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A Comparison of IMRT and VMAT treatment techniques on centrally-located lung tumors and their effects on V5 lung doseAmber Mehr, B.S.; Andrew Edel, B.S; Jenny Huang, B.S., R.T.(T); Ruha Siddiqui, B.S.;Ashley Hunzeker, M.S., C.M.D.; Nishele Lenards, R.T.(R)(T), M.S., C.M.D., FAAMD;?Alyssa Olson, M.S., R.T.(T), C.M.DUniversity of Wisconsin – La Crosse Medical Dosimetry ProgramABSTRACT???The goal of this study was to determine if there was a difference in the percentage of lung receiving 5 Gy or more (V5) in patients planned with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) treatment planning. Fourteen patients with centrally-located lung tumors and planning target volumes (PTVs) between 100-1500cc were selected for this research study. The heart, lungs, spinal cord and esophagus were contoured by the medical dosimetrist for dose tracking purposes to organs at risk (OAR). Each patient was planned using IMRT and VMAT techniques for comparison purposes. When creating the VMAT and IMRT treatment plans, the medical dosimetrist used similar optimization techniques to ensure that the planning objectives were met. Upon plan completion, a paired t-test was used to determine if there was a significant difference between the planning techniques with regard to the V5. The t-test score for the lung V5 dose was 3.02 and was considered statistically significant. Therefore, when comparing IMRT and VMAT techniques for lung treatments, the results of this study demonstrated that IMRT provided an advantage of sparing lung V5 dose.Keywords: Intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), V5, centrally-located lung tumors??????Introduction???In the past, three-dimensional conventional radiation therapy (3DCRT) planning was primarily used for lung treatments.1 The onset of 3DCRT first began in the early 1980s and was an important advancement in the world of radiation therapy. Using 3DCRT allowed the manipulation of spatial orientation, field number selection, beam energy selection, field weighting and more.2 Through innovative developments in technology, 3DCRT is slowly being replaced by IMRT and VMAT. Intensity modulated radiation therapy allows radiation dose to conform precisely to the 3D shape of the tumor by allowing the manipulation of radiation intensity through modulation of various static beams in small volume.3 On the contrary, VMAT is a different type of IMRT where the linear accelerator continuously rotates around the patient while simultaneously reshaping and altering the intensity of the beam. Both are more advanced types of planning techniques that utilize more sophisticated dose computation algorithms and optimization methods.5 These optimization methods allow both techniques to target lung tumors specifically while minimizing radiation exposure to OAR via inverse planning. Khalil et al4 proposed that 3DCRT posed difficulties when treating large lung volumes, whereas IMRT was able to successfully treat large tumor volumes without increasing standard lung dose constraints such as mean lung dose (MLD) and the percentage of lung receiving 20 Gy or more (V20). Additional research by Liao et al3 further supported this claim. Radiation therapy is an integral component of treating lung cancer; however, as with many treatment modalities, the potential side-effects, such as radiation pneumonitis (RP) must be considered. A comparative study between IMRT and 3DCRT techniques found that RP was present in 3.5% and 7.9% of the studied population respectively.3 Dosimetric parameters such as lung V5, V20, the percentage of lung receiving 30 Gy or more (V30), and MLD, have been considered variables used to predict the occurrence of RP.4 Recent research on V5 indicated an association with RP and should be kept as low as possible in addition to the aforementioned dosimetric parameters.4 Yet, a study by Lievens et al6 stated that lung V5 dose is not predictive of RP nor indicated if lung V5 dose levels are higher when dose is delivered with dynamic arcs. While there still remains some uncertainty regarding the importance of V5, it should still be considered when creating a treatment plan as it can be an indicator of RP.7,8?The purpose of this study was to determine if there was a difference in the lung V5 dose between IMRT and VMAT treatment planning. Materials and Methods????Patient Selection & Setup?????To prevent bias of the data set, plans chosen for comparison purposes had to meet certain criteria to be considered eligible for data collection. These criteria included a specified range for the prescription dose, PTV volume (cm3), PTV vertical length (cm), and lung volume (cm3). Fourteen patients with centrally-located lung tumors, treated at 3 different cancer centers, were selected for this study. Patients were prescribed a dose range between 45 to 70 Gy. The tumor pathology varied among patients, which included non-small cell lung carcinoma (NSCLC), small cell lung carcinoma (SCLC), and squamous cell carcinoma (SCC). All patients had centrally-located lung tumors with a PTV between 100-1500 cm3.?The lung PTV volume of the studied patients varied from 1900-3350 cm3 with the vertical lengths of 6-17 cm (Table 1). Patients excluded from this study included those with a laterally-located lung tumor or prior radiation treatment. All 14 patients were positioned in a similar fashion.?Patients were set up in the supine head-first position using immobilization devices during their CT simulation (Figure 1). The patient’s arms were placed over their heads using a T-Bar device to remove the upper extremities from the treatment field. A vacuum bag was placed underneath the patient’s chest and arms to provide stability and comfort. Once the CT scan was performed, the isocenter was established by the radiation oncologist and medical dosimetrist within the clinic. The radiation therapist placed marks on the patient’s skin surface to denote the isocenter position; these marks were used for daily treatment for alignment purposes.????Contouring?????After the CT simulation was performed, the patient CT images were imported into the treatment planning system (TPS) to be contoured by the radiation oncologist and medical dosimetrist. The radiation oncologist contoured the gross tumor volume (GTV), clinical target volume (CTV) and PTV. The GTV was created around the cancerous tissue as visualized from the CT. The GTV was then expanded by 0.7 cm in all directions to create the CTV. To create the PTV, the CTV was expanded 1.0 cm superiorly and inferiorly and 0.5 cm anteriorly, posteriorly, laterally and medially coinciding with the Radiation Therapy Oncology Group (RTOG) 0617 Protocol.9 Once the target volumes were completed, the medical dosimetrist contoured the thoracic OAR following RTOG 0617 protocol and included the spinal cord, lungs, esophagus, and heart.9???Treatment Planning?????Planning was performed using Pinnacle 9.8, Pinnacle 14.0 and Eclipse 13.7 TPS and was used to plan 4, 10, and 4 patients respectively. Patient treatments were completed on Varian 21iX, Trilogy and Elekta Infinity and Synergy linear accelerators. All 14 patients received thoracic radiation treatment using a VMAT technique and were re-planned using the IMRT technique with the same TPS and linear accelerator.The original prescription for each patient ranged between 45 –70 Gy and was maintained for the purpose of this study. Applying proper technique depended on the tumor size, tumor location, OAR and dose tolerance criteria. Both techniques used the same planning objectives. None of the 28 plans were optimized using a V5 objective to allow for an effective evaluation of the metric. Other required planning objectives included the ipsilateral lung V20 < 30-35%, ipsilateral lung V30 < 20-25%, a maximum dose to the spinal cord ≤ 50 Gy, spinal cord dose to 0.03 cm3 < 44 Gy, and to reduce the volume of both the esophagus receiving 45 Gy (V45) and the volume of the heart receiving 60 Gy (V60) to 33% (Table 2). The VMAT beams were arranged as either 2 partial arcs, 2 full arcs, 3 partial arcs or 3 full arcs with varying collimator angles to create a conformal plan around the PTV (Figure 2). The beams were planned with different rotational directions, clockwise (CW) and counter clockwise (CCW) and utilized a photon energy of 6 MV. The plans were optimized according to the desired constraints for the lung(s), spinal cord, esophagus, and heart, while simultaneously optimizing to obtain PTV coverage. All VMAT plans were normalized so that 95% of the PTV received prescription dose.For the IMRT plan, the treatment planning objectives, target volumes, isocenter and prescription were kept the same as the initial VMAT plan. For each IMRT plan, the medical dosimetrists used a median of 6 coplanar beams (range 4-9 beams) and 6 MV to deliver the prescribed dose to 95% of the PTV. The beams were placed at optimal angles to avoid the OAR and create a conformal plan (Figures 3 and 4).ResultsFor comparison purposes, data was collected on the V5 lung dose metric in all IMRT and VMAT plans. A paired t-test was conducted to evaluate the significance of V5 and a statistical analysis was performed to compare the 2 techniques. A visual evaluation of the 5 Gy isodose distribution in each plan as was also completed for trend mapping. With regard to the lung V5 dose, VMAT plans averaged 6% higher V5 dose than IMRT. Upon further evaluation, a statistical analysis concluded that the noticed variance was statistically significant. The mean V5 value for the VMAT plans was 69.57% ± 17.16%, while the mean V5 value for the IMRT plan was 63.5% ± 14.73%. A 2-tailed t-test for paired samples resulted in a t13-score of 3.02 and a p-value of .001. For each plan, a visual inspection of the isodose distribution for 5 Gy was compared. A common trend when comparing the isodose distributions for both techniques was an increased amount of 5 Gy dose inferiorly within the lungs for the VMAT treatment technique. However, as the dose progressed more superiorly, less lung tissue was spared from the 5 Gy dose for both techniques. In addition, it was noticed that the 5 Gy isodose line for both techniques was found close to the entry points for each treatment beam. Therefore, due to the delivery method of the VMAT technique, the lung V5 dose for the VMAT plans looked more continuous due to the constant supply of dose as the beam traveled around the patient (Figure 5). Discussion??Notable differences between IMRT and VMAT treatment planning techniques were observed throughout this study. For VMAT, the mean V5 lung dose percentage across patient cases was greater than 65% which has been suggested as a reasonable lung V5 dose constraint.?For both planning techniques the standard deviation was over 15% between patients. This high degree of variance limits the conclusions that can be drawn. It is unclear if the results were skewed by a few cases with large changes in V5 dose. Large standard deviations between patients could be due to differences in anatomy, prescription dose, beam arrangement, PTV size, and PTV location.5,9 Planning technique is one of many factors that also can affect lung V5 dose.?Further research is needed to discover which of these factors are covariable. Upon completion of this study, it was obvious that IMRT and VMAT have different characteristic dosimetric outcomes that become more pronounced at the lower doses. With VMAT, the dose entered the patient continuously while the gantry moved along the arc. This resulted in dose being delivered to the patient’s body through more entry points when compared to IMRT treatment planning. With this change in dose delivery, patients received a higher integral dose due to the increased access to the lungs.5 Therefore, as concluded in this study, the lung V5 dose for IMRT was lower when compared to the VMAT plans.?When comparing the isodose distribution, the 5 Gy isodose line was greater superiorly within the lung and greater for VMAT plans. This makes sense because the volume of lung becomes greater when traveling more inferiorly through the patient’s thorax. The V5 dose was also more prominently observed wherever the beams entered the patient. This meant that when the patients were treated using IMRT, there was less V5 dose because there were gaps between beam entry points unlike when the patient was treated with VMAT.In a study conducted by Boyle et al,10 the IMRT technique produced lower doses to the lungs, heart, and esophagus in comparison to 3DCRT. However, few studies have analyzed the significance of lung dose when comparing newer methods such as VMAT versus IMRT. This study aimed to present the statistical difference and evaluate results in lung dose received in IMRT and VMAT planning. Therefore, IMRT may provide better results for physicians inquiring into the significance of lung dose, especially V5, with regards to treatment planning. Conclusion???Through advancements in technology, IMRT and VMAT planning are now being used to treat lung cancer. With the introduction of these techniques, the difference in lung V5 dose between planning techniques required more analysis due to the risk of RP. Patients with centrally-located lung tumors were selected and IMRT and VMAT comparison plans were created to determine the differences in lung V5 dose. When comparing IMRT and VMAT treatment plans, there were differences found among both techniques. The VMAT planning technique resulted in higher lung V5 dose in patients. Both techniques, however, were beneficial to the medical dosimetrists because coverage was maintained while limiting dose to critical structures due to blocking and beam placement. The limitations of this study included a small sample size and the exclusion of 3DCRT as one of the planning techniques for comparison.?For future studies, a larger sample size of IMRT and VMAT treatments plans should be created focusing solely on laterally-located lung tumors to see if there is a difference in overall lung dose. The beams should also be limited to the side of the patient’s body that contains the tumor to control dose to the contralateral lung.7 Medical dosimetrists should also look at IMRT planning versus 3DCRT and VMAT versus 3DCRT to identify changes in lung dose with planning technique advancements. ReferencesGraham MV, Purdy JA, Emami B, et al. Clinical dose-volume histogram analysis for pneumonitis after 3D treatment for non-small cell lung cancer (NSCLC). Int J Radiat Oncol Biol Phys. 1999;45(2):323-329. (99)00183-2??Cai J, Malhotra HK, Orton CJ. A 3D- conformal technique is better than IMRT or VMAT for lung SBRT. Med Phys. 2014;41(4):040601-040602.? SG, Hu C, Choy H, et al. Impact of intensity-modulated radiation therapy technique for locally advanced non-small-cell lung cancer: a secondary analysis of the NRG Oncology RTOG 0617 randomized clinical trial. J Clin Oncol. 2017;35(1):56-62. AA, Hoffman L, Moeller DS, et al. New dose constraint reduces radiation-induces fatal pneumonitis in locally advanced non-small cell lung cancer patients treated with intensity-modulated radiotherapy. Acta Oncologica. 2015;54:1343-1349. Y, Wang J, Tan L, et al. Dosimetric comparison between IMRT and VMAT in irradiation for peripheral and central lung cancer. Oncol Lett. 2018;15(3):3735-3745. A, Czerminska M, J?nne P, et al. Fatal pneumonitis associated with intensity-modulated radiation therapy for mesothelioma. Int J Radiat Oncol Biol Phys. 2006;65(3):640–645. H, Jauregui M, Zhang X, et al. Beam angle optimization and reduction for intensity-modulated radiation therapy of non–small-cell lung cancers. Int J Radiat Oncol Biol Phys. 2006;65(2):561–572. Y, Nulens A, Gaber MA, et al. Intensity-modulated radiotherapy for locally advanced non-small-cell lung cancer: a dose-escalation planning study. Int J Radiat Oncol Biol Phys. 2011;80(1):306-313. J, Choy H, Komaki R, et al. RTOG 0617: A randomized phase III comparison of standard-dose (60 Gy) versus high dose (74 Gy) conformal radiotherapy with concurrent and consolidation carboplatin/paclitaxel +/- cetuximab (IND #103444) in patients with stage IIIA/IIIB non-small cell lung cancer. Lancet Oncol. 2015(2):187-199. (14)71207-0??Boyle J, Ackerson B, Gu L, et al. Dosimetric advantages of intensity modulated radiation therapy in locally advanced lung cancer. Adv Radiat Oncol. 2017;2(1):6-11. 1. ?Demonstration of patient positioning for CT simulation and treatment delivery.??Figure 2. The conformal dose distribution for a lung plan treated to 61.2 Gy (red) with 3-partial arcs using a VMAT treatment planning technique.Figure 3. An example of a lung plan with a 5-field beam arrangement using an IMRT treatment technique.Figure 4. The conformal dose distribution, for a 61.2 Gy prescription dose, of a 7-field beam arrangement using an IMRT treatment technique.Figure 5. A comparison of the lung V5 dose (red) distribution for VMAT (left) and IMRT (right) treatments within the lung. TablesTable 1. Patient qualifiers which included the following: prescription (Gy), lung volume (cm3), PTV volume (cm3), PTV length (cm).Patient No. Prescription(Gy) Lung Volume (cm3)PTV Volume(cm3)PTV length(cm)1?602728.13362.813.52?702690.28629.29.33?603211.21087.912.34?603044.3916.514.05?61.202086724.111.76?502816.82582.310.27?453347.971409.517.38?61.201868.36535.617.79?602682.9320.812.610?602851.84120.26.011?502364.43340.611.812?50.401916.4273.28.013?502521.7336.210.914?602325.9240.88.7Table 2. The thoracic constraints used for patient treatment planningin IMRT and VMAT.?Organ at risk??Objectives??Spinal Cord??Maximum Dose (point dose) < 50 Gy??Maximum Dose (0.03 cm3) < 44 Gy??Lung??V20 < 30-35%??V30 < 20-25%??Esophagus??V45 < 33%??Heart??V60 < 33%?? ................
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