Dosimetric characterization and optimization of a customized Stanford total skin electron irradiation (TSEI) technique

Evaluation of Dose Distribution in Optimized Stanford Total Skin Electron Therapy (TSET) Technique in Rando Anthropomorphic Phantom using EBT3 Gafchromatic Films

Background:

The Total Skin Electron Therapy (TSET) targets the whole of skin using 6 to 10 MeV electrons in large field size and large Source to Surface Distance (SSD). Treatment in sleeping position leads to a better distribution of dose and patient comfort.

Objective:

This study aims to investigate the uniformity of absorbed dose in the sleeping Stanford technique on the Rando phantom using dosimetry.

Material and Methods:

It is an experimental study which was performed using 6 MeV electron irradiation produced by Varian accelerator in the AP and PA positions with gantry angles of 318/3, 0 and 41/5 degrees, and RAO, LAO, RPO and LPO with 291/4 gantry angle and 45 degrees of collimator angle in the sleeping position.

Results:

The results show that the dose uniformity achieved in this technique is in the range of (100 ± 25%) and, the dose accuracy was 6%.

Conclusion:

Total Skin Electron Therapy (TSET) technique in sleeping position is very suitable for elderly and disabled patients, and meets the required dose uniformity. Furthermore, the use of a flattening filter is recommended for the more dose distribution uniformity.

Fabrication of anthropomorphic phantoms for use in total body irradiations studies

Purpose:

The aim of this study was to produce a low-cost anatomical model of adult male including lower limbs to evaluate the three-dimensional dose distribution for dosimetry measurements, especially in total body irradiation (TBI) and total skin electron therapy (TSET).

Materials and methods:

Computed tomography (CT) scan images of the atomic energy organisation RANDO phantom and lower limb CT scan images of 20 healthy persons were averaged. Selections of different body tissues substitute materials and phantom validation were performed according to previous studies worked on construction of radiation therapy phantoms.

Results:

The dosimetry aspect of the selected substitute materials from all considered methods showed that they were in good agreement with real human tissue, especially bone, with a percentage error of 0·5%. The results show that the electron densities obtained from the linear attenuation coefficient (reDLAC) for the tissue equivalent material used in the phantom is a better option for validation.

Conclusions:

This validated phantom has numerous advantages over the origin type of RANDO phantom. Therefore, using it in TBI and TSET dosimetry is recommendable.

Total skin electron beam therapy rationalization and utility of in vivo dosimetry in a high-volume centre

OBJECTIVE

This paper reports on the rationalization of a substantial pool of in vivo dosimetry (IVD) data from patients treated with total skin electron beam therapy (TSEBT) and the application of this to verify the accurate delivery of TSEBT when changing linac manufacturer.

METHODS

Thermoluminescent dosimeter IVD data from 149 patients were analyzed comparing the population mean and standard deviation for each site. The number of sites required to confirm the prescribed dose were reviewed considering both dosimetric and clinical relevance. The reduced sites were then used to assess the continued dosimetric accuracy on new equipment and the results were compared statistically using the Mann-Witney test.

RESULTS

The trunk dose measurement points were reduced from nine to six and five extra trunk sites were identified and reviewed clinically prior to removal.Following change in manufacturer the trunk dose points showed no statistically significant change and confirmed that patients had received within 1.3% of the intended mean trunk dose using both delivery methods.A statistically significant change in 4 out of the 13 extra trunk sites was seen following the move to the new centre. However, all but one site showed a change of less than 1 standard deviation.

CONCLUSION

The total number of measurement points per patient were reduced from 27 to 19 which constituted a 25% saving in preparation and read out.Accurate delivery of prescribed dose was confirmed following measurement point reduction for treatments delivered on linacs from two different manufacturers.

ADVANCES IN KNOWLEDGE

Proven methodology for rationalization of IVD measurements for TSEBT.

Three dimensional printed electron beam modifier for total skin electron treatments

Total Skin Electron Beam (TSEB) treatment, despite its proven effectiveness in skin malignancies, is a rather exhausting irradiation method, especially for feeble patients. In an effort to reduce treatment time by creating a clinically acceptable single TSEB field, various beam modifiers of different materials and shapes were tested. Using the TSEB immobilization device of our department and 3D printing technology, aluminum and thermoplastic modifiers were designed and constructed, according to the resulting profiles at treatment distance. Electron beam characteristics were measured and calculated both at SSD = 100 cm and at treatment level. Aluminum scatterers of the same thickness caused different modification according to the area of blocking. Aluminum modifiers reduced significantly central dose deposition for the same amount of MUs and therefore they expanded treatment time in undesirable levels. Plastic modifiers offer a good combination of field dimensions and treatment time. The final 3D printed modifier shaped the electron beam as desired resulting to a clinically acceptable 6 MeV field of 176 × 70 cm field with 10% inhomogeneity in vertical and 3% in the lateral dimension with adequate skin coverage at SSD = 400 cm. This modification offered approximately a two-minute treatment time reduction compared to the current technique. Underdosed areas appear near the edge of the field, but in regions that are far from the torso of the patient. Bremsstrahlung radiation was kept at clinically accepted levels (< 5%). This modification of the original six dual-field technique of our hospital could probably benefit fragile patients who could not easily tolerate a twenty-minute standing position without compromising the quality of their treatment.

Moving gantry method for electron beam dose profile measurement at extended source-to-surface distances

A novel method has been put forward for very large electron beam profile measurement. With this method, absorbed dose profiles can be measured at any depth in a solid phantom for total skin electron therapy. Electron beam dose profiles were collected with two different methods. Profile measurements were performed at 0.2 and 1.2 cm depths with a parallel plate and a thimble chamber, respectively. 108 cm×108 cm and 45 cm×45 cm projected size electron beams were scanned by vertically moving phantom and detector at 300 cm source‐to‐surface distance with 90° and 270° gantry angles. The profiles collected this way were used as reference. Afterwards, the phantom was fixed on the central axis and the gantry was rotated with certain angular steps. After applying correction for the different source‐to‐detector distances and incidence of angle, the profiles measured in the two different setups were compared. Correction formalism has been developed. The agreement between the cross profiles taken at the depth of maximum dose with the ‘classical’ scanning and with the new moving gantry method was better than 0.5 % in the measuring range from zero to 71.9 cm. Inverse square and attenuation corrections had to be applied. The profiles measured with the parallel plate chamber agree better than 1%, except for the penumbra region, where the maximum difference is 1.5%. With the moving gantry method, very large electron field profiles can be measured at any depth in a solid phantom with high accuracy and reproducibility and with much less time per step. No special instrumentation is needed. The method can be used for commissioning of very large electron beams for computer‐assisted treatment planning, for designing beam modifiers to improve dose uniformity, and for verification of computed dose profiles.

PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.jf

Total skin electron beam therapy using an inclinable couch on motorized table and a compensating filter

Total skin electron beam is a specialized technique that involves irradiating the entire skin from the skin surface to only a few millimetres in depth. In the Stanford technique, the patient is in a standing position and six different directional positions are used during treatment. Our technique uses large electron beams in six directions with an inclinable couch on motorized table and a compensating filter was also used to spread the electron beam and move its intensity peak. Dose uniformity measurements were performed using Gafchromic films which indicated that the surface dose was 2.04 ± 0.05 Gy. This technique can ensure the dose reproducibility because the patient is fixed in place using an inclinable couch on a motorized table.