Martin G Carolan

A new radiotherapy surface dose detector: The MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOS-FET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-microns in diameter and consists of a 0.5-microns Al electrode on top of a 1-microns SiO2 and 300-microns Si substrate. Results for % surface dose were within +/- 2% compared to the Attix chamber and within +/- 3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10 x 10-cm and 40 x 40-cm field size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

Comparison of prostate IMRT and VMAT biologically optimised treatment plans.

Recently, a new radiotherapy delivery technique has become clinically available--volumetric modulated arc therapy (VMAT). VMAT is the delivery of IMRT while the gantry is in motion using dynamic leaf motion. The perceived benefit of VMAT over IMRT is a reduction in delivery time. In this study, VMAT was compared directly with IMRT for a series of prostate cases. For 10 patients, a biologically optimized seven-field IMRT plan was compared with a biologically optimized VMAT plan using the same planning objectives. The Pinnacle RTPS was used. The resultant target and organ-at-risk dose-volume histograms (DVHs) were compared. The normal tissue complication probability (NTCP) for the IMRT and VMAT plans was calculated for 3 model parameter sets. The delivery efficiency and time for the IMRT and VMAT plans was compared. The VMAT plans resulted in a statistically significant reduction in the rectal V25Gy parameter of 8.2% on average over the IMRT plans. For one of the NTCP parameter sets, the VMAT plans had a statistically significant lower rectal NTCP. These reductions in rectal dose were achieved using 18.6% fewer monitor units and a delivery time reduction of up to 69%. VMAT plans resulted in reductions in rectal doses for all 10 patients in the study. This was achieved with significant reductions in delivery time and monitor units. Given the target coverage was equivalent, the VMAT plans were superior.

Simultaneous macro and micro dosimetry with MOSFETs

The application of MOSFET dosimeters in complicated mixed radiation fields for measurement of absorbed dose distribution in tissue equivalent phantoms has been studied. The spectra of secondary charged particles have been measured simultaneously with average absorbed doses by the same MOSFET dosimeter. A good correlation has been observed between neutron depth dose distribution in a water phantom obtained using MOSFETs in integral mode and a tissue equivalent (T.E.) ionisation chamber. Such MOSFET dosimeters are a promising tool for micro-macro dosimetry in Boron Neutron Capture Therapy (BNCT) and Fast Neutron Therapy (FNT). Paired MOSFETs with one of the dosimeters covered by /sup 10/B have been applied for measuring of average boron dose distribution and microdosimetric spectra due to alpha particles and /sup 7/Li ions throughout a perspex phantom exposed in the epithermal neutron beam at the Brookhaven Medical Research Reactor (BMRR).

Gastric cancer stem cells: evidence, potential markers, and clinical implications

Abstract Gastric cancer is a significant global health problem. It is the fifth most common cancer and third leading cause of cancer-related death worldwide (Torre et al. in CA Cancer J Clin 65(2):87–108, 2015). Despite advances in treatment, overall prognosis remains poor, due to tumour relapse and metastasis. There is an urgent need for novel therapeutic approaches to improve clinical outcomes in gastric cancer. The cancer stem cell (CSC) model has been proposed to explain the high rate of relapse and subsequent resistance of cancer to current systemic treatments (Vermeulen et al. in Lancet Oncol 13(2):e83–e89, 2012). CSCs have been identified in many solid malignancies, including gastric cancer, and have significant clinical implications, as targeting the CSC population may be essential in preventing the recurrence and spread of a tumour (Dewi et al. in J Gastroenterol 46(10):1145–1157, 2011). This review seeks to summarise the current evidence for CSC in gastric cancer, with an emphasis on candidate CSC markers, clinical implications, and potential therapeutic approaches.

A silicon strip detector dose magnifying glass for IMRT dosimetry

PURPOSE Intensity modulated radiation therapy (IMRT) allows the delivery of escalated radiation dose to tumor while sparing adjacent critical organs. In doing so, IMRT plans tend to incorporate steep dose gradients at interfaces between the target and the organs at risk. Current quality assurance (QA) verification tools such as 2D diode arrays, are limited by their spatial resolution and conventional films are nonreal time. In this article, the authors describe a novel silicon strip detector (CMRP DMG) of high spatial resolution (200 microm) suitable for measuring the high dose gradients in an IMRT delivery. METHODS A full characterization of the detector was performed, including dose per pulse effect, percent depth dose comparison with Farmer ion chamber measurements, stem effect, dose linearity, uniformity, energy response, angular response, and penumbra measurements. They also present the application of the CMRP DMG in the dosimetric verification of a clinical IMRT plan. RESULTS The detector response changed by 23% for a 390-fold change in the dose per pulse. A correction function is derived to correct for this effect. The strip detector depth dose curve agrees with the Farmer ion chamber within 0.8%. The stem effect was negligible (0.2%). The dose linearity was excellent for the dose range of 3-300 cGy. A uniformity correction method is described to correct for variations in the individual detector pixel responses. The detector showed an over-response relative to tissue dose at lower photon energies with the maximum dose response at 75 kVp nominal photon energy. Penumbra studies using a Varian Clinac 21EX at 1.5 and 10.0 cm depths were measured to be 2.77 and 3.94 mm for the secondary collimators, 3.52 and 5.60 mm for the multileaf collimator rounded leaf ends, respectively. Point doses measured with the strip detector were compared to doses measured with EBT film and doses predicted by the Philips Pinnacle treatment planning system. The differences were 1.1% +/- 1.8% and 1.0% +/- 1.6%, respectively. They demonstrated the high temporal resolution capability of the detector readout system, which will allow one to investigate the temporal dose pattern of IMRT and volumetric modulated are therapy (VMAT) deliveries. CONCLUSIONS The CMRP silicon strip detector dose magnifying glass interfaced to a TERA ASIC DAQ system has high spatial and temporal resolution. It is a novel and valuable tool for QA in IMRT dose delivery and for VMAT dose delivery.

The use of a silicon strip detector dose magnifying glass in stereotactic radiotherapy QA and dosimetry

PURPOSE Stereotactic radiosurgery/therapy (SRS/SRT) is the use of radiation ablation in place of conventional surgical excision to remove or create fibrous tissue in small target volumes. The target of the SRT/SRS treatment is often located in close proximity to critical organs, hence the requirement of high geometric precision including a tight margin on the planning target volume and a sharp dose fall off. One of the major problems with quality assurance (QA) of SRT/SRS is the availability of suitable detectors with the required spatial resolution. The authors present a novel detector that they refer to as the dose magnifying glass (DMG), which has a high spatial resolution (0.2 mm) and is capable of meeting the stringent requirements of QA and dosimetry in SRS/SRT therapy. METHODS The DMG is an array of 128 phosphor implanted n+ strips on a p-type Si wafer. The sensitive area defined by a single n+ strip is 20 x 2000 microm2. The Si wafer is 375 microm thick. It is mounted on a 0.12 mm thick Kapton substrate. The authors studied the dose per pulse (dpp) and angular response of the detector in a custom-made SRS phantom. The DMG was used to determine the centers of rotation and positioning errors for the linear accelerators gantry, couch, and collimator rotations. They also used the DMG to measure the profiles and the total scatter factor (S(cp)) of the SRS cones. Comparisons were made with the EBT2 film and standard S(cp) values. The DMG was also used for dosimetric verification of a typical SRS treatment with various noncoplanar fields and arc treatments when applied to the phantom. RESULTS The dose per pulse dependency of the DMG was found to be < 5% for a dpp change of 7.5 times. The angular response of the detector was investigated in the azimuthal and polar directions. The maximum polar angular response was 13.8% at the gantry angle of 320 degrees, which may be partly due to the phantom geometry. The maximum azimuthal angular response was 15.3% at gantry angles of 90 degrees and 270 degrees. The angular response at the gantry angle of 180 degrees was 6.3%. A correction function was derived to correct for the angular dependence of the detector, which takes into account the contribution of the azimuthal and polar angular response at different treatment couch positions. The maximum positioning errors due to collimator, gantry, and couch rotation were 0.2 +/- 0.1, 0.4 +/- 0.1, and 0.4 +/- 0.2 mm, respectively. The SRS cone S(cp) agrees very well with the standard data with an average difference of 1.2 +/- 1.1%. Comparison of the relative intensity profiles of the DMG and EBT2 measurements for a simulated SRS treatment shows a maximum difference of 2.5%. CONCLUSIONS The DMG was investigated for dose per pulse and angular dependency. Its application to SRS/SRT delivery verification was demonstrated. The DMG with its high spatial resolution and real time capability allows measurement of dose profiles for cone applicators down to 5 mm in diameter, both accurately and rapidly as required in typical SRS/SRT deliveries.

VERIFICATION OF BRACHYTHERAPY DOSIMETRY WITH RADIOCHROMIC FILM

The aim of this work is to empirically validate the optimized dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3) at a distance of 1.0 cm from a stepping source of high-dose-rate-iridium 192 (192Ir). The longitudinal dose distribution at 1.0 cm from a straight pathway of multiple-source positions is measured using radiochromic film and compared with the planning systems calculated results. The optical density of the exposed films was determined with a modified Scanditronix film scanner, and the film was calibrated with 192Ir using manually calculated exposure times. A calibration equation was used to convert scanner output to dose. Our results illustrate the significance of exacting geometry in the experimental setup due to the inverse square law and the small distances involved. The dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3), at a distance of 1.0 cm, is validated to within +/-4% of the measured dose distribution. The advantages and limitations of radiochromic film as a dosimetry tool are also addressed in this work.

Characterization of an Innovative p-type Epitaxial Diode for Dosimetry in Modern External Beam Radiotherapy

Due to the ever-increasing complexity of treatment modalities in radiation therapy, there has been a greater need for detectors to perform quality assurance to ensure patients are treated correctly and safely. Modern radiation therapy techniques involve small field sizes, high dose gradients, and varying intensity of energy and rate. The ideal dosimeter for this treatment should display high spatial resolution, high linearity, accuracy, and radiation hardness. Silicon detectors have been widely used for radiotherapy measurements and have many attractive qualities as a dosimeter; weaknesses of silicon detectors are, however, decreases in sensitivity with accumulated dose. The Centre for Medical Radiation Physics has developed a new technology with an unusual charge collection efficiency variation with accumulated dose which stabilizes the response of the detector within ±5% after 120 kGy photon irradiation. The sensor has been also characterized by irradiation by an 18 MV medical LINAC with sensitivity to a photoneutron-induced damage of less than 0.5%/100 Gy. The radiation damage mechanism has been validated by TCAD simulations which confirmed the mechanism behind the CCE increase as a function of the accumulated dose.

Direct and pulsed current annealing of p-MOSFET based dosimeter: The "MOSkin"

Contemporary radiation therapy (RT) is complicated and requires sophisticated real-time quality assurance (QA). While 3D real-time dosimetry is most preferable in RT, it is currently not fully realised. A small, easy to use and inexpensive point dosimeter with real-time and in vivo capabilities is an option for routine QA. Such a dosimeter is essential for skin, in vivo or interface dosimetry in phantoms for treatment plan verification. The metal-oxide-semiconductor-field-effect-transistor (MOSFET) detector is one of the best choices for these purposes, however, the MOSFETs sensitivity and its signal stability degrade after essential irradiation which limits its lifespan. The accumulation of positive charge on the gate oxide and the creation of interface traps near the silicon–silicon dioxide layer is the primary physical phenomena responsible for this degradation. The aim of this study is to investigate MOSFET dosimeter recovery using two proposed annealing techniques: direct current (DC) and pulsed current (PC), both based on hot charged carrier injection into the gate oxide of the p-MOSFET dosimeter. The investigated MOSFETs were reused multiple times using an irradiation-annealing cycle. The effect of the current-annealing parameters was investigated for the dosimetric characteristics of the recovered MOSFET dosimeters such as linearity, sensitivity and initial threshold voltage. Both annealing techniques demonstrated excellent results in terms of maintaining a stable response, linearity and sensitivity of the MOSFET dosimeter. However, PC annealing is more preferable than DC annealing as it offers better dose response linearity of the reused MOSFET and has a very short annealing time.

IMRT treatment Monitor Unit verification using absolute calibrated BEAMnrc and Geant4 Monte Carlo simulations

Intensity Modulated Radiation Therapy (IMRT) treatments are some of the most complex being delivered by modern megavoltage radiotherapy accelerators. Therefore verification of the dose, or the presecribed Monitor Units (MU), predicted by the planning system is a key element to ensuring that patients should receive an accurate radiation dose plan during IMRT. One inherently accurate method is by comparison with absolute calibrated Monte Carlo simulations of the IMRT delivery by the linac head and corresponding delivery of the plan to a patient based phantom. In this work this approach has been taken using BEAMnrc for simulation of the treatment head, and both DOSXYZnrc and Geant4 for the phantom dose calculation. The two Monte Carlo codes agreed to within 1% of each other, and these matched very well to our planning system for IMRT plans to the brain, nasopharynx, and head and neck.