Accuracy of the dose delivery in prostate cancer patients-using an electronic portal imaging device [EPID]

Background: To correct patient positioning errors [setup errors] during prostate cancer treatment using EPID and fiducial gold markers, to improve the accuracy of the dose delivery in these patients

Materials and Methods: Fifteen patients with localized prostate carcinoma after implantation of fiducial gold markers in their prostate gland underwent the five-field IMRT planning technique. The plan was prepared in accordance with ICRU 50 guidance [PTV to receive 95-107% dose]. The software program reconstructed the three-dimensional position of the markers from the different Beams Eye Views [BEV]. The discrepancies of the seeds' positions [prostate surrogate] between plan and daily images were calculated three dimensionally. Then, necessary corrections were applied to match the prostate fiducial markers in the portal image with the BEV image in the planned one by moving the couch in the X, Y and Z directions

Results: Data from 15 patients and 469 fractions of radiotherapy were analyzed in this study. Two sets of data were available from EPID software before and after 3D set-up corrections. The mean of the population displacement in Left /Right [L/R], Anterior/Posterior [A/P] and Crania/Caudal [C/C] directions were 0.5, -1.0 and 2.4mm before, and -0.1, -0.5 and 0.9mm after corrections, respectively. The systematic and random errors for the measured populations in the three mentioned directions were 2.4, 2.7 and 2mm and 6.4, 5.9 and 6.1mm before corrections, and 1.1, 2.4 and 1.4mm and 3.8, 3.9 and 3.6mm after corrections, correspondingly

Conclusion: This study provides further evidence that using gold markers in the prostate improves dose delivery to the prostate. Also, it has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments

Effect of uniform magnetic field on dose distribution in the breast radiotherapy

To reduce the dose to normal tissues surrounding the treated breast, a uniform magnetic field was used within a humanoid phantom in breast radiotherapy. Monte Carlo simulations were performed with GEANT4, irradiating humanoid phantoms in a magnetic field. To reconstruct phantoms, computed tomography [CT] data slices of four patients were used for the Monte Carlo simulations. All of them had left breast cancer either or not mastectomy. In the simulations, the planning and methods of chest wall irradiation were similar to the actual clinical planning. Utilizing magnetic field will help to produce uniform dose distribution to the breast with a sharp dose-volume histogram [DVH] curve for the planning target volume [PTV], however, for the ipsilateral lung and chest wall skin the mean dose was reduced by a mean of 16% and 12% at 1.5 T, and 9% and 7% at 3 T, respectively. The magnetic field was shown to restrict the lateral spread of secondary electrons to the contralateral organs, resulting in significient dose reductions to the contralateral breast [CB] and contralateral chest wall skin [CCWS] by a mean [range] of 28% [21-37%] and 58% [44-75%] at 1.5 T, and 48% [32-81] and 66% [54-73%] at 3 T, respectively. The simulations established that the magnetic field can reduce the dose to the internal and contralateral tissues and increase it to the PTV with sharper edge DVH curve

Monte Carlo study on beam hardening effect of physical wedges

Physical wedges are still widely used as beam modifiers in external beam radiotherapy. However the presence of them in the beam trace may cause beam hardening which may not be considered in many treatment planning systems. The aim of this study is to investigate the beam hardening effect generated by physical wedges via different beam quality indexes as photon spectrum, half value layer, mean energy and tissue-phantom ratio. The effect of physical wedges on the photon beam quality of a 6-18MV Varian 2100C/D accelerator was studied with the BEAMnrc Monte Carlo code. Good agreements were obtained between measured and calculated depth doses and beam profiles for open and wedged photon beams at both energies. It was noticed that for 6 MV photon beams, physical wedges have more significant effects on beam quality than for 18 MV. Also it was obtained that at 18 MV photon beam as the wedge angle increased, the effect of wedge on beam quality becomes reversed and beam softening occurred. According to these results, it is recommended that beam hardening and softening of physical wedges should be considered in treatment planning systems in order to increase the accuracy in dose delivery

Dosimetric characterization of a high dose rate 192I source for brachytherapy application using Monte Carlo simulation and benchmarking with thermoluminescent dosimetry

The purpose of this project was to derive the brachytherapy dosimetric functions described by American Association of Physicists in Medicine [AAPM] TG-43 U1 based on high dose rate [192]I sources. The method utilized included both simulation of the designed Polymethyl methacrylate [PMMA] phantom using the Monte Carlo of MCNP4C and benchmarking of the simulation with thermoluminescent [TL] dosimeters. The obtained results for the radial dose function and anisotropy function showed nominal errors of less than 3% between TL measurements and the MCNP4C results. It may be concluded that due to small observed errors and the large uncertainty associated with the high dose gradients near the source point the simulation results can be used for dose estimation

Optimization of clinical target volume delineation using magnetic resonance spectroscopic imaging [MRSI] in 3D conformal radiotherapy of prostate cancer

For the purpose of individual clinical target volume assessment in radiotherapy of prostate cancer, MRSI was used as a molecular imaging modality with MRI and CT images. The images of 20 prostate cancer patients were used in this study. The MR and MRSI images were registered with CT ones using non-rigid registration technique. The CT based planning [BP], CT/MRI BP and CT/MRSI BP was performed for each patient. For plan evaluation, Dose Volume Histograms [DVHs] data were used. A paired sample T-test was used for the analysis of the obtained data. The percentage of variation of CTVMRI to CTVCT and PTVMRI to PTVCT were 12.83% and 8.97%, respectively. CTVMRSI and PTVMRSI were 21% and 27.41% more than their corresponding values of CT volumes. The mean percentage of variation in rectum volume that received 60% of the prescribe dose [V60R] in MRSI/CT BP relative to CT BP was 14.66%. The use of MRSI in detecting of prostate adenocarcinoma could provide some decisive information to determine optimum volume and safe margin for target definition to improve adaptive radiotherapy in prostate cancer

Megavoltage dose enhancement of gold nanoparticles for different geometric set-ups: Measurements and Monte Carlo simulation

Gold nanoparticles [GNPs] have been shown as a good radiosensitizer. In combination with radiotherapy, several studies with orthovoltage X-rays have shown considerable dose enhancement effects. This paper reports the dose enhancement factor [DEF] due to GNPs in 18 megavoltage [MV] beams. Different geometrical 50-nm GNPs configurations at a concentration of 5 mg/ml were used by both experimental and Monte Carlo [MC] simulation in a deep-seated tumor-like insertion within a phantom. Using MCNP repeated structure capability; a large number of gold nanospheres with a semi-random distribution were applied to simulate this phantom based study. Thermoluminescence dosimetries were used to verify the process of irradiation and MC simulation. Under geometries with different probable combinations of water and GNPs distribution in the tumor, the percentage depth dose and DEF were calculated. Incorporation of GNPs into the radiation field in our set-ups showed a 12% DEF. We show that the method of nanoparticles, distribution, and orientation can effectively change the DEF value

Comparing the results of 3D treatment planning and practical dosimetry in craniospinal radiotherapy using Rando phantom

Craniospinal radiotherapy faces technical challenges which are due to the sensitivity of the location in which the gross tumor is, and to organs at risk around planning target volume. Using modern treatment planning systems causes a reduction in the complexities of the treatment techniques. The most effective method to assess the dosimetric accuracy and the validity of the software used for treatment planning is to investigate the radiotherapy and treatment planning by means of a anthropomorphic Rando phantom which was used here for treatment planning and practical dosimetry for craniospinal radiotherapy. Studying the absorbed dose by the organs at risk was the secondary objective discussed in this paper. Treatment planning in craniospinal radiotherapy was done using CorePlan 3D treatment planning software. Radiotherapy was administered on a anthropomorphic Rando phantom and practical dosimetry was done using GR-200 TLDs. Varian Clinac 2100C/D was used for radiotherapy. The absorbed dose by regions of interest was separately calculated for treatment planning and radiotherapy. Except the conjunction areas of the cranial and spinal radiation fields, the difference among the results was not more than 5%. Full comparison of the results for each part has been presented. The comparison the results of practical dosimetry and treatment planning software supports the validity of CorePlan treatment planning system. Also analysis of the absorbed dose through organs at risk showed that the absorbed dose by organs at risk have an acceptable value with respect to tolerance dose of these organs. The only unacceptable result was related to thyroid

Monte Carlo estimation of electron contamination in a 18 MV clinical photon beam

The electron contamination may reduce or even diminish the skin sparing property of the megavoltage beam. The detailed characteristics of contaminant electrons are presented for different field sizes and cases. The Monte Carlo code, MCNPX, has been used to simulate 18 MV photon beam from a Varian Linac-2300 accelerator. All dose measurements were carried out using a PTW-MP2 scanner with an ionization chamber [0.6 CC] at the water phantom. The maximum electron contaminant dose at the surface ranged from 6.1% for 5 x 5 cm[2]to 38.8% for 40 x 40 cm[2] and at the depth of maximum dose was 0.9% up to 5.77% for the 5 x 5 cm[2] to the 40 x 40 cm[2] field sizes, respectively. The additional contaminant electron dose at the surface for the field with tray increased 2.3% for 10 x 10 cm[2], 7.3% for 20 x 20 cm[2], and 21.4% for 40 x 40 cm[2] field size comparing to the standard field without any accessories. This increase for field with tray and shaping block was 5.3% and 13.3% for 10 x 10 and 20 x 20 cm[2], respectively, while, the electron contamination decreased for the fields with wedge, i.e. 2.2% for the 10 x 10 cm[2] field. The results have provided more comprehensive knowledge of the high-energy clinical beams and may be useful to develop the accurate treatment planning systems capable of taking the electron contamination in to account

[Dosimetry, control and irradiation of rat cervical spinal cord]

In radiobiology, the most important physical radiation quantity to predict the effect of irradiation of a biological specimen is the absorbed dose to the tissue of interest. In this project an irradiation set up was designed and verification of set up and dosimetry procedure was performed to deliver a precise X-ray dose to the small field of rat cervical spinal cord. AAPM-TG61 [American association and physician in medicine radiation therapy committee task group 61] protocol was used for dose measurement and we tailored a rat like phantom from polyethylene for dosimetry verification. The ionization chamber in this study was a farmer type and the X-ray generator was an orthovoltage Siemens machine working at 200 KVp potential. Dosimetry was done in air and phantom. An special jig was also built to fix the animals during irradiation which could help the treatments to be reproducible. Simulation and portal films were obtained to verify the irradiation field. The average value of dose rate in specified geometries by measurement was 146.54 cGy/min [SD=0.109]. While the dose determined by calculation was 95.145 cGy/min [SD=0.105], the comparison between these 2 methods shown a small discrepancy of 0.50% [P<0.001], which lies within the error limit of +/- 5.3% as mentioned by ICRU. Using protocol of AAPM TG-61 can provide an accurate dosimetry with minimum ambiguity. Application of appropriate correction factors and protocols can increase the accuracy and decrease the irradiation errors in radiobiological studies