% were insignificant for all sites except brain and nasopharynx. Dose maximum differences were within 2% for the majority of critical structures and target volumes. 3D gamma analysis results revealed that majority of plans satisfied the 2% and 2 mm criteria. CCT may be acquired for VMAT radiotherapy planning purposes instead of NCCT, since there is no clinically significant difference in dose calculations based on either image set.

Comprehensive nodal breast VMAT: solving the low-dose wash dilemma using an iterative knowledge-based radiotherapy planning solution.

IntroductionAimed to develop a simple and robust volumetric modulated arc radiotherapy (VMAT) solution for comprehensive lymph node (CLN) breast cancer without increase in low‐dose wash.MethodsForty CLN‐breast patient data sets were utilised to develop a knowledge‐based planning (KBP) VMAT model, which limits low‐dose wash using iterative learning and base‐tangential methods as benchmark. Another twenty data sets were employed to validate the model comparing KBP‐generated ipsilateral VMAT (ipsi‐VMAT) plans against the benchmarked hybrid (h)‐VMAT (departmental standard) and bowtie‐VMAT (published best practice) methods. Planning target volume (PTV), conformity/homogeneity index (CI/HI), organ‐at‐risk (OAR), remaining‐volume‐at‐risk (RVR) and blinded radiation oncologist (RO) plan preference were evaluated.ResultsIpsi‐ and bowtie‐VMAT plans were dosimetrically equivalent, achieving greater nodal target coverage (P < 0.05) compared to h‐VMAT with minor reduction in breast coverage. CI was enhanced for a small reduction in breast HI with improved dose sparing to ipsilateral‐lung and humeral head (P < 0.05) at immaterial expense to spinal cord. Significantly, low‐dose wash to OARs and RVR were comparable between all plan types demonstrating a simple VMAT class solution robust to patient‐specific anatomic variation can be applied to CLN breast without need for complex beam modification (hybrid plans, avoidance sectors or other). This result was supported by blinded RO review.ConclusionsA simple and robust ipsilateral VMAT class solution for CLN breast generated using iterative KBP modelling can achieve clinically acceptable target coverage and OAR sparing without unwanted increase in low‐dose wash associated with increased second malignancy risk.

The Advantage of Proton Therapy in Hypothalamic-Pituitary Axis and Hippocampus Avoidance for Children with Medulloblastoma.

PurposeCraniospinal irradiation (CSI) improves clinical outcomes at the cost of long-term neuroendocrine and cognitive sequelae. The purpose of this pilot study was to determine whether hypothalamic-pituitary axis (HPA) and hippocampus avoidance (HPA-HA) with intensity-modulated proton therapy (IMPT) can potentially reduce this morbidity compared with standard x-ray CSI.Materials and MethodsWe retrospectively evaluated 10 patients with medulloblastoma (mean, 7 years; range, 4-14 years). Target volumes and organs at risk were delineated as per our local protocol and the ACNS0331 atlas. An experienced neuroradiologist verified the HPA and hippocampus contours. The primary objective was CSI and boost clinical target volume (CTV) covering 95% of the volume (D95) > 99% coverage with robustness. Described proton therapy doses in grays are prescribed using a biological effectiveness relative to photon therapy of 1.1. The combined prescribed dose in the boost target was 54 Gy. Secondary objectives included the HPA and hippocampus composite average dose (Dmean ≤ 18 Gy). For each patient, volumetric modulated arc radiotherapy (VMAT) and tomotherapy (TOMO) plans existed previously, and a new plan was generated with 3 cranial and 1 or 2 spinal beams for pencil-beam scanning delivery. Statistical comparison was performed with 1-way analysis of variance.ResultsCompared with standard CSI, HPA-HA CSI had statistically significant decreases in the composite doses received by the HPA (32.2 versus 17.9 Gy; P < .001) and hippocampi (39.8 versus 22.8 Gy; P < .001). The composite HPA Dmean was lower in IMPT plans (17.9 Gy) compared with that of VMAT (21.8 Gy) and TOMO (21.2 Gy) plans (P = .05). Hippocampi composite Dmean was also lower in IMPT plans (21 Gy) compared with that of VMAT (27.5 Gy) and TOMO (27.2 Gy) plans (P = .02). The IMPT CTV D95 coverage was lower in IMPT plans (52.8 Gy) compared with that of VMAT (54.6 Gy) and TOMO (54.6 Gy) plans (P < .001) The spared mean volume was only 1.35% (19.8 cm3) of the whole-brain CTV volume (1476 cm3).ConclusionWe found that IMPT has the strong potential to reduce the dose to the HPA and hippocampus, compared with standard x-ray CSI while maintaining target coverage. A prospective clinical trial is required to establish the safety, efficacy, and toxicity of this novel CSI approach.

Challenges of radical chemoradiation planning in Stage III non-small-cell lung cancer: Can volumetric modulated arc radiotherapy overcome an unfavourable location?

Radiotherapy treatment planning of radical doses for concurrent chemoradiation in Stage III non-small-cell lung cancer (NSCLC) presents many challenges. This dosimetric study aimed to analyse the impact of spatial location of tumour and nodal burden in limiting the achievement of normal organ constraints and the use of appropriate radiotherapy technique to address it. Fifteen Stage III NSCLC patients underwent 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET)/computed tomography (CT) based treatment planning. VMAT (Volumetric Modulated Arc Radiotherapy) plans were made for all patients treated by 3D-CRT (3-Dimensional Conformal Radiotherapy). A binomial logistic regression was performed to ascertain the tumour and nodal characteristics that decreased the likelihood of being planned to 60Gy. Inability to achieve normal tissue constraints, particularly spinal cord dose to less than 50Gy, during initial planning by the assigned treatment technique was the primary dose limiting factor in four patients (p=0.02). Alternate VMAT plans could achieve the dose constraints where 3D-CRT was unsuccessful in patients with bulky central disease in two patients. This technique fell short when there was gross vertebral body erosion. For tumours with bulky central disease, VMAT should be preferred. With gross vertebral body erosion, even VMAT falls short if the planning target volume includes the spinal cord. In a subset of Stage III NSCLC upfront chemoradiation to radical doses may not be feasible.

A dosimetric study using split x-jaw planning technique for the treatment of endometrial carcinoma

The aim of this retrospective study was to determine if the split x-jaw planning technique could be used with Varian linear accelerators to improve plan conformity and limit dose to organs at risk (OAR) for planning target volumes that require field sizes larger than the 15 cm extent of the multileaf collimator in the x-jaw position. Traditional planning techniques include limited and open x-jaw methods. The study population included 20 randomly selected patients with endometrial carcinoma. Treatment plans for each patient were designed using split, limited, and open x-jaw volumetric modulated arc radiotherapy for comparison purposes. Dose statistics including the PTV conformity index and dose to OAR were used to evaluate plan performance. The split x-jaw planning method had the most consistent conformity index (0.98 ± 0.02), followed by the open (01 ± 0.03), and the limited (1.04 ± 0.05) techniques. On average, the split method better spared the OAR. In comparison to the limited and open techniques, the split method reduced the dose to the bowel by 3.8%, rectum by 3.2%, sigmoid by 2.1%, right femoral head by 3.5%, and left femoral head by 3.9%. The split and open techniques showed comparable bladder results and were superior over the limited method. The monitor units were highest with the split method leading to increased treatment times. The split x-jaw planning technique should be used with Varian linear accelerators to produce superior volumetric modulated arc radiotherapy plans for planning target volumes larger than the maximum extent of the multileaf collimator in the x-jaw direction.

EP1.18-14 Algorithm for Deciding Radiotherapy Technique in Stage III Non-Small Cell Lung Cancer

Concurrent chemoradiotherapy has shown superior survival outcomes in inoperable, Stage III non-small cell lung cancer (NSCLC) with good performance status. Usage of 3-dimensional conformal radiotherapy (3D-CRT) technique is commonest with intensity modulated radiotherapy (IMRT) gaining popularity owing to its ability to spare normal tissues. In our study we analysed the tumour and nodal characteristics of patients to predict which technique maybe beneficial by generating optimised plans of both 3D-CRT and volumetric modulated arc radiotherapy (VMAT), a type of IMRT. A total of 27 Stage III NSCLC patients underwent PET-CT (in the treatment position) co-registered treatment planning using either 3D-CRT or VMAT. The total prescription dose was 60 Gy in 30 fractions over 6 weeks with concurrent cisplatin based chemotherapy. Retrospectively alternate treatment plans were generated to obtain two sets of plans (3D-CRT and VMAT) of each patient for dosimetric comparison. The tumour and nodal characteristics and treatment technique were analysed to identify predictive factors limiting total dose delivery. The angle of contact of the PTV with the circumference of the vertebral body was calculated for patients where the tumour or nodal mass was in contact with the vertebral body (Figure 1). Of the 27 patients enrolled, 6 patients could not be planned to the complete dose of 60 Gy in a single phase by 3D-CRT. Dose limiting factors were high spinal cord doses due to vertebral body erosion (2 out of 6) and bilateral bulky mediastinal lymphadenopathy (4 out of 6). Alternate VMAT plans for these patients could achieve the dose constraints in all but the two patients with vertebral body erosion. Differences in the normal tissue doses of bilateral normal lungs, esophagus, heart and spinal cord were compared between both techniques to ascertain statistical significance. The spatial location of the tumour and nodal burden rather than the volume limited the maximum dose delivered. For tumours with vertebral body contact of more than 90 degrees, VMAT should be used. With gross vertebral body erosion, even VMAT falls short if the PTV includes the spinal cord. In view of high patient volume and limited resources, VMAT is not an economical option for all patients. A simple clinical algorithm is outlined (Figure 1) which will help select patients who will best benefit from VMAT.

Dosimetric comparison of jaw tracking in intensity modulated and volumetric modulated arc radiotherapy for carcinoma of cervix

Abstract Aim: To study the dosimetric advantages of the jaw tracking technique in intensity-modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) for carcinoma of cervix patients. Materials and Methods: We retrospectively selected ten previously treated cervix patients in this study. All the ten patients underwent CT simulation along with immobilization and positional devices. Targets and organ at risks (OARs) were delineated slice by slice for all the patients. All the patients were planned for IMRT and VMAT with intend to deliver 50 Gy in 25 fractions. All the plans were planned with 6 MV photon beam using millennium-120 multi leaf collimator (MLC) using the TrueBeam linear accelerator. IMRT and VMAT plans were performed with jaw tracking (JT) and with static jaw (SJ) techniques by keeping the same constraints and priorities for the target volumes and critical structures for a particular patient. For standardization, all the plans were normalized to the target mean of the planning target volume. All the plans were accepted with the criteria of bladder mean dose &lt; 40 Gy and rectum mean dose &lt; 40 Gy without compromising the target volumes. Target conformity, dose to the critical structures and low dose volumes were recorded and analyzed for IMRT and VMAT plans with and without jaw tracking for all the patients. Results: The conformity index average of all patients followed by standard deviation (̄x± σ̄x) for JT-IMRT, SJ-IMRT, JT-VMAT and SJ-VMAT were 1.176 ± 0.139, 1.175 ± 0.139, 1.193 ± 0.220 and 1.228 ± 0.192 and homogeneity index were 0.089 ± 0.022, 0.085 ± 0.024, 0.102 ± 0.016 and 0.101 ± 0.016. In low dose volume J,T-IMRT shows a 5.4% (p-value &lt; 0.001) overall reduction in volume receiving at least 5 Gy (V5) compared to SJ-IMRT, whereas 1.2% reduction was observed in V5 volume in JT-VMAT compared to SJ-VMAT. JT-IMRT showed mean reduction in rectum and bladder of 1.34% (p-value &lt; 0.001) and 1.46% (p-value &lt; 0.001) compared to SJ-IMRT, while only 0.30% and 0.03% reduction were observed between JT-VMAT and SJ-VMAT. JT-IMRT plans also showed considerable dose reduction to inthe testine, right femoral head, left femoral head and cauda compared to the SJ-IMRT plans. Conclusion: Jaw tracking resulted in decreased dose to critical structures in IMRT and VMAT plans. But significant dose reductions were observed for critical structures in the JT-IMRT compared to SJ-IMRT technique. In JT-VMAT plans dose reduction to the critical structures were not significant compared to the JT-IMRT due to relatively lesser monitor units in the VMAT 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.

Preserving Endocrine Function in Premenopausal Women Undergoing Whole Pelvis Radiation for Cervical Cancer.

Whole pelvis radiation therapy (WPRT) in premenopausal women with cervical cancer can cause permanent ovarian damage, resulting in premature menopause. Oophoropexy, often considered as an initial step, demonstrates safety of sparing 1 ovary at the cost of delay in initiating WPRT. Therefore, we dosimetrically compared volumetric modulated arc radiotherapy (VMAT) and intensity modulated proton therapy (IMPT) techniques to allow for ovarian-sparing WPRT. Ten patients previously treated for cervical cancer at our institution were included in this institutional review board-approved analysis. A modified clinical treatment volume (CTV) was designed, sparing 1 ovary (left or right), as determined by the physician (ovarian-sparing CTV) and disease extent, including physical exam, positron emission tomography/computed tomography and magnetic resonance imaging. An ovarian-sparing planning target volume was determined as the ovarian-sparing CTV+5 mm for patients who were supine and 7 mm for those who were prone. All plans were calculated to a dose of 45 Gy with specific optimization goals for target volumes, while attempting to maintain a mean ovary dose (Dmean) < 15 Gy. Dosimetric goals were compared across the 2 modalities using the Mann-Whitney U test. Both treatment modalities were able to achieve primary clinical goal coverage to the uterus/cervix (P = .529, comparing VMAT versus IMPT), ovarian-sparing CTV (P = .796) and ovarian-sparing planning target volume (P = .004). All 10 IMPT plans were able to accomplish the ovary objective (14.0 ± 1.66 Gy). However, only 4 of the 10 VMAT plans were able to achieve a Dmean < 15 Gy to the prioritized ovary, with an average dose of 15.3 ± 4.10 Gy. Sparing an ovary in women undergoing WPRT for cervical cancer is dosimetrically feasible with IMPT without sacrificing coverage to important clinical targets. Future work will incorporate the brachytherapy dose to the ovarian-sparing CTV and assess the clinical response of this technique as a means to preserve ovarian endocrine function.

Dosimetric and radiobiological comparison of external beam radiotherapy using simultaneous integrated boost technique for esophageal cancer in different location.

e15505 Background: Based on dosimetry and radiobiology to compare treatment plans for esophageal cancer (EC) in different location using intensity modulated radiotherapy (IMRT), volumetric modulated arc radiotherapy (VMAT) and helical tomotherapy(HT) with simultaneous integrated boost (SIB) technique. Methods: A total of 20 patients including 5 cases respectively located in the cervix, upper, middle and lower thorax were generated for IMRT, VMAT and HT plans. The dose volume histogram statistics, conformity index (CI), homogeneity index (HI), tumor control probability (TCP) and normal tissues control probability (NTCP) were analyzed to evaluate treatment plans. Results: HT showed significantly improvement over IMRT and VMAT in terms of CI(0.93±0.03), HI(0.07±0.03) and TCP(88.08±0.82%) in cervical EC(p&lt;0.05). IMRT greatly developed TCP(88.29±1.79%;85.11±0.79%), and offered superior CIs (0.87±0.04;0.90±0.01) and HIs(0.10±0.01; 0.06±0.01) compared with VMAT and HT in upper and middle thoracic EC(p&lt;0.05). Meanwhile, the V30(33.30±6.49%), mean dose (2559.00±219.64cGy) and NTCP(0.50±0.61%) of heart for IMRT were significantly reduced than other two techniques in middle thoracic EC. Patients with lower thoracic EC yielded the similar CIs and HIs(all p&gt;0.05) for the 3 techniques, but VMAT showed the lowest NTCP of lungs (0.01±0.01%) with improved TCP (84.84±1.13%). Conclusions: HT was a good option with little lung and heart involvement as it achieved superior dose conformality and uniformity. IMRT was a perfect strategy with large thoracic involvement. It significantly improved tumor local control and reduced heart dose and complications with acceptable dose to lungs. VMAT was preferred with a smaller target volume but surrounded by more heart and less lungs. Individually choosing optimal technique for EC in different location will be warranted.

Trajectory modulated arc therapy using quasi-continuous couch motion layered on top of volumetric modulated arc therapy in left breast and chest wall irradiation: a feasibility study

AbstractAimTo investigate the dosimetric advantage of quasi-continuous couch motion-enabled trajectory modulated arc radiotherapy therapy (TMAT) over the coplanar tangential partial arcs volumetric modulated arc radiotherapy (VMAT) for treating left breast and chest wall patients.MethodTreatment plans of 43 patients who received radiotherapy for left breast (17) or for left chest wall (26) using coplanar partial tangential arcs VMAT (reference plan) were considered for this study. For each patient, in addition to the treatment plan, a TMAT plan was also generated using quasi-continuous couch rotation. The TMAT plan consisted of original two 30° tangential arc beams and two supplementary beams having a couch rotation of ±10°, ±20° and ±30°, respectively. The difference in PTV volume coverage (PTV V95%) between TMAT plan and VMAT plan was calculated for all the cases and normalised to the plan’s prescription dose. Similarly, differences in PTV_V105% and several dose-volume parameters related to organs at risk (OAR) were also computed and tabulated.ResultTMAT shows an increment in the PTV dose coverage V95% with respect to reference plan by 4·7±2·5% when averaged overall prescription dose levels. Mean PTV dose (averaged overall prescription levels) for reference and TMAT plan was 4638·6±423·8 and 4793·5±447·2 cGy, respectively, and statistically insignificant (p=0·06). However mean PTV_V105% values for TMAT and for reference plans were 6·7±4·8 and 7·2±5·2%, respectively, and were not statistically different (p=0·85). Mean heart dose in TMAT was less than in VMAT plans, but not significantly. As regarding D1% to heart, TMAT plan was again found to be better with a mean difference of 137·1 cGy over VMAT plan. Other parameters evaluated were: mean dose and D1% to contralateral breast, and V20 Gy and V5 Gy for lung.ConclusionTMAT plans were found to be better than VMAT plans in terms of PTV coverage and D1% for heart. For evaluated dose parameters apart from PTV coverage and D1% to the heart, no significant differences were observed. Thus, TMAT plans yielded better dose distribution in terms of PTV dose coverage, hot spots and OAR doses.

Actual gains in dosimetry and treatment delivery efficiency from volumetric modulated arc radiotherapy for inoperable, locally advanced lung cancer over five-field forward-planned intensity-modulated radiotherapy

Volumetric modulated arc radiotherapy (VMAT) is used for inoperable, locally advanced nonsmall cell lung cancer, where three-dimensional conformal radiotherapy (3D-CRT) cannot yield an acceptable plan. The planning and treatment data were prospectively collected on the first 18 patients treated using VMAT plans. We analyzed the actual dosimetric gain and impact on treatment, compared with complex multisegment 3D-CRT (five-field forward-planned intensity-modulated radiotherapy [IMRT]) that were generated for treatment. Proportion of planning target volume (PTV) receiving 95% dose (PTV-V95%) conformity index (CI), conformity number (CN), dose homogeneity index (DHI), monitor units (MUs), and treatment time were also analyzed. The PTV coverage (PTV-V95%) was improved from a median of 91.41% for 5-F forward-IMRT to 98.25% for VMAT (P < 0.001). The CI improved with a mean of 1.12 for VMAT and 1.31 for 5-F forward-IMRT (P < 0.001). The mean DHI improved from 1.15 for forward-IMRT to 1.08 for VMAT (P < 0.001). The mean CN improved from 0.62 for forward-IMRT to 0.87 for VMAT (P < 0.001). No significant increase in the low-dose bath (V5, V10 and mean lung dose) to the lung was seen. Significantly higher number of MUs (P < 0.001) and shorter treatment delivery times (P = 0.03) were seen with VMAT. VMAT resulted in improvement in target volume coverage, demonstrated by PTV-V95%, CI, CN, and DHI, without any increase in the low-dose bath to the lung. For conventional fractionation, VMAT requires more MUs (P < 0.001) but has a shorter treatment delivery time (P = 0.03) per fraction.

Non-coplanar trajectories to improve organ at risk sparing in volumetric modulated arc therapy for primary brain tumors

Background and purposeTo evaluate non-coplanar volumetric modulated arc radiotherapy (VMAT) trajectories for organ at risk (OAR) sparing in primary brain tumor radiotherapy. Materials and methodsFifteen patients were planned using coplanar VMAT and compared against non-coplanar VMAT plans for three trajectory optimization techniques. A geometric heuristic technique (GH) combined beam scoring and Dijkstra’s algorithm to minimize the importance-weighted sum of OAR volumes irradiated. Fluence optimization was used to perform a local search around coplanar and GH trajectories, producing fluence-based local search (FBLS) and FBLS+GH trajectories respectively. ResultsGH, FBLS, and FBLS+GH trajectories reduced doses to the contralateral globe, optic nerve, hippocampus, temporal lobe, and cochlea. However, FBLS increased dose to the ipsilateral lens, optic nerve and globe. Compared to GH, FBLS+GH increased dose to the ipsilateral temporal lobe and hippocampus, contralateral optics, and the brainstem and body. GH and FBLS+GH trajectories reduced bilateral hippocampi normal tissue complication probability (p=0.028 and p=0.043, respectively). All techniques reduced PTV conformity; GH and FBLS+GH trajectories reduced homogeneity but less so for FBLS+GH. ConclusionsThe geometric heuristic technique best spared OARs and reduced normal tissue complication probability, however incorporating fluence information into non-coplanar trajectory optimization maintained PTV homogeneity.

SU-F-T-377: Monte Carlo Re-Evaluation of Volumetric-Modulated Arc Plans of Advanced Stage Nasopharygeal Cancers Optimized with Convolution-Superposition Algorithm

Background: Commercial treatment planning system Pinnacle3 (Philips, Fitchburg, WI, USA) employs a convolution-superposition algorithm for volumetric-modulated arc radiotherapy (VMAT) optimization and dose calculation. Study of Monte Carlo (MC) dose recalculation of VMAT plans for advanced-stage nasopharyngeal cancers (NPC) is currently limited. Methods: Twenty-nine VMAT prescribed 70Gy, 60Gy, and 54Gy to the planning target volumes (PTVs) were included. These clinical plans achieved with a CS dose engine on Pinnacle3 v9.0 were recalculated by the Monaco TPS v5.0 (Elekta, Maryland Heights, MO, USA) with a XVMC-based MC dose engine. The MC virtual source model was built using the same measurement beam dataset as for the Pinnacle beam model. All MC recalculation were based on absorbed dose to medium in medium (Dm,m). Differences in dose constraint parameters per our institution protocol (Supplementary Table 1) were analyzed. Results: Only differences in maximum dose to left brachial plexus, left temporal lobe and PTV54Gy were found to be statistically insignificant (p> 0.05). Dosimetric differences of other tumor targets and normal organs are found in supplementary Table 1. Generally, doses outside the PTV in the normal organs are lower with MC than with CS. This is also true in the PTV54-70Gy doses but higher dose in the nasal cavity near the bone interfaces is consistently predicted by MC, possibly due to the increased backscattering of short-range scattered photons and the secondary electrons that is not properly modeled by the CS. The straight shoulders of the PTV dose volume histograms (DVH) initially resulted from the CS optimization are merely preserved after MC recalculation. Conclusion: Significant dosimetric differences in VMAT NPC plans were observed between CS and MC calculations. Adjustments of the planning dose constraints to incorporate the physics differences from conventional CS algorithm should be made when VMAT optimization is carried out directly with MC dose engine.