Purpose:
The purpose of this study is to access VMAT‐SABR plan using flattening filter (FF) and flattening filter‐free (FFF) beam, and compare the verification results for all pretreatment plans.
...Methods:
SABR plans for 20 prostate patients were optimized in the Eclipse treatment planning system. A prescription dose was 42.7 Gy/7 fractions. Four SABR plans for each patient were calculated using Acuros XB algorithm with both FF and FFF beams of 6‐ and 10‐MV. The dose‐volume histograms (DVH) and technical parameters were recorded and compared. A pretreatment verification was performed and the gamma analysis was used to quantify the agreement between calculations and measurements.
Results:
For each patient, the DVHs are closely similar for plans of four different beams. There are small differences showed in dose distributions and corresponding DVHs when comparing the each plan related to the same patient. Sparing on bladder and rectum was slightly better on plans with 10‐MV FF and FFF than with 6‐MV FF and FFF, but this difference was negligible. However, there was no significance in the other OARs. The mean agreement of 3%/3mm criteria was higher than 97% in all plans. The mean MUs and deliver time employed was 1701±101 and 3.02±0.17 min for 6‐MV FF, 1870±116 and 1.69±0.08 min for 6‐MV FFF, 1471±86 and 2.68±0.14 min for 10‐MV FF, and 1619±101 and 0.98±0.04 min for 10‐MV FFF, respectively.
Conclusion:
Dose distributions on prostate SABR plans using FFF beams were similar to those generated by FF beams. However, the use of FFF beam offers a clear benefit in delivery time when compared to FF beam. Verification of pretreatment also represented the acceptable and comparable results in all plans using FF beam as well as FFF beam. Therefore, this study suggests that the use of FFF beam is feasible and efficient technique for prostate SABR.
Purpose:
Total body irradiation (TBI) uses large parallel‐opposed radiation fields to suppress the patient's immune system and eradicate the residual cancer cells in preparation of recipient for bone ...marrow transplant. The manual placement of lead compensators has conventionally been used to compensate for the varying thickness through the entire body in large‐field TBI. The goal of this study is to pursue utilizing the modern electronic compensation technique to more accurately and efficiently deliver dose to patients in need of TBI.
Methods:
Treatment plans utilizing electronic compensation to deliver a total body dose were created retrospectively for patients for whom CT data had been previously acquired. Each treatment plan includes two, specifically weighted, pair of opposed fields. One pair of open, large fields (collimator=45°), to encompass the patient's entire anatomy, and one pair of smaller fields (collimator=0°) focused only on the thicker midsection of the patient. The optimal fluence for each one of the smaller fields was calculated at a patient specific penetration depth. Irregular surface compensators provide a more uniform dose distribution within the smaller opposed fields.
Results:
Dose‐volume histograms (DVH) were calculated for the evaluating the electronic compensation technique. In one case, the maximum body doses calculated from the DVH were reduced from the non‐compensated 195.8% to 165.3% in the electronically compensated plans, indicating a more uniform dose with the region of electronic compensation. The mean body doses calculated from the DVH were also reduced from the non‐compensated 120.6% to 112.7% in the electronically compensated plans, indicating a more accurate delivery of the prescription dose. All calculated monitor units were well within clinically acceptable limits.
Conclusion:
Electronic compensation technique for TBI will not substantially increase the beam on time while it can significantly reduce the compensator setup time and the potential risk of errors in manually placing lead compensators.
Purpose:
When Bragg peaks from proton PBS beams are overlaid from a large number of directions, it creates a localized dose distribution with rapid falloff. By positioning these overlaid Bragg peak ...(OBP) spots strategically throughout the targeted volume, a uniform and conformal dose distribution can be achieved. When PBS spots from all the directions are delivered in an arc fashion, it becomes the Volume Modulated Proton Therapy (VMPT) technique which corresponds to the VMAT technique in photon therapy.
Methods:
PBS beam from an IBA machine and a 20cm radius cylindrical phantom were simulated with GATE/GEANT4. To position the OBP spot at the center of the phantom, single energy PBS beam from 10/36/360 equally‐spaced directions around the phantom was used. To position the OBP spot at an off‐center position, PBS beams with various energies were used. To form a uniform dose distribution in 1D/2D, OBP spots are positioned in arrays with spot intervals and weight optimized.
Results:
Dose profile for single energy OBP spot improved when the number of beam angles increased from 10 to 36, but remained same from 36 to 360 angles. Multi‐energy OBP spot had a slightly non‐symmetrical profile due to the different energy proton beams used. By optimizing each OBP spot's weight, 11 OBP spots with 10mm interval size created a uniform dose distribution covering a length of 10cm. Similarly, 11×11 OBP spots in an array of 10mm interval size created a uniform dose distribution covering an area of 10×10cm2.
Conclusion:
We demonstrated the technique of using OBP spots to deliver uniform dose in 1D/2D. This technique can be easily extended through the optimization of position and weight of OBP spots in 3D and becomes the arc based VMPT technique. Further studies are needed to explore the optimization methods and full potential of the VMPT technique.
Purpose:
End‐of‐exhale (EOE) phase is generally preferred for gating window because tumor position is more reproducible. However, other gating windows might be more appropriate for dose distribution ...perspective. In this pilot study, we proposed to utilize overlap volume histogram (OVH) to search optimized gating window and demonstrated its feasibility.
Methods:
We acquired 4DCT of 10 phases for 3 lung patients (2 with a target at right middle lobe and 1 at right upper lobe). After structures were defined in every phase, the OVH of each OAR was generated to quantify the three dimensional spatial relationship between the PTV and OARs (bronchus, esophagus, heart and cord etc.) at each phase. OVH tells the overlap volume of an OAR according to outward distance from the PTV. Relative overlap volume at 20 mm outward distance from the PTV (ROV‐20) was also defined as a metric for measuring overlap volume and obtained. For dose calculation, 3D CRT plans were made for all phases under the same beam angles and objectives (e.g., 95% of the PTV coverage with at least 100% of the prescription dose of 50 Gy). The gating window phase was ranked according to ROV‐20, and the relationship between the OVH and dose distribution at each phase was evaluated by comparing the maximum dose, mean dose, and equivalent uniform dose of OAR.
Results:
OVHs showed noticeable difference from phase to phase, implying it is possible to find optimal phases for gating window. For 2 out of 3 patients (both with a target at RML), maximum dose, mean dose, and EUD increased as ROV‐20 increased.
Conclusion:
It is demonstrated that optimal phases (in dose distribution perspective) for gating window could exist and OVH can be a useful tool for determining such phases without performing dose optimization calculations in all phases.
This work was supported by the Radiation Technology R&D program (No. 2013M2A2A7043498) and the Mid‐career Researcher Program (2012‐007883) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea.
Purpose:
The ability of pencil beam scanning (PBS) to deliver highly conformal dose distributions may be affected by patient‐ and physics‐related uncertainties. In clinical practice, selection of ...proton beam angles is determined qualitatively. This study investigates whether an optimal proton PBS beam angle could be quantitatively determined to ensure robust planning for pelvic targets.
Methods:
PBS beam angles were optimized based on two independent criteria; shortest and most homogeneous path from the patient surface to the distal edge of the target. The beam angle optimization criteria for gantry angles between 90°‐270° were quantified in 10° increments for each ray, calculated as the straight line distance from the surface of the skin to the CTV's distal edge. The goal was to minimize the path length of a proton PBS beam from the patient surface to the distal edge of the CTV, relative to the entry point, while minimizing HU inhomogeneity along the ray. HU homogeneity (i.e. HU variation) was quantitatively defined as the standard deviation of the average intra‐ray HU intensity distribution of the rays comprising a single beam. This method was validated relative to inter‐fraction changes on ten consecutive, locally advanced, rectal cancer patients, who underwent an average 4 verification CTs. The displacement of the 95–98% isodose lines was determined from forward calculated dose distributions on verification CTs.
Results:
The posterior beam (180°) had the average shortest path length, 132.7±17.2mm, and the most homogenous path, 31.9±4.3HU. The 95–98% isodose lines from all plans verified our path length to within 2.3±1.2% and HU homogeneity to within 1.2±0.5%.
Conclusion:
The proposed optimization algorithm determined the posterior beam dose distribution as the most robust relative to inter‐fraction variation for large pelvic targets treated with PBS and was validated via verification CT for our patient cohort. Future work will focus on further algorithm development.
Purpose:
The planar average dose in a C‐arm Cone Beam CT (CBCT) acquisition had been estimated in the past by averaging the four peripheral dose measurements in a CTDI phantom and then using the ...standard 2/3rds peripheral and 1/3 central CTDIw method (hereafter referred to as Dw). The accuracy of this assumption has not been investigated and the purpose of this work is to test the presumed relationship.
Methods:
Dose measurements were made in the central plane of two consecutively placed 16cm CTDI phantoms using a 0.6cc ionization chamber at each of the 4 peripheral dose bores and in the central dose bore for a C‐arm CBCT protocol. The same setup was scanned with a circular cut‐out of radiosensitive gafchromic film positioned between the two phantoms to capture the planar dose distribution. Calibration curves for color pixel value after scanning were generated from film strips irradiated at different known dose levels. The planar average dose for red and green pixel values was calculated by summing the dose values in the irradiated circular film cut out. Dw was calculated using the ionization chamber measurements and film dose values at the location of each of the dose bores.
Results:
The planar average dose using both the red and green pixel color calibration curves were within 10% agreement of the planar average dose estimated using the Dw method of film dose values at the bore locations. Additionally, an average of the planar average doses calculated using the red and green calibration curves differed from the ionization chamber Dw estimate by only 5%.
Conclusion:
The method of calculating the planar average dose at the central plane of a C‐arm CBCT non‐360 rotation by calculating Dw from peripheral and central dose bore measurements is a reasonable approach to estimating the planar average dose.
Research Grant, Siemens AG
Purpose:
In this project, the possibility of utilizing the BEBIG 60Co HDR system for AccuBoostTM treatment has been evaluated.
Methods:
Dose distributions in various breast sizes have been calculated ...for both Co‐60 and Ir‐192 sources using the MCNP5 code. These calculations were performed in breast tissues with thicknesses of 4cm, 6cm, and 8cm. The initial calculations were performed with the same applicator dimensions as the existing applicators used with the HDR Ir‐192 system. The activity of the Co‐60 source was selected such that the dose at the breast center was the same as the values from 192Ir. Then, the applicator thicknesses were increased to twice of those used with HDR Ir‐192 system, for reducing skin and chest doses by Co‐60 system. Dose to breast skin and chest wall were compared for both applicators types, with and without inclusion of a focusing cone at the applicator center.
Results:
The results showed that loading HDR Co‐60 source inside the thin applicators impose higher doses to breast skin and chest wall compared to the 192Ir source. The area of the chest wall covered by 10Gy when treated by Co‐60 with the thin and thick applicators, or treated by Ir‐192 with thin applicator are 79cm2, 39cm2, and 3.8cm2, respectively. These values are reduced to 34cm2, 0cm2, and 0cm2 by using the focusing cone. It is worth noting that the breast skin areas covered by the 60Gy isodose line are 9.9cm2 and 7.8cm2 for Co‐60 with the thin and thick applicators, respectively, while it is 20cm2 for Ir‐192 when no focusing cone is present. These values are 0cm2, 0cm2, and 11cm2 in the presence of the focusing cone.
Conclusion:
The results indicate that using Co‐60 with the thicker applicators is beneficial because of the higher half‐life of Co‐60, and the reduced maximum skin dose when compared with Ir‐192.
Purpose:
Currently, planning systems allow robustness calculations to be performed, but a generalized assessment methodology is not yet available. We introduce and evaluate a methodology to quantify ...the robustness of a plan on an individual patient basis.
Methods:
We introduce the notion of characterizing a treatment instance (i.e. one single fraction delivery) by describing the dose distribution within an organ as an alpha‐stable distribution. The parameters of the distribution (shape(α), scale(γ), position(δ), and symmetry(β)), will vary continuously (in a mathematical sense) as the distributions change with the different positions. The rate of change of the parameters provides a measure of the robustness of the treatment. The methodology is tested in a planning study of 25 patients with known residual errors at each fraction. Each patient was planned using Eclipse with an IBA‐proton beam model. The residual error space for every patient was sampled 30 times, yielding 31 treatment plans for each patient and dose distributions in 5 organs. The parameters’ change rate as a function of Euclidean distance from the original plan was analyzed.
Results:
More than 1,000 dose distributions were analyzed. For 4 of the 25 patients the change in scale rate (γ) was considerably higher than the lowest change rate, indicating a lack of robustness. The sign of the shape change rate (α) also seemed indicative but the experiment lacked the power to prove significance.
Conclusion:
There are indications that this robustness measure is a valuable tool to allow a more patient individualized approach to the determination of margins. In a further study we will also evaluate this robustness measure using photon treatments, and evaluate the impact of using breath hold techniques, and the a Monte Carlo based dose deposition calculation. A principle component analysis is also planned.
Purpose:
To verify the dose accuracy of Acuros XB (AXB) dose calculation algorithm at air‐tissue interface using inhomogeneous phantom for 6‐MV flattening filter‐free (FFF) beams.
Methods:
An ...inhomogeneous phantom included air cavity was manufactured for verifying dose accuracy at the air‐tissue interface. The phantom was composed with 1 and 3 cm thickness of air cavity. To evaluate the central axis doses (CAD) and dose profiles of the interface, the dose calculations were performed for 3 × 3 and 4 × 4 cm2 fields of 6 MV FFF beams with AAA and AXB in Eclipse treatment plainning system. Measurements in this region were performed with Gafchromic film. The root mean square errors (RMSE) were analyzed with calculated and measured dose profile. Dose profiles were divided into inner‐dose profile (>80%) and penumbra (20% to 80%) region for evaluating RMSE. To quantify the distribution difference, gamma evaluation was used and determined the agreement with 3%/3mm criteria.
Results:
The percentage differences (%Diffs) between measured and calculated CAD in the interface, AXB shows more agreement than AAA. The %Diffs were increased with increasing the thickness of air cavity size and it is similar for both algorithms. In RMSEs of inner‐profile, AXB was more accurate than AAA. The difference was up to 6 times due to overestimation by AAA. RMSEs of penumbra appeared to high difference for increasing the measurement depth. Gamma agreement also presented that the passing rates decreased in penumbra.
Conclusion:
This study demonstrated that the dose calculation with AXB shows more accurate than with AAA for the air‐tissue interface. The 2D dose distributions with AXB for both inner‐profile and penumbra showed better agreement than with AAA relative to variation of the measurement depths and air cavity sizes.
Purpose:
In this study, we aim to evaluate the effect of dose grid size on the accuracy of calculated dose for small lesions in intracranial stereotactic radiosurgery (SRS), and to verify dose ...calculation accuracy with radiochromic film dosimetry.
Methods:
15 intracranial lesions from previous SRS patients were retrospectively selected for this study. The planning target volume (PTV) ranged from 0.17 to 2.3 cm3. A commercial treatment planning system was used to generate SRS plans using the volumetric modulated arc therapy (VMAT) technique using two arc fields. Two convolution‐superposition‐based dose calculation algorithms (Anisotropic Analytical Algorithm and Acuros XB algorithm) were used to calculate volume dose distribution with dose grid size ranging from 1 mm to 3 mm with 0.5 mm step size. First, while the plan monitor units (MU) were kept constant, PTV dose variations were analyzed. Second, with 95% of the PTV covered by the prescription dose, variations of the plan MUs as a function of dose grid size were analyzed. Radiochomic films were used to compare the delivered dose and profile with the calculated dose distribution with different dose grid sizes.
Results:
The dose to the PTV, in terms of the mean dose, maximum, and minimum dose, showed steady decrease with increasing dose grid size using both algorithms. With 95% of the PTV covered by the prescription dose, the total MU increased with increasing dose grid size in most of the plans. Radiochromic film measurements showed better agreement with dose distributions calculated with 1‐mm dose grid size.
Conclusion:
Dose grid size has significant impact on calculated dose distribution in intracranial SRS treatment planning with small target volumes. Using the default dose grid size could lead to under‐estimation of delivered dose. A small dose grid size should be used to ensure calculation accuracy and agreement with QA measurements.
