Automated treatment planning and/or optimization systems (ATPS) are in the process of broad clinical implementation aiming at reducing inter-planner variability, reducing the planning time allocated ...for the optimization process and improving plan quality. Five different ATPS used clinically were evaluated for advanced head and neck cancer (HNC).
Three radiation oncology departments compared 5 different ATPS: 1) Automatic Interactive Optimizer (AIO) in combination with RapidArc (in-house developed and Varian Medical Systems); 2) Auto-Planning (AP) (Philips Radiation Oncology Systems); 3) RapidPlan version 13.6 (RP1) with HNC model from University Hospital A (Varian Medical Systems, Palo Alto, USA); 4) RapidPlan version 13.7 (RP2) combined with scripting for automated setup of fields with HNC model from University Hospital B; 5) Raystation multicriteria optimization algorithm version 5 (RS) (Laboratories AB, Stockholm, Sweden). Eight randomly selected HNC cases from institution A and 8 from institution B were used. PTV coverage, mean and maximum dose to the organs at risk and effective planning time were compared. Ranking was done based on 3 Gy increments for the parallel organs.
All planning systems achieved the hard dose constraints for the PTVs and serial organs for all patients. Overall, AP achieved the best ranking for the parallel organs followed by RS, AIO, RP2 and RP1. The oral cavity mean dose was the lowest for RS (31.3 ± 17.6 Gy), followed by AP (33.8 ± 17.8 Gy), RP1 (34.1 ± 16.7 Gy), AIO (36.1 ± 16.8 Gy) and RP2 (36.3 ± 16.2 Gy). The submandibular glands mean dose was 33.6 ± 10.8 Gy (AP), 35.2 ± 8.4 Gy (AIO), 35.5 ± 9.3 Gy (RP2), 36.9 ± 7.6 Gy (RS) and 38.2 ± 7.0 Gy (RP1). The average effective planning working time was substantially different between the five ATPS (in minutes): < 2 ± 1 for AIO and RP2, 5 ± 1 for AP, 15 ± 2 for RP1 and 340 ± 48 for RS, respectively.
All ATPS were able to achieve all planning DVH constraints and the effective working time was kept bellow 20 min for each ATPS except for RS. For the parallel organs, AP performed the best, although the differences were small.
Purpose:
In 2008, a national intensity modulated radiation therapy (IMRT) dosimetry intercomparison was carried out for all 23 radiation oncology institutions in Switzerland. It was the aim to check ...the treatment chain focused on the planning, dose calculation, and irradiation process.
Methods:
A thorax phantom with inhomogeneities was used, in which thermoluminescence dosimeter (TLD) and ionization chamber measurements were performed. Additionally, absolute dosimetry of the applied beams has been checked. Altogether, 30 plan-measurement combinations have been used in the comparison study. The results have been grouped according to dose calculation algorithms, classified as “type a” or “type b,” as proposed byKnöös
et al.
“Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations,” Phys. Med. Biol.
51, 5785–5807 (2006).
Results:
Absolute dosimetry check under standard conditions: The mean ratio between the dose derived from the single field measurement and the stated dose, calculated with the treatment planning system, was
1.007
±
0.010
for the ionization chamber and
1.002
±
0.014
(
mean
±
standard
deviation) for the TLD measurements. IMRT Plan Check: In the lung tissue of the planning target volume, a significantly better agreement between measurements (TLD, ionization chamber) and calculations is shown for type b algorithms than for type a
(
p
<
0.001
)
. In regions outside the lungs, the absolute differences between TLD measured and stated dose values, relative to the prescribed dose,
|
(
D
m
−
D
s
)
/
D
prescribed
|
, are
1.9
±
0.4
%
and
1.4
±
0.3
%
, respectively. These data show the same degree of accuracy between the two algorithm types if low-density medium is not present.
Conclusions:
The results demonstrate that the performed intercomparison is feasible and confirm the calculation accuracies of type a and type b algorithms in a water equivalent and low-density environment. It is now planned to offer the intercomparison on a regular basis to all Swiss institutions using IMRT techniques.
The quantity of interest for high-energy photon beam therapy recommended by most dosimetric protocols is the absorbed dose to water. Thus, ionization chambers are calibrated in absorbed dose to ...water, which is the same quantity as what is calculated by most treatment planning systems (TPS). However, when measurements are performed in a low-density medium, the presence of the ionization chamber generates a perturbation at the level of the secondary particle range. Therefore, the measured quantity is close to the absorbed dose to a volume of water equivalent to the chamber volume. This quantity is not equivalent to the dose calculated by a TPS, which is the absorbed dose to an infinitesimally small volume of water. This phenomenon can lead to an overestimation of the absorbed dose measured with an ionization chamber of up to 40% in extreme cases. In this paper, we propose a method to calculate correction factors based on the Monte Carlo simulations. These correction factors are obtained by the ratio of the absorbed dose to water in a low-density medium □D(w,Q,V1)(low) averaged over a scoring volume V₁ for a geometry where V₁ is filled with the low-density medium and the absorbed dose to water □D(w,QV2)(low) averaged over a volume V₂ for a geometry where V₂ is filled with water. In the Monte Carlo simulations, □D(w,QV2)(low) is obtained by replacing the volume of the ionization chamber by an equivalent volume of water, according to the definition of the absorbed dose to water. The method is validated in two different configurations which allowed us to study the behavior of this correction factor as a function of depth in phantom, photon beam energy, phantom density and field size.
Purpose: The use of small fields in radiotherapy has become widespread with techniques such as IMRT and stereotactic radiosurgery. Dose measurement in small fields in a low‐density medium must be ...corrected for the replacement perturbation effect. This effect occurs when measurements are performed with an ionization chamber whose walls are made of water equivalent materials. Electron fluence is locally perturbated by the detector itself and the measured absorbed dose is overestimated by a factor which depends on the field size. The goal of this work is to investigate the behavior of the detector reading as a function of the detector size. Method and Materials: Replacement perturbation factors of an A1SL ionization chamber (Standard Imaging, USA) were calculated as a function of the field size and the medium density. Monte Carlo simulations were performed with the OMEGA/BEAMnrc code, previously validated in heterogeneous conditions. The absorbed dose to water was calculated at 10cm depth in a phantom composed of slabs of 5cm water, 13cm lung and 10cm water (5cm in the heterogeneity) and normalized by the value at 2.5cm. Results: The replacement perturbation factor is negligible for large fields, but increases for small fields and as the density decreases. In a lung‐equivalent material (relative density = 0.2), the error due to the replacement perturbation effect can be as high as 20% with a 1 × 1 cm2 field measured by a A1SL ionization chamber (suitable for small field measurements). Conclusion: The replacement perturbation factor becomes very important as the field size becomes smaller and must be taken into account when measuring in low‐density media.
Purpose: A helical tomotherapy accelerator has been commissioned last year in Switzerland at Lausanne University Hospital (CHUV). Despite the fact that more than 150 such instruments have already ...been sold around the world, this technique presents a dosimetric challenge and there is no internationally accepted protocol for the reference dose yet. The goal of the present study is to investigate different alternatives to have an independent method to determine the dose reference of the accelerator. Method and Materials: Several dosimetric techniques with various metrological traceabilities were tested in a number of phantoms in static and helical modes. The first measurements were performed with the A1SL ionization chamber, which is delivered by the vendor as a reference instrument: it is traceable to the American national metrology institute (NIST) in absorbed dose to water in a Co‐60 beam quality through a graphite calorimeter. In Switzerland, each radiotherapy department is directly traceable to the national standard (METAS) in absorbed dose to water through a water calorimeter. A NE 2611A ionization chamber calibrated by METAS was therefore used to determine the reference dose as well. In order to have another fully independent way of measurement, the reference dose was also determined by mean of alanine dosimeters provided by the British national laboratory (NPL) and calibrated in absorbed dose to water through a graphite calorimeter. Finally, in order to take into account one of the chamber that is widely used in the clinical practice, the reference dose was also measured using a Farmer‐type instrument (NE 2571). Conclusion: Compared to our standard (NE 2611A), the A1SL, alanine and Farmer chamber (NE 2571) showed differences of 1.2 %, −0.4 % and −1.7 % respectively. These values are within the measurements uncertainty of the different methods and can be partially explained by the design of the chambers.
Introduction : Tomographic techniques in diagnostic and interventional radiology or radiation therapy are in rapid development. The CTDI formalism that is currently used to assess dose has been ...challenged by several groups. The goal of this contribution is to evaluate the required scan length to reach equilibrium on various MDCT units and estimate the average dose delivered within a slice when using systems having a cone beam geometry.
Method and Materials: Measurements were performed on three MDCT systems (GEMS 8, 16 and 64‐row), a flat panel fluoroscopy (Allura — Philips), an IGRT (Synergy, Elekta ltd) and on a tomotherapy (TomoTherapy Inc.) using two standard CTDI phantoms and a home made phantoms (PMMA cylinder of 30 cm filled with water) with a conventional small volume ion chamber (0.6 cc Farmer type) and a standard pencil ion chamber. Dose profiles were also recorded at various positions within the slice to study the impact of scatter as a function of the distance between the centre of the phantom and its periphery. Results: For CT units a theoretical length of 400 mm is required to reach the equilibrium at the center of the phantom. The measurements show that a dose plateau was reached after 420 mm. The average dose within a slice measured in our home made phantom for standard abdominal CT (120 kV, 210 effective mAs) was 15.3 mGy; 26.9 mGy for the fluoro‐CT (122kV, 274mAs, 20.7s); 39.0mGy for IGRT system (120kV, 219mAs, 27×27cm2 field size, one rotation) and 13.1 mGy for the Tomotherapy system (3.5 MV, pitch=0.8). Conclusion: The CTDI concept should be replaced by a more generic methodology, such as the one presented here, that could be used in all cone beam geometries. Measurements should be done using water equivalent phantoms.
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