Skin dose measurements using radiochromic films, TLDS and ionisation chamber and comparison with Monte Carlo simulation

Graphite foils as potential skin and epithelium dosimeters at therapeutic photon energies

This work builds upon a prior study, examining the dosimetric utility of pencil lead and thin graphitic sheets, focusing upon the measurement of skin doses within the mammographic regime. In recognizing the near soft-tissue equivalence of graphite and the earlier-observed favourable thermoluminescence yield of thin sheets of graphite, this has led to present study of 50 μm thick graphite for parameters typical of external beam fractionated radiotherapy and skin dose evaluations. The graphite layers were annealed and then stacked to form an assembly of 0.5 mm nominal thickness. Using a 6 MV photon beam and delivering doses from 2- to 60 Gy, irradiations were conducted, the assembly first forming a superficial layer to a solid water phantom and subsequently underlying a 1.5 cm bolus, seeking to circumvent the build-up to electronic equilibrium for skin treatments. Investigations were made of several dosimetric properties arising from the thermoluminescence yield of the 50 μm thick graphite slabs, in particular proportionality and sensitivity to dose. The results show excellent sensitivity within the dose range of interest, the thermoluminescence response varying with increasing depth through the stacked graphite layers, obtaining a coefficient of determination of 90%. Acknowledging there to be considerable challenge in accurately matching skin thickness with dose, the graphite sheets have nevertheless shown considerable promise as dosimeters of skin, sensitive in determination of dose from the surface of the graphite through to sub-dermal depth thicknesses.

Carbon rich media for luminescence-based surface dosimetry and study of associated surface defects

The present study continues research into the utilisation of carbonaceous media for medical radiation dosimetry, focusing on the effects of surface area-to-volume ratio and carbon content on structural interaction alterations and dosimetric properties in sheet- and bead-type graphitic materials (with the respective carbon content of ∼98 wt% and ∼90 wt%). Using ⁶⁰Co gamma-rays and doses from 0.5 Gy to 20 Gy, the study has been made of the response of commercially available graphite in the form of 0.1 mm, 0.2 mm, 0.3 mm and 0.5 mm thick sheets, also of activated carbon beads. Confocal Raman and photoluminescence spectroscopy have been employed, examining radiation-induced structural interaction alterations. Dose-dependent variation in the Raman intensity ratio I_D/I_G relates to the varying dominance of defect generation and dose-driven defect annealing. Of the various thickness graphite sheets, the 0.1 mm thick medium possesses the greatest surface area-to-volume ratio. Perhaps unsurprisingly, it also exhibits the greatest thermoluminescence (TL) yield compared to that of the other carbonaceous sheet foils used herein. Moreover, the second greatest mass-normalised TL yield has been observed to be that of the porous beads, reflected in the greater defect density (I_D/I_G > 2) when compared to the other media, due in part to their inherent feature of large internal surface area. Considering the challenge posed in matching skin thickness with skin dose, the near tissue equivalent graphite sheets show particular promise as a skin dosimeter, sensitive as a function of depth.

Experimental determination of breast skin dose using volumetric modulated arc therapy and field-in-field treatment techniques

Introduction:

The use of volumetric modulated arc therapy (VMAT) on the breast has several dosimetric advantages but its impact on skin dose should be evaluated and compared to well-established treatment techniques using tangential fields. The aim of this work is to contrast the skin dose for VMAT and field-in-field (FIF) and to estimate the magnitude of the skin dose involved.

Method:

The skin dose was measured, without build-up, using thermoluminescent dosimeter (TLD) and optically stimulated luminescence dosimeter (OSLD) in breast radiotherapy by an in-house anthropomorphic phantom. Two different treatment techniques were used: FIF and VMAT, based on the planning strategy proposed by Nicolini et al. The dose levels were 4300 cGy, 4600 cGy and 5600 cGy in 20 fractions. In vivo dosimetry with TLD for VMAT was performed for different breast sizes in the same locations as phantom measurements.

Results:

The ipsilateral phantom breast skin dose using both treatment techniques was equivalent. TLD measured doses by the VMAT technique were up to 5% higher than OSLD, although they agree if we consider the geometry uncertainty of the TLD. In accordance with in vivo dosimetry, the mean dose of the ipsilateral breast skin was 62 ± 6% (51%, 75%) relative to the prescribed dose, regardless of the breast size for the volumes considered with this small population (n = 9) as shown by Mann–Whitney U-test (Z = 1·9, 95% confidence). The uncertainty expected in this region due to geometry (volume) changes is up to 9% higher for volumes from 225·9 cc to 968·8 cc. According to the treatment techniques and in vivo dosimetry, the contralateral breast skin dose was 1·0% in FIF and 2·5% in VMAT concerning the prescribed dose.

Conclusion:

There is no difference in skin dosimetry between VMAT and FIF techniques on the ipsilateral breast. It provides useful support for the use of VMAT as a planning technique for breast irradiation. The work describes the importance of quantifying potential differences in skin dosimetry.

Skin Dosimetry with EBT3 Radiochromic Film in Radiotherapy of Parotid Cancer

Background

Skin is a sensitive organ and should be spared in radiotherapy and irradiation of skin in radiotherapy can cause to acute and late skin effects such as erythema, desquamation, epilation, color change, or even necrosis.

Objective

The aim of the present study is to do skin dosimetry in radiotherapy of parotid cancer using Gafchromic EBT3 radiochromic film. EBT3 radiochromic films were calibrated in 0.2-5 Gy dose range.

Material and Methods

This is an experimental study in the field of radiotherapy physics. Treatment planning was performed on a RANDO phantom for treatment of parotid cancer by a clinical oncologist. Based on the treatment planning, the skin dose at various points in the overlapping region of right anterior-oblique and right posterior-oblique fields were measured using EBT3 radiochromic film.

Results

The minimum and maximum skin doses in a fraction (with 2.0 Gy prescribed dose) were 0.50 Gy and 0.97 Gy, respectively. Based on these values, the total skin dose in 30 treatment fractions (for removed tumor) or in 35 treatment fractions (for unremoved tumor) was in the range of 15-33 Gy.

Conclusion

Based on the skin dosimetry results of parotid cancer radiotherapy using EBT3 films, it is predicted that there will occur mild skin reactions and these reactions can be neglected due to being mild.

Effect of auto flash margin on superficial dose in breast conserving radiotherapy for breast cancer

Purpose

To investigate the dose‐effect of Auto Flash Margin (AFM) on breast cancer's superficial tissues based on the Treatment Planning System (TPS) in the breast‐conserving radiotherapy plan.

Methods

A total of 16 breast‐conserving patients with early stage breast cancer were selected, using the X‐ray Voxel Monte Carlo (XVMC) algorithm. Then, every included case plan was designed using a 2 cm‐AFM (the value of AFM is 2 cm) and N‐AFM (without AFM). Under the condition of ensuring the same configuration of #MU and collimator, the absorbed dose after a simulated inspiratory motion was calculated again using the new plan center, which moved backward to the linac source. The dose difference between the measurement points between AFM and N‐AFM groups was compared.

Results

In the dose results, PTV_V50Gy of the AFM group was superior to that of the N‐AFM group, PTV_D2%, PTV_Dmean, Lung_Ipsi_V20Gy, Lung_Ipsi_Dmean, and Body_Dmax. Also, the dose results of the N‐AFM group were significantly higher than those of the AFM group. However, there was no significant difference between Lung_Contra_V5Gy, Heart_Dmean, and Breast_Contra_V10Gy in the two groups. In the collimator alignments at the same angle between groups, the AFM group formed an apparent air region outside the collimator compared with the N‐AFM group. In the XVMC algorithm feature parameter, the AFM group had less #MU, higher QE, and slightly longer optimization time. The #segments of both groups were close to the 240 control points preset by the plan. The validation results of EBT3 film in both groups were more significant than 95%, meeting the clinical plan's application requirements. The difference in film results between groups was mainly reflected in the dose distribution at the near‐source. 4DCT was used to summarize the maximum and minimum inspiratory motion distances of 7.31 ± 0.45 and 3.42 ± 0.91 mm respectively.

Conclusions

These results suggest that the AFM function application could significantly reduce the possibility of insufficient tumor target caused by inspiratory motion and ensure sufficient tumor target exposure.