N.M. Ung

Characterization of MOSkin detector for in vivo skin dose measurement during megavoltage radiotherapy

In vivo dosimetry is important during radiotherapy to ensure the accuracy of the dose delivered to the treatment volume. A dosimeter should be characterized based on its application before it is used for in vivo dosimetry. In this study, we characterize a new MOSFET‐based detector, the MOSkin detector, on surface for in vivo skin dosimetry. The advantages of the MOSkin detector are its water equivalent depth of measurement of 0.07 mm, small physical size with submicron dosimetric volume, and the ability to provide real‐time readout. A MOSkin detector was calibrated and the reproducibility, linearity, and response over a large dose range to different threshold voltages were determined. Surface dose on solid water phantom was measured using MOSkin detector and compared with Markus ionization chamber and GAFCHROMIC EBT2 film measurements. Dependence in the response of the MOSkin detector on the surface of solid water phantom was also tested for different (i) source to surface distances (SSDs); (ii) field sizes; (iii) surface dose; (iv) radiation incident angles; and (v) wedges. The MOSkin detector showed excellent reproducibility and linearity for dose range of 50 cGy to 300 cGy. The MOSkin detector showed reliable response to different SSDs, field sizes, surface, radiation incident angles, and wedges. The MOSkin detector is suitable for in vivo skin dosimetry. PACS number: 87.55.Qr

Sensitive Fibre-Based Thermoluminescence Detectors for High Resolution In-Vivo Dosimetry.

With interest in the potential of optical fibres as the basis of next-generation thermoluminescence dosimeters (TLDs), the development of suitable forms of material and their fabrication has become a fast-growing endeavour. Present study focuses on three types of Ge-doped optical fibres with different structural arrangements and/or shapes, namely conventional cylindrical fibre, capillary fibre, and flat fibre, all fabricated using the same optical fibre preform. For doses from 0.5 to 8 Gy, obtained at electron and photon energies, standard thermoluminescence (TL) characteristics of the optical fibres have been the subject of detailed investigation. The results show that in collapsing the capillary fibre into a flat shape, the TL yield is increased by a factor of 5.5, the yield being also some 3.2 times greater than that of the conventional cylindrical fibre fabricated from the same perform. This suggests a means of production of suitably sensitive TLD for in-vivo dosimeter applications. Addressing the associated defects generating luminescence from each of the optical fibres, the study encompasses analysis of the TL glow curves, with computerized glow curve deconvolution (CGCD) and 2nd order kinetics.

Thermoluminescence responses of photon- and electron-irradiated lithium potassium borate co-doped with Cu+Mg or Ti+Mg

New glasses Li2CO3-K2CO3-H3BO3 (LKB) co-doped with CuO and MgO, or with TiO2 and MgO, were synthesized by the chemical quenching technique. The thermoluminescence (TL) responses of LKB:Cu,Mg and LKB:Ti,Mg irradiated with 6 MV photons or 6 MeV electrons were compared in the dose range 0.5-4.0 Gy. The standard commercial dosimeter LiF:Mg,Ti (TLD-100) was used to calibrate the TL reader and as a reference in comparison of the TL properties of the new materials. The dependence of the responses of the new materials on (60)Co dose is linear in the range of 1-1000 Gy. The TL yields of both of the co-doped glasses and TLD-100 are greater for electron irradiation than for photon irradiation. The TL sensitivity of LKB:Ti,Mg is 1.3 times higher than the sensitivity of LKB:Cu,Mg and 12 times less than the sensitivity of TLD-100. The new TL dosimetric materials have low effective atomic numbers, good linearity of the dose responses, excellent signal reproducibility, and a simple glow curve structure. This combination of properties makes them suitable for radiation dosimetry.

Motion corrected LV quantification based on 3D modelling for improved functional assessment in cardiac MRI.

Cine MRI is a clinical reference standard for the quantitative assessment of cardiac function, but reproducibility is confounded by motion artefacts. We explore the feasibility of a motion corrected 3D left ventricle (LV) quantification method, incorporating multislice image registration into the 3D model reconstruction, to improve reproducibility of 3D LV functional quantification. Multi-breath-hold short-axis and radial long-axis images were acquired from 10 patients and 10 healthy subjects. The proposed framework reduced misalignment between slices to subpixel accuracy (2.88 to 1.21 mm), and improved interstudy reproducibility for 5 important clinical functional measures, i.e. end-diastolic volume, end-systolic volume, ejection fraction, myocardial mass and 3D-sphericity index, as reflected in a reduction in the sample size required to detect statistically significant cardiac changes: a reduction of 21-66%. Our investigation on the optimum registration parameters, including both cardiac time frames and number of long-axis (LA) slices, suggested that a single time frame is adequate for motion correction whereas integrating more LA slices can improve registration and model reconstruction accuracy for improved functional quantification especially on datasets with severe motion artefacts.

On the Use of Optically Stimulated Luminescent Dosimeter for Surface Dose Measurement during Radiotherapy

This study was carried out to investigate the suitability of using the optically stimulated luminescence dosimeter (OSLD) in measuring surface dose during radiotherapy. The water equivalent depth (WED) of the OSLD was first determined by comparing the surface dose measured using the OSLD with the percentage depth dose at the buildup region measured using a Markus ionization chamber. Surface doses were measured on a solid water phantom using the OSLD and compared against the Markus ionization chamber and Gafchromic EBT3 film measurements. The effect of incident beam angles on surface dose was also studied. The OSLD was subsequently used to measure surface dose during tangential breast radiotherapy treatments in a phantom study and in the clinical measurement of 10 patients. Surface dose to the treated breast or chest wall, and on the contralateral breast were measured. The WED of the OSLD was found to be at 0.4 mm. For surface dose measurement on a solid water phantom, the Markus ionization chamber measured 15.95% for 6 MV photon beam and 12.64% for 10 MV photon beam followed by EBT3 film (23.79% and 17.14%) and OSLD (37.77% and 25.38%). Surface dose increased with the increase of the incident beam angle. For phantom and patient breast surface dose measurement, the response of the OSLD was higher than EBT3 film. The in-vivo measurements were also compared with the treatment planning system predicted dose. The OSLD measured higher dose values compared to dose at the surface (Hp(0.0)) by a factor of 2.37 for 6 MV and 2.01 for 10 MV photon beams, respectively. The measurement of absorbed dose at the skin depth of 0.4 mm by the OSLD can still be a useful tool to assess radiation effects on the skin dermis layer. This knowledge can be used to prevent and manage potential acute skin reaction and late skin toxicity from radiotherapy treatments.

Treatment outcome for nasopharyngeal carcinoma in University Malaya Medical Centre from 2004-2008.

BACKGROUND Nasopharyngeal carcinoma (NPC) is the commonest radiocurable cancer in Malaysia. This study aimed to determine the treatment outcomes and late effects of radiotherapy for NPC patients treated in University Malaya Medical Centre (UMMC). MATERIALS AND METHODS All newly diagnosed patients with NPC referred for treatment to the Oncology unit at UMMC from 2004-2008 were retrospectively analyzed. Treatment outcomes were 5 years overall survival (OS), disease free survival (DFS), cause-specific survival (CSS), loco- regional control (LRC) and radiotherapy-related late effects. The Kaplan-Meier method was used for survival analysis and differences in survival according to AJCC stage was compared using the log-rank test. RESULTS A total of 176 patients with newly diagnosed NPC were treated in UMMC during this period. Late presentation was common, with 33.5% presenting with T3-4 disease, 84.7% with N1-3 disease and 75.6% with AJCC stage 3-4 disease. Radical RT was given to 162 patients with 22.7% having RT alone and 69.3% having CCRT. The stipulated OTT was 7 weeks and 72.2% managed to complete their RT within this time period. Neoadjuvant chemotherapy was given to 14.8% while adjuvant chemotherapy was administered to 16.5%. The 5 years OS was 51.6% with a median follow up of 58 months. The 5 years OS according to stage were 81.8% for stage I, 77.9% for stage II, 47.4% for stage III and 25.9% for stage IV. The 5 years overall CSS, DFS and LRC were 54.4%, 48.4% and 70.6%, respectively. RT related late effects were documented in 80.2%. The commonest was xerostomia (66.7%). Other documented late effects were hearing deficit (17.3%), visual deficit (3.1%), neck stiffness (3.1%) , dysphagia (3.4%), cranial nerve palsy (2.5%), pneumonitis (0.6%) and hypothyroidism (1.2%). CONCLUSIONS The 5 years OS and LRC in this study are low compared to the latest studies especially those utilizing IMRT. Implementation of IMRT for NPC treatment should be strongly encouraged.

Dose planning study of target volume coverage with intensity- modulated radiotherapy for nasopharyngeal carcinoma: Penang General Hospital experience.

BACKGROUND To compare the dosimetric coverage of target volumes and organs at risk in the radical treatment of nasopharyngeal carcinoma (NPC) between intensity-modulated radiotherapy (IMRT) and three- dimensional conformal radiotherapy (3DCRT). MATERIALS AND METHODS Data from 10 consecutive patients treated with IMRT from June-October 2011 in Penang General Hospital were collected retrospectively for analysis. For each patient, dose volume histograms were generated for both the IMRT and 3DCRT plans using a total dose of 70Gy. Comparison of the plans was accomplished by comparing the target volume coverage (5 measures) and sparing of organs at risk (17 organs) for each patient using both IMRT and 3DCRT. The means of each comparison target volume coverage measures and organs at risk measures were obtained and tested for statistical significance using the paired Student t-test. RESULTS All 5 measures for target volume coverage showed marked dosimetric superiority of IMRT over 3DCRT. V70 and V66.5 for PTV70 showed an absolute improvement of 39.3% and 24.1% respectively. V59.4 and V56.4 for PTV59.4 showed advantages of 18.4% and 16.4%. Moreover, the mean PTV70 dose revealed a 5.1 Gy higher dose with IMRT. Only 4 out of 17 organs at risk showed statistically significant difference in their means which were clinically meaningful between the IMRT and 3DCRT techniques. IMRT was superior in sparing the spinal cord (less 5.8Gy), V30 of right parotid (less 14.3%) and V30 of the left parotid (less 13.1%). The V55 of the left cochlea was lower with 3DCRT (less 44.3%). CONCLUSIONS IMRT is superior to 3DCRT due to its dosimetric advantage in target volume coverage while delivering acceptable doses to organs at risk. A total dose of 70Gy with IMRT should be considered as a standard of care for radical treatment of NPC.

CT Based 3-Dimensional Treatment Planning of Intracavitary Brachytherapy for Cancer of the Cervix : Comparison between Dose-Volume Histograms and ICRU Point Doses to the Rectum and Bladder

BACKGROUND CT based brachytherapy allows 3-dimensional (3D) assessment of organs at risk (OAR) doses with dose volume histograms (DVHs). The purpose of this study was to compare computed tomography (CT) based volumetric calculations and International Commission on Radiation Units and Measurements (ICRU) reference-point estimates of radiation doses to the bladder and rectum in patients with carcinoma of the cervix treated with high-dose-rate (HDR) intracavitary brachytherapy (ICBT). MATERIALS AND METHODS Between March 2011 and May 2012, 20 patients were treated with 55 fractions of brachytherapy using tandem and ovoids and underwent post-implant CT scans. The external beam radiotherapy (EBRT) dose was 48.6 Gy in 27 fractions. HDR brachytherapy was delivered to a dose of 21 Gy in three fractions. The ICRU bladder and rectum point doses along with 4 additional rectal points were recorded. The maximum dose (DMax) to rectum was the highest recorded dose at one of these five points. Using the HDR plus 2.6 brachytherapy treatment planning system, the bladder and rectum were retrospectively contoured on the 55 CT datasets. The DVHs for rectum and bladder were calculated and the minimum doses to the highest irradiated 2cc area of rectum and bladder were recorded (D2cc) for all individual fractions. The mean D2cc of rectum was compared to the means of ICRU rectal point and rectal DMax using the Students t-test. The mean D2cc of bladder was compared with the mean ICRU bladder point using the same statistical test .The total dose, combining EBRT and HDR brachytherapy, were biologically normalized to the conventional 2 Gy/fraction using the linear-quadratic model. (α/β value of 10 Gy for target, 3 Gy for organs at risk). RESULTS The total prescribed dose was 77.5 Gy α/β10. The mean dose to the rectum was 4.58 ± 1.22 Gy for D 2cc, 3.76 ± 0.65 Gy at D ICRU and 4.75 ± 1.01 Gy at DMax. The mean rectal D 2cc dose differed significantly from the mean dose calculated at the ICRU reference point (p<0.005); the mean difference was 0.82 Gy (0.48 -1.19 Gy). The mean EQD2 was 68.52 ± 7.24 Gy α/β3 for D 2cc, 61.71 ± 2.77 Gy α/β3 at D ICRU and 69.24 ± 6.02 Gy α/β3 at DMax. The mean ratio of D 2cc rectum to D ICRU rectum was 1.25 and the mean ratio of D 2cc rectum to DMax rectum was 0.98 for all individual fractions. The mean dose to the bladder was 6.00 ± 1.90 Gy for D 2cc and 5.10 ± 2.03 Gy at D ICRU. However, the mean D 2cc dose did not differ significantly from the mean dose calculated at the ICRU reference point (p=0.307); the mean difference was 0.90 Gy (0.49-1.25 Gy). The mean EQD2 was 81.85 ± 13.03 Gy α/β3 for D 2cc and 74.11 ± 19.39 Gy α/β3 at D ICRU. The mean ratio of D 2cc bladder to D ICRU bladder was 1.24. In the majority of applications, the maximum dose point was not the ICRU point. On average, the rectum received 77% and bladder received 92% of the prescribed dose. CONCLUSIONS OARs doses assessed by DVH criteria were higher than ICRU point doses. Our data suggest that the estimated dose to the ICRU bladder point may be a reasonable surrogate for the D 2cc and rectal DMax for D 2cc. However, the dose to the ICRU rectal point does not appear to be a reasonable surrogate for the D 2cc.

Photon irradiation response of photonic crystal fibres and flat fibres at radiation therapy doses

Radiation effects of photon irradiation in pure Photonic Crystal Fibres (PCF) and Flat fibres (FF) are still much less investigated in thermoluminescense dosimetry (TLD). We have reported the TL response of PCF and FF subjected to 6 MV photon irradiation. The proposed dosimeter shows good linearity at doses ranging from 1 to 4 Gy. The small size of these detectors points to its use as a dosimeter at megavoltage energies, where better tissue-equivalence and the Bragg-Gray cavity theory prevails.

Thermoluminescence Response of Ge-Doped Cylindrical-, Flat- and Photonic Crystal Silica-Fibres to Electron and Photon Radiation

Study has been made of the thermoluminescence (TL) response of silica-based Ge-doped cylindrical, flat and photonic crystal fibres (referred to herein as PCF-collapsed) to electron (6, 12 and 20 MeV) and photon (6, 10 MV) irradiation and 1.25 MeV γ-rays, for doses from 0.1 Gy to 100 Gy. The electron and photon irradiations were delivered through use of a Varian Model 2100C linear accelerator located at the University of Malaya Medical Centre and γ-rays delivered from a 60Co irradiator located at the Secondary Standard Dosimetry Laboratory (SSDL), Malaysian Nuclear Agency. Tailor-made to be of various dimensions and dopant concentrations (6–10% Ge), the fibres were observed to provide TL yield linear with radiation dose, reproducibility being within 1–5%, with insensitivity to energy and angular variation. The sensitivity dependency of both detectors with respect to field size follows the dependency of the output factors. For flat fibres exposed to 6 MV X-rays, the 6% Ge-doped fibre provided the greatest TL yield while PCF-collapsed showed a response 2.4 times greater than that of the 6% Ge-doped flat fibres. The response of cylindrical fibres increased with core size. The fibres offer uniform response, high spatial resolution and sensitivity, providing the basis of promising TL systems for radiotherapy applications.