Increase In Maximum Dose Research Articles

A Monte Carlo study on electron and neutron contamination caused by the presence of hip prosthesis in photon mode of a 15 MV Siemens PRIMUS linac

Several investigators have pointed out that electron and neutron contamination from high‐energy photon beams are clinically important. The aim of this study is to assess electron and neutron contamination production by various prostheses in a high‐energy photon beam of a medical linac. A 15 MV Siemens PRIMUS linac was simulated by MCNPX Monte Carlo (MC) code and the results of percentage depth dose (PDD) and dose profile values were compared with the measured data. Electron and neutron contaminations were calculated on the beam's central axis for Co‐Cr‐Mo, stainless steel, Ti‐alloy, and Ti hip prostheses through MC simulations. Dose increase factor (DIF) was calculated as the ratio of electron (neutron) dose at a point for 10×10 cm2 field size in presence of prosthesis to that at the same point in absence of prosthesis. DIF was estimated at different depths in a water phantom. Our MC‐calculated PDD and dose profile data are in good agreement with the corresponding measured values. Maximum dose increase factor for electron contamination for Co‐Cr‐Mo, stainless steel, Ti‐alloy, and Ti prostheses were equal to 1.18, 1.16, 1.16, and 1.14, respectively. The corresponding values for neutron contamination were respectively equal to: 184.55, 137.33, 40.66, and 43.17. Titanium‐based prostheses are recommended for the orthopedic practice of hip junction replacement. When treatment planning for a patient with hip prosthesis is performed for a high‐energy photon beam, attempt should be made to ensure that the prosthesis is not exposed to primary photons.PACS numbers: 87.56.bd, 87.55.kh, 87.55.Gh

Assessment of the robustness of volumetric-modulated arc therapy for lung radiotherapy.

Volumetric-modulated arc therapy (VMAT) is increasingly popular as a treatment method in radiotherapy owing to the speed with which treatments can be delivered. However, there has been little investigation into the effect of increased modulation in lung plans with regard to interfraction organ motion. This is most likely to occur where the planning target volume (PTV) lies within areas of low density. This paper aims to investigate the effect of modulation on the dose distribution using simulated patient movement and to propose a method that is less susceptible to such movement. Simulated interfraction motion is achieved by moving the plan isocentre in steps of 0.5 cm and 1.0 cm in six directions for five clinical VMAT patients. The proposed planning method involves optimisation using a density override of 1 g cm(-3), within the PTV in lung, to reduce segment boosting in the periphery of the PTV. This investigation shows that modulation can result in an increase in the maximum dose of >25%, an increase in PTV near-maximum dose of 17% and a reduction in near-minimum dose by 46%. Unacceptable organ at risk (OAR) doses are also seen. The proposed method reduces modulation, resulting in a maximum dose increase of 10%. Although safeguards are in place to prevent the increased dose to OARs from patient movement, there is nothing to prevent the increased dose as a result of modulation in lung. A simple planning method is proposed to safeguard against this effect. Investigation suggests that, where modulation exists in a plan, this method reduces it and is clinically viable.

SU‐E‐T‐451: Is Rotational IMRT More Susceptible to Delivery Uncertainties than Dynamic IMRT?

Purpose: To investigate whether rotational IMRT is susceptible to delivery uncertainties. Methods: Dosimetric effects of variation in dose rate, gantry position and MLC leaf positions were evaluated by incorporating their corresponding uncertainties into recalculated RapidArc treatment plans for four head‐and‐neck (HN) and four prostate test cases via modifications in gantry angles or MLC leaf positions. Total monitor units per aperture and therefore per plan were held fixed. The dose distributions and dose‐value histograms of the original and modified plans were compared using 3D Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results reported below used 2%–2 mm Gamma test pass rate criterion. Results: For a 50% normally distributed random variation in dose rate (sigma = 1.3° in gantry angle), all HN plans had a 99.7% Gamma test pass rate. For HN PTVs, an increase in dose rate variation resulted in a decrease in minimum dose and an increase in maximum dose; for nearly all OARs, no more than a 0.4% dose difference was observed. For HN plans, a 5° rotational shift resulted in a 96.2% average pass rate. Dose distributions were sensitive to systematic variations especially above 1 mm; HN plans modified with a −2 mm shift in the X1 bank resulted in a 97.3% pass rate, and PTVs and OARs received a 3.8% higher dose on average. Uniformly distributed random shifts of +/−2 mm in aperture MLC leaf positions showed no significant change in mean dose (<0.5%) and resulted in a 100% pass rate Conclusions: Rotational IMRT deliveries were found to be very tolerant to random errors in dose rate, gantry position and MLC leaf position. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.

Effect of Breathing Motion on Dose Accumulation in the Lung Bronchial Tree

To improve the accuracy of the calculated dose to the bronchial tree during stereotactic body radiation therapy (SBRT) for non–small-cell lung cancer (NSCLC), to better understand the relationship between radiation dose and bronchial complications and to aid in SBRT eligibility decisions. Ten NSCLC patients treated under a 60 Gy in 3 fractions or 48 Gy in 4 fractions SBRT protocol were retrospectively evaluated. Inhale and exhale reconstructions from 4D-CT images at treatment planning were obtained. The external contour, lungs, and tumor were contoured in both images for each patient. MORFEUS, a finite element model-based deformable registration algorithm with the capability to model the sliding interface between the lungs and the rib cage was used to model the breathing motion. The model was expanded to include the bronchial tree, which was segmented from the 4D-CT using a threshold technique. Triangular elements with a thickness of 0.1 cm were used to model the bronchial trees, while tetrahedral elements were used to model the lungs. Radiation dose was calculated in a commercial treatment planning system on 4D-CT inhale and exhale images and accumulated over the breathing cycle using MORFEUS. Dose results for the breathing model were compared to standard treatment planning (exhale images). Six patients showed a maximum dose deviation larger than 30 Gy to an individual surface element within the bronchial tree. The average maximum change to a surface element over all patients was 32 Gy (range, 9–61 Gy), with an average maximum dose reduction of 30 Gy (range, 6–61 Gy) and an average maximum dose increase of 21 Gy (range, 5–40 Gy). Four patients showed changes in dose larger than 5 Gy to more than 10% of the bronchial tree (of treated lung) when breathing motion and deformation are considered. These same 4 patients showed changes in dose larger than 10 Gy to more than 3.5% of total treated lung bronchial tree. One patient showed a change in dose larger than 20 Gy to 4% of the bronchial tree within the treated lung. The change in overall maximum dose to a 2 cc volume in the bronchial tree exceeded 2 Gy. The change in mean lung dose (range, 0–0.05 Gy) and lung V20 (range, 0.05–1.4%) was insignificant. Results indicate that large changes in dose to the bronchial tree can occur due to breathing during SBRT treatment compared to standard (static) treatment planning. Furthermore, large changes in local maxima within the bronchial tree can occur without similarly large changes in mean lung dose or V20, indicating that the structures should be separately considered since maximum dose to the bronchial tree is predictive of late complications. Biomechanical modeling may help to develop a more quantitative tool for determining eligibility for SBRT treatment and improve understanding of late bronchial complications.

Comparison of conventional inserts and an add-on electron MLC for chest wall irradiation of left-sided breast cancer

Background. Collimation of irregularly shaped clinical electron beams is currently based on electron inserts made of low melting point alloys. The present investigation compares a conventional electron applicator with insert and add-on eMLC-based dose distributions in the postoperative chest wall irradiation of left-sided breast cancer. Material and methods. Voxel Monte Carlo++ (VMC++) calculated dose distributions related to electron fields were compared with 10 left-sided breast cancer patients after radical mastectomy. The prescription dose was 50 Gy at a build-up maximum. The same dose was prescribed for the ipsilateral axillary, parasternal and supraclavicular lymph nodes that were treated with photons and calculated with a pencil beam algorithm. The insert beams were shaped with 1.5 cm thick Wood's metal electron inserts in an electron applicator of a Varian 2100 C/D linac. Doses for the eMLC-shaped beams were calculated for an eMLC prototype with 2 cm thick and 5 mm wide steel leaves. The same collimator-to-surface distance (CSD) of 5.8 cm was used for both collimators. Results. The mean PTV dose was slightly higher for the eMLC plans (50.7 vs 49.5 Gy, p<0.001, respectively). The maximum doses assessed by D5% for the eMLC and insert were 60.9 and 59.1 Gy (p<0.001). The difference was due to the slightly higher doses near the field edges for the eMLC. The left lung V20 volumes were 34.5% and 34.0% (p<0.001). There was only a marginal difference in heart doses. Discussion: Despite a slight increase of maximum dose in PTV the add-on electron MLC for chest wall irradiation results in practically no differences in dose distributions compared with the present insert-based collimation.

Target Coverage in Head and Neck Cancer Treated with Intensity-Modulated Radiotherapy: A Comparison between Conventional and Conformal Techniques

We designed a comparative planning study aimed at quantifying the advantages of intensity-modulated radiotherapy (IMRT) over the conventional 3-field technique (3FT) and a 5-field conformal technique (5FCT) for head and neck (HN) cancer. We selected 9 patients treated at our institution with curative radiotherapy for a HN cancer. For all cases 4 plans were generated: 2 plans using the "standard" techniques (3FT and 5FCT), a third plan using IMRT, and a fourth "mixed" plan using IMRT followed by a conventional boost. Our study confirmed literature data on the ability of IMRT to significantly decrease the dose received by organs at risk, compared with previous techniques. Target coverage was systematically better with 5FCT and IMRT than with 3FT. However, the increase in coverage of both PTV2 and PTV1 was only about 3-5% and this was achieved at the price of a similar increase in maximum dose (D1%). Volumetric parameters (V100%, V95%) were much more sensitive in detecting the improvement with IMRT. The improvement of target coverage attained by IMRT, as compared with conventional and conformal techniques, might be overestimated by data currently available in the medical literature. If treatment with conventional techniques is planned using all tools provided by currently available fully 3-D planning systems, excellent target coverage can be obtained.

Adverse effect of a distended rectum in intensity-modulated radiotherapy (IMRT) treatment planning of prostate cancer

Background and purposeThe retrospective planning study for intensity-modulated radiotherapy (IMRT) of prostate cancer evaluated whether proximal rectum and supra-anal rectum/anal canal should be delineated as separated organs-at-risk (OARs) to achieve optimal dose distributions to the anorectal region. Patients and methodsFor 10 patients with localized prostate cancer IMRT plans were generated with the rectum and anal canal as separated OARs (Rec-sep) and as one single OAR (Rec-tot). Two different treatment planning systems (TPS) were utilized. Influence on dose distributions to target and OARs was analyzed. ResultsResults from both TPS showed significantly increased doses to the distal rectum/anal canal for plans Rec-tot compared with Rec-sep in case of a distended rectum in the planning CT study: doses were increased by up to mean 31% (P=0.02) and 18% (P=0.03), respectively, in both TPS. For the patient with the largest rectum, the maximum dose increase was 61%. No significant differences in doses to target, bladder, femoral head and proximal rectum were seen. ConclusionsFor patients with a distended rectum in the planning CT, delineation of separated OARs for proximal rectum and distal rectum/anal canal resulted in superior dose distributions to the anorectal region and therefore, we recommend this as standard procedure for IMRT planning of prostate cancer.

Exposure variations under error conditions in automatic exposure controlled film–screen projection radiography

Improper automatic exposure control (AEC) termination may result in high overexposures on some radiographic systems. Under AEC, X-ray factors are adjusted automatically to compensate for differences in patient thickness and density. In radiography, AEC is implemented using ionization chambers placed in the film bucky. In this study we deliberately chose incorrect set-up conditions and assessed the response of the AEC system. Two types of incorrect set-up were studied: (1) incorrect selection of bucky radiation detector and (2) simulated misalignment between the X-ray field and light field. The systems tested varied in age from 1 year to in excess of 10 years. In the first test, overexposures of 90 mGy were recorded. Two systems did not meet EC guidelines for improper AEC termination. The second test, misalignment of the X-ray field, was observed to affect the exposure delivered by approximately +/-22%. The maximum dose increase observed, with a chest phantom in the beam, was 165 microGy. Misalignments also resulted in reduced exposures, which may impact on image quality.

Effect of tissue inhomogeneity on dose distribution of continuous activity of low‐energy electrons in bone marrow cavities with different topologies

Monte Carlo calculations have previously been performed by Eckerman to evaluate the absorbed fractions of continuous sources of monoenergetic electrons in marrow cavities of human bone. The difference in scattering power of electrons in cortical bone (CB) and the red marrow (RM) was neglected. In the present work the Integrated Tiger Series and Electron-Gamma-Shower Monte Carlo codes were used to investigate the effect of topology of the bone and bone marrow interface on backscatter dose increase to the marrow. Planar, cylindrical, and spherical geometries were included. For the planar geometry, a maximum dose increase of 9 +/- 1 (S.E. of the mean) % was obtained in the region within 12 mg/cm2 from the interface due to a semi-infinite source of electrons with energy greater than 0.5 MeV. An increase of 7 +/- 1% was observed experimentally in the same region due to a semi-infinite source of 32P. This was in good agreement with Monte Carlo calculation. Averaged over the region of RM embedding electron sources between two planar CB/RM interfaces 1000 microns apart, a dose enhancement of 10 +/- 2% was predicted for electron energies from 1 to 1.75 MeV. For the cylindrical interface with 500-microns radius of curvature, the maximum dose increase averaged over the whole cylinder due to an isotropic distribution of monoenergetic electrons inside the cylinder was 12 +/- 1%.(ABSTRACT TRUNCATED AT 250 WORDS)