MOSFET Dosimeters Research Articles

SU-D-103-03: Converting Size Specific Dose Estimate (SSDE) to Patient Specific Organ Dosimetry: Application to a Pediatric Population

Purpose: Develop a methodology to convert size specific dose estimate (SSDE) to pediatric patient specific organ dosimetry. Methods: Each thoracic and abdominal-pelvic organ within a family of four anthropomorphic phantoms (ATOM, CIRS) developed to emulate a wide range of pediatric patient body habitus was measured for absolute dose using MOSFET dosimeters; e.g., a series of 884 measurements were made in 21 different organs spanning the thorax of an anthropomorphic phantom developed to emulate a 10 yr patient. A Scout scan was performed for each phantom wherein the phantoms thoracic and abdominal-pelvic effective diameters were calculated pursuant to the AAPM Report 204 methodology. The phantoms effective diameter was matched with a pediatric patient population average effective diameter, and imaged according to that patient populations weight based CT scan parameters; thus, normalizing CTDIvol between phantom and representative patient populations. A correction factor was developed for each phantoms organ dose as it related to the phantoms calculated SSDE (both thoracic and abdominal-pelvic). The SSDE-to-Absolute dose correction factors were then applied to an IRB approved patient cohort of 492 patients (7.6 ± 5.4 yrs; range 2 mo to 28 yrs and mean weight was 35 ± 33 kg; range 4–147 kg) to convert patient specific SSDE to organ dose. Results: Organ dose linearly correlated with patient effective diameter with an average R2 of 0.96. In general, chest organ doses were greater than the chest SSDE by 10–40%, the abdomen organ doses were greater than the abdomen SSDE by 10–20%, but organ doses in the pelvis were lower by 0–20%. Conclusion: Correction factors were established to convert SSDE to absolute organ dose. Organ dose values linearly correlated with patient size. SSDE, when calculated based on Report 204, generally underestimates actual patient chest and abdomen organ dosimetry from 10–40%, but overestimates pelvic organ dose by 0–20%.

Modeling gan dosimetric response to therapeutic photon beam radiation

Introduction The use of Gallium Nitride (GaN), a direct-gap semiconductor, has been recently pro-posed as a radioluminescent transducer for real-time in-vivo dosimetric applications [1]. GaN has the following advantages: a lower e–h pair creation energy than conventional scintillators, a high prompt radioluminescence yield resulting from the high probability of band edge radiative recombination. Nevertheless, it shares the common drawback of silicon diodes or MOSFET dosimeters with an over response relative to water for low energy scattered photons. The aim of our study is to compensate for this dependence through modelling, which may lead us to improve the accuracy of GaN-based dosimetric systems. Our modeling work presented here is focused on GaN dosimetric response for therapeutic photo beam irradiation from medical linacs. Experimental The dosimetric response of a GaN transducer was first modeled by combining large cavity theory and the Spencer– Attix small cavity theory respectively for the low and high energy components of the local spectrum according the approach proposed in [2]. The local spectra were calculated from fluence pencil kernels, and Monte Carlo simulations were performed to determine some key parameters included into the model. Results and discussion The developed model was used to compute TMR curves of a implantable GaN-based probe. A good agreement with measurements is achieved which validates the modeling work. Furthermore, we used the model to calculate GaN response factor at different depths in water for 6 MV photon beam. The obtained results (not shown here) confirm that GaN response factor is proportional to the square field aperture (with a proportionality coefficient which varies with depth). This linear relationship was experimentally observed in clinical conditions [3].

Dosimetry Investigation of MOSFET for Clinical IMRT Dose Verification

In IMRT, patient-specific dose verification is followed regularly at each centre. Simple and efficient dosimetry techniques play a very important role in routine clinical dosimetry QA. The MOSFET dosimeter offers several advantages over the conventional dosimeters such as its small detector size, immediate readout, immediate reuse, multiple point dose measurements. To use the MOSFET as routine clinical dosimetry system for pre-treatment dose verification in IMRT, a comprehensive set of experiments has been conducted, to investigate its linearity, reproducibility, dose rate effect and angular dependence for 6 MV x-ray beam. The MOSFETs shows a linear response with linearity coefficient of 0.992 for a dose range of 35 cGy to 427 cGy. The reproducibility of the MOSFET was measured by irradiating the MOSFET for ten consecutive irradiations in the dose range of 35 cGy to 427 cGy. The measured reproducibility of MOSFET was found to be within 4% up to 70 cGy and within 1.4% above 70 cGy. The dose rate effect on the MOSFET was investigated in the dose rate range 100 MU/min to 600 MU/min. The response of the MOSFET varies from -1.7% to 2.1%. The angular responses of the MOSFETs were measured at 10 degrees intervals from 90 to 270 degrees in an anticlockwise direction and normalized at gantry angle zero and it was found to be in the range of 0.98 ± 0.014 to 1.01 ± 0.014. The MOSFETs were calibrated in a phantom which was later used for IMRT verification. The measured calibration coefficients were found to be 1 mV/cGy and 2.995 mV/cGy in standard and high sensitivity mode respectively. The MOSFETs were used for pre-treatment dose verification in IMRT. Nine dosimeters were used for each patient to measure the dose in different plane. The average variation between calculated and measured dose at any location was within 3%. Dose verification using MOSFET and IMRT phantom was found to quick and efficient and well suited for a busy radiotherapy department.

Monte Carlo investigation of I‐125 interseed attenuation for standard and thinner seeds in prostate brachytherapy with phantom validation using a MOSFET

In permanent seed implant prostate brachytherapy the actual dose delivered to the patient may be less than that calculated by TG-43U1 due to interseed attenuation (ISA) and differences between prostate tissue composition and water. In this study the magnitude of the ISA effect is assessed in a phantom and in clinical prostate postimplant cases. Results are compared for seed models 6711 and 9011 with 0.8 and 0.5 mm diameters, respectively. A polymethyl methacrylate (PMMA) phantom was designed to perform ISA measurements in a simple eight-seed arrangement and at the center of an implant of 36 seeds. Monte Carlo (MC) simulation and experimental measurements using a MOSFET dosimeter were used to measure dose rate and the ISA effect. MC simulations of 15 CT-based postimplant prostate treatment plans were performed to compare the clinical impact of ISA on dose to prostate, urethra, rectum, and the volume enclosed by the 100% isodose, for 6711 and 9011 seed models. In the phantom, ISA reduced the dose rate at the MOSFET position by 8.6%-18.3% (6711) and 7.8%-16.7% (9011) depending on the measurement configuration. MOSFET measured dose rates agreed with MC simulation predictions within the MOSFET measurement uncertainty, which ranged from 5.5% to 7.2% depending on the measurement configuration (k = 1, for the mean of four measurements). For 15 clinical implants, the mean ISA effect for 6711 was to reduce prostate D90 by 4.2 Gy (3%), prostate V100 by 0.5 cc (1.4%), urethra D10 by 11.3 Gy (4.4%), rectal D2cc by 5.5 Gy (4.6%), and the 100% isodose volume by 2.3 cc. For the 9011 seed the mean ISA effect reduced prostate D90 by 2.2 Gy (1.6%), prostate V100 by 0.3 cc (0.7%), urethra D10 by 8.0 Gy (3.2%), rectal D2cc by 3.1 Gy (2.7%), and the 100% isodose volume by 1.2 cc. Differences between the MC simulation and TG-43U1 consensus data for the 6711 seed model had a similar impact, reducing mean prostate D90 by 6 Gy (4.2%) and V100 by 0.6 cc (1.8%). ISA causes the delivered dose in prostate seed implant brachytherapy to be lower than the dose calculated by TG-43U1. MC simulation of phantom seed arrangements show that dose at a point can be reduced by up to 18% and this has been validated using a MOSFET dosimeter. Clinical simulations show that ISA reduces DVH parameter values, but the reduction is less for thinner seeds.

Poster - Thur Eve - 03: LDR to HDR: RADPOS applications in brachytherapy.

The RADPOS in vivo dosimetry system combines an electromagnetic positioning sensor and either one or five MOSFET dosimeters. The feasibility of using the system for quality control has been explored for a range of radiotherapy treatment techniques including most recently transperineal interstitial permanent prostate brachytherapy and high dose rate (HDR) treatments. Dose and position information was collected by a RADPOS array detector inside a Foley catheter within patients' urethra during permanent seed implantation. Ten patients were studied, and average displacement during implantation was Δr = (1.4-5.1) mm, with movements up to 9.7 mm due to the removal of the transrectal ultrasound probe. Maximum integral dose in the prostatic urethra ranged from 110-195 Gy, and it was found that the dose can change up to 63 cGy (62.0%) depending on whether the rectal probe is in place. For HDR, a RADPOS detector was first calibrated with an Ir-192 source. A treatment was then simulated using a total of 50 dwell positions in 5 catheters in an acrylic phantom. Dwell positions ranged from 1 to 10 cm away from the RADPOS detector and dose was measured for each source position. An average calibration coefficient of 0.74±0.11 cGy/mV was calculated for the detector and the average absolute difference between measured values and expected dose was 0.7±5.4 cGy (5±20%). The demonstrated accuracy of RADPOS dose measurements along with its ability to simultaneously measure displacement makes it a powerful tool for brachytherapy treatments, where high dose gradients can present unique in vivo dosimetry challenges.

SU‐E‐T‐110: Small Electron Field Surface Dosimetry Using Solid State Detectors

Solid state detectors from two independent systems have shown similar over-responses to small electron fields used for treatment of surface lesions. We propose to evaluate the accuracy of solid state detector systems for highly blocked electron fields as a method for clinical dosimetry and determine the cause for any deviations from expected dose-delivery calculations. A QED Diode detector system and a Best-Medical MOSFET dosimeter system were used to measure relative output factors (ROF's) for a range of blocked electron fields with 3 different electron energies. Measurements were taken on the surface of a Solid-Water phantom as well as under varying thicknesses of bolus (surface to dmax) to determine the extent of this effect with increasing depth. Measurements were compared with expected results obtained during machine commissioning with an A-14 micro-ionization chamber. Field sizes larger than 4×4 cm2 showed little significant deviation from expected output factor values. However, fields smaller than this threshold showed as much as a 30% over-response of the solid state detectors compared to ion chamber measurements on the surface. This deviation was consistent over multiple measurements using both systems. The degree of over-response is both energy and effective field size dependent. As the effective depth of measurement increases, the relative differences in responses decrease and eventually converge. Surface dosimetry using solid state detectors is a straightforward method of verifying dosimetric calculations. Care must be taken when using these systems for surface dosimetry with highly blocked fields as their non-tissue equivalence makes these detectors overly sensitive to low-energy scattered electrons and bremsstrahlung photons produced from the block edge. Measurements taken near dmax will not show a significant deviation from the actual dose and do not need to account for this effect.

In vivo dose measurement using TLDs and MOSFET dosimeters for cardiac radiosurgery

In vivo measurements were made of the dose delivered to animal models in an effort to develop a method for treating cardiac arrhythmia using radiation. This treatment would replace RF energy (currently used to create cardiac scar) with ionizing radiation. In the current study, the pulmonary vein ostia of animal models were irradiated with 6 MV X‐rays in order to produce a scar that would block aberrant signals characteristic of atrial fibrillation. The CyberKnife radiosurgery system was used to deliver planned treatments of 20–35 Gy in a single fraction to four animals. The Synchrony system was used to track respiratory motion of the heart, while the contractile motion of the heart was untracked. The dose was measured on the epicardial surface near the right pulmonary vein and on the esophagus using surgically implanted TLD dosimeters, or in the coronary sinus using a MOSFET dosimeter placed using a catheter. The doses measured on the epicardium with TLDs averaged 5% less than predicted for those locations, while doses measured in the coronary sinus with the MOSFET sensor nearest the target averaged 6% less than the predicted dose. The measurements on the esophagus averaged 25% less than predicted. These results provide an indication of the accuracy with which the treatment planning methods accounted for the motion of the target, with its respiratory and cardiac components. This is the first report on the accuracy of CyberKnife dose delivery to cardiac targets.PACS numbers: 87.53.Ly, 87.53.Bn

Breast radiotherapy in the prone position primarily reduces the maximum out‐of‐field measured dose to the ipsilateral lung

To quantify the potential advantages of prone position breast radiotherapy in terms of the radiation exposure to out-of-field organs, particularly, the breast or the lung. Several dosimetric studies have been reported, based on commercial treatment planning software (TPS). These TPS approaches are not, however, adequate for characterizing out-of-field doses. In this work, relevant out-of-field organ doses have been directly measured. The authors utilized an adult anthropomorphic phantom to conduct measurements of out-of-field doses in prone and supine position, using 50 Gy prescription dose intensity modulated radiation therapy (IMRT) and 3D-CRT plans. Measurements were made using multiple MOSFET dosimeters in various locations in the ipsilateral lung, the contralateral lung and in the contralateral breast. The closer the organ (or organ segment) was to the treatment volume, the more dose sparing was seen for prone vs supine positioning. Breast radiotherapy in the prone position results in a marked reduction in the dose to the proximal part of the ipsilateral lung, compared with treatment in the conventional supine position. This is true both for 3D-CRT and for IMRT. For IMRT, the maximum measured dose to the lung was reduced from 4 to 1.6 Gy, while for 3D-CRT, the maximum measured lung dose was reduced from 5 to 1.7 Gy. For the proximal part of the ipsilateral lung, as well as for the contralateral lung and the contralateral breast, there is little difference in the measured organ doses whether the treatment is given in the prone or the supine-position. The decrease in the maximum dose to the proximal part of the ipsilateral lung produced by prone position radiotherapy is of potentially considerable significance. The dose-response relation for radiation-induced lung cancer increases monotonically in the zero to 5-Gy dose range, implying that a major decrease in the maximum lung dose may result in a significant decrease in the radiation-induced lung cancer risk.

Peripheral doses in patients undergoing Cyberknife treatment for intracranial lesions. A single centre experience

BackgroundStereotactic radiosurgery/radiotherapy procedures are known to deliver a very high dose per fraction, and thus, the corresponding peripheral dose could be a limiting factor for the long term surviving patients. The aim of this clinical study was to measure the peripheral dose delivered to patients undergoing intracranial Cyberknife treatment, using the MOSFET dosimeters. The influence of the supplemental shielding, the number of monitor units and the collimator size to the peripheral dose were investigated.MethodsMOSFET dosimeters were placed in preselected anatomical regions of the patient undergoing Cyberknife treatment, namely the thyroid gland, the nipple, the umbilicus and the pubic symphysis.ResultsThe mean peripheral doses before the supplemental shielding was added to the Cyberknife unit were 51.79 cGy, 13.31 cGy and 10.07 cGy while after the shielding upgrade they were 38.40 cGy, 10.94 cGy, and 8.69 cGy, in the thyroid gland, the umbilicus and the pubic symphysis, respectively. The increase of the collimator size corresponds to an increase of the PD and becomes less significant at larger distances, indicating that at these distances the PD is predominate due to the head leakage and collimator scatter.ConclusionWeighting the effect of the number of monitor units and the collimator size can be effectively used during the optimization procedure in order to choose the most suitable treatment plan that will deliver the maximum dose to the tumor, while being compatible with the dose constraints for the surrounding organs at risk. Attention is required in defining the thyroid gland as a structure of avoidance in the treatment plan especially in patients with benign diseases.

Modification of an Implantable MOSFET Dosimeter to Act as a Fiducial Marker

In radiation therapy, the tumor and normal tissue response is highly dependent on accurate absorbed dose delivery to the intended target. Although ex vivo techniques are widely used to improve the accuracy of the dose delivered, it is impossible to verify that the dose has been delivered correctly without measuring the dose within the patient. The Dose Verification System (Sicel Technologies, Inc., Morrisville, NC), which consists of an implantable MOSFET dosimeter and a wireless reader, is currently the only commercial dosimetry system capable of measuring this in situ dose. Since the MOSFET dosimeter is implanted in the patient, it has been suggested that it could be used as not only a dosimeter but also a fiducial marker for image-guided radiation therapy (IGRT) provided that it could be seen on radiographs. Though visible on kilovoltage radiographs, the current design of the dosimeter is not visible on megavoltage radiographs limiting its usefulness in IGRT treatment. In this work, we modified the MOSFET dosimeter to increase its visibility on megavoltage radiographs. We then compared the visibility of the modified dosimeter with that of the commercial (unmodified) dosimeter and a traditional gold fiducial marker in an anthropomorphic phantom on an IGRT system. To ensure that the modification did not affect the dosimeter's ability to measure and transmit dose data, we compared the modified dosimeter's wireless read range, energy dependence, and angular dependence with those of the commercial dosimeter. We conclude that modifying the MOSFET dosimeter increased its visibility without affecting its dosimetric properties, thereby allowing a single implanted device to serve two critical roles in IGRT.

Clinical use of a novel in vivo 4D monitoring system for simultaneous patient motion and dose measurements

PurposeA new 4D invivo dosimetry tool, RADPOS, has been used on lung cancer patients to evaluate the feasibility of using the detectors to characterize variations in patient breathing patterns as well as to monitor daily variations in dose. Methods and materialsThe RADPOS system combines a MOSFET dosimeter with an electromagnetic positioning sensor for simultaneous measurement of real-time dose and spatial coordinates. Three RADPOS sensors were placed on patients’ chest and abdomen during a 4DCT and daily treatments. A fourth detector was also placed on the couch as reference. Position data were collected in real-time and total dose was read at the end of each fraction. ResultsSignificant deviations in surface motion have been found between the day of 4DCT and treatment fractions in 9 of 10 patients. Variations in daily dose ranged from 2.5 to 13.7cGy (2.8–14.0%) and results agreed with treatment plan values for all but three points. ConclusionsChanges in breathing motion have been found that emphasize a need for continued position monitoring. RADPOS measurements can be used to monitor such variations as well as to measure surface dose without any disruption to the treatment schedule or discomfort to patients.

Evaluation of an implantable MOSFET dosimeter designed for use with hypofractionated external beam treatments and its applications for breast and prostate treatments

An implantable metal-oxide semiconductor field effect transistors-based dosimeter has recently been developed for the in vivo monitoring of hypofractionated radiotherapy. This DVS-HFT dosimeter is designed for fraction sizes of 340-950 cGy and can also be used for bis in die fraction monitoring. The current work reports on the testing and evaluation of this dosimeter, including both its basic characteristics as well as its performance during simulated clinical treatment plans. The authors tested the dose rate dependence of this dosimeter (300 MU/min versus 600 MU/min), the treatment time dependence (4 min per treatment versus up to 60 min per treatment), and the dose and energy dependence (6 and 18 MV irradiations of 700-900 cGy per fraction). Additionally, they irradiated the detectors in-phantom with breast and prostate hypofractionated treatments. The detectors showed no significant dose rate, treatment time, energy, or dose dependence. Furthermore, the detectors were found to perform within manufacturer tolerances for all hypofractionated treatments examined, accurately reporting the measured dose (average disagreement of - 0.65%). These dosimeters appear well suited for in vivo monitoring of hypofractionated radiotherapy doses, and thereby, have the potential to improve patient care.

TH‐C‐301‐03: Utilization of MOSFET Dosimeters for Clinical Measurements in Radiology

Purpose: MOSFET dosimeters are attractive dosimeters for a number of applications in diagnostic radiology due primarily to their small size and the ability to rapidly read the resulting x‐ray dose. It is important to understand the characteristics of these unique dosimeters in order to accurately interpret the output and successful predict patient dose.Methods: The characteristics of MOSFET dosimeters have been evaluated for a variety of diagnostic radiology applications. Specific tests were designed to evaluate important MOSFET responses over the x‐ray energy range of interest (20–150 keV). Responses of these devices have been characterized for energy response, angular dependence, temperature, fading, dose linearity and sensitivity.Results: MOSFET dosimeters are available in configurations having differing physical geometry, sizes, sensitivity and absorbed dose limited usefulness. These dosimeters typically exhibit good angular response around their longitudinal axis and the variation of response as a function of ambient temperature is small. Some of the parameters that need to be carefully considered include the variation in energy response and sensitivity limits inherent to MOSFET dosimeters. These dosimeters require an accurate calibration exposure (or dose) to mV calibration for the energy source corresponding to the intended application. For example, a separate calibration needs to be developed for mammography applications for different accelerating potential and target/filter combinations. The overall sensitivity of these dosimeters can be a limitation for accurate dose assessment at clinical exposure levels. Examples of MOSFET applications for clinical dosimetry, when integrated with anthropomorphic phantoms are presented for mammography, radiography and computed tomography examples.Conclusions : MOSFET dosimeters can provide a convenient dosimetry tool capable of simultaneous measurement for multiple dosimetry points, usually up to twenty individual locations. Careful energy calibration is essential to accurate dosimetry interpretation and care should be exercised to ensure that exposures are large enough to provide consistent radiation measurement.

SU‐E‐T‐347: A Cost‐Effective Technique for Treating Penis Cancer with Delivery Accuracy Verified with In‐Vivo Measurement

Purpose: To report an in‐house cost‐effective, easily made and set up, yet accurate technique for treating penis cancer at a community freestanding cancer. Methods: Recently, a patient with penis cancer was under radiotherapy at our institution. The treated site was his penis only. The patient was simulated in supine position with his penis inserted in a custom supporting tool. This supporting tool is made of a regular candle that can be bought at any retail store. It measures about 10 cm in diameter and has a HU about −100. A hole slightly larger than the patientˈs penis was drilled in the middle with enough buildup wall left for a 6 MV beam used for the treatment plan. For the remaining space not occupied by the patientˈs penis, regular flour was filled in tightly, which has a HU about −200. The plan was done with an opposing lateral conformal field setup to get a uniform dose distribution across the treatment site. It makes essential no difference whether the heterogeneity is turned on or not in the dose calculation with Varianˈs Eclipse TPS. Four MOSFET dosimeters were placed on the surface of the penis for in‐vivo measurement for two consecutive fractions. The prescription was 180 cGy/fx. The mean dose for the skin (3 mm deep region from the surface contoured in Eclipse) was 182.9 cGy with a minimum of 179.3 cGy and a maximum of 185.8 cGy. Results: The in‐vivo measurement was 183.2 cGy +/− 1.4 cGy, including a port film with 2 MU. The measurement was in good agreement with the calculation. Conclusions: In‐vivo measurement proved the in‐house developed supporting tool to be cost‐effective, easily made, and easy to set up on a daily basis for accurately treating penis cancer.

TH‐A‐214‐11: Dose to Implantable Cardiac Devices from Cone Beam CT

Purpose: To quantify the cumulative dose contribution to implantable cardiac rhythm management devices (ICRMDs) from cone beam CT (CBCT) for patients undergoing radiotherapy and to investigate dose reduction strategies. Methods: An implantable pacemaker (IP) and implantable cardioverter‐defibrillator (ICD) with internal MOSFET dosimeters were used to measure the cumulative CBCT dose to the IP and ICD. Specifically, the MOSFET threshold voltage shifts resulting from standard clinical CBCT imaging protocols were recorded and compared to shifts caused by peripheral megavoltage dose associated with 6 MV IMRT treatment delivery. The dose assessments were performed on an anthropomorphic phantom with the IP or ICD placed in the left clavicular area both inside and outside of the CBCT field of view (FOV). Two dose reduction strategies were evaluated: the use of a 2 mm copper plate to shield the device during imaging and the use of partial‐angle scanning. Results: The average internal IP and ICD doses due to standard clinical CBCT imaging protocols with the device inside the FOV were in the 2.2 – 4.3 cGy and 0.8 – 1.9 cGy range, respectively, but could reach as high as 11.2 cGy per scan. Five centimeters outside the FOV, the dose can reach up to 10% of the in‐field dose in the cranio‐caudal direction and up to 70% in the medio‐ lateral direction. Copper shielding resulted in a 3‐fold reduction in total CBCT dose and a posterior 180 degree scan reduced the dose by a factor of 8 in comparison to a full 360 degree scan. Conclusions: The CBCT imaging dose to ICRMDs can be substantial and needs to be accounted for. Use of a MOSFET dosimeter during the first treatment fraction is an effective way of obtaining a reasonable upper limit of the average cumulative ICRMD dose. Partial posterior scans can greatly reduce this dose.

SU‐E‐T‐238: OnedosePlus MOSFET Dosimeter Performance in Tangential Field in Vivo Dosimtery

Purpose: To investigate the incident angle dependence of the OnedosePlus MOSFET dosimeter and to evaluate the dosimeterˈs performance in vivo dosimetry for tangential fields treatments. Methods: Two types of external beam radiation plan were generated based on a solid water phantom: one with a group of oblique beams directed toward the anterior surface of the phantom and other with two opposing half blocked beams directed parallel to the anterior surface. The second plan represented an extreme case of tangential field treatments. In the deliveries of the plans, OnedosePlus dosimeters were placed at various locations on the phantom anterior surface. Calibrated Radiochromic EBT2 film dosimeters were placed at the same locations of the phantom anterior surface and at the corresponding locations 1cm away from the anterior surface. The measured doses from the OnedosePlus dosimeters were compared against the corresponding doses in the plans and the film measurements. Results: As the incident beam angle increased from 0 to 90 degree to the CAX, the OnedosePlus reading increased 15%. The doses measured using the OnedosePlus dosimeters in the extreme tangential field plan did not have a clear correspondence to either the surface dose or the doses at 1 cm depth. The film readings were in close agreement with the TPS calculated doses with an average dose difference less than 5%. The OnedosePlus measured doses were higher than the TPS calculated doses at the 1cm depth by a constant (average: 31.45+/− 2.71cGy), and much higher than the surface film measurements by an average of 246%. Conclusions: OnedosePlus dosimeter readings have a strong dependence on the incident beam angle, especially when the beam angle is larger than 45 degree. The point of measurement for OnedosePlus dosimeter is unclear for the treatments involving tangential beams and it overestimates doses for the assumed point of measurement (1cm depth).

SU‐E‐T‐317: MOSFET Assessment of Radiation Dose Delivered to Mice Using the Small Animal Radiation Research Platform (SARRP)

Purpose: MOSFETS were employed in this study for in situ assessment of radiation dose delivered by the Small Animal Radiation Research Platform (SARRP) to mouse anatomical target volumes: brain, left kidney, left lung, liver, pancreas and prostate. Method and Materials:MOSFET detectors were calibrated with conversion factors (CF) determined for each MOSFET as a function of depth in tissue equivalent material. Calibrated MOSFET was surgically implanted post‐euthanasia into the mouse target volume, with minimal organ disruption. Using CBCT imaging and robotic motion, the mouse was then positioned with implanted MOSFET at the isocenter of the SARRP. The depth of the MOSFET from the CBCT image and corresponding CF were used in the SARRP dose calculation program to calculate the target dose for 220 kVp and 13 mA, without heterogeneity correction. Irradiations were conducted using a 0.15 cm copper filter. Three readings per MOSFET were obtained for each organ of three euthanized male mice. The measured dose was determined by averaging the MOSFET readings for that organ. The percentage difference between the measured and calculated dose was determined. Results: The results showed differences between the calculated and measured dose of between: +1.2 to 4.6 %, −3.3 to +4.8 %, +0.9 to +5.1 %, −5.9 to +2.5 %, ‐2.3 to +1.0 %, and −4.3 to +1.4 %, for the left kidney, liver, pancreas, prostate, left lung, and brain, respectively. Conclusions: The study indicates that, for 220 kVp x‐rays, the radiation dose delivered by the SARRP to characteristic target volumes in mice is within 6% of the calculated dose. The results provide a useful assessment of the need for tissue heterogeneity corrections in SARRP dose calculations for clinically relevant tumor model sites. The work also demonstrates the feasibility of using MOSFET dosimeters for in situ assessment of SARRP radiation dose delivery.

SU‐E‐T‐198: Evaluation of Dosimetric Characteristics of MOSFET Dosimeter for the Quality Assurance of Photon and Electron Beams

Purpose: To study the basic dosimetric characteristics of MOSFET dosimeter for its suitability on constructing the quality assurance phantom. Methods: For all the measurements, the high sensitivity MOSFET with standard bias settings (3mV/cGy) was used. Solid water phantoms were used for both photon and electron beams. Tests were performed for 60Co beams, 6 and 15MV photon beams and 9MeV electrons. The dose‐linearity of MOSFET was studied up to 500 cGy with small increment at lowerdoses. Results were compared with the farmer chamber for photons and PP chamber for electrons. Reproducibility of the MOSFET was studied for the absorbed dose of 100cGy. Beam symmetry was measured at 5‐cm off‐axis position for 15×15cm2 field and comparisons were made with the farmer chamber and EDR‐2 film for photons and PP chamber and Kodak Xomat‐V for electrons. Also, the beam quality index was derived by measuring the dose at 10 and 20cm depth for photon beams. Measurements were done for five different days and compared with ion chamber measurements. Results: The correlation co‐efficient (R2) for MOSFET and ion chamber linearity curve is 0.9999, 0.9999, 0.9998 and 0.9999 for 6MV, 15MV and 60Co beams and 9MeV electrons. High sensitivity MOSFET showed good linearity at lower doses even with standard bias settings. Reproducibility of the MOSFET is 1.680%, 1.815%, 1.7194% and 1.754% for 6MV, 15MV and 60Co beams and 9MeV electrons. Beam Symmetry measured with MOSFET showed maximum variation up to 4.2% where as ion chamber and EDR‐2 was within 2.7% and 3.1%. MOSFET over‐responded consistently at the off‐axis position. Quality index measured with MOSFET was within 0.701% and 1.355% for 6 and 15MV photons, whereas the ion chamber was within 0.450%. Conclusions: The high sensitivity MOSFET may be used as a dosimeter with proper cross‐calibration for the quality assurance phantom, which is under construction.

Thermal drift reduction with multiple bias current for MOSFET dosimeters

New thermal compensation methods suitable for p-channel MOSFET (pMOS) dosimeters with the usual dose readout procedure based on a constant drain current are presented. Measuring the source–drain voltage shifts for two or three different drain currents and knowing the value of the zero-temperature coefficient drain current, IZTC, the thermal drift of source–drain or threshold voltages can be significantly reduced. Analytical expressions for the thermal compensation have been theoretically deduced on the basis of a linear dependence on temperature of the parameters involved. The proposed thermal modelling has been experimentally proven. These methods have been applied to a group of ten commercial pMOS transistors (3N163). The thermal coefficients of the source–drain voltage and the threshold voltage were reduced from −3.0 mV °C−1, in the worst case, down to −70 µV °C−1. This means a thermal drift of −2.4 mGy °C−1 for the dosimeter. When analysing the thermal drifts of all the studied transistors, in the temperature range from 19 to 36 °C, uncertainty was obtained in the threshold voltage due to a thermal drift of ±9mGy (2 SD), a commonly acceptable value in most radiotherapy treatments. The procedures described herein provide thermal drift reduction comparable to that of other technological or numerical strategies, but can be used in a very simple and low-cost dosimetry sensor.

Comment on "The Clinical Features and Pathophysiology of Acute Radiation Dermatitis in Patients Receiving Tomotherapy"

Dear Editor: We read with interest the article by Lee et al.1, where a significantly more severe dermatitis was reported in patients receiving tomotherapy when compared to patients receiving radiation. However, the authors do not provide any reasons for their findings. Unfortunately, there is no information on contouring of the planning target volume (PTV); therefore, we can only speculate that grade 2 dermatitis in breast cancer patients might occur due to inclusion of voxels with low computer tomography (CT) numbers (air) as the target. Low density materials are less attenuating and require higher intensity of radiation for delivery of the same dose as to more dense tissues. If this is the case, the actual dose delivered to the surface layer might be higher than the prescription. In our institution, tomotherapy has been used clinically since September 2004, and we have not observed serious dermatitis in our tomotherapy patients, compared to conventional techniques. In order to verify the correspondence of calculated and actually delivered doses, either MOSFET dosimeters OneDose™ (Sicel Technologies, Morrisville, NC)2 or optically stimulated luminescence (OSL) dosimeters InLight® NanoDot™ (Landauer Inc., Glenwood, IL)3 are used during the first fraction of the treatment for lesions close to the surface. To stress the importance of careful contouring of the target and planning, we refer to one of our recent cases involving a 73-year-old male with a history of squamous cell carcinoma (SCC) on his scalp since 2004. He had undergone at least two surgical excisions with skin flaps in 2006 and 2008. He has had multiple recurrences of these lesions, mainly around flaps involving a large area of his scalp. The patient refused additional surgical procedures. He underwent photodynamic therapy (PDT) in 2007 for treatment of actinic keratosis of his scalp as an attempt at possible control of his SCC. However, due to skin irritation and pain from treatment, the patient did not tolerate PDT. Tomotherapy treatment was recommended in order to avoid critical organs, such as optic nerves, lens, and brain tissue. He underwent CT simulation and planning for tomotherapy treatment of his scalp lesion of 60 Gy in 1.8 Gy fractions over a period of 6 ½ weeks, resulting in the dose-volume histogram shown in Fig. 1A with good sparing of organs at risk. The PTV was contoured 2 mm below the surface in order to avoid air voxels in the target volume, as shown in Fig. 1B together with the planned dose distribution. During the course of tomotherapy, grade 1 radiation dermatitis developed after 4 weeks into treatment. The treating physician reviewed this patient during his course of treatment and recommended a skin moisturizing gel (hypo-allergenic Aloe Vera). Dry skin desquamation was observed in the irradiated area of the scalp during the last week of the therapy. He tolerated the course of the therapy without any treatment interruption. Two months after tomotherapy, the patient showed a clinically complete response, as shown in Fig. 2, and recovered from radiation dermatitis on follow-up assessment. To date, 5 months after completion of tomotherapy the patient has shown no disease recurrence. MRI of the brain showed no acute radiation effect on brain tissue. This example, with a very large superficial target, shows that careful contouring of targets close to the skin surface is necessary in order to avoid significant dermatitis. Fig. 1 (A) Cumulative relative dose-volume histogram and (B) dose distribution for the patient with large superficial target volume. GTV: gross tumor volume, PTV: planning target volume, unlabeled dose-volume histograms correspond to the right eye (purple), ... Fig. 2 Clinical photographs of the patient's scalp: (A) before and (B) 2 months after tomotherapy treatment.