Optimization of a dosimetric procedure in total body irradiation

The experimental dosimetry of 2 radiotherapy beams produced by a 60Co Picker unit and by a Siemens 4 MV unit, respectively, was analyzed to verify the use of tissue air ratio (TAR) and tissue maximum ratio (TMR) in the computerized planning of total body irradiation (TBI). The use of a small ionization chamber PRO5P Capintec in anthropometric phantoms allowed us to test a computed calculation procedure adopted to reduce both experimental uncertainties and time consumption. The experimental test on the computed procedure was also useful to identify the equivalent fields the patient's body had to be divided into for dosimetric planning. Such dosimetric specifications as average dose to the patient and degree of dose inhomogeneity are calculated when the thickness of compensator filters in perspex is optimized. Following the guidelines reported in ICRU 29, a dosimetric record is presented. In page 1 the target volume is described, in page 2 the provisional treatment planning, and in page 3 the actual treatment planning, checked with in vivo TLD measurements, and the dose specifications for TBI.     

Treatment technic and clinical dosimetry in whole-body irradiation

At the Istituto Nazionale Tumori, Milan, total-body irradiation (TBI) is delivered by a 15 MV linear accelerator, with two lateral opposed beams. Maximum build-up at the skin is achieved by lateral slabs of perspex 3 cm thick. Attenuation filters or bolus are used for dose compensation, or reduction, to the head and lungs. The dose delivered to clinically relevant anatomic regions is determined by "in vivo" dosimetry. For this purpose, calibrated diodes are employed, which are positioned at the entrance and at the exit of the beams. "In vivo" dosimetry data show our TBI technique to allow an homogeneous irradiation of all body areas, with maximum deviation of the mean dose value from reference point dose of -11% in the posterior abdomen, at the spinal cord shielded by arms.     

Forward Scatter Dose Effect at Metallic Interfaces Irradiated by X and Gamma Ray Therapy Beams

AIM: In this study forward scattering effects near different metallic interfaces are measured for Co-60 gamma and 6 and 18 MV photon beams. The studied effects are the transport of secondary electrons across the metallic interface and the scattering of photons by the metallic inhomogeneity. MATERIALS AND METHODS: All measurements were carried out with a PTW thin-window, parallel plate ionisation chamber (B 23344-036) and an RDM-1F electrometer with digital readout. Thin sheets of aluminium, mild steel, copper, cadmium and lead were used as inhomogeneities. The inhomogeneities were placed between the polystyrene phantom and the front window of the chamber which was maintained at 100 cm SSD. RESULTS: It was noticed that for a high energy photon beam (18 MV) the forward scatter dose factor (FSDF) increases rapidly as the thickness of the metallic inhomogeneity increases. For low energy photons, there is a sharp initial decrease of the FSDF until a minimum value is reached followed by a slow increase with increasing thickness of the inhomogeneity. It was also noted that the FSDF variation at off-axis distances has slightly more slope compared with the ionization ratio (IR) curves for both 6 MV and 18 MV photons. However, the variation in slope is prominent for 18 MV compared with 6 MV photon beam. CONCLUSION: The sharp dose decrease observed downstream of a metallic inhomogeneity at relatively low photon energies (Co-60, 6 MV) is attributed to the internal scattering of secondary electrons within the metal. The dose enhancement observed for high energy photon beams is attributed to the domination of the pair production process, increasing with atomic number. Since FSDF is dependent on the photon beam spectra, it can be used as a measure of beam quality across the beam.     

Clinical application of GAFCHROMIC EBT film for in vivo dose measurements of total body irradiation radiotherapy.

The GAFCHROMIC EBT film model is a fairly new film product designed for absorbed dose measurements of high-energy photon beams. In vivo dosimetry for total body irradiation (TBI) remains a challenging task due to the extended source-to-surface distance (SSD), low dose rates, and the use of beam spoilers. EBT film samples were used for dose measurements on an anthropomorphic phantom using a TBI setup. Additionally, in vivo measurements were obtained for two TBI patients. Phantom results verified the suitability of the EBT film for TBI treatment in terms of accuracy, reproducibility, and dose linearity. Doses measured were compared to conventional dosimeter measurements using thermoluminescent dosimeters (TLDs), resulting in an agreement of 4.1% and 6.7% for the phantom and patient measurements, respectively. Results obtained from the phantom and patients confirm that GAFCHROMIC EBT films are a suitable alternative to TLDs as an in vivo dosimeter in TBI radiotherapy.     

Investigation of dose homogeneity in paediatric anthropomorphic phantoms for a simple total body irradiation technique

The technique for treating total body irradiation patients used at the centre involves no compensation for the inhomogeneity of patient shape. Dose is prescribed to the lung, and monitor units are derived from standard data depending on the external dimensions of the patient at nipple level. Dose measurements were made during standard treatments on three paediatric anthropomorphic phantoms representing children of 5, 10 and 15 years of age. The results confirmed that the measured dose to the lung was within 4% of the prescribed dose, and dose homogeneity was within +/- 5%, excluding the neck, where the higher measured doses were still within tissue tolerance.     

Benefits of online in vivo dosimetry for single-fraction total body irradiation

Use of a patient test dose before single-fraction total body irradiation (TBI) allows review of in vivo dosimetry and modification of the main treatment setup. However, use of computed tomography (CT) planning and online in vivo dosimetry may reduce the need for this additional step. Patients were treated using a supine CT-planned extended source-to-surface distance (SSD) technique with lead compensators and bolus. In vivo dosimetry was performed using thermoluminescent dosimeters (TLDs) and diodes at 10 representative anatomical locations, for both a 0.1-Gy test dose and the treatment dose. In total, 28 patients were treated between April 2007 and July 2013, with changes made in 10 cases (36%) following test dose results. Overall, 98.1% of measured in vivo treatment doses were within 10% of the prescribed dose, compared with 97.0% of test dose readings. Changes made following the test dose could have been applied during the single-fraction treatment itself, assuming that the dose was delivered in subportions and online in vivo dosimetry was available for all clinically important anatomical sites. This alleviates the need for a test dose, saving considerable time and resources.     

Polarity effects of ionization chambers used in tbi dosimetry due to cable irradiation.

This paper presents the results of an investigation on polarity effects in total-body irradiation (TBI) dosimetry. Thimble (NE2571, 0.6 cc) and plane-parallel (Markus NE2534 0.055 cc) chambers were investigated in a 30 x 30 x 30-cm3 acrylic phantom in TBI conditions (6-MV x-rays). The thimble chamber was positioned at the midline and at the entrance and exit Dmax (1.5 cm from the phantom surface) levels. The Markus chamber, which is generally used for skin dose estimations, was positioned at various depths from the entrance surface of the phantom (from 0- to 2-cm depth). The polarity factor (Ppol) was defined as (Q+ + Q-)/2Q-, where Q+ and Q- were the collected charges at positive and negative bias voltage, respectively. The variations of Ppol with many parameters (absorbed dose, dose rate, the presence or absence of a 1-cm acrylic spoiler, irradiated cable length) were investigated. Results show that Ppol is quite small (within 1.002 for on-axis measurements and 1.005 for off-axis measurements) for the NE2571 chamber when the beam spoiler is placed. Ppol was significantly higher without the beam spoiler (within 1.008 for on-axis measurements, up to 1.02 for off-axis measurements). Concerning the Markus chamber, for on-axis skin dose measurements, Ppol was found to be less than unity (around 0.988) or more than unity (around 1.0035), respectively, with and without the beam spoiler. Possible "directional effects" of the currents generated in the cable were investigated for both chambers and found to be insignificant. This shows that the application of Ppol correction has to be considered a reliable procedure in minimizing these effects. When the beam spoiler is placed, the cable has to be drawn to minimize the portion of cable just outside the beam; if this is not the case, Ppol may significantly vary (for the NE2571 chamber values up to 1.0035 were found for on-axis measurements).     

In vivo dosimetry. Assessment of exit dose correction factors]

INTRODUCTION: In vivo dosimetry allows to verify dose delivering accuracy in radiotherapy treatments. Exit dose measurements add more information about delivered dose than entrance dose evaluations. MATERIALS AND METHODS: Commercial semiconductor diodes are used for exit dose measurements. The diodes are calibrated by comparison with an ionization chamber at a reference condition. Diode reading was compared with the dose measured by the ionization chamber at the exit point. The exit point is defined as the point on the central axis of the beam, at a distance equal to the maximum dose from the exit surface of a homogeneous water-like phantom. As clinical irradiation conditions are always different from reference conditions, exit dose correction factors have been investigated as a function of phantom thickness, field size at the isocenter, source-surface distance, wedge and tray. Measurements have been performed by irradiating a set of p-type semiconductor detectors with 6 MV photon beam (four diodes--mod. EDP10--Scanditronix) and 18 MV photon beam (three diodes--mod. EDP20--Scanditronix) from a Clinac 1800 linear accelerator (Varian, Palo Alto, CA, USA). RESULTS: The most relevant exit dose correction factors are related to field size and phantom thickness for 6 MV photons. The variation of these factors as a function of field size may be greater than 1% with a standard deviation of the same order. On the contrary, the correction factors for field, thickness and tray photons are negligible for 18 MV. CONCLUSIONS: Applying exit dose correction factors may require a great effort, particularly when many silicon diodes must be used. The actual effectiveness of each calibration factor is evaluated through the statistical analysis of experimental data. In this way, the usefulness of correction factor calculation, as depending from both experimental conditions and diode responses, can be derived from its effects on the exit dose value.     

Measured inhomogeneity correction factors for lung in electron beam treatments of the chest wall

A set of clinically relevant measurements of percentage depth dose and inhomogeneity correction factors for electron beam irradiation of the chest wall and underlying lung is given. Electron beam nominal energies of 9, 12, 15, and 18 MeV and lung densities of 0.22 g/cm3 and 0.404 g/cm3 are considered. This data can serve in treatment planning to indicate the penetration of the beam into the lung and serve as a comparison for calculation algorithms which are used to calculate electron dose absorption in heterogeneous phantoms.     

Investigation of irradiation conditions for recurrent breast cancer in JRR-4

Clinical trials of boron neutron capture therapy (BNCT) for recurrent breast cancers are considered at Japan Research Reactor No. 4 (JRR-4). In this study, the irradiation technique for a total mastectomy patient with recurrent cancer was optimized by dosimetric calculations using JAEA computational dosimetry system (JCDS). The evaluation was performed using an en face technique and a tangents technique with thermal neutron beam mode at JRR-4. The results revealed that equivalent doses of lung, heart, liver and skin were similar in each irradiation technique due to the isotropic scattering of thermal neutrons in the body. On the other hand, the irradiation time with the tangents technique was a few times longer than with the en face technique. We concluded that the en face technique was an optimal irradiation technique for recurrent breast cancers using thermal neutron beam mode in terms of shorter irradiation time and easier patient positioning.     

Total skin electron irradiation: evaluation of dose uniformity throughout the skin surface.

In this study, in vivo dosimetic data of 67 total skin electron irradiation (TSEI) treatments were analyzed. Thermoluminescent dosimetry (TLD) measurements were made at 10 different body points for every patient. The results demonstrated that the dose inhomogeneity throughout the skin surface is around 15%. The homogeneity was better at the trunk than at the extratrunk points, and was worse when a degrader was used. There was minimal improvement of homogeneity in subsequent days of treatment.     

In vivo ESR dosimetry in total body irradiation

Total body irradiation (TBI) using high doses (about 10 Gy) with photons in the range between 1 and 10 MV combined with intensive chemotherapy has been used successfully in treatment of acute and chronic leukemia before bone marrow transplantation. One of the principal international guidelines in TBI is to use in vivo dosimetry in order to compare the prescribed dose with that absorbed. The use of in vivo dosimetry is also useful as a retrospective evaluation of any deviation from the prescribed dose greater than +/- 5% for relevant parts of the body, especially in the lung and in other organs at risk. In this paper, Electron Spin Resonance (ESR), using alanine dosimeters, is demonstrated to be a powerful tool for absorbed dose evaluation in TBI by detection of free radicals produced in alanine by ionizing radiation. In this study, we present the results obtained using ESR dosimetry in eleven patients undergoing TBI. The major advantages appear to be: 1. the ESR signal in alanine dosimetry is stable for years without fading: 2. the detection of the ESR signal does not destroy the information and so enables a retrospective judgment of the TBI plan adopted.     

Dosimetric errors during treatment of centrally located lung tumors with stereotactic body radiation therapy: Monte Carlo evaluation of tissue inhomogeneity corrections

Early experience with stereotactic body radiation therapy (SBRT) of centrally located lung tumors indicated increased rate of high-grade toxicity in the lungs. These clinical results were based on treatment plans that were computed using pencil beam–like algorithms and without tissue inhomogeneity corrections. In this study, we evaluated the dosimetric errors in plans with and without inhomogeneity corrections and with planning target volumes (PTVs) that were within the zone of the proximal bronchial tree (BT). For 10 patients, the PTV, lungs, and sections of the BT either inside or within 2 cm of the PTV were delineated. Two treatment plans were generated for each patient using the following dose-calculation methods: (1) pencil beam (PB) algorithm without inhomogeneity correction (IC) (PB ? IC) and (2) PB with inhomogeneity correction (PB + IC). Both plans had identical beam geometry but different beam segment shapes and monitor units (MU) to achieve similar conformal dose coverage of PTV. To obtain the baseline dose distributions, each plan was recalculated using a Monte Carlo (MC) algorithm by keeping MUs the same in the respective plans. The median maximum dose to the proximal BT and PTV dose coverage in the PB + IC plans were overestimated by 8% and 11%, respectively. However, the median maximum dose to the proximal BT and PTV dose coverage in PB ? IC plans were underestimated by 15% and 9%. Similar trends were observed in low-dose regions of the lung within the irradiated volume. Our study indicates that dosimetric bias introduced by unit tissue density plans cannot be characterized as underestimation or overestimation of dose without taking the tumor location into account. This issue should be considered when analyzing clinical toxicity data from early lung SBRT trials that utilized unit tissue density for dose calculations.     

半身照射条件下以微核估计全身等效剂量可能性的探讨

作者探讨了半身照射条件下以微核（ＭＮ）率估计相当于全身一次均匀照射剂量的可能性，并与人体模型以相同条件照射后的剂量计算结果及临床反应相验证，结果显示：以ＭＮ率所估算的剂量与人体模型所计算出的相当于一次全身均匀照射的红骨髓干细胞活存计权剂量及临床反应基本一致，照后无或仅有白细胞、血小板计数的轻微下降，多数有恶心呕吐，可能与全腹照射有关。因此，在以下半身为主的高度不均匀照射条件下，ＭＮ检测所估计的生物剂量可用以表示全身等效剂量及反映全身损伤程度。     

Lung compensator design using an electronic portal imaging device.

Using a liquid filled electronic portal imaging device (EPID) installed on a linear accelerator and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the pixel values of the imaging detector and the corresponding equivalent thickness of the overlying phantom material. Results for 6 and 10 MV photons show that the relationship depends on the so-called input/output characteristics of the imaging device for a particular photon energy. For a chest irradiation, an EPID image obtained under treatment geometry provides the pixel value information that is used to calculate the tissue deficit over the lung region. The compensators are made of lead whose thickness is calculated from the established empirical relationship to replace the tissue deficit over lungs. The effectiveness of the method is demonstrated with thermoluminescent dosimetry (TLD) for 6 and 10 MV beams. With compensators in place, the dose uniformity was found to be within +/- 5%.     

Cone-beam computed tomography-derived adaptive radiotherapy

The purpose of this study was to evaluate the reliability of cone-beam computed tomography (CBCT)-derived adaptive radiotherapy. We evaluate planning computed tomography (pCT) and CBCT in 50 patients who had undergone image guided radiotherapy (IGRT) with CBCT. Irradiated sites included head, neck, chest, abdomen, and pelvis; there were 10 patients in each group. Treatment plans including 153 beam data were recalculated based on CBCT. To compare between pCT and CBCT, we estimated CT values of normal tissues, body contour, effective depth, and monitor units (MU) calculation. The maximum difference in CT values was observed in lung estimation. The 5 mm or more differences in depth were observed in 2 beams of 2 pelvic cases, but CBCT also demonstrated a shift of abdominal wall due to intestinal motility. There were downward trends for the effective depth and MU based on CBCT, especially in lung cases. However, the differences in prescribed dose due to MU calculation were less than 5% because all patients were treated with a multifield irradiation plan. CBCT provides not only precise daily setup but also accurate anatomical information on body contour. In addition, CBCT may be considered as a useful tool for dose calculation.     

Study of a radiation technic with an anterior mantle-shaped field and 10 MV photons

The authors present a preliminary dosimetry study of 10 MV X-rays for irradiation technique with only one anterior mantle-field. The results of the studied beam for SSD = 120 cm are: 40 X 40 cm2 fields, build-up at 2.5 cm, penumbra (85% divided by 15%) variable between 8 divided by 9 mm, 50% of the dose at depth of 18 cm, homogeneity +/- 4%. These dosimetry results are suitable for beginning the clinical research.     

Study on the dose distribution of 8-MeV bremsstrahlung in mantle field techniques (author's transl)]

The dose distribution within the patient was studied with 8-MeV bremsstrahlung from a linear accelerator during mantle field irradiation using molded shielding blocks. Doses and dose distributions in the different layers of a modified Alderson phantom were measured by means of film dosimetry and related to the dose in the central ray beam at the middle of the body. Dose distribution within unshielded regions perpendicular to the central ray beam generally being relatively homogeneous, the highest relative doses, amounting to ca. 115%, are found in the region of the mandibular angle and in the supraclavicular region; the dose to superficial lymph nodes at the supraclavicular region reaches 100% of the dose in the central ray beam. As a cause for these important doses near the surface of the body are discussed the extension of mantle fields as well as the increased exit dose of the opposed field and the oblique incidence of radiation.