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An evaluation of adaptive planning by assessing the dosimetric impact of weight loss throughout the course of radiotherapy in bilateral treatment of head and neck cancer patientsKayla Tedrick, BS, RT(T); Zach Stauch, BS, RT(T); Wes Zoller, CMD; Steve Walston, MD, PhD; Daniel Christ, MS, DABR; Ashley Hunzeker, MS, CMD; Nishele Lenards PhD, CMD, RT(R)(T), FAAMD; Lee Culp, MS, CMD; Dukagjin Blakaj, MD, PhDIntroductionMost head and neck (H&N) cancers are squamous cell carcinomas and associated with risk factors such as tobacco and alcohol use.1 Head and neck cancers are common, with the incidence being twice as high in males as in females. In total, there will be an estimated 51,540 new cases of oral cavity and pharyngeal cancer in the United States in 2018.2 For cancers of the oral cavity and pharynx, the 5-year survival rate is 65%. Standard treatments for H&N cancers often include radiation therapy and surgery, combined or individually, with chemotherapy being used when appropriate.Head and neck radiotherapy treatments have progressed dramatically over the past several years. Technological advancements, such as the utilization of contrast-enhanced diagnostic-quality computed tomography (CT) during radiation treatment planning, has significantly contributed to this improvement. Likewise, the use of Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) treatment planning and delivery have become commonplace with external beam treatments.1 The advanced treatment techniques have led to dose escalation and reduction of the associated toxicity for many H&N patients. Despite these advancements, H&N radiation therapy continues to cause significant short and long term side effects, such as mucositis and dysphagia, for many patients throughout the course of treatment. Due to the toxicities that commonly frequent H&N patients under treatment, patient health is monitored closely. Weight loss is a common occurrence with H&N patients due to the associated side effects. Specifically, dysphagia and mucositis can often make it difficult for patients to maintain the proper intake without assistance or intervention leading to an increased propensity for weight loss.3 The volumetric and anatomic changes that may occur as a result, have opened discussion on how the dosimetric distribution may be affected to both the target areas and surrounding normal tissues. There are numerous organs at risk (OAR) in the H&N region that are critical to avoid during radiation treatment planning in order to maintain proper function. However, weight loss causes planned dose in normal structures to deviate as well as the dose distribution to the defined target volumes.4 Due to these findings, re-planning of patients who have experienced weight loss during treatment has become increasingly common. Castelli et al5?concluded that patients with a change in anatomy during treatment received an overdose of more than 2.5?Gy?to the parotid glands and weekly re-planning reduced the parotid gland mean dose by 4?Gy.?Similarly, Zhao et al6 found an increase in the dose to 1% (D1) of the spinal cord and brainstem volumes of 5.6 Gy and 2.5 Gy, respectively. As for target volumes, changes in dose distribution have been found to be more negligible due to the use of a planning target volume (PTV) that provides margin for error.4 Despite these dosimetric findings, the possibility exists that re-planning is not always required and practical. Hunter et al7 concluded that re-planning is unlikely to improve salivary output after treatment in most cases, even though re-planning can reduce the mean dose to the parotids. Additionally, there has been no definitive method established to define the amount of weight loss in the H&N region that necessitates a re-plan. El-Sayed et al8 claimed that patients should be re-simulated and re-planned if more than 10% of the weight loss occurs within the first three weeks of radiation treatment. However, patients who were re-planned in the El-Sayed study were selected based upon inadequate setup for daily treatment rather than specifically weight loss alone. The incorporation of setup-related error introduces alternate dosimetric variables into consideration. As a result, a true recommendation on when to initiate re-planning based on true anatomic loss remains arbitrary.Given the many resources required for re-planning and potential for treatment breaks, there is a possibility that re-planning may not be the best treatment option for patients with mild weight loss based on the significance of dosimetric changes. An investigation could provide useful steps in determining when re-planning becomes necessary due to weight loss alone, ultimately allowing for the establishment of weight loss parameters and re-simulation guidelines. The objective of this retrospective study was to assess the dosimetric effects of weight loss in H&N patients who were defined as having “acceptable daily positioning,” which was quantified based on anatomic landmarks. The patients were all planned with VMAT and IMRT treatment techniques and the resulting deviations in dose distribution were evaluated. From the dosimetric data, the need for re-planning was assessed. Additionally, the dosimetric impact of weight loss for both VMAT and IMRT techniques was evaluated to determine if there was a difference in plan robustness between the delivery methods as patient weight decreased. Methods and MaterialsPatient Selection & SetupData was collected from 22 H&N cancer patients diagnosed primarily with stage II and III squamous cell carcinoma with regional lymph node involvement.4 Each was treated definitively or postoperatively including primary disease as well as associated lymph node levels bilaterally. Patients with disease of the nasopharynx were excluded from this study due to increased emphasis on alignment of sinus cavities and base of skull versus spinal column. In the patient population, each experienced weight loss in the H&N area throughout the course of treatment as identified by the attending physician and treatment team on daily imaging. To evaluate whether the weight loss was resulting in loss of target coverage or increased dose to organs at risk (OAR), patients received an assessment CT positioned in standard treatment setup at the request of an attending physician. In order to narrow the focus to weight loss and eliminate other dosimetric variables involving the setup, patients were only selected for this study if they were defined as having “acceptable daily positioning” based on the registration between the original CT and the assessment CT. Specifically, “acceptable daily positioning” was defined as having no translational shift greater than 6 mm, concentrating on the spinal column from C1 and extending to the vertebral body representing the most inferior in-field region of the supraclavicular target volume. The translational shifts included the left to right, anterior to posterior, and superior to inferior directions, which were each assessed independently.Each patient was simulated in a supine position on a GE Discovery CT scanner with a setting of 2.5 mm slice thickness. A conformal H&N board was used for each patient. Immobilization included either a Q-fix “Q2” with a custom headrest or a Silverman “C” headrest with two 1 mm shims placed underneath. Long thermoplastic masks were formed for each patient with shoulders placed inferiorly with arms at sides grasping indexed pegs placed into the lateral aspect of the conformal H&N board (Figure 1). Patients with primary disease involving the oral cavity were simulated with custom mouthpieces to immobilize the maxilla per physician’s request. A knee sponge was placed underneath the legs for comfort.ContouringFollowing simulation, the patient’s CT dataset was imported into the Eclipse treatment planning system (TPS). Organs at risk were contoured by a certified medical dosimetrist per Radiation Therapy Oncology Group (RTOG) 1016 with few noted exceptions. For the purposes of this study, the exceptions included the segmentation of the esophagus, larynx, and pharynx structures. Specifically, the esophagus was contoured from the distal end of the pharynx to 1 cm below the inferior portion of the PTV to include all in-field contents. The larynx was contoured to include suprahyoid epiglottis superiorly and extend inferiorly to the level of the cricoid cartilage. The larynx contour extended from the anterior commissure to include the arytenoids, and the infra-hyoid region was segmented as a triangular prism shape. The pharynx included the posterior pharyngeal wall from the base of the skull to the cricoid cartilage, including adjacent constrictor muscles. No OAR were cropped or subtracted from target volumes for the dose evaluation portion of this study. Immobilization contents, such as the posterior mask and head rest, were contoured when appropriate such that attenuation would be included in the dose calculation. The normal tissue contours were reviewed by attending physicians, and 10 specific OAR were selected to be evaluated on both the original CT and assessment CT for dosimetric evaluation in this study. These OAR were chosen per physician preference based on common replanning considerations. This evaluation criteria included: spinal cord, spinal cord planning risk volume (PRV), brainstem, brainstem PRV, oral cavity, parotid glands, pharynx, larynx, and esophagus. For consistency of evaluation, nomenclature was templated as _R_SpinalCord, _R_SpinalCord_05, _R_Brainstem, _R_Brainstem_03, _R_Esophagus, _R_Layrnx, _R_Pharynx, _R_Cavity_Oral, _R_Parotid_L, and _R_Parotid_R corresponding to each associated OAR.All target volumes were contoured by attending physicians. The gross tumor volume (GTV) and clinical target volumes (CTV) were initially contoured based on the registration between the contrast-enhanced planning CT and a diagnostic PET/CT. Segmentation of the GTV was defined to include the primary tumor and clinically positive lymph nodes seen either on the planning CT or pre-treatment PET imaging with a standardized uptake value (SUV) greater than 3 g/mL. The CTV was defined to include appropriate lymph node levels based on risk-assessment by attending physicians, as well as a standard uniform margin of 6 mm applied to GTV delineations where appropriate. Multiple CTVs were created and separated into a high risk volume, intermediate risk volume, and low risk volume corresponding to the prescribed dose to each; the high risk volume being prescribed the highest dose. To establish segmentation consistency for this study, the low risk volume included both the intermediate risk volume and the high risk volume in addition to the low risk volume (Figure 2). Likewise, the intermediate risk volume contained the high risk volume in addition to the intermediate. The planning target volumes (PTV) were separated in the same manner as CTVs and were created by uniform expansion of the CTVs 3-5 mm in all directions based on imaging frequency. Per the physician, the PTV and CTV volumes were permitted to be extracted 3 mm from the external contour of the patient surface to exclude the dermis and epidermis layers of the skin. For the evaluation purpose of this study, duplicate targets contours were created and labeled _R_PTV_High, _R_PTV_Int, _R_PTV_Low, _R_CTV_High, _R_CTV_Int, and _R_CTV_Low corresponding to the originally defined risk levels and target types (Figure 2).For segmentation of the assessment CT, the 10 identified OAR were delineated in the same defined manner as employed during original creation including the same contouring exceptions to the R_Larynx, R_Pharynx, and R_Esophagus contours as previously annotated. In addition to OAR, the original target volumes were transferred from the original CT to the assessment CT and extracted 3 mm from the patient’s external surface to account for skin build-up. Normal structure contours and target volumes were edited as needed and physician-approved. This included adjustments to R_CTV and R_PTV structures where applicable. Support structures—including immobilization—and external surface were also re-contoured on the assessment CT to include in dose calculation. Treatment PlanningThe original treatment plans were all planned with a VMAT method in the Eclipse TPS and approved by physicians. With every patient dataset including 2-3 PTVs, the plans all involved the utilization of a simultaneous integrated boost (SIB) technique. To extend the scope of this study, as well as to compare the robustness of different planning techniques, a physician approved 9-field IMRT sliding window IMRT plan was created on the original planning CT for each patient, but not used clinically. The patients selected were prescribed definitive doses by attending physicians. Of the 22 patients, 20 were prescribed 70 Gy in 35 fractions to the PTV_High, 63 Gy to the R_PTV_Int, and 56 Gy to the R_PTV_Low to be delivered simultaneously. The remaining two patients were prescribed a dose regimen of 69.96 Gy in 33 fractions to the PTV_High, 59.4 Gy to the R_PTV_Int, and 54.12 Gy to the R_PTV_Low to be delivered simultaneously. Due to the bilateral involvement of disease in each patient, the arc angles chosen for the VMAT plans were all a variation of a full arc. Each treatment plan involved at least 3 arcs but no more than 4, utilizing 6 MV as the designated energy. The collimator angles and sizes chosen for each arc were selected to maximize the efficiency of the multi leaf collimators (MLCs) in blocking out OAR along with achieving an adequate dose conformity. For IMRT treatment planning, 9 fields were arranged equidistant around the patient with 6 MV energy selection. Collimation of 90°at 200°, 0° and 160° allowed for efficient MLC blocking of midline structures. The other 6 beams employed a 0° collimator setting. All treatment plans were executed on Varian Truebeam linear accelerators.The planning constraints used were established from suggestions stated in the RTOG 1016 protocol along with departmental objectives. All plans on the original CT datasets were normalized to ensure that 100% of each prescription dose covered 95% of each of the individual PTVs. The maximum dose, defined as dose to 0.03cc, was not to exceed 110% of the prescription dose with an acceptable variation of 115%.Each VMAT plan was optimized in either the progressive resolution optimizer (PRO) version 13.6.23 or the photon optimizer (PO) version 13.6.23. Similarly, IMRT plans were optimized in PO version 13.6.23 of Eclipse. All plans were calculated with the Acuros External Beam (AcurosXB) version 13.6.23.Quantification of Weight LossThe primary focus of this study was to examine the variation in dose distribution for patients that experience weight loss in the H&N area during their course of radiation therapy. In order to do so, it was required that weight loss be quantified in geometric measurements such that they could be replicated with treatment imaging. As a result, weight loss was defined as the measured difference in separation between the original planning CT and the assessment CT at various landmarks. Separations were measured by reviewing three diameters at 3 vertebral levels in the H&N: C1, C3, and the interspace of C4/C5. To collect these measurements, viewing planes were positioned at midline of the vertebral body in the sagittal and coronal planes and positioned at the most anterior apex of the vertebral body in the axial plane (Figure 3). On the viewing plane axis for each vertebral level, 3 vectors were placed to measure separation. The first 2 vectors were drawn diagonally, 60 degrees apart from the central axis bifurcating the anterior apex of the vertebral body (Figure 3). These vectors were chosen to ensure that the difference in diameter measured from the original CT to the assessment CT was representative of the total circumferential loss in the H&N area. The third vector was drawn by bifurcating the anterior apex of the vertebral body extending left to right along the horizontal axis (Figure 3). This process was repeated at all 3 vertebral levels for both the original CT and the assessment CT, measuring to the external contour of the patient habitus (Figure 3). The measured difference between corresponding vectors of the original and assessment CT were then recorded for all 9 vectors, and averaged as a quantifiable metric of weight loss for the H&N region (Table 1). Likewise, the percentage weight loss at the time of the assessment CT was recorded for each patient to establish magnitude of weight loss (Table 1). Plan ComparisonsTo assess the effects of the patients’ weight loss on the dose distribution, a verification plan of the original VMAT and 9-field IMRT plans were calculated on the assessment CT using all original plan parameters. This was done by forming a rigid registration in Eclipse (TPS) between the original planning CT and the assessment CT for each patient. In the same manner as defined in patient selection, qualifiers of “acceptable daily positioning” were applied. This consisted of an imaging review to ensure that the rigid registration did not include a translational shift greater than 0.6 centimeters, concentrating on the spinal column from C1 and extending to the vertebral body representing the most inferior in-field region of the PTV. Each vertebral body was independently analyzed to ensure that all criteria were satisfied. Following review, the translational shifts from the rigid registration were applied to the verification plan performed on the assessment CT prior to calculation. Shift application was performed to mimic the methodology associated with daily imaging and treatment on the linear accelerator.Following the calculation of the verification plans on the assessment CTs, coverage to the target volumes and dose to OAR included in the structure set were recorded and compared to the original treatment plan. The metrics evaluated were chosen with reference to RTOG 1016. They included the dose to 95% of the target [D95] and dose to 5% of the target [D5] for PTVs, dose to 95% of the target [D98] for CTVs, dose to 99% of the _R_GTV_High [D99], maximum dose to _R_SpinalCord, _R_SpinalCord_05, _R_Brainstem, _R_Brainstem_03, mean dose for the parotid glands, _R_Larynx, and _R_Esophagus, mean dose and percent volume receiving 65Gy [V65] for _R_Pharynx, and lastly mean and maximum dose for the _R_Cavity_Oral. The percent difference between the metrics on the original plan CTs and the metrics on the verification plans were then calculated. This was completed for both the VMAT and 9 field IMRT plans. ResultsUsing values generated by a Dose-Volume Histogram (DVH), statistical analysis was performed to test for a significant correlation between all dose metrics and the average diameter loss at the corresponding level in the head and neck area using Pearson correlation. Specifically, the spinal cord, brainstem, pharynx, and all target metrics were assessed by total average diameter loss, the means of the larynx and esophagus by average diameter loss at C4/5, the mean of the oral cavity by average diameter loss at C3, and the mean of the parotids by average diameter loss at C1. For both the VMAT and IMRT plans, a significant correlation was found for the D5 of the _R_PTV_High, D95 for the _R_PTV_Int and _R_CTV_Int, and the V65 of the _R_Pharynx. The significant Pearson correlation coefficients for the IMRT plans were 0.012, 0.001, 0.017, and 0.006, respectively (Table 2). As for the VMAT plans, the coefficients were as follows: 0.012, 0.013, 0.037, and 0.005 (Table 2). Regarding D5 of the _R_PTV_High, an average increase of 3.29% was found for the VMAT plans, and an average increase of 2.64% for the IMRT plans (Table 3). Coverage to D95 of the _R_PTV_Int escalated on average 1.37% and 0.87% for the VMAT and IMRT plans, correspondingly (Table 3). The D95 of the _R_CTV_Int increased on average by 1.58% for VMAT plans and 1.12% for IMRT plans (Figures 4 & 5). The V65 of the _R_Pharynx saw an average increase of 7.7% and 6.2% for the VMAT and IMRT plans, respectively (Tables 3).An independent t-test was also performed to test for a significant difference between the VMAT and IMRT plans for each metric. The single significant difference found was the D95 for the _R_PTV_Int, with an average increase of 1.37% for VMAT versus 0.87% for IMRT plans.With a total average diameter loss of 1.06 cm and largest tissue loss at a single vector of 5.55 cm, the following statistics are notable. Coverage of all targets improved on average regarding both VMAT and IMRT. Specifically, D95 of the _R_PTV_ High increased 0.77% and 0.60% for VMAT and IMRT plans, respectively (Table 3). Also, an average increase of 1.25% and 1.01% was found for D95 of the _R_CTV_High (Table 3). Concerning OAR, the majority of these structures also received additional dose. The largest increase was that of the _R_Pharynx. V65 for VMAT went up 7.7% as well as for IMRT with a 6.2% increase (Table 3). _R_SpinalCord saw an average increase of 1.8 Gy and 1.5 Gy for the VMAT and IMRT plans, respectively, while _R_Brainstem saw an increase of 0.4 Gy for VMAT and 0.2 Gy for IMRT (Table 4). Lastly, _R_Parotids saw an increase of 4.1 Gy and 3.9 Gy for the VMAT and IMRT plans, respectively (Table 4).DiscussionAs expected from reviewing previous research, this study supports the finding that weight loss causes deviation in planned dose distribution to normal structures and target volumes.5 Patients with separation loss in the head and neck region and no setup discrepancies experience a general increase of dose to the measured structures.6-9 The results of this study were in line with these findings in terms of the loss of tissue the patients in this study underwent and the increased coverage of D95 and D5 for target volumes and increase in dose to OAR.When strictly dealing with weight loss, an increase in SSD can lead to less attenuation and therefore an increase in coverage for VMAT and IMRT plans.10 This study supported this statement with a decrease of 1.06 cm in total average diameter loss resulting in improvement in average target coverage. The decrease of average tissue attenuation led to an increase in coverage of the D95 by 0.77% and 0.66% for the _R_PTV_High for the VMAT and IMRT plans respectively. The increase in SSD and average reduction in attenuation due to weight loss could also be linked to the increase of D5.10 The correlation between separation loss and an increase to the V65 for the _R_Pharynx could have been influenced by inter-fractional motion and volume-based changes. A significant volume of _R_Pharynx was located within target structures for many patients in this study. Structures such as the _R_Parotids, _R_Esophagus, _R_Cavity_Oral, and _R_Larynx were frequently displaced internally resulting in increased overlap with target volumes. These volumetric transformations on the assessment CTs yielded increased dose to these overlapping portions of measured OAR. The _R_Pharynx consistently remained anterior to the cervical vertebral column, which was the focal alignment structure when registering the CT’s. It is debatable why _R_SpinalCord did not also have a strong correlation between total average diameter loss and increase in dose but one limitation could be the testing for the correlation having been done using total average diameter loss instead of the defined level of the maximum point dose. There was also no significant difference found between the VMAT and IMRT plans aside from D95 of _R_PTV_Int. Due to IMRT having less unique entrance angles and control points than VMAT plans, it could be expected that IMRT would be less robust regarding weight loss and plan quality.11 As the amount of control points is decreased, perhaps a larger variation is seen, however that could not be proven in this study and requires more research. Other factors could explain the minimal significant difference between the VMAT and IMRT plans in this study, such as the asymmetric diameter loss seen in the head and neck patients, with a majority of the loss seen on the lateral aspect. The weighting of the lateral control points in VMAT plans is perhaps similar to the weighting of the lateral 280° fields in the IMRT plans. While an average loss of tissue resulting in less tissue attenuation has been found to cause an increase in dose to targets and OAR, significant correlation was not found for most metrics in this study. The desired independent variable for this study was average tissue diameter loss in total or at C1, C3, or C4/5, however, the possibility of confounding variables exists. For instance, internal anatomy variations were seen in most patients, likely consequential from the radiation itself. Specifically, parotid glands have been found to decrease in volume during a radiation therapy course.12-14 This deemed apparent in most patients in this study. Volume reduction caused the mean parotid dose outcome to be affected by the amount of parotid overlap with the target volumes (Figure 4a & 4b). In the cases where mean parotid dose increased due to volume reduction and increased overlap with target volumes, re-planning may not always be advantageous. If the dose distribution remains robust in cases of patient weight loss, the increased overlap of the parotids may not allow for substantial mean parotid dose reduction, if target coverage is the physician’s priority.No quantitative method for finding the threshold of separation loss that requires a re-plan was established. Nonetheless, the data in this study suggests that patients’ weight loss during their course of treatment may not always require a re-plan, but physician preference still prevails. It is essential to consider the changes in dose distribution found in this study are the maximum projections. The verification plans that were calculated were prescribed the full course of 35 fractions. This study is representative of a case where the weight loss occurred before the first fraction and what the dose distribution would appear as if the patient were not re-planned for their entire course of treatment. Realistically, dose variations would be much less than stated in this study since most patients underwent their assessment CT after they had already completed over half of their course of radiation treatments. For example, one patient’s mean parotid dose increased by 444 cGy and 425 cGy in their VMAT and IMRT plans, respectively, but it can be presumed the actual increase would be minimal because the assessment CT was performed on the day of the 23rd fraction of the total 35 fractions. The collected data serves as a reference of the average dosimetric changes to structures when weight loss occurs in head and neck patients.ConclusionIn conclusion, if a patient has experienced weight loss in the head and neck region, but is setting up well otherwise, re-planning may not always be necessary. While marginal changes in the dose to OAR were seen, the differences recorded were collected by projecting the full course of treatments. The reality however, is that weight loss is gradual throughout a course of treatment, and the actual differences were less. Even with using the maximum projection, a majority of the patients were still within their tolerance objectives, showing the robustness of these VMAT and IMRT plans.Additionally, no major difference between the robustness of VMAT and IMRT was found. Both techniques displayed an increase in coverage and marginal increase in dose to OAR.No method for establishing a threshold of weight loss and the need to re-plan was established. While patients experiencing setup issues were excluded from this study, the internal anatomy changes that were seen in most patients were a factor in this study. This limitation would have to be further addressed in future studies, to get a more accurate dose measurement caused strictly by weight loss. A larger sample size would also be needed to further validate the findings of this study.ReferencesArgiria A, Karamouzis MV, Raben D, Ferris RL. Head and neck cancer. Lancet. 2008;371(9625):17-23. (08)60728-XAmerican Cancer Society. Cancer facts & figures 2018. . Revised June 2018. Accessed July 26, 2018.Dawson P, Taylor A, Bragg C. Exploration of risk factors for weight loss in head and neck cancer patients. J Radiother Pract. 2015;14(4):343-352. WM, Patel SG, O’Sullivan B, et al. Head and Neck cancers-major changes in the American Joint Committee on cancer eighth edition cancer staging manual. CA Cancer J Clin. 2017;67:122-137. CL, Steenbakkers RJHM, Langendijk JA, Sijtsema NM. Identifying patients who may benefit from adaptive radiotherapy: Does the literature on anatomic and dosimetric changes in head and neck organs at risk during radiotherapy provide information to help? Radiother Oncol. 2015;115(3):285-294. Castelli J, Simon A, Rigaud B, et al. A nomogram to predict parotid gland overdose in head and neck IMRT.?Radiat?Oncol. 2016;11(79):1-11.? L, Wan Q, Zhou Y, Deng X, Xie C, Wu S. The role of replanning in fractionated intensity modulated radiotherapy for nasopharyngeal carcinoma. Radiother Oncol. 2011;99(2):23-27. K, Fernandes L, Vineberg K, et al. Parotid glands dose–effect relationships based on their actually delivered doses: implications for adaptive replanning in radiation therapy of head-and-neck cancer. Int J Radiat Oncol Biol Phys. 2013;87(4):676-682. S, Yemchuk S, Broomfield J, Bahn J. Is percentage of weight loss predictive of the need for re-planning of patients with head and neck cancer treated with IMRT radiotherapy? Results of a prospective study. Int J Radiat Onc Biol Phys. 2011;81(2):S540-S541. ML, Du W, Rojas HD, et al. Dosimetric effects of weight loss or gain during volumetric modulated arc therapy and intensity-modulated radiation therapy for prostate cancer. Med Dosim. 2013;38(3):251-254. DJ, Beasley WJ, Garcez K, et al. Relative plan robustness of step-and-shoot vs rotational intensity-modulated radiotherapy on repeat computed tomographic simulation for weight loss in head and neck cancer. Med Dosim. 2016;41(2):154-158. K, Murakami R, Tomitaka E, et al. Radiation-induced Parotid Gland Changes in Oral Cancer Patients: Correlation Between Parotid Volume and Saliva Production. Jpn J Radiat Oncol. 2010;40(1):42-46. JL, Garden AS, Ang KK, et al. Quantification of volumetric and geometric changes occurring during fractionated radiotherapy for head-and-neck cancer using an integrated CT/linear accelerator system. Int J Radiat Oncol Biol Phys. 2004;59(4):960-970. Y, Nakamatsu K, Shibata T, et al. Importance of the Initial Volume of Parotid Glands in Xerostomia for Patients with Head and Neck Cancers Treated with IMRT. Jpn J Clin Oncol. 2005;35(7):375-379. 1. Example of patient setup lying supine on the conformal head and neck board with a long aquaplast mask formed to H&N area, hands holding onto pegs, and knee sponge for comfort.Figure 2. Example of a high risk volume in red (_R_PTV_High), intermediate risk volume in blue (_R_PTV_Int), and low risk volume in green (_R_PTV_Low). Figure 3. Example of the assessment CT overlaying the original CT with both external contours demonstrating change in tissue. The viewing planes are positioned for C3 measurement including the two diagonal vectors drawn. Also shown are the three measurements taken at C3. Figure 4a. Example of the volume change seen in the _R_Parotids (pink), and the increased overlap with the target volumes (red, blue) that parotids undergo between the original CT (Figure 5a) and the assessment CT (Figure 5b).left127000Figure 4b. Example of parotid shrinkage and overlap with target volumes on an assessment CT.TablesTable 1. Percent weight loss for each patient as well as average diameter loss at each of the designated vertebral bodies; C1, C3, and C4/5.PatientWeight Loss, %C1 Avg Diameter Loss, cmC3 Avg Diameter Loss, cmC4/5 Avg Diameter Loss, cm1111.281.231.602121.211.520.903120.630.890.93470.590.611.09571.011.012.37670.700.681.11770.660.661.07880.941.051.25930.650.771.101081.021.100.8011171.161.350.6612100.810.630.7013120.830.801.191480.651.021.8215100.761.011.1716130.711.082.0417101.601.731.721840.630.800.701980.851.242.852080.620.891.672170.750.870.97Table 2. Pearson correlation coefficients of each dose metric for IMRT and VMAT plans. StructureDose MetricArea of AssessmentIMRT CoefficientsVMAT CoefficientsPTV_HighD95Total Average Diameter Loss0.3580.325PTV_HighD5Total Average Diameter Loss0.0120.012PTV_IntD95Total Average Diameter Loss0.0010.013PTV_LowD95Total Average Diameter Loss0.1450.087CTV_HighD95Total Average Diameter Loss0.3630.409CTV_IntD95Total Average Diameter Loss0.0170.037CTV_LowD95Total Average Diameter Loss0.2220.057Spinal CordD-maxTotal Average Diameter Loss0.6990.098Spinal Cord + 5mmD-maxTotal Average Diameter Loss0.7910.500Brain StemD-maxTotal Average Diameter Loss0.6010.194Brain Stem + 5mmD-maxTotal Average Diameter Loss0.3300.384ParotidMeanAverage Diameter Loss C10.3430.337EsophagusMeanAverage Diameter Loss C4/50.5350.854Oral CavityMeanAverage Diameter Loss C30.8700.727LarynxMeanAverage Diameter Loss C4/50.8510.907PharynxV65Total Average Diameter Loss0.0060.005Table 3. Average percent differences for target and pharynx dose metrics for IMRT and VMAT plans and corresponding standard deviations.StructureDose MetricAverage Difference IMRT, %Standard Deviation IMRT, %Average Difference VMAT, %Standard Deviation VMAT, %PTV_HighD950.600.950.771.30PTV_HighD52.640.743.291.34PTV_IntD950.870.621.370.86PTV_LowD952.081.132.200.96CTV_HighD951.010.701.251.09CTV_IntD951.120.781.580.94CTV_LowD952.361.202.520.90PharynxV656.245.367.676.71Table 4. Average differences for OAR dose metrics for IMRT and VMAT plans and corresponding standard deviations.StructureDose MetricAverage Difference IMRT, cGyStandard Deviation IMRT, cGyAverage Difference VMAT, cGyStandard Deviation VMAT, cGySpinal CordD-max151121178152Spinal Cord + 5mmD-max338294388353Brain StemD-max420424171Brain Stem + 5mmD-max5936141305ParotidMean394394410401EsophagusMean95193124214Oral CavityMean-251565.8158LarynxMean144239175240 ................
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