Snout Position Research Articles

Technical Note: Dosimetric characteristics of the ocular beam line and commissioning data for an ocular proton therapy planning system at the Proton Therapy Center Houston.

To systematically analyze and present the properties of a small-field, double-scattering proton beam line intended to be used for the treatment of ocular cancer, and to provide configuration data for commission of the Eclipse Ocular Proton Planning System. Measurements were made using ionization chambers, diodes, and films to determine dose profiles and output factors of the proton beams of the beam line at the Proton Therapy Center Houston. In parallel, Monte Carlo simulations were performed to validate the measured data and to provide additional insight into detailed beam parameters that are hard to measure, such as field size factors and a comparison of output factors as a function of circular and rectangular fields. The presented data comprise depth dose profiles, including distal and proximal profiles used to configure the Eclipse Ocular Proton Planning system, distal fall-off widths, lateral profiles and penumbrae sizes, as well as output factors as a function of field size, SOBP width, range shifter thickness, snout position, and source-to-surface distance. We have completed a comprehensive characterization of the beam line. The data will be useful to characterize proton beams in clinical and experimental small-field applications.

Decadal Change in Glacier Area in the Chorabari Sub Watershed

This area looks at glacier area change in Chorabari Sub Watershed for 1962-1990 and 2000-2016 periods. Study area of Chorabari Sub Watershed extends between the latitudes of 28o-31oN to longitudes 77o-81°E. Chorabari Sub Watershed is the part of Mandakini basin and it has total number of 40 glaciers covered an area of 81.64 km 2 with the ice reserve of 5.9856 km 2 . The reduction in the glacier area has been observed on the lateral side of Chorabari glacier and not on the snout position. Overall reduction in the basin glacier area was observed 1.23 km 2 during the year 1990 to 2016. In addition, this paper describes a method for estimating the ice surface elevation changes using the SRTM (2000) and elevation data generated from topographic maps (1962) to quantify the ice thickness change for the 1962-2000 periods.

How embryos escape from danger: the mechanism of rapid, plastic hatching in red-eyed treefrogs.

Environmentally cued hatching allows embryos to escape dangers and exploit new opportunities. Such adaptive responses require a flexibly regulated hatching mechanism sufficiently fast to meet relevant challenges. Anurans show widespread, diverse cued hatching responses, but their described hatching mechanisms are slow, and regulation of timing is unknown. Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, escape from snake attacks and other threats by very rapid premature hatching. We used videography, manipulation of hatching embryos and electron microscopy to investigate their hatching mechanism. High-speed video revealed three stages of the hatching process: pre-rupture shaking and gaping, vitelline membrane rupture near the snout, and muscular thrashing to exit through the hole. Hatching took 6.5-49 s. We hypothesized membrane rupture to be enzymatic, with hatching enzyme released from the snout during shaking. To test this, we displaced hatching embryos to move their snout from its location during shaking. The membrane ruptured at the original snout position and embryos became trapped in collapsed capsules; they either moved repeatedly to relocate the hole or shook again and made a second hole to exit. Electron microscopy revealed that hatching glands are densely concentrated on the snout and absent elsewhere. They are full of vesicles in embryos and release most of their contents rapidly at hatching. Agalychnis callidryas' hatching mechanism contrasts with the slow process described in anurans to date and exemplifies one way in which embryos can achieve rapid, flexibly timed hatching to escape from acute threats. Other amphibians with cued hatching may also have novel hatching mechanisms.

SU‐E‐T‐566: Neutron Dose Cloud Map for Compact ProteusONE Proton Therapy

Purpose:To establish the base line of neutron cloud during patient treatment in our new compact Proteus One proton pencil beam scanning (PBS) system with various beam delivery gantry angles, with or without range shifter (RS) at different body sites. Pencil beam scanning is an emerging treatment technique, for the concerns of neutron exposure, this study is to evaluate the neutron dose equivalent per given delivered dose under various treatment conditions at our proton therapy center.Methods:A wide energy neutron dose equivalent detector (SWENDI‐II, Thermo Scientific, MA) was used for neutron dose measurements. It was conducted in the proton therapy vault during beam was on. The measurement location was specifically marked in order to obtain the equivalent dose of neutron activities (H). The distances of 100, 150 and 200 cm at various locations are from the patient isocenter. The neutron dose was measured of proton energy layers, # of spots, maximal energy range, modulation width, field radius, gantry angle, snout position and delivered dose in CGE. The neutron dose cloud is reproducible and is useful for the future reference.Results:When distance increased the neutron equivalent dose (H) reading did not decrease rapidly with changes of proton energy range, modulation width or spot layers. For cranial cases, the average mSv/CGE was about 0.02 versus 0.032 for pelvis cases. RS will induce higher H to be 0.10 mSv/CGE in average.Conclusion:From this study, neutron per dose ratio (mSv/CGE) slightly depends upon various treatment parameters for pencil beams. For similar treatment conditions, our measurement demonstrates this value for pencil beam scanning beam has lowest than uniform scanning or passive scattering beam with a factor of 5. This factor will be monitored continuously for other upcoming treatment parameters in our facility.

Evaluating the schmidt hammer as a method for distinguishing the relative age of late holocene moraines: svellnosbreen, jotunheimen, norway

In this study we used the chmidt hammer to assess the relative exposure ages of previously dated ittle ce ge moraines deposited by vellnosbreen glacier in otunheimen southern orway. chmidt hammer measurements were taken on moraine surfaces which have been deglaciated for 79–259 years since the ittle ce ge, to reconstruct and date previous snout positions. Dating used a typical linear ittle ce ge age‐calibration curve and the ages of unknown surfaces were predicted based on chmidt hammer measurements with the support of lichenometric dates. Calibrated ages exhibited potential dating errors of ±63–103 years, which were estimated using 95% confidence intervals associated with two control points based on moraines of known ages. chmidt hammer R‐value variability was identified and sources of significant errors were attributed to variations in boulder surface texture and the incorporation of weathered boulders into moraines through push mechanisms and paraglacial reworking.

Contrasting Signals of Glacier Changes in Zanskar Valley, Jammu & Kashmir, India Using Remote Sensing and GIS

The study of advancement and recession of the glaciers in the Himalayas is essential due to their contrasting response towards climatic change. In the present study, Survey of India (SOI) topographical maps of 1962, IRS: LISS-III image of 2001 and LANDSAT-5: TM (Thematic Mapper) image of 2009 were used to analyze the glacier fluctuations in a part of Zanskar valley. The analysis carried out on 212 glaciers indicated decrease of 57 km2 (8 %) of glacier area over many glacier which was partly compensated with area increase by 42 km2 (6 %) in other glaciers, resulting an overall glacier area decrease by only 15 km2 (2 %) from 1962–2001. Due to glacier fragmentation the number of glaciers increased from 212 in 1962 to 238 by 2001. Although majority of glaciers (88 %) exhibited retreat (up to 13 my−1), minor advancement ( 5 km2) located over wider altitudinal range (700 m–1,000 m) whereas smaller glaciers ( 5 km2). During 2001–2009 majority of the glaciers were apparently stable in terms of their area and snout position indicating less impact of climate forcing in parts of Zanskar valley as compared to other parts of the Himalaya.

Are the Himalayan glaciers retreating

The Himalayan mountain system to the north of the Indian land mass with arcuate strike of NW-SE for about 2400 km holds one of the largest concentration of glaciers outside the polar regions in its high-altitude regions. Perennial snow and ice-melt from these frozen reservoirs is used in catchments and alluvial plains of the three major Himalayan river systems, i.e. the Indus, Ganga and Brahmaputra for irrigation, hydropower generation, production of bio-resources and fulfilling the domestic water demand. Also, variations in the extent of these glaciers are understood to be a sensitive indicator of climatic variations of the earth system and might have implications on the availability of water resources in the river systems. Therefore, mapping and monitoring of these freshwater resources is required for the planning of water resources and understanding the impact of climatic variations. Thus a study has been carried out to find the change in the extent of Himalayan glaciers during the last decade using IRS LISS III images of 2000/01/02 and 2010/11. Two thousand and eighteen glaciers representing climatically diverse terrains in the Himalaya were mapped and monitored. It includes glaciers of Karakoram, Himachal, Zanskar, Uttarakhand, Nepal and Sikkim regions. Among these, 1752 glaciers (86.8%) were observed having stable fronts (no change in the snout position and area of ablation zone), 248 (12.3%) exhibited retreat and 18 (0.9%) of them exhibited advancement of snout. The net loss in 10,250.68 sq. km area of the 2018 glaciers put together was found to be 20.94 sq. km or 0.2% (±2.5% of 20.94 sq. km).

Recent changes in the snout position and surface velocity of Gangotri glacier observed from space

Glacier mass variations have a direct impact on some of the key components of the global water cycle, including sea level rise and freshwater availability. Apart from being one of the largest Himalayan glaciers, Gangotri is one of the sources of water for the Ganges river, which has a considerable influence on the socioeconomic structure of a largely over-populated catchment area accounting for ∼26% of India’s landmass. In this study, we present the most recent assessment of the Gangotri glacier dynamics, combining the use of interferometric techniques on synthetic aperture radar data and sub-pixel offset tracking on Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite imagery. Results show that on average, the Gangotri glacier snout has receded at a rate of 21.3 ± 3 m year−1 over a period of 6 years (2004–2010). While glacier surface velocity near the snout is estimated to be between 24.8 ± 2.3 and 28.9 ± 2.3 m year−1, interior portions of the glacier recorded velocities in the range of 13.9 ± 2.3 to 70.2 ± 2.3 m year−1. Further, the average glacier surface velocity in the northern (lower) portions (28.1 ± 2.3 m year−1) is observed to be significantly lower than in the southern (higher) portions (48.1 ± 2.3 m year−1) of the Gangotri glacier. These values are calculated with an uncertainty of less than 5 m year−1. Results also highlight a consistent retreat and non-uniform dynamics of the Gangotri glacier.

Glacier response to current climate change and future scenarios in the northwestern Italian Alps

We analyze longtime series of annual snout positions of several valley glaciers in the northwestern Italian Alps, together with a high-resolution gridded dataset of temperature and precipitation available for the last 50 years. Glacier snout fluctuations are on average negative during this time span, albeit with a period of glacier advance between about 1970 and 1990. To determine which climatic variables best correlate with glacier snout fluctuations, we consider a large set of seasonal predictors, based on our climatic dataset, and determine the most significant drivers by a stepwise regression technique. This in-depth screening indicates that the average glacier snout fluctuations strongly respond to summer temperature and winter precipitation variations, with a delay of 5 and 10 year, respectively. Snout fluctuations display also a significant (albeit weak) response to concurrent (same year) spring temperature and precipitation conditions. A linear regressive model based on these four climatic variables explains up to 93 % of the variance, which becomes 89 % when only the two delayed variables are taken into account. When employed for out-of-sample projections, the empirical model displays high prediction skill, and it is thus used to estimate the average glacier response to different climate change scenarios (RCP4.5, RCP8.5, A1B), using both global and regional climate models. In all cases, glacier snout fluctuations display a negative trend, and the glaciers of this region display an accelerated retreat, leading to a further regression of the snout position. By 2050, the retreat is estimated to be between about 300 and 400 m with respect to the current position. Glacier regression is more intense for the RCP8.5 and A1B scenarios, as it could be expected from the higher severity of these emission pathways.

Study of temporal changes in snout position and wet snow line for Gangotri glacier using remote sensing, ground observations and meteorological data

The temporal changes (1891-2010) in snout position of Gangotri glacier were monitored using Landsat images, ground observations and SoI (Survey of India) toposheets. Temporal changes (2001-09) in wet snow line (wsl) altitude were also observed using LISS-III images and high resolution (6 m pixel size) DEM generated using Cartosat stereopair. These observations were correlated and justified with available snow meteorological data. OCGM (Oerlemans Coarse Grained Model) was run in parallel using meteorological inputs to support the obtained glacier retreat using Landsat data during 2001-10. The retreats observed were 73 m and 54 m respectively using Landsat data and OCGM. The difference was less than 20 m (less than a pixel of Landsat image) thus supporting the usefulness of remote sensing observations for monitoring of rough and hard to access terrain. The wsl altitude showed an overall ascending trend during melting season (May to September). Some atypical fluctuations in this trend were observed for different years which could be explained using available meteorological records.

Chronology of late Quaternary glaciation in the Pindar valley, Alaknanda basin, Central Himalaya (India)

Palaeoglacial reconstruction based on geomorphological mapping in the Pindari glacier valley, Alaknanda basin (Central Himalaya), has revealed five glacial stages with decreasing magnitude. The oldest and most extensive stage-I glaciation deposited sediments at ∼2200masl (Khati village). The stage-II glaciation was around 7km long and luminescence dated to 25±2ka, and has deposits at 3200masl (Phurkia village). Stage-III glaciation is represented by degraded linear moraine ridges and is dated to 6±1ka and its remnants can be found around 3850masl. A sharp crested crescentic moraine extending from around 3650 masl to 3900 masl is attributed to stage-IV glaciation and is dated to 3±1ka. Following this, there appears to have been a gradual recession in Pindari glacier as indicated by four sharp crested unconsolidated moraines (stage-V) on the valley floor which abuts the stage-IV moraine.We suggest that the stage-I glaciation occurred during the cool and wet Marine Isotopic Stage 3/4 (MIS-3/4), stage-II glaciations began with the onset of MIS-2, whereas the stage-III and IV glaciations occurred during the mid-to late Holocene (MIS-1). We speculate that the first sharp crested unconsolidated moraines around 3600masl correspond to the later phase of the Little Ice Age (LIA). Historical data suggests that the remaining three ridges represent Pindari glacier snout positions at 1906, 1958 and 1965. We argue that the late Quaternary glaciations in the Pindar valley were modulated by changing insolation and summer monsoon intensity including the LIA, whereas the 20th century recessional trends can be attributed to post-LIA warming.

A clinically feasible method for the detection of potential collision in proton therapy

Potential collision between the patient∕couch and the gantry could delay the start of the treatment and reduce clinical efficiency. The ability to accurately detect possible collisions during the treatment planning phase is desired. Such collision detection should account for the specific proton gantry design, the treatment beam configuration, couch orientation, and the patient specific geometry. In this paper the authors developed an approach to detect possible patient-machine collisions using patient treatment plan data. The geometry of the machine and the patient was reconstructed relative to the isocenter of the proton treatment room. The surface contour of the gantry was first captured from the proton computer aided design and reconstructed to account for specific gantry rotation, snout position, collimator rotation, and range compensator dimensions based on the patient treatment plan data. The patient body and couch contours were captured from the patient's CT DICOM structure file. They were reconstructed relative to the isocenter taking into account treatment couch rotation. For potential collision that occurs at body portions where no CT images exist, scout images are used to construct the body contour. A software program was developed using a ray casting algorithm that was applied to detect collisions by determining if any of the patient and couch contour points fall into the spatial polygons formed by the proton gantry surfaces. Twenty-four patient plans with or without potential collisions were retrospectively identified and analyzed using the collision detection software. In addition, five collision cases were artificially generated using an anthropomorphic phantom. The program successfully detected the collisions in all cases. The calculation time for each case was within 20 s. The software program was implemented in the authors' clinic to detect patient-gantry or gantry-couch collisions in the treatment planning phase. The authors developed a fast and clinically feasible patient-specific collision detection program for proton therapy based on a ray casting algorithm. If incorporated during the treatment planning phase it may lead to improved clinical efficiency. This methodology could also be applied to patient collision detection in photon therapy.

SU-E-T-45: Can We Model Proton Beam Output Factors Accurately without Measurements?

Purpose: Currently, no commercial protontreatment planning systems have been FDA‐approved for Monitor Unit (MU) prediction. Patient and field specific MU numbers and output factors are therefore determined by either measurements or in‐house modeling. The purpose of this project is to assess the modeling accuracy and determine in which cases measurements are necessary. Methods: We developed an in‐house analytical model to predict output factors for patient specific proton fields, taking into account the effect of proton range, modulation width, and field size on the dose output. A refined model, which incorporated more commissioning data and accounted for the effects of scanning area and field size, was developed later and retrospectively applied to the fields. Model‐predicted and measured output factors were compared for 1074 proton fields. Causes for the differenceswere analyzed for differences of 2.5% or more, which would be used for future improvement and determine in which cases where measurements are needed. Results: For the 1074 patient fields we analyzed, the refined model predicted output factors within 2% for 91.6% of fields, 2.5% for 96.9% for fields, and 3% for 99.7% of fields, compared to 85.2%, 93.6 and 98.2% respectively for the old model. Large differences between modeling and measurements (>2.5%) typically occurred for shallow range, large modulation, and/or irregular shaped proton fields. Causes for differences were analyzed, which mainly include dose rate dependence, off axis fields, data interpolation, and snout position effects. Conclusion: The study demonstrated that the analytical model can predict output factors accurately for most patient specific proton fields. For over 90% of proton fields, measurements may not be necessary, thus improving work flow and saving valuable proton beam time. It is possible to further improve modeling accuracy by incorporating more commissioning data, dose rate effects and other factors into the output factor model.

Measurements of neutron dose equivalent for a proton therapy center using uniform scanning proton beams

Neutron exposure is of concern in proton therapy, and varies with beam delivery technique, nozzle design, and treatment conditions. Uniform scanning is an emerging treatment technique in proton therapy, but neutron exposure for this technique has not been fully studied. The purpose of this study is to investigate the neutron dose equivalent per therapeutic dose, H/D, under various treatment conditions for uniform scanning beams employed at our proton therapy center. Using a wide energy neutron dose equivalent detector (SWENDI-II, ThermoScientific, MA), the authors measured H/D at 50 cm lateral to the isocenter as a function of proton range, modulation width, beam scanning area, collimated field size, and snout position. They also studied the influence of other factors on neutron dose equivalent, such as aperture material, the presence of a compensator, and measurement locations. They measured H/D for various treatment sites using patient-specific treatment parameters. Finally, they compared H/D values for various beam delivery techniques at various facilities under similar conditions. H/D increased rapidly with proton range and modulation width, varying from about 0.2 mSv/Gy for a 5 cm range and 2 cm modulation width beam to 2.7 mSv/Gy for a 30 cm range and 30 cm modulation width beam when 18 × 18 cm(2) uniform scanning beams were used. H/D increased linearly with the beam scanning area, and decreased slowly with aperture size and snout retraction. The presence of a compensator reduced the H/D slightly compared with that without a compensator present. Aperture material and compensator material also have an influence on neutron dose equivalent, but the influence is relatively small. H/D varied from about 0.5 mSv/Gy for a brain tumor treatment to about 3.5 mSv/Gy for a pelvic case. This study presents H/D as a function of various treatment parameters for uniform scanning proton beams. For similar treatment conditions, the H/D value per uncollimated beam size for uniform scanning beams was slightly lower than that from a passive scattering beam and higher than that from a pencil beam scanning beam, within a factor of 2. Minimizing beam scanning area could effectively reduce neutron dose equivalent for uniform scanning beams, down to the level close to pencil beam scanning.

Evaluating patterns of temporal glacier changes in Greater Himalayan Range, Jammu & Kashmir, India

To account for the variable response of the Himalayan glaciers towards climatic warming during the recent past, an attempt has been made in the present study to evaluate the changes in glacier area and shift in glacier snout position of selected glaciers in a part of the Greater Himalayan Range (GHR), Jammu & Kashmir (J&K), India. Multi-temporal satellite images of different years viz. 1975, 1989, 1992, 2001 and 2007 were used for mapping the boundaries of glaciers. Among the three observation periods (1975–1989/1992, 1989/1992–2001 and 2001–2007), during 1989/1992–2001 the majority of the glaciers exhibited considerable decrease in area. In contrast during 2001–2007, some glaciers exhibited increase in area indicating comparatively cooler climatic conditions as compared to the previous period. With reference to snout retreat, all the glaciers had a fluctuating trend of retreat during the observation periods although the retreat rate was higher during 1989/1992–2001 in some glaciers.

Change of Tipra Glacier in the Garhwal Himalaya, India, between 1962 and 2008

Systematic observations of Himalayan glaciers over the last few decades provide reliable indications of continuous shrinkage of most of the glaciers. Changes in mass, volume, area and length of glaciers are reported, but an up-to-date regional assessment of glacier changes is lacking. In the present study, satellite data, maps and ground-based measurements have been used to obtain the snout retreat and surface changes of the Tipra Glacier in the Alaknanda river basin of the Garhwal Himalaya for the period 1962–2008. The study reveals that a large part of the glacier has been detached from the main trunk and separated into the Tipra (7.5 km2) and Rataban (7.4 km2) Glaciers as it had one outlet (snout) in 1987. Between 1962 and 2002 estimated surface area reduced by ~18% and snout retreat was ~535 m with an average rate of 13.4 m a−1. Measurement of snout positions of the Tipra and Rataban Glaciers from 2002 to 2008 indicates an enhanced annual retreat of 21.3 and 21.2 m a−1, respectively. Total frontal area vacated during this period was calculated to be 0.084 km2 for Tipra Glacier and 0.028 km2 for Rataban Glacier. The estimated Equilibrium Line Altitude (ELA) rise was 76 m for the Tipra Glacier and 57 m for the Rataban Glacier. Accumulation Area Ratio (AAR) was calculated as 0.47 for the Tipra Glacier and 0.49 for the Rataban Glacier, during the study period. The observations compared with the other studies carried out in the region show a significant reduction in glacier areas. The increased retreat rate of glacier snouts is probably a direct consequence of global warming. The present snouts of the Tipra and Rataban Glaciers are located at altitudes of 3865 and 4120 m, respectively.

MO‐F‐BRA‐02: Measurements of Neutron Dose Equivalent for Uniform Scanning Proton Beams

Purpose: Neutron exposure is of concern in proton therapy, and varies with beam delivery technique and treatment conditions. The purpose of this study is to investigate the neutron dose equivalent per therapeutic dose, H/D, under various treatment conditions for uniform scanning beams employed at our proton therapy center. Methods: Using a wide energy neutron dose equivalent detector (SWENDI‐ II, ThermoScientific, MA), we measured the neutron dose equivalent as a function of proton range, modulation width, beam scanning area, collimation aperture size and snout position. The influence of other factors on neutron dose equivalent, such as aperture material and the presence of a compensator were also evaluated. Neutron dose equivalents for various treatment sites were estimated. Comparison among different proton facilities and modalities (passive scattering, uniform scanning and pencil beam scanning) was performed. Results: The H/D values for uniform scanning beams varied from about 0.2 mSv/Gy for a 5 cm range and 2 cm SOBP width beam to 2.3 mSv/Gy for 30 cm range and 30 cm modulation width beam. H/D increased rapidly with proton range and modulation width, and decreased slowly with aperture size and snout retraction. H/D increased almost proportionally to the beam scanning area. The presence of a compensator actually reduced the H/D compared to that without a compensator present. Aperture material and compensator material also has an influence on neutron dose equivalent, but the influence is relatively small. Conclusions: This study presents H/D as a function of treatment parameters for uniform scanning proton beams. For similar treatment conditions, the H/D value was slightly lower than that from the passive scattering beam but higher than that from a spot scanning beam. Minimizing the beam scanning area could effectively reduce neutron dose equivalent for uniform scanning beams, down to the level of that generated in spot scanning.

SU-E-T-51: Patient Collision Detection in Proton Therapy during Treatment Planning Phase

Purpose: Collision detection of the patient with the proton gantry during treatment planning phase can reduce re‐planning effort and patient setup time. The collision detection in proton therapy should account for specific proton gantry design, treatment beam configuration, and patient geometry. Methods: The coordinates of the surface contour of the gantry head were captured from the proton CAD design and reconstructed relative to the isocenter. This reconstruction accounts for patient specific gantry rotation, snout position, collimator rotation and compensator dimensions based on patient's treatment plan. The patient's body contour and couch, captured from the CT structure DICOM file and CAD design, were also reconstructed relative to the isocenter while considering the couch angle from the treatment plan. The ray‐casting algorithm was applied to monitor collisions by determining if any of the patient and couch body points falls into the polygon formed by the proton gantry spatial contour. Results: A software program was developed for patient/couch‐gantry collision detection in proton therapy. We have successfully predicted collisions in real and artificially constructed treatment plan cases. The collision detection program is being clinically validated as an accurate and quick method to predict patient‐gantry collision in proton therapy. Conclusions: An accurate and quick patient‐specific collision detection program for proton therapy can be implemented and applied in treatment planning phase using proton CAD design and patient CTimages.

Commissioning of output factors for uniform scanning proton beams

Current commercial treatment planning systems are not able to accurately predict output factors and calculate monitor units for proton fields. Patient-specific field output factors are thus determined by either measurements or empirical modeling based on commissioning data. The objective of this study is to commission output factors for uniform scanning beams utilized at the ProCure proton therapy centers. Using water phantoms and a plane parallel ionization chamber, the authors first measured output factors with a fixed 10 cm diameter aperture as a function of proton range and modulation width for clinically available proton beams with ranges between 4 and 31.5 cm and modulation widths between 2 and 15 cm. The authors then measured the output factor as a function of collimated field size at various calibration depths for proton beams of various ranges and modulation widths. The authors further examined the dependence of the output factor on the scanning area (i.e., uncollimated proton field), snout position, and phantom material. An empirical model was developed to calculate the output factor for patient-specific fields and the model-predicted output factors were compared to measurements. The output factor increased with proton range and field size, and decreased with modulation width. The scanning area and snout position have a small but non-negligible effect on the output factors. The predicted output factors based on the empirical modeling agreed within 2% of measurements for all prostate treatment fields and within 3% for 98.5% of all treatment fields. Comprehensive measurements at a large subset of available beam conditions are needed to commission output factors for proton therapy beams. The empirical modeling agrees well with the measured output factor data. This investigation indicates that it is possible to accurately predict output factors and thus eliminate or reduce time-consuming patient-specific output measurements for proton treatments.

Reconstruction of the moraine dammed lake, based on field evidence and paleo-history, Samudra Tapu Glacier, Chandra Basin, Himachal Pradesh

Peri-glacial studies of the Samudra Tapu glacier reveal three stages of well preserved moraines which are also marked by fluvial overprinting. The signatures of fluvial outwashes over the otherwise morainal deposits have been utilized in estimating the extent of palaeo-lake in the valley of Samudra Tapu glacier of the Chandra basin. Distance and thickness of snout of the glacier and its morainal deposits is estimated by the Laser Range Finder. With the change in the position of the snout there is a change in the extent and orientation of the lake. Recent expedition to the glacier confirms this process to be continuing. The study is important in reconstructing formation of the lake with respect to snout position of the glacier. Based on ground truth, fast melting of the snout is confirmed from year 2004 onwards, with the development of fresh end/terminal moraine. The study reveals that the snout region of the Samudra Tapu glacier is thinning and retreating at an alarming rate and is enlarging the lake extent. The development of fresh terminal moraine has the potential to impound the fast melting glacial waters which in turn can pose serious threat to the downstream regions of the Chandra valley.