Verification of treatment planning system parameters in tomotherapy using EBT Radiochromic Film E.B.Rajmohan¹, Pratik Kumar¹, Bhudatt Paliwal,² David Westerly², N.Gopishankar³, R.K.Bisht³, D.Tewatia², K.S.Jothi Basu 4, and G.K.Rath 4. ¹ Department of Medical Physics Unit, BRAIRCH, All India Institute of Medical Sciences New Delhi, N.D-110029. ² Department Of Human Oncology, University of Wisconsin, Highland Avenue Madison, WI 53792-0001 ³ Department of Neurosurgery, All India Institute of Medical Sciences New Delhi, N.D-110029. 4 Department of Radiotherapy, All India Institute of Medical Sciences New Delhi, N.D-110029 Objectives: To verify the planning parameters and dose distribution calculated with the Tomotherapy treatment planning system for a prostate patient using external beam therapy (EBT) radiochromic film. Introduction: The recent development of intensity modulated radiotherapy (IMRT) using a linear accelerator (linac) offers a major advancement in radiation therapy by allowing for more conformal dose distributions to be delivered to target volumes; thereby allowing for greater sparing of normal tissues. Tomotherapy represents the next step in IMRT treatments. Tomotherapy, which literally m eans slice therapy, uses a megavoltage linac mounted on a CT-like ring gantry which rotates and delivers fan beam of radiation as a patient is translated on a treatment couch through the bore of the machine. Intensity modulation is achieved using a binary multi-leaf collimator. In addition, a bank of megavoltage-ct (MVCT) detectors is located on the gantry, opposite the linac, to provide onboard MVCT imaging capabilities. While both conventional IMRT and Tomotherapy treatments offer significant advantages in terms of the ability to shape delivered dose distributions to the target, they do so at the cost of increasing the complexity of treatment delivery. This increased complexity in turn requires more rigorous and precise methods of quality assurance to ensure that the dose distribution delivered by the machine agrees with the distribution calculated at the time of planning. Current methods for the verification of the dose delivery of the tomotherapy involve delivering the treatment beams from an accepted patient plan to a solid-water phantom containing radiographic film(s). The use of radiographic films however presents various difficulties, including a strong deviation from tissue equivalence, cost escalation and time-consuming film processing. In order to avoid these difficulties, this study seeks to asses the potential of EBT radochromic film for the verification Tomotherapy dose delivery. Materials and methods: Both EDR2 and EBT films were characterized by their response to a series of nine known dose values ranging from 0-400 cgy delivered with a Varian Clinac 2300 linear accelerator. The treatment plan for single patient, previously treated for prostate cancer with Tomotherapy, was selected and the treatment beams from the selected plan were applied to a cylindrical solid-water phantom, allowing for the dose to solid-water to be calculated. The treatment was then delivered a total of four times, twice with EDR2 films placed within the phantom and twice with EBT films. For each type of film, treatments were delivered with the films oriented in first the coronal and then the saggittal plane. Additional measurements were made during each treatment delivery using an A1SL ion chamber. The ion-chamber measurements were used for both point dose comparisons
Scan Value as well as a means of scaling the relative film values to an absolute dose measurement. Following treatment delivery, films were allowed to wait overnight before being developed and then scanned using a Vidar VXR-16 Dosimetry-Pro Film Scanner. The scanned films were then transferred to the TomoTherapy treatment planning system for analysis. Results and conclusions: Film response curves for both EDR2 and EBT films are shown in Figure 1. This figure clearly shows the sensitivity of the EBT film to be considerably lower across the entire measured dose range as compared with the radiographic films. Figure 3 shows iso-dose curves and film profiles taken with both EDR2 and EBT films in the coronal plane. This figure shows good agreement between the calculated and measured dose distributions in the high dose region using both films; however, the EBT film shows a large discrepancy in the low-dose region due to the reason that radiochromic films are sensitive and has a better peak of absorption in the range of 600 to 670+10nm range of light. In addition to the iso-dose curves and film profiles, the TomoTherapy treatment planning software also provides a means for calculating a distance to agreement parameter, gamma. Gamma maps for both EDR2 and EBT films taken in the coronal plane are shown in Figure 4. The low dose discrepancy is likely also a result of low sensitivity. The low sensitivity required that large scaling factors (based on ion-chamber measurements made in the high dose region) be applied From this study the low sensitivity of EBT, the shape of the response curve for the EBT and scaling to ion-chamber readings taken in the high dose region rectifies the problem in the high dose regions, but fails to do the same for the low dose regions. The MATLAB analysis for the same is underway. Film Response vs. Dose 40000 35000 EDR2 30000 EBT 25000 20000 15000 10000 5000 0 0 50 100 150 200 250 300 350 400 450 Dose (cgy) Figure 1
Figure 2
Figure 3
Figure 4