RapidArc Treatment Plans Research Articles

Monte Carlo based, patient‐specific RapidArc QA using Linac log files

A Monte Carlo (MC) based QA process to validate the dynamic beam delivery accuracy for Varian RapidArc (Varian Medical Systems, Palo Alto, CA) using Linac delivery log files (DynaLog) is presented. Using DynaLog file analysis and MC simulations, the goal of this article is to (a) confirm that adequate sampling is used in the RapidArc optimization algorithm (177 static gantry angles) and (b) to assess the physical machine performance [gantry angle and monitor unit (MU) delivery accuracy]. Ten clinically acceptable RapidArc treatment plans were generated for various tumor sites and delivered to a water-equivalent cylindrical phantom on the treatment unit. Three Monte Carlo simulations were performed to calculate dose to the CT phantom image set: (a) One using a series of static gantry angles defined by 177 control points with treatment planning system (TPS) MLC control files (planning files), (b) one using continuous gantry rotation with TPS generated MLC control files, and (c) one using continuous gantry rotation with actual Linac delivery log files. Monte Carlo simulated dose distributions are compared to both ionization chamber point measurements and with RapidArc TPS calculated doses. The 3D dose distributions were compared using a 3D gamma-factor analysis, employing a 3%/3 mm distance-to-agreement criterion. The dose difference between MC simulations, TPS, and ionization chamber point measurements was less than 2.1%. For all plans, the MC calculated 3D dose distributions agreed well with the TPS calculated doses (gamma-factor values were less than 1 for more than 95% of the points considered). Machine performance QA was supplemented with an extensive DynaLog file analysis. A DynaLog file analysis showed that leaf position errors were less than 1 mm for 94% of the time and there were no leaf errors greater than 2.5 mm. The mean standard deviation in MU and gantry angle were 0.052 MU and 0.355 degrees, respectively, for the ten cases analyzed. The accuracy and flexibility of the Monte Carlo based RapidArc QA system were demonstrated. Good machine performance and accurate dose distribution delivery of RapidArc plans were observed. The sampling used in the TPS optimization algorithm was found to be adequate.

Dosimetric Validation of RapidArc for Stereotactic Radiotherapy for Peripheral Lung Tumors

Purpose/Objective(s): In hypofractionated stereotactic treatments of Stage I lung tumors, highly conformal treatment techniques are required in order to minimize normal tissue toxicity. RapidArc, a new volumetric intensity modulated arc therapy (Varian Medical Systems, Palo Alto, USA), offers this possibility within very short treatment times, the latter being important to reduce the risk of intra-fraction baseline shifts of tumor position. Due to the small field sizes, tissue inhomogeneities and possible occurrence of interplay effects between MLC motion and tumor motion, the dosimetry of RapidArc in these cases is challenging. We examined the dosimetric accuracy of RapidArc in a heterogeneous phantom, with and without motion of the target.

Sci—Wed PM: Delivery—08: Monte Carlo Based RapidArc QA Using LINAC Log Files

Purpose/Objective(s): To present our Monte Carlo based RapidArc quality assurance (QA) process to validate both the dose calculation and dynamic beam delivery accuracy using the planning MLC control files and the post‐delivery MLC diagnostic files. Materials/Methods: Ten clinically acceptable RapidArc treatment plans were generated with a clinical version of the planning system for various tumor sites. Monte Carlo dose calculations were performed in a water equivalent phantom for each plan using both DynaLog files and the planning control (DVA) files. Results were compared to measurements using a calibrated Farmer ionization chamber with an active volume of . Comparison of RapidArc and Monte Carlo 3D doses was performed using a 3 dimensional Gamma‐factor analysis with a 3%/3mm DTA criteria. A thorough analysis of the DynaLog files was performed to evaluate the treatment delivery accuracy. Results: Good agreement was observed between chamber measurements and MC dose calculations and between RapidArc and MC dose distributions with Gamma values below 1 in over 90% of the points considered for all plans. The analysis of the MLC DynaLog files indicated that the leaf position errors were lower than 1 mm in more than 94% of the time with none above 2.5 mm and that few beam hold‐offs occurred Conclusions The accuracy and flexibility of our Monte Carlo based RapidArc QA system was demonstrated. Good machine performance and accurate dose distributions delivery of RapidArc plans was observed.

PSYCHOSOCIAL FACTORS AND HEALTH-RELATED QUALITY OF LIFE IN HEAD AND NECK CANCER PATIENTS

At the Department of Radiation Oncology at the University Hospital, Rigshospital, Copenhagen, IMRT has been used for head and neck cancers, including nasopharyngeal cancers, since October 2000. Replacing the 3D-CRT both simplified and complicated the treatment. The advantages were immense, both for the radiation oncologists, dosimetrists, treatment staff and not least for the patient. IMRT can result in superior dose distributions compared with classic 3D-CRT. Furthermore, using template plans and constraints can save preparation time for a treatment plan. A certain amount of research had to be made to confirm that IMRT was the treatment of choice, and an accurate quality assurance had to be implemented. Currently Rapid Arc is being introduced as a new treatment technique. Single or dual arcs can result in superior dose distributions compared with IMRT. Treatment time and the total amount of monitor units can also be drastically decreased. The same investigation procedures as for IMRT must be performed. IMRT and rapid arc treatment plans contain steep dose gradients, and therefore a good immobilisation device which should be accurate and reproducible, but with as much comfort as possible, is paramount. Nevertheless, the daily treatment position of the patient is variable and must be taken into account by using a good image verification strategy and adequate set-up margins. In our clinic the Exactrac, orthogonal kV and CBCT verification systems are used. Many head and neck patients have advanced disease, are undernourished, have a poor performance status and prefer the pub to the kitchen! They are at risk of heavy weight loss, which can lead to inadequate immobilisation of the patientduring the course of treatment. This could result in an under-dose to the tumour or an overdose to organs at risk, especially the spinal cord. Similarly, the treatment response or presence of oedema can cause alterations in the body contours and therefore monitoring of changes in anatomy is advised. Monitoring can be done by using CBCT. It is often necessary to re-plan the treatment, and the problems involved in this will be discussed.The RTT should play an active role in the innovation of these new techniques and in assessment and evaluation of treatment variations.

Volumetric Modulated Arc Therapy (VMAT) Treatment Planning for Superficial Tumors

The physician's planning objective is often a uniform dose distribution throughout the planning target volume (PTV), including superficial PTVs on or near the surface of a patient's body. Varian's Eclipse treatment planning system uses a progressive resolution optimizer (PRO), version 8.2.23, for RapidArc dynamic multileaf collimator volumetric modulated arc therapy planning. Because the PRO is a fast optimizer, optimization convergence errors (OCEs) produce dose nonuniformity in the superficial area of the PTV. We present a postsurgical cranial case demonstrating the recursive method our clinic uses to produce RapidArc treatment plans. The initial RapidArc treatment plan generated using one 360° arc resulted in substantial dose nonuniformity in the superficial section of the PTV. We demonstrate the use of multiple arcs to produce improved dose uniformity in this region. We also compare the results of this superficial dose compensation method to the results of a recursive method of dose correction that we developed in-house to correct optimization convergence errors in static intensity-modulated radiation therapy treatment plans. The results show that up to 4 arcs may be necessary to provide uniform dose to the surface of the PTV with the current version of the PRO.

MO‐FF‐A2‐06: Effects of Lung Tumor Motion On Rapid Arc Treatment Technique

Purpose/Objective: To determine the error due to lung tumor motion on dose distribution delivered by Rapid Arc (RA) treatment technique, a procedure also known as volumetric modulated arc therapy. Materials/Methods: An indigenously developed dynamic phantom was set in linear sinusoidal motion in cranio‐caudal direction with amplitude of 2cm and at a frequency of 15 cycles/minute. A RA treatment plan was optimized for and delivered to the phantom containing a simulated target and critical organs. The treatment plan was executed for 30 fractions to the dynamic phantom, started at random initial breathing phases. GAF‐Chromic films embedded in the moving phantom in the coronal plane at the isocenter level were used to capture the dose distribution. Results: (i) ROI as PTV: When the daily dose distributions were compared against the static distribution, dose variation of 10% to 20% was observed near the field edges and 5% to 10% dose difference was observed elsewhere. Similar results were observed when comparing the dose distribution averaged for 30 fractions against the static distribution. (ii) ROI as CTV (simulated target): It is observed that on a day‐to‐day basis the standard deviation of the dose to a given pixel could be as high as 4.5% to 6.5%. Also, the mean, median, and modal dose varies inter‐fractionally with a standard deviation of 5.62%, 5.61%, and 5.73% respectively. When comparing the averaged dose distribution with the static distribution, dose variation ranging from 5% to 10% was observed. Also, the mean, median and modal doses were reduced by 6.35%, 6.45% and 6.87% respectively, compared with static distribution. Conclusion: Our study indicates that, if respiratory management techniques were not implemented, RA procedures for lung cancer treatment can result in underdosage, so this error should be measured and accounted for as part of the patient specific quality assurance.

SU-FF-T-253: Use of a Commercial Linac QA Device for RapidArc Routine Quality Assurance

Purpose: Intensity modulated arc therapy is entering clinical use; however, quality assurance standards for this technology are still evolving. We investigated the QA BeamChecker Plus (Standard Imaging, Madison, WI) linear accelerator QA tool, for use with RapidArc (Varian Medical Systems, Palo Alto, CA). Method and Materials: The QA BeamChecker Plus (BC+) is comprised of 5 parallel plate ionization chambers embedded within buildup material. One of the chambers is located at the center and the remaining 4 are equidistant along orthogonal axes. The device has a rotational mode for use with helical tomotherapy. We obtained a CT of the device, contoured the ionization chambers and two phantom avoidance regions, and optimized a RapidArc treatment plan to deliver a uniform dose to each of the chambers. The dose to the avoidance regions was constrained to produce significant gantry and dose rate modulation. The plan was delivered to the BC+ in rotational mode 11 times over 6 days. To monitor the machine output independent of RapidArc, a static reference field was also delivered. The treatment table has movable support rails that have non-negligible attenuation, so the plan was always delivered with the rails in the center. Results: The standard deviation of the center detector for the static field was 0.1%. The mean difference for all 5 detectors was 0.0%, with a standard deviation of 0.2%. The range was −0.5% to 0.5%. If the rails are not in the center, dose measurements change by up to 5%. Conclusion: The BC+ can be used to verify consistent RapidArc delivery for daily system QA. Future work is necessary to evaluate the types of errors that the BC+ can identify. Conflict of Interest: Research sponsored by Standard Imaging.

RapidArc volumetric modulated therapy planning for prostate cancer patients

Purpose. Recently, Varian Medical Systems have announced the introduction of a new treatment technique, RapidArc, in which dose is delivered over a single gantry rotation with dynamically variable MLC positions, dose rate and gantry speed. At Rigshospitalet, the RapidArc technique was brought into clinical practice in May 2008 for treatment of prostate cancer patients. We report here our experiences with performing treatment planning using the Eclipse RapidArc optimization software for this patient group. Material and methods. A stand-alone installation of Eclipse 8.5 with RapidArc optimization capability was performed at Rigshospitalet. Patient data for 8 prostate cancer patients were imported, most of whom were previously treated at Rigshospitalet using IMRT. Three of the patients were treated at Rigshospitalet using the RapidArc technique. Treatment plans were optimized using objectives as given by standard guidelines for clinical treatment planning. RapidArc plans were compared to the IMRT plans by which the patients were actually treated or in the three cases treated with the RapidArc technique to IMRT plans achieved using standard guidelines. Comparison was done with respect to target coverage, doses to rectum and bladder, over-all maximum dose and number of monitor units. Results. Overall, the RapidArc treatment plans gave better or equal sparing of the organs at risk than the IMRT treatment plans. The number of monitor units was lower in most cases, by up to approximately 75%. However, the target dose homogeneity was not as high as for IMRT. The low-dose bath was larger than for IMRT. Conclusion. RapidArc optimization is very promising, especially regarding the potential of reducing the number of monitor units, while providing good sparing of organs at risk. Some improvement is still warranted with respect to achieving high target dose homogeneity.

Monte Carlo simulation of RapidArc radiotherapy delivery

RapidArc radiotherapy technology from Varian Medical Systems is one of the most complex delivery systems currently available, and achieves an entire intensity-modulated radiation therapy (IMRT) treatment in a single gantry rotation about the patient. Three dynamic parameters can be continuously varied to create IMRT dose distributions—the speed of rotation, beam shaping aperture and delivery dose rate. Modeling of RapidArc technology was incorporated within the existing Vancouver Island Monte Carlo (VIMC) system (Zavgorodni et al 2007 Radiother. Oncol. 84 S49, 2008 Proc. 16th Int. Conf. on Medical Physics). This process was named VIMC-Arc and has become an efficient framework for the verification of RapidArc treatment plans. VIMC-Arc is a fully automated system that constructs the Monte Carlo (MC) beam and patient models from a standard RapidArc DICOM dataset, simulates radiation transport, collects the resulting dose and converts the dose into DICOM format for import back into the treatment planning system (TPS). VIMC-Arc accommodates multiple arc IMRT deliveries and models gantry rotation as a series of segments with dynamic MLC motion within each segment. Several verification RapidArc plans were generated by the Eclipse TPS on a water-equivalent cylindrical phantom and re-calculated using VIMC-Arc. This includes one ‘typical’ RapidArc plan, one plan for dual arc treatment and one plan with ‘avoidance’ sectors. One RapidArc plan was also calculated on a DICOM patient CT dataset. Statistical uncertainty of MC simulations was kept within 1%. VIMC-Arc produced dose distributions that matched very closely to those calculated by the anisotropic analytical algorithm (AAA) that is used in Eclipse. All plans also demonstrated better than 1% agreement of the dose at the isocenter. This demonstrates the capabilities of our new MC system to model all dosimetric features required for RapidArc dose calculations.