Introduction
Flattening Filter Free (FFF) modalities have been widely used for Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT) procedures, because of the high dose rate, sharp penumbra, reduced leaf transmission and head scatter associated with FFF beams [
1‐
3]. Modern radiation therapy modalities, including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), produce SRS/SBRT plans with highly irregular and steep dose gradient distributions. IMRT/VMAT treatment beams are often heterogeneous and complex, consisting of many small beam apertures realized by a multi-leaf collimator (MLC). Accurate verification of such complex treatment fields is challenging. The conventional method for patient specific quality assurance (PSQA) of measuring a point dose using an ionization chamber (IC) or fluence using a two-dimensional (2D) array is inadequate for highly modulated treatment fields with sharp dose gradient fall off [
4‐
6].
Gafchromic™ EBT3 film has been introduced to eliminate measurement orientation effects as well as Newton rings formed during film scanning [
7]. EBT3 Film dosimetry has been used most commonly for relative dose analysis [
8]. The absolute dose map is built by normalizing a selected reference point to the reading of a dosimeter, which is inadequate for SRS and SBRT PSQA considering the volume size of the dosimeters in general. There has been no study to evaluate the characteristics of EBT3 Gafchromic™ films on FFF beams and high dose range in the SRS/SBRT dose regimen. This study presents a practical and efficient film dosimetry protocol for SRS and SBRT PSQA using FFF photon beams. Uncertainties associated with the film channels coupled with a flatbed scanner were evaluated in the absolute dose analysis.
Discussion
Gafchromic™ film has excellent spatial resolution and is independent of beam angle, energy and dose rate [
19]. This makes it suitable for a variety of commissioning and PSQA tasks. However, several sources of uncertainties associated with film dosimetry, such as scanner, background, film uniformity, impact the dosimetric accuracy. Film dosimetry has been used most commonly for relative dose analysis. We systematically investigated the dosimetric uncertainty of EBT3 films. Instead of normalizing the film dose based on ion chamber measurement, we were able to analyze the film based on absolute dose directly. To improve the efficiency of using Gafchromic™ film routinely for patient specific QA, we have developed an integrated film dosimetry protocol that converts OD values to dose, registers film and planar dose and analyzes the profiles and gamma values. The protocol ensures the consistency of film dosimetric results with a clear understanding of the uncertainties of the film dosimetry protocol.
We used the reference doses measured with the MatriXX detector arrays to calculate the scanner pixel-by-pixel non-uniform response correction. The spatial resolution of the MatriXX was limited by the center-to-center distance between the ion chambers. This problem and its potential impact on quality assurance have been investigated by several groups to validate the 2D detector array’s performance [
20,
21]. Poppe et al. investigated the sample frequency provided by different 2D arrays and the Nyquist frequency for 1 × 1 cm
2 field size and complex IMRT fields. They illustrated that the spatial frequency was not extended beyond 0.1 mm
−1 and very low beyond 0.06 mm
−1 [
4]. Since the spatial frequency of the sampled dose distribution did not exceed one half of the MatriXX sampling frequency 0.085 mm
−1, i.e. the Nyquist frequency limitation was not violated, the spatial resolution of the MatriXX was appropriate to measure the penumbra region and sharp dose gradient drop off region for small field sizes investigated in this study.
As part of our film dosimetry protocol OD to dose response curve was applied directly. For both the red and green channels, there were negligible differences between the OD and netOD methods, implying that the OD and netOD methods produce the same dosimetric accuracy. However, the profiles from the red channel were skewed due to the scanner lateral dependence artifact at the higher dose levels [
22]. For the blue channel, lower doses (e.g. 1 Gy) were insufficient for capturing differences between OD and netOD. Therefore, the images from green channel were extracted and converted to dose using pre-established calibration curves. The dose maps were compared to the treatment planning dose matrix for subsequent profile and gamma analysis for routine SRS/SBRT PSQA.
Film non-uniformity is one of the largest sources of uncertainty for EBT film-based dosimetry. The uncertainties of the Gafchromic
TM EBT model have been previously reported in the literature. Battum et al. reported that the uncertainty of 1.8 % was achievable up to a dose range of 2.3 Gy using the red channel when the films were scanned at the central location of the scanner bed [
23]. Devic et al. reported 1.5 % uncertainty using the red channel in the dose range of 0~4 Gy [
24]. Saur and Frengen reported 2.5 % uncertainty at 2 Gy exposure when scanned in the landscape mode [
11] and uncertainty was reduced to 1.7 % when films were scanned in the portrait mode. Papaconstadopoulos et al. suggested a two-color reflection scanning protocol for EBT3 film dosimetry in the dose range of 0 to 8 Gy [
25]. Red channel was recommended for doses less than 2 Gy with an accuracy level of 3.7 % and the green channel for higher doses at an accuracy level of 5 %. However, there have been lack of studies to evaluate the uncertainties of EBT3 films for SRS/SBRT treatment. Our study showed that the red channel has very high sensitivity and low uncertainty in the low dose region (<10 Gy) and that the green channel is best suited for most SRS/SBRT prescription dose levels and the overall uncertainty can be controlled at 1.5 %. This uncertainty does not account for the dose response changes of the films as a function of time.
In the time dependency study, the dose difference was greater than 10 % if the film was scanned in the first 2 h after delivery and within 3 % within 1 day thereafter. Calibration curves should be generated at different time periods considering PSQA workflow, such as 4 h for same day delivery and scan, 20 h for the next day scan and 2.5 days if the delivery is done on Friday night and the film is scanned on Monday morning. In our study, four cases did not pass 90 % gamma passing rate, which triggered further evaluation. The passing rate based on absolute dose analysis was 83.4 % (T6 spine), 71.3 % (T9 spine), and 87.6 % (lung, right upper lobe). When the film results were normalized to ion chamber measurements, the corresponding gamma passing rates based on the relative dose analysis were 99.8, 99.7, and 99.6 %. The failure of absolute dose analysis for all the cases was due to the different development time between the calibration and patient films. It is very important to incorporate the impact of time dependent change on OD for absolute dose analysis using GafchromicTM film.
Our film dosimetry protocol can improve QA efficiency with a single measurement for both point and planar, absolute dose distribution analysis. Our measurements encompassed a variety of complexity in the SRS/SBRT cases. No variables (volume size, treatment site, treatment delivery technique) were found to have a statistically significant impact on the gamma index based on by the one-way ANOVA test. Very limited data have been reported on the SRS/SBRT PSQA using FFF beams [
26]. The established confidence limits reported from a large SRS/SBRT patient cohort can be subsequently applied for patient specific verification. The excellent spatial resolution (0.2 mm) of the films, makes it ideal for absolute dose profile analysis, as a means evaluate plan delivery quality for highly modulated dose distributions or small target volumes, as commonly encountered in SRS/SBRT treatments.
Acknowledgements
The authors thank American Cancer Society for funding this project.