2013 American College of Radiology /13/$36.00

Transcription

1 Treatment Efficiency of Volumetric Modulated Arc Therapy in Comparison With Intensity-Modulated Radiotherapy in the Treatment of Prostate Cancer William A. Hall, MD, Timothy H. Fox, PhD, Xiaojun Jiang, MS, Roshan S. Prabhu, MD, Peter J. Rossi, MD, Karen Godette, MD, Ashesh B. Jani, MD, MSEE Purpose: Treatment with intensity-modulated radiation therapy (IMRT) is increasingly standard for prostate cancer. Volume-modulated arc therapy (VMAT) to deliver IMRT potentially enables shorter treatment time. The aim of this study was to test this hypothesis by measuring the average patient in-room time with VMAT versus dynamic multileaf collimator (DMLC) IMRT. Methods: Custom institutional software (RTMetrix) was used to mine the treatment times from the record-and-verify database. The in-room time (the time between patient entry and exit) was computed for each patient using RTMetrix. Average room time was compared between VMAT patients (n 44) and IMRT patients (n 99). Subgroup comparisons (1-arc or 2-arc VMAT, 5-field or 7-field IMRT, and electromagnetic transponder [Calypso] or gold-marker tracking) were performed. For all comparisons, 2-tailed, 2-sample, equal variance Student s t-tests were used. Results: Average room time was significantly shorter for all VMAT versus DMLC IMRT (P.0014) procedures, along with VMAT versus 7-field DMLC IMRT (P.001), but not VMAT versus 5-field DMLC IMRT (P.81). Room time was longer for Calypso versus gold seed patients (P.001), but VMAT reduced treatment time in Calypso patients (P.01). This resulted in Calypso VMAT patients having similar treatment times to non-calypso DMLC IMRT patients (P.220). Conclusions: These data show that VMAT can shorten room times and improve patient throughput over 7-field DMLC IMRT. Additionally, the data demonstrate that treatment with VMAT permits the use of advanced prostate tracking (Calypso), resulting in similar room times as with standard 7-field DMLC IMRT with conventional tracking. Key Words: Treatment efficiency prostate cancer, VMAT, IMRT treatment efficiency J Am Coll Radiol 2013;10: Copyright 2013 American College of Radiology INTRODUCTION Prostate cancer is the most common cancer in men, and changing demographics over the coming years will make it increasingly more common [1]. Current treatment Department of Radiation Oncology and the Winship Cancer Institute, Emory University, Atlanta, Georgia. These data were presented as a digital poster discussion at the 2011 meeting of the American Society for Radiation Oncology in Miami, Florida. Corresponding author and reprints: William A. Hall, MD, Emory University, Department of Radiation Oncology, 1365 Clifton Road NE, Atlanta, GA 30322; whall4@emory.edu. 128 modalities for adenocarcinoma of the prostate include watchful waiting, surgical resection, and definitive radiation therapy (RT) with external-beam RT, brachytherapy, or a combination of both. Dose-escalated external-beam RT is the current standard of care for patients with early, locally advanced, or high-risk disease [2,3]. Historically, the standard of care for the delivery of external-beam RT to the prostate was 3-D conformal RT. The development of the dynamic multileaf collimator (DMLC) and inverse treatment planning software brought the ability to deliver intensity-modulated RT (IMRT) [4]. The use of IMRT lends itself extremely well 2013 American College of Radiology /13/$

2 Hall et al/vmat vs IMRT in Prostate Cancer 129 to the treatment of prostate cancer because of its ability to conform its dose distribution to avoid normal critical structures. The use of IMRT in the treatment of prostate cancer has been shown to result in lower toxicity to normal tissues [5,6]. Although the benefits of IMRT in the treatment of prostate cancer are ample, it remains a complex and time-consuming treatment modality that requires numerous gantry positions, a large number of monitor units, and considerable daily treatment time commitments by patients. In addition to the traditional DMLC IMRT delivery, novel methods of more efficiently delivering IMRT have been recently developed, including volume-modulated arc therapy (VMAT) [7], a new approach to the delivery of IMRT that uses dynamic modulated arcs. Through the simultaneous coordination of multileaf collimator motion, gantry rotation, and dose-rate modulation, highly conformal treatment plans similar to traditional IMRT can be generated that optimally spare critical normal structures while improving treatment efficiency [8]. The dosimetric equivalence and potential advantages of VMAT compared with traditional DMLC IMRT in the treatment of prostate cancer have recently been demonstrated [9]. Given the time requirement by patients for daily prostate cancer treatment, we aimed to measure the true efficiency of VMAT treatment compared with DMLC IMRT using custom institutional software to record the actual in-room times. Another aim was to consider the treatment time added when devices such as Calypso electromagnetic localization beacons (Calypso, Seattle, Washington) or gold seeds are used to monitor intrafractional motion or assist in prostate localization. One additional aim was to determine if advances in the delivery of IMRT using VMAT could offset the increased treatment time required when using electromagnetic localization beacons. The aim of our overall analysis was to precisely determine the most efficient manner by which IMRT could be delivered in the treatment of adenocarcinoma of the prostate. METHODS Patient Selection The in-room times for a total of 44 patients who were undergoing definitive VMAT for the treatment of their prostate cancer were compared with those for 99 patients undergoing definitive DMLC IMRT for treatment. All patients had tissue-proven diagnoses of adenocarcinoma of the prostate without known distant metastasis. Patient characteristics were evenly distributed between the two different groups (Table 1). The clinical practice at Emory University is such that patients are treated with a variety of different IMRT delivery modalities. This is due to the multiple hospital locations (4 in total) that constitute the Emory University Department of Radiation Oncology and the differing technologies available for treatment Table 1. Patient characteristics DMLC Patient IMRT VMAT Characteristic (n 99) (n 44) P Age (y) Race.51 White 52.5% 50.0% Black 43.4% 36.4% Other 4.1% 13.6% Height (cm) Weight (kg) T stage.402 T1c 3 0 T1c T2 2 0 T2a 11 6 T2b 13 0 T2c 1 3 T2c 4 7 Gleason score Prostate-specific antigen (ng/ml) Hormone Therapy.646 No Yes Prostate volume (cm 3 ) Region treated.437 Whole pelvis 10 0 Prostate SVs Prostate SVs LNs Prostate SVs 1 1 rectum Prostate LNs 10 0 Note: Data are expressed as means, as numbers, or as percentages. DMLC dynamic multileaf collimation; IMRT intensity-modulated radiation therapy; LN lymph nodes; SV seminal vesicles; VMAT volume-modulated arc therapy. Pearson s chi-square test. Fisher s exact test. Student s t-test. delivery at each of these hospitals. As a consequence of these different IMRT delivery methods, patients in the central Emory prostate database have been treated with an assortment of IMRT delivery technologies. It was this difference in delivery that enabled this analysis. Software Development and Treatment Time Measurement Our department has developed an efficiency analysis software platform, RTMetrix, for analyzing various performance metrics. RTMetrix is a Web-based platform with customized query reports for quickly giving information to physicians and clinical staff members on daily, weekly, and annual bases for patient treatment planning and delivery statistics. In addition, RTMetrix automates

3 130 Journal of the American College of Radiology/ Vol. 10 No. 2 February 2013 Fig 1. Schematic representation of how our measurement of room time was defined. The treatment room time in this case was the data point that was mined for each patient. the data collection and review process of the record-andverify system (ARIA; Varian Medical Systems, Palo Alto, California). For this project, RTMetrix implemented a database query with customized data extraction of the time measurements for treatment delivery (Fig. 1). Because a centralized record-and-verify system is used for the treatment delivery process, data for all delivered treatments are accessible in the system. The start and end of treatment delivery are obtained from a time stamp in the record-and-verify system. The RTMetrix software extracted 99 IMRT and 44 VMAT patients. The software calculated the clinical treatment times (including localization times) for these patients. RTMetrix used the time stamps in the record-and-verify database for treatment field history to calculate the time each patient was on the treatment table for actual treatment fields, kilovoltage or megavoltage images, and immobilization. In addition to the actual treatment time s comparisons (1-arc or 2-arc VMAT, 5-field or 7-field IMRT, and electromagnetic transponder [Calypso, Seattle, Washington] or goldmarker tracking) were performed. For Calypso patients, intrafraction motion tracking is used during the treatment delivery process. When using Calypso, treatment delivery is stopped, and patients are repositioned when the intrafraction motion exceeds predefined limits. Overall, this custom software was able to generate accurate times in minutes for the entire treatment process of both VMAT and DMLC IMRT, including patients with gold-seed fiducial implants or Calypso markers. A schematic representation of the software, and the measured time point, can be seen in Fig. 1. Patient Setup and Immobilization All patients were treated in the supine position, most patients had individually designed immobilization devices (Vac-Lok pelvic cushions; Civco Medical Solutions, Kalona, Iowa), and endorectal balloons were not used. Patients were instructed to have a full bladder at the time of CT simulation along with each fraction of treatment with either VMAT or IMRT. The CT simulation data sets were used to design the treatment plans for each patient with either a VMAT technique or an IMRT technique. Two different types of tracking devices were used, namely, either gold seeds or Calypso beacons. If Calypso was not used, an on-board imaging (OBI) system (Varian Medical Systems) was used for performing image-guided RT and assessing interfraction motion on a daily basis. The OBI system is composed of an x-ray tube with 0.4-mm and 0.8-mm focal spots, a 14 anode angle, and 800 kj/h (model G242; Varian Medical Systems) and an amorphous-silicon imaging panel (PaxScan 4030CB; Varian Medical Systems) attached to the gantry of the linear accelerator with Exact robotic arms (Varian Medical Systems). The OBI system acquires kilovoltage x-ray images to support the verification process for target localization of the gold seeds compared with the digitally reconstructed radiograph created by the treatment planning system. A quartet of images, 2 reference digitally reconstructed radiographic images and 2 newly acquired kilovoltage images, are used for the imaging matching of the gold seeds, which can be performed manually or automatically with the system. In Fig. 1, the image acquisition and analysis time of kilovoltage or megavoltage portal imaging can be made between the end of the first acquired OBI image and the start of the treatment delivery process. VMAT Plan and Treatment Delivery The VMAT plans were designed using the Eclipse treatment planning system (Varian Medical Systems). A Varian 23EX linear accelerator (Varian Medical Systems) was used to deliver the VMAT treatment plan, which is commercially referred to as RapidArc treatment. The RapidArc treatment technique has been previously described [7]. Either a single or double coplanar arc treatment plan was used. For patients who had a single coplanar clockwise arc, the starting gantry angle was 181 and this finished at a gantry angle of 179, the arc consisted of 178 control points, and the collimator angle was fixed at 0 throughout the arc. In patients who had a double coplanar arc treatment plan, the first arc s gantry angle started at 190 and moved clockwise to a stop angle

4 Hall et al/vmat vs IMRT in Prostate Cancer 131 Table 2. Absolute room-time measurements Category Room Time (min) All IMRT patients (n 99) All VMAT patients (n 44) field IMRT (n 18) field IMRT (n 59) field IMRT (n 22) arc VMAT (n 21) arc VMAT (n 23) All Calypso (n 52) All gold seeds (n 47) VMAT with gold seeds (n 25) IMRT with gold seeds (n 22) Calypso IMRT (n 24) Calypso VMAT (n 28) Non-Calypso IMRT Note: Data are expressed as mean SD. IMRT intensity-modulated radiation therapy; VMAT volume-modulated arc therapy. of 170, with a collimator angle of 30. The second arc moved counterclockwise with a starting gantry angle of 170, a stop angle of 190, and a collimator angle of 330. Each arc used 178 control points for VMAT delivery. DMLC IMRT Treatment Plan and Delivery The DMLC IMRT plans were designed using the Eclipse treatment planning system. The Eclipse treatment planning system uses an inverse planning algorithm for creating segmented multileaf collimator patterns that are delivered using the sliding-windows DMLC delivery technique. Patients were treated with either 5-field or 7-field IMRT treatment plans, which indicates that static gantry angles are used, as opposed to a continuous arc as in VMAT plans. For certain patients, the number of fields delivered at the treatment console was recorded as 14. This was due to the planned treatment field having a larger length than the multileaf collimator carriage permitted, in which case the 7-field plan had to be split into 14 fields for accurate treatment delivery. This process divides each planned treatment field into 2 delivery treatment fields, and this is referred to as split-carriage multileaf collimator fields. All the treatment parameters, such as table position and gantry angle, remained the same between the split-carriage fields; however, from a roomtime perspective, these 7 planned fields that had to be split into 14 delivery fields took longer to deliver because the treating therapist had to mode up and start each individual treatment delivery field. Statistical Analysis All statistics were calculated using either Microsoft Excel (Microsoft Corporation, Redmond, Washington) or Stata (StataCorp LP, College Station, Texas). Statistical comparisons were made between the two groups of patients with either Student s t-tests, Fisher s exact tests, or 2 tests. Student s t-tests were used to compare treatment times between all VMAT patients and all IMRT patients. A multivariate analysis was performed using Stata data analysis and statistical software. Univariate and multivariate regression analyses were performed using all major demographic, disease, and treatment factors; all variables were entered into the model without any stepwise variable deletion or addition. In all cases, P values.05 were considered statistically significant. RESULTS Treatment Time Comparison The average room times for all DMLC IMRT patients are summarized in Table 2. The average room time for all DMLC IMRT patients was found to be min, and the average room time for all VMAT patients was found to be min. The average room times for the various permutations of the VMAT and IMRT treatment deliveries are listed in Table 2. Table 3 shows a comparison of the various treatment modalities. The average room time was significantly shorter for all VMAT patients versus all DMLC IMRT patients (P.0014). There was a statistically significantly longer treatment time between the 7-field DMLC IMRT and VMAT treatments (P.001). Additionally, it was shown that patients treated with VMAT and gold seeds had shorter treatment times than all patients treated with VMAT (P.03). Factors that did not reach statistical significance are also shown in Table 3. A visual representation of the patient average treatment times in various categories appears in Fig. 2. The univariate analysis is shown in Table 4. Factors that reached statistical significance on univariate analysis were the use of VMAT, Gleason score, and region treated. Additional factors that did not reach statistical significant are listed in Table 4. Multivariate analysis (Table 4) demonstrated that the only factor reaching statistical significance was the use of VMAT. The characteristics of the patients that were used in the multivariate analysis, along with statistical comparisons of these two groups, are shown in Table 1. Table 3. Treatment time comparisons between various IMRT delivery modalities Comparison P All IMRT vs all VMAT field IMRT vs all VMAT.81 7-field IMRT vs all VMAT arc vs 2 arcs.14 Calypso vs gold seeds.001 All VMAT vs VMAT gold seeds.03 Calypso IMRT vs Calypso VMAT.002 Non-Calypso IMRT vs Calypso VMAT.220 Note: IMRT intensity-modulated radiation therapy; VMAT volumemodulated arc therapy. Student s t-test.

5 132 Journal of the American College of Radiology/ Vol. 10 No. 2 February Treatment Times Treatment Times (minutes) Fig 2. Graphical relationship of the average treatment times and how various treatment factors influence treatment time. Statistical comparison of these factors is shown in Table 3. DISCUSSION The use of RT in the definitive treatment of adenocarcinoma of the prostate is widespread, and with an aging population, rates of prostate cancer will continue to rise in the future [1]. The role of dose-escalated radiation along with the use of DMLC IMRT using static gantry angles in the definitive management of adenocarcinoma of the prostate is well established [2,3]. Establishing an understanding as to the most efficient way to deliver RT to the prostate is important given the increasingly common occurrence of this disease and the need to optimize RT delivery. Currently, a variety of modalities exist by which IMRT treatment plans can be delivered, including DMLC and VMAT plan designs. Volume-modulated arc therapy has been shown in several different dosimetric comparison studies to improve the conformality and decrease the radiation dose to critical normal structures, including the rectum [8-10]. In addition to technological advances in the delivery of IMRT, monitoring the motion of the prostate during treatment with electromagnetic localization beacons is increasingly done. Devices such as Calypso beacons have recently demonstrated the advantages of intrafractional monitoring of prostate movement on margins and the feasibility of prostate localization [11-13]. In addition, several analyses have recently been published showing potential dosimetric advantages to the use of intrafraction prostate electromagnetic localization beacons such as Calypso [14-16]. Our analysis involved custom institutional software to measure the actual treatment efficiency of two different IMRT delivery modalities for adenocarcinoma of the prostate: VMAT and DMLC static-gantry angle IMRT. Importantly, we focused on incorporating commonly used localization devices into our efficiency analysis to determine if the use of VMAT can indeed offset the additional treatment time requirements of new localization devices such as Calypso. Our analysis confirmed that the use of VMAT results in an overall reduction in treatment time of approximately 14% compared with 7-field IMRT. A treatment Table 4. Univariate and multivariate results of factors potentially influencing treatment time Treatment Variable Coefficient P 95% Confidence Interval Univariate analysis Age to 0.09 Race to 1.26 Body mass index 1, to T stage to 1.20 Prostate volume to 0.05 Prostate-specific antigen to 0.06 Hormone therapy to Localization technique to 0.73 VMAT use to 1.39 Region treated to 2.54 Gleason score to 1.48 Multivariate analysis Age to 0.05 Race to 1.31 T stage to 0.99 Gleason score to 1.24 Prostate-specific antigen to 0.03 Hormone therapy to 1.03 Localization technique to 2.12 Region treated to 3.58 VMAT use to 1.26 Note: VMAT volume-modulated arc therapy. Calypso, gold seeds, or weekly port films. Whole pelvis vs prostate only.

6 Hall et al/vmat vs IMRT in Prostate Cancer 133 time of min for all IMRT patients was measured, compared with min for all VMAT patients. This overall reduction is similar to other analyses that have compared the overall treatment times of VMAT with static-gantry angle IMRT [8,17-19]. Wolff et al [17] recently presented an analysis in which treatment time differences are reported among the CT data sets of 9 patients, and their conclusions were that the use of VMAT reduced overall treatment time by approximately 3 min [17]. Important to note is that these were only estimated treatment time reductions, not actually measured reductions. Tsai et al [8] presented a similar series in which the CT data sets for 12 patients were examined and the mean treatment times estimated. The difference between the treatment times in the analysis by Tsai et al was approximately 1.2 min less with VMAT compared with IMRT, slightly lower than our measured difference of 2.01 min. Treatment time was defined in Tsai et al s analysis as the time from the first beam on to the last beam off and may not have accounted for patient positioning and total treatment delivery as accurately as the in-room treatment time measurement presented in our analysis. Treatment time was also the focus of an analysis by Aznar et al [18], who examined a group of 46 patients who were compared with a group of 50 patients treated with 5-field IMRT. In this analysis, treatment time was found to be approximately 3.5 min faster using VMAT compared with 5 field IMRT. The treatment times in this analysis were measured in the same manner used by Tsai et al, defined as the amount of time from the first beam on to the last beam off. These results showed a slightly larger time savings with VMAT than demonstrated by Tsai et al. A third analysis focusing on the treatment efficiency of VMAT compared with DMLC IMRT was conducted by Davidson et al [20]. Unique to their study was the measurement of treatment times using a stopwatch at the treatment consul as opposed to calculated times with the treatment planning system. Davidson et al concluded that VMAT reduced the average treatment times by 70% to 86%. Important to note is that this analysis also measured treatment time as the difference between first beam on and last beam off. The reductions in treatment times provided through the use of VMAT compared with DMLC IMRT in the analysis of Davidson et al were larger than those found in our analysis or other similar analysis in the published literature. To further demonstrate the value of VMAT compared with DMLC IMRT, we performed a univariate analysis along with a multivariate analysis on the clinical factors that could influence treatment times. It was shown in the univariate analysis that the only factors that seemed to influence treatment times were the use of VMAT; the region treated, which consisted of either lymph nodes or no lymph nodes; and the Gleason score. We expected that the region treated would have affected the total treatment time; understandably, a treatment volume that includes the pelvic lymph nodes would take longer than one that does not. It should also be noted that in our analysis, the percentage of patients treated to the lymph nodes or pelvis in each of the groups (VMAT vs DMLC- IMRT) were relatively similar, with approximately 35% of patients in the VMAT group (16 of 44) and 31.3% of patients in the IMRT group (31 of 99). The impact of Gleason score and the trend that was observed with the use of hormone therapy were logical given the close correlation of these factors to the region treated. Our conclusion that the use of VMAT does affect overall treatment time was confirmed in the multivariate analysis, as the only variable that was found to be statistically significant. In addition to the conclusions of VMAT compared with IMRT, we were able to make several subgroup comparisons looking at different ways of delivering VMAT (Table 3). The statistically significant decrease in treatment time that was shown for patients treated with gold seeds is likely due to the clinical practice of using more frequent cone-beam CT scans in patients without gold seeds, along with a more efficient method for therapists to localize and correctly position patients using gold seeds as reference points. In an era of increasing scrutiny and calls for the validation of novel medical technologies, the importance of substantiating and documenting the advantages of new technologies is paramount. Although 2 to 3 min of added treatment efficiency on a daily basis for each individual patient could seem trivial, this time savings quickly becomes significant in a high-volume RT clinic with 25 to 30 patients undergoing treatment for prostate cancer in a single day. Furthermore, this time savings can enable the implementation of more advanced patient positioning devices (such as Calypso) and not affect the overall treatment time requirement. Finally, probably the most important component to consider when evaluating the significance of decreased treatment time is the decrease in the possibility for intrafraction prostate motion during treatment. It has been shown in multiple different analysis that the longer a patient remains on the treatment table, the greater the possibility of intrafraction motion. Greater intrafraction motion invariably leads to less accurate treatment delivery and the potential for greater normal tissue toxicity [21,22]. CONCLUSIONS Our analysis is the first of its kind to measure the actual room time of a variety of patients treated for adenocarcinoma of the prostate. We have presented novel data that show the calculated treatment times and provide valuable information regarding the efficiency of the two different IMRT delivery modalities. Unique to this analysis is the inclusion of a large number of patients with a variety of localization devices, treatment volumes, and permutations of IMRT delivery. With an aging population and rapidly rising rates of prostate cancer, the ability to de-

7 134 Journal of the American College of Radiology/ Vol. 10 No. 2 February 2013 termine the most efficient modality by which patients with adenocarcinoma of the prostate should be treated is critical. It seems from our analysis that 5-field IMRT and VMAT are the most efficient modalities by which to deliver prostate IMRT. Furthermore, our analysis shows that if more advanced methods of IMRT delivery are used, localization devices can be incorporated without any statistically significant differences in the overall treatment time. This may provide important information to prostate cancer practices looking to improve their efficiency while maximizing the quality of their RT delivery. TAKE-HOME POINTS Delivery of IMRT using VMAT is more efficient with regard to treatment time than traditional 7-field DMLC IMRT. Volume-modulated arc therapy does not seem to provide a statistically significant improvement in treatment time compared with 5-field DMLC IMRT. Delivering IMRT with VMAT enables the use of more advanced positioning devices (such as Calypso) without statistically influencing overall treatment time compared with 7-field DMLC IMRT. The improved treatment efficiency seen when using VMAT may enable improvements in both overall treatment time and clinical throughput, along with less opportunity for intrafraction patient motion due to prolonged treatment time. ACKNOWLEDGMENT The authors thank Sherrie Cooper for assistance with data collection and preparation. REFERENCES 1. Jemal A, Siegel R, Xu J, Ward E. Cancer statistics CA Cancer J Clin 2010;60: Zietman AL, DeSilvio ML, Slater JD, et al. Comparison of conventionaldose vs high-dose conformal radiation therapy in clinically localized adenocarcinoma of the prostate: a randomized controlled trial. JAMA 2005; 294: Peeters ST, Heemsbergen WD, Koper PC, et al. Dose-response in radiotherapy for localized prostate cancer: results of the Dutch multicenter randomized phase III trial comparing 68 Gy of radiotherapy with 78 Gy. J Clin Oncol 2006;24: Ling CC, Burman C, Chui CS, et al. Conformal radiation treatment of prostate cancer using inversely-planned intensity-modulated photon beams produced with dynamic multileaf collimation. Int J Radiat Oncol Biol Phys 1996;35: Zelefsky MJ, Fuks Z, Happersett L, et al. Clinical experience with intensity modulated radiation therapy (IMRT) in prostate cancer. Radiother Oncol 2000;55: Jani AB, Su A, Correa D, Gratzle J. Comparison of late gastrointestinal and genitourinary toxicity of prostate cancer patients undergoing intensity-modulated versus conventional radiotherapy using localized fields. Prostate Cancer Prostatic Dis 2007;10: Otto K. Volumetric modulated arc therapy: IMRT in a single gantry arc. Med Phys 2008;35: Tsai CL, Wu JK, Chao HL, Tsai YC, Cheng JC. Treatment and dosimetric advantages between VMAT, IMRT, and helical tomotherapy in prostate cancer. Med Dosim 2011;36: Zhang P, Happersett L, Hunt M, Jackson A, Zelefsky M, Mageras G. Volumetric modulated arc therapy: planning and evaluation for prostate cancer cases. Int J Radiat Oncol Biol Phys 2010;76: Palma D, Vollans E, James K, et al. Volumetric modulated arc therapy for delivery of prostate radiotherapy: comparison with intensity-modulated radiotherapy and three-dimensional conformal radiotherapy. Int J Radiat Oncol Biol Phys 2008;72: Balter JM, Wright JN, Newell LJ, et al. Accuracy of a wireless localization system for radiotherapy. Int J Radiat Oncol Biol Phys 2005;61: Litzenberg DW, Balter JM, Hadley SW, et al. Influence of intrafraction motion on margins for prostate radiotherapy. Int J Radiat Oncol Biol Phys 2006;65: Kupelian P, Willoughby T, Mahadevan A, et al. Multi-institutional clinical experience with the Calypso system in localization and continuous, real-time monitoring of the prostate gland during external radiotherapy. Int J Radiat Oncol Biol Phys 2007;67: Rajendran RR, Plastaras JP, Mick R, McMichael Kohler D, Kassaee A, Vapiwala N. Daily isocenter correction with electromagnetic-based localization improves target coverage and rectal sparing during prostate radiotherapy. Int J Radiat Oncol Biol Phys 2010;76: Li HS, Chetty IJ, Enke CA, et al. Dosimetric consequences of intrafraction prostate motion. Int J Radiat Oncol Biol Phys 2008;71: Langen KM, Lu W, Willoughby TR, et al. Dosimetric effect of prostate motion during helical tomotherapy. Int J Radiat Oncol Biol Phys 2009; 74: Wolff D, Stieler F, Welzel G, et al. Volumetric modulated arc therapy (VMAT) vs. serial tomotherapy, step-and-shoot IMRT and 3D-conformal RT for treatment of prostate cancer. Radiother Oncol 2009;93: Aznar MC, Petersen PM, Logadottir A, et al. Rotational radiotherapy for prostate cancer in clinical practice. Radiother Oncol 2010;97: Rao M, Yang W, Chen F, et al. Comparison of Elekta VMAT with helical tomotherapy and fixed field IMRT: plan quality, delivery efficiency and accuracy. Med Phys 2010;37: Davidson MT, Blake SJ, Batchelar DL, Cheung P, Mah K. Assessing the role of volumetric modulated arc therapy (VMAT) relative to IMRT and helical tomotherapy in the management of localized, locally advanced, and post-operative prostate cancer. Int J Radiat Oncol Biol Phys 2011; 80: Kotte AN, Hofman P, Lagendijk JJ, van Vulpen M, van der Heide UA. Intrafraction motion of the prostate during external-beam radiation therapy: analysis of 427 patients with implanted fiducial markers. Int J Radiat Oncol Biol Phys 2007;69: Shelton J, Rossi PJ, Chen H, Liu Y, Master VJ, Jani AB. Observations on prostate intrafraction motion and the effect of reduced treatment time using volumetric modulated arc therapy. Pract Radiat Oncol 2011;1: