Exit Dose Research Articles

Dosimetric comparison between proton and photon beams in the moving gap region in cranio-spinal irradiation (CSI)

Purpose. To investigate the moving gap region dosimetry in proton beam cranio-spinal irradiation (CSI) to provide optimal dose uniformity across the treatment volume. Material and methods. Proton beams of ranges 11.6 cm and 16 cm are used for the spine and the brain fields, respectively. Beam profiles for a 30 cm snout are first matched at the 50% level (hot match) on the computer. Feathering is simulated by shifting the dose profiles by a known distance two successive times to simulate a 2 × feathering scheme. The process is repeated for 2 mm and 4 mm gaps. Similar procedures are used to determine the dose profiles in the moving gap for a series of gap widths, 0–10 mm, and feathering step sizes, 4–10 mm, for a Varian iX 6MV beam. The proton and photon dose profiles in the moving gap region are compared. Results. The dose profiles in the moving gap exhibit valleys and peaks in both proton and photon beam CSI. The dose in the moving gap for protons is around 100% or higher for 0 mm gap, for both 5 and 10 mm feathering step sizes. When the field gap is comparable or larger than the penumbra, dose minima as low as 66% is obtained. The dosimetric characteristics for 6 MV photon beams can be made similar to those of the protons by appropriately combining gap width and feathering step size. Conclusion. The dose in the moving gap region is determined by the lateral penumbras, the width of the gap and the feathering step size. The dose decreases with increasing gap width or decreasing feathering step size. The dosimetric characteristics are similar for photon and proton beams. However, proton CSI has virtually no exit dose and is beneficial for pediatric patients, whereas with photon beams the whole lung and abdomen receive non-negligible exit dose.

Proton therapy for non-small cell lung cancer: Current evidence and future directions.

Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related death worldwide. Radiation dose escalation can improve survival in NSCLC patients but is often limited by adverse effects. One promising radiotherapy modality is proton radiotherapy, which, because of its physical characteristics, delivers minimal exit dose beyond the target volume and thus results in better sparing of normal tissues than does photon radiotherapy. Passive-scattering proton therapy and intensity-modulated proton therapy have shown promise in the treatment of early-stage and locally advanced non-small cell lung cancer. However, more studies are needed to optimize proton therapy, particularly intensity-modulated proton therapy, to address motion and density changes and to guide appropriate patient selection.

A simplified technique for delivering total body irradiation (TBI) with improved dose homogeneity

Total body irradiation (TBI) with megavoltage photon beams has been accepted as an important component of management for a number of hematologic malignancies, generally as part of bone marrow conditioning regimens. The purpose of this paper is to present and discuss the authors' TBI technique, which both simplifies the treatment process and improves the treatment quality. An AP/PA TBI treatment technique to produce uniform dose distributions using sequential collimator reductions during each fraction was implemented, and a sample calculation worksheet is presented. Using this methodology, the dosimetric characteristics of both 6 and 18 MV photon beams, including lung dose under cerrobend blocks was investigated. A method of estimating midplane lung doses based on measured entrance and exit doses was proposed, and the estimated results were compared with measurements. Whole body midplane dose uniformity of ±10% was achieved with no more than two collimator-based beam modulations. The proposed model predicted midplane lung doses 5% to 10% higher than the measured doses for 6 and 18 MV beams. The estimated total midplane doses were within ±5% of the prescribed midplane dose on average except for the lungs where the doses were 6% to 10% lower than the prescribed dose on average. The proposed TBI technique can achieve dose uniformity within ±10%. This technique is easy to implement and does not require complicated dosimetry and/or compensators.

Strict X-ray beam collimation for facial bones examination can increase lens exposure.

It is well accepted that collimation is a cost-effective dose-reducing tool for X-ray examinations. This phantom-based study investigated the impact of X-ray beam collimation on radiation dose to the lenses of the eyes and thyroid along with the effect on image quality in facial bone radiography. A three-view series (occipitomental, occipitomental 30 and lateral) was investigated, and radiation doses to the lenses and thyroid were measured using an Unfors dosemeter. Images were assessed by six experienced observers using a visual grading analysis and a total of 5400 observations were made. Strict collimation significantly (p<0.0001) reduced the radiation dose to the lenses of the eyes and thyroid when using a fixed projection-specific exposure. With a variable exposure technique (fixed exit dose, to simulate the behaviour of an automatic exposure control), while strict collimation was again shown to reduce thyroid dose, higher lens doses were demonstrated when compared with larger fields of exposure. Image quality was found to significantly improve using strict collimation, with observer preference being demonstrated using visual grading characteristic curves. The complexities of optimising radiographic techniques have been shown and the data presented emphasise the importance of examining dose-reducing strategies in a comprehensive way.

Calculation of midplane dose for total body irradiation from entrance and exit dose MOSFET measurements

This work is the development of a MOSFET based surface in vivo dosimetry system for total body irradiation patients treated with bilateral extended SSD beams using PMMA missing tissue compensators adjacent to the patient. An empirical formula to calculate midplane dose from MOSFET measured entrance and exit doses has been derived. The dependency of surface dose on the air-gap between the spoiler and the surface was investigated by suspending a spoiler above a water phantom, and taking percentage depth dose measurements (PDD). Exit and entrances doses were measured with MOSFETs in conjunction with midplane doses measured with an ion chamber. The entrance and exit doses were combined using an exponential attenuation formula to give an estimate of midplane dose and were compared to the midplane ion chamber measurement for a range of phantom thicknesses. Having a maximum PDD at the surface simplifies the prediction of midplane dose, which is achieved by ensuring that the air gap between the compensator and the surface is less than 10 cm. The comparison of estimated midplane dose and measured midplane dose showed no dependence on phantom thickness and an average correction factor of 0.88 was found. If the missing tissue compensators are kept within 10 cm of the patient then MOSFET measurements of entrance and exit dose can predict the midplane dose for the patient.

The measurement of entrance and exit doses with in-vivo dosimetry in head and neck cancers and comparison with treatment planning doses

The measurement of entrance and exit doses with in-vivo dosimetry in head and neck cancers and comparison with treatment planning doses

Technical advances and pitfalls in head and neck radiotherapy.

Intensity Modulated Radiotherapy (IMRT) is the standard of care in the treatment of head and neck squamous cell carcinomas (HNSCC) based on level 1 evidence. Technical advances in radiotherapy have revolutionized the treatment of HNSCC, with the most tangible gain being a reduction in long term morbidity. However, these benefits come with a serious and sobering price. Today, there is a greater chance of missing the target/tumor due to uncertainties in target volume definition by the clinician that is demanded by the highly conformal planning process involved with IMRT. Unless this is urgently addressed, our patients would be better served with the historically practiced non conformal radiotherapy, than IMRT which promises lesser morbidity. Image guided radiotherapy (IGRT) ensures the level of set up accuracy warranted to deliver a highly conformal treatment plan and should be utilized with IMRT, where feasible. Proton therapy has a theoretical physical advantage over photon therapy due to a lack of “exit dose”. However, clinical data supporting the routine use of this technology for HNSCC are currently sparse. The purpose of this article is to review the literature, discuss the salient issues and make recommendations that address the gaps in knowledge.

Proton therapy in pediatric brain tumors

Proton beam therapy (PBT) was initiated in the particle accelerator located in the physics laboratory. As this modality moves from the research laboratory to hospital-based facilities, there has been a rapid growth in the number of proton medical facilities worldwide. The clinical advantage of proton radiotherapy over photon radiotherapy is the marked reduction in the radiation dose to the normal tissues due to the absence of an exit dose with a proton Bragg peak. Pediatric patients are considered to have the maximum clinical gain with the use of protons because their normal tissues will keep developing and growing with an anticipated long remaining life. In pediatric brain tumors, PBT is used aiming to preserve neurocognitive function as much as possible. In this article, the basic principles and benefits of PBT are introduced with the latest available clinical data on PBT for pediatric brain tumors.

Malignant induction probability maps for radiotherapy using X-ray and proton beams

The aim of this study was to display malignant induction probability (MIP) maps alongside dose distribution maps for radiotherapy using X-ray and charged particles such as protons. Dose distributions for X-rays and protons are used in an interactive MATLAB® program (MathWorks, Natick, MA). The MIP is calculated using a published linear quadratic model, which incorporates fractionation effects, cell killing and cancer induction as a function of dose, as well as relative biological effect. Two virtual situations are modelled: (a) a tumour placed centrally in a cubic volume of normal tissue and (b) the same tumour placed closer to the skin surface. The MIP is calculated for a variety of treatment field options. The results show that, for protons, the MIP increases with field numbers. In such cases, proton MIP can be higher than that for X-rays. Protons produce the lowest MIPs for superficial targets because of the lack of exit dose. The addition of a dose bath to all normal tissues increases the MIP by up to an order of magnitude. This exploratory study shows that it is possible to achieve three-dimensional displays of carcinogenesis risk. The importance of treatment geometry, including the length and volume of tissue traversed by each beam, can all influence MIP. Reducing the volume of tissue irradiated is advantageous, as reducing the number of cells at risk reduces the total MIP. This finding lends further support to the use of treatment gantries as well as the use of simpler field arrangements for particle therapy provided normal tissue tolerances are respected.

A feasibility study of using couch‐based real time dosimetric device in external beam radiotherapy

Measurement of actual dose delivered during radiotherapy treatment aids in checking the accuracy of dose delivered to the patient. In this study, a couch-based real time dosimetric device has been proposed to measure the exit or entrance dose to a patient during external beam radiotherapy. The utility and feasibility of such a device using a 2D array of diodes has been demonstrated. Two MAPCHECK devices: MAPCHECK (1175) and MAPCHECK 2 (both SunNuclear) were embedded in a foam block in the treatment couch of a Varian 21iX linear accelerator. The angular dependence of the detector response for both devices was studied before implementing the MAPCHECKs for experimental purposes. An Alderson Rando head phantom was scanned with the MAPCHECK and MAPCHECK 2 devices separately and four different treatment plans were generated with target volumes at three different positions simulating typical clinical situations. The analytical anisotropic algorithm (AAA) was used to compute the doses in an Eclipse treatment planning system (Varian Medical Systems). The Rando phantom with the MAPCHECK device was exposed in Clinac 21iX linear accelerator. The measured dose distribution was compared with the calculated dose distribution to check for the accuracy in dose delivery. Measured and computed dose distribution were found to agree with more than 93% of pixels passing at 3% and 3 mm gamma criteria for all the treatment plans. The couch-based real time dosimetry system may also be applied for noncoplanar beams where electronic portal imaging device (EPID) is not practical to measure the dose. Other advantages include checking the beam stability during the patient treatment, performing routine morning quality assurance (QA) tests in the linear accelerator, and to perform pretreatment verification of intensity modulated radiation therapy (IMRT). One of the drawbacks of this system is that it cannot be used for measuring the dose at 90° or 270° gantry angles. This preliminary study shows that a 2D array of detectors may be used as part of the treatment couch for real time patient dosimetry in studying the dose delivered to the patient in real time and also for performing routine quality assurance.

Doses under automatic exposure control (AEC) for direct digital radiographic (DDR) X-ray systems

Current guidelines quote tolerances for automatic exposure control (AEC) device performance for X-ray systems as 'Baseline ± X %'. However, in the situation where a baseline figure has not yet been achieved, as in the case of commissioning assessments, this tolerance is not relevant. The purpose of this work is to provide mean doses for direct digital radiography (DDR) X-ray system, operating in AEC, against which comparisons can be made. Dose measurements have been recorded under AEC operation on 29 DDR detectors from three different manufacturers. Two different testing protocols were examined: (1) water equivalent phantoms in front of the DDR detector and (2) aluminium block at the tube head. The average patient exit dose, using the aluminium block was 4.6 μGy with the antiscatter grid in place and 4.0 μGy with the grid removed. Using the water phantoms, the average dose was measured at 17.1 μGy with the antiscatter grid in place and 5.4 μGy with grid removed. Based on these results, it is clear that different testing configurations significantly impact on the measured dose.

Optimizing the accuracy of a helical diode array dosimeter: A comprehensive calibration methodology coupled with a novel virtual inclinometer

The goal of any dosimeter is to be as accurate as possible when measuring absolute dose to compare with calculated dose. This limits the uncertainties associated with the dosimeter itself and allows the task of dose QA to focus on detecting errors in the treatment planning (TPS) and/or delivery systems. This work introduces enhancements to the measurement accuracy of a 3D dosimeter comprised of a helical plane of diodes in a volumetric phantom. We describe the methods and derivations of new corrections that account for repetition rate dependence, intrinsic relative sensitivity per diode, field size dependence based on the dynamic field size determination, and positional correction. Required and described is an accurate "virtual inclinometer" algorithm. The system allows for calibrating the array directly against an ion chamber signal collected with high angular resolution. These enhancements are quantitatively validated using several strategies including ion chamber measurements taken using a "blank" plastic shell mimicking the actual phantom, and comparison to high resolution dose calculations for a variety of fields: static, simple arcs, and VMAT. A number of sophisticated treatment planning algorithms were benchmarked against ion chamber measurements for their ability to handle a large air cavity in the phantom. Each calibration correction is quantified and presented vs its independent variable(s). The virtual inclinometer is validated by direct comparison to the gantry angle vs time data from machine log files. The effects of the calibration are quantified and improvements are seen in the dose agreement with the ion chamber reference measurements and with the TPS calculations. These improved agreements are a result of removing prior limitations and assumptions in the calibration methodology. Average gamma analysis passing rates for VMAT plans based on the AAPM TG-119 report are 98.4 and 93.3% for the 3%/3 mm and 2%/2 mm dose-error/distance to agreement threshold criteria, respectively, with the global dose-error normalization. With the local dose-error normalization, the average passing rates are reduced to 94.6 and 85.7% for the 3%/3 mm and 2%/2 mm criteria, respectively. Some algorithms in the convolution/superposition family are not sufficiently accurate in predicting the exit dose in the presence of a 15 cm diameter air cavity. Introduction of the improved calibration methodology, enabled by a robust virtual inclinometer algorithm, improves the accuracy of the dosimeter's absolute dose measurements. With our treatment planning and delivery chain, gamma analysis passing rates for the VMAT plans based on the AAPM TG-119 report are expected to be above 91% and average at about 95% level for γ(3%/3 mm) with the local dose-error normalization. This stringent comparison methodology is more indicative of the true VMAT system commissioning accuracy compared to the often quoted dose-error normalization to a single high value.

Intensity modulated radiation therapy planning for patients with a metal hip prosthesis based on class solutions

PurposeWith the aging of the population, an increasing number of patients with metallic hip implants are referred for radiotherapy treatment. Class solutions for intensity modulated radiation therapy (IMRT) treatment planning are generally not applicable for these patients due to the required avoidance of dose delivery through prostheses. In this work a new approach for IMRT planning is presented, allowing the use of a default beam setup. Methods and MaterialsFor IMRT planning, Monaco (Elekta; CMS Software, Maryland Heights, MO) was used. In addition to the target and organs at risk, so-called prosthesis avoidance volumes (PAVs) were delineated in the beam's eye view projection for beams in which the prosthesis was partially in front of the target. By putting strict constraints on these virtual organs at risk, entrance dose delivery through a prosthesis is avoided while exit dose delivery is allowed. In this way, uncertainties in the dose delivery to the target and organs at risk, as derived by the treatment planning system, are largely minimized. To show the advantages of this IMRT-PAV technique, for 2 prostate cancer patients, 1 with bilateral and the other with unilateral metallic hip prostheses, obtained IMRT plans were compared with conventional IMRT plans using a prosthesis-avoiding beam setup. ResultsFor both IMRT techniques a similar planning target volume coverage was achieved, but with the IMRT-PAV technique the mean doses to the bladder and the rectum were reduced by up to 25%. While the IMRT-PAV technique required more time for delineation, the time for treatment planning reduced because the default beam setup could be applied. The number of segments needed for dose delivery was comparable for both techniques. ConclusionsWith the new IMRT-PAV technique IMRT class solutions can safely be applied for cancer patients with metallic hip prostheses, generally yielding a reduced dose delivery to organs at risk or improved target coverage.

SU‐E‐T‐177: RPLGD: A Navel Tool for TBI in Vivo Dosimetry

Purpose: After implementing a new total body irradiation (TBI) technique in clinic, in vivo dosimetry check is essential. We use a navel tool, radiophotoluminescent glass dosimeter (RPLGD), for in vivo dose measurement. It has superior physical and dosimetric characteristics than thermoluminenscent dosimeter (TLD). Method and Materials:Bilateral TBI is performed with the Siemens Prius Linac with a nominal energy of 6 MV photon beam. The patient is at the lying position and the source to midpoint of the patient is 415 cm. Water bags are used to compensate the change in body thickness and arms are positioned laterally at the mid AP‐thickness level to block the lungs. In vivo dose measurements were performed with RPLGDs placed bilaterally at five levels to obtain the entrance dose and exit dose : on brain, chest, abdomen, thigh and lower legs. The midline dose is the average of entrance dose and exit dose. The prescribed dose is 120 cGy per fraction to the umbilicus to a total of 1200 cGy. Results: The absolute dose measured on dose prescribed point is 123.6 cGy and the difference between the prescription and measurement is 3%. The dose at five different levels are normalized to that at the abdomen level. The relative doses of brain, chest, thigh and lower legs are 89%, 99%, 1.03% and 88% respectively. The absorbed doses at brain and lower leg levels may be overcompensated and these can be adjusted by modifying the thickness of water bags. Conclusions: In our preliminary result, RPLGD is an ideal dosimeter to perform the in vivo dose measurement, not only the better physical and dosimetric characteristics but the easy process without annealing and the ability to be readout repeatedly. Conclusion, the RPLGD is a useful tool to for in vivo dosimetry in clinic.

SU-E-T-496: Dosimetric Comparison between Protons and Photons in the Field Junction in Craniospinal Irradiation (CSI)

Purpose: In this study, we investigated the dosimetry in the moving gap region in proton CSI for various combinations of field junction widths and feathering step sizes. We have also compared the dosimetry in the field junction between proton and photon beams. Methods: Dose profiles for two proton ranges, 11.6cm and 16cm, both with 10cm SOBP for the 30cm snout are used. Feathering of the junction is simulated by shifting the profiles by two successive steps. Three junction widths (0, 2, 4mm) and two feathering step sizes (5, 10 mm) are investigated. Similar simulations (but also include larger gap widths due to larger penumbra) are performed for 6 and 15 MV x‐ rays. Dose profiles at the field junctions are then compared for the proton and photon fields for various gaps and feathering step sizes. Results: For protons, full dose is achieved in the field junction for 0mm gap for both 5mm and 10mm featherings. However, with even a 2mm gap, the dose falls to below 90% in the moving gap due to the steep proton penumbra. For photons, even 4mm gap still results in >90% dose in the moving gap due to the large penumbra and the ‘tail’ in the profiles. Despite the different dose characteristics, it is possible to produce similar dose profiles in the moving gap for proton and photon fields by varying the gap and the feathering sizes for photon fields. Conclusion: Full dose can be safely achieved in the junction by ‘hot‐matching’ the spine and the brain fields in proton CSI and feathering the junctions. The steep proton penumbra results in a large dose gradient even with 2mm gap between fields. It is possible to produce similar dosimetry in the junction with photon CSI. The difference is that there is no exit dose with protons.

SU‐E‐T‐444: Feasibility Study of In‐Vivo Transit Dosimetry for IMRT Patients

Purpose: The ultimate goal of IMRT QA should be to verify that the correct dose has been delivered to the patient. The purpose of this study is to evaluate for feasibility and reliability of volumetric transit dosimetry as a routine in‐vivo dosimetry method during the actual patient treatment for different sites. Methods: To evaluate volumetric transit dosimetry for IMRT delivery, prostate, H&amp;N and lung IMRT cases were selected. During patient treatment, the EPID imager was extended to collect transit radiation through the patient. Collected transit dose images are imported and reconstructed as a volumetric dose distribution using the Dosimetry Check (Math Resolutions LLC, Maryland, USA) Exit Dose module. Dosimetry Check program to compare to the reconstructed volumetric dose including tissue heterogeneous correction from the transit EPID images. Also, to investigate the influence of daily setup or anatomy change, multiple transit dosimetry measurements were performed on different treatment days up to 5 fractions for prostate case. In addition, patientˈs verification plan was generated on a homogeneous CIRS IMRT phantom to verify the absolute dose and transit dosimetry algorithm check. Collected QA results were evaluated with 1) gamma passing rate (3 mm DTA and 3% dose) comparing calculated dose from exit dosimetry and treatment planning system (TPS) dose prediction, 2) difference of absolute point dose from exit dosimetry module compared to ion chamber measurements in the phantom. Results: The current implementation shows promise at predicting point doses from exit dose images acquired during patient treatment. However, 3D dose distributions calculated from exit dose images are not consistent in their agreement with the planned dose distribution. Conclusions: Further study is needed to explain the differences. Areas to consider include offsets in imager position, differences in dose calculation algorithm, couch attenuation effects and anatomy changes.

SU-E-T-221: An In-House Developed Resettable MOSFET Dosimeter for Radiotherapy

Purpose: To report the feasibility and clinical validation of an in-house developed MOSFET dosimetry system and describe an integrated nondestructive reset procedure. Methods: Off-the-shelf MOSFETs are connected to a PC using an 18 bit/analogue-input and 16 bit/output data acquisition card. A reading algorithm was developed defining the zero-temperature-coefficient point (ZTC) to determine the threshold voltage. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0—9 V) to the gate. Dosimetric properties have been evaluated for 6 MV and 15 MV clinical photon beams and in vivo benchmarking was performed against TLD for conventional treatments and total body irradiation (TBI). Results: Sensitivity of 0.08 mV cGy−1 can be obtained for 200 cGy irradiations at 5 V bias voltage. Ten consecutive measurements at 200 cGy yield a SD of 2.08 cGy (1.05%). Increasing the dose in steps from 5 cGy to 1000 cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160–800 cGy min−1) was within 2.5%, temperature dependence within 2.0% (25–37° C). Dose response is stable up to 50 Gy (saturation occurs at approximately 90 Gy), which is used as threshold dose before resetting the MOSFET. An average measured-over-calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane-dose assessed by entrance and exit dose measurements agreed within 1.9% with ionization chamber in phantom, and within 1.0% with TLD in vivo. Conclusions: An in-house developed resettable MOSFET-based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for open field treatment configurations.

SU‐E‐T‐879: Dosimetric Analysis of Proton Passive‐Scattering Stereotactic Body Radiotherapy (SBRT) of Treated Spine Lesions versus Photon SBRT

Purpose: To investigate the trade‐off between spinal cord sparing and target coverage in spine patients treated with Proton Passive‐scattering Stereotactic Body Radiotherapy (SBRT) compared to Photon SBRT plans with IMRT and 3D‐CRT. Methods: Ten patients with spinal and paraspinal lesions have treated with Proton SBRT in our institution. Afterward, they were re‐planned with photon IMRT and 3D‐CRT SBRT protocol considering sufficient target coverage and sharp dose fall‐off at the margin of target and spinal cord. Dosimetric data obtained from the plans and were compared in target volume, spinal cord and other adjacent critical organs such as thecal sac, Cauda equina, esophagus. To test the validity of dose calculation in heterogeneous tissues for photon and proton, in house developed Monte Carlo has been used. Results: IMRT plans showed better coverage and conformity of the target volume compared to proton plans and 3D‐CRT. Dose fall‐off at the margin of spinal cord and target volume were comparable for IMRT, Proton and 3D‐CRT plans, however IMRT showed a better result. On the other hand, photon plans including IMRT and 3D‐CRT showed more exit dose to the beyond normal tissues. Conclusions: Proton plans showed wider beam penumbra at the margin of spinal cord and target volume due to multiple proton scatter events. The effect can leading to increase of spinal cord dose or decrease of target coverage and therefore risk of treatment failure.

SU-E-T-545: Assessing the Impact of Proton Range Uncertainties on NSCLC Lung Patients Treated with Proton Beam-Based SBRT

Purpose: Proton therapy has been proposed as an alternative to photon beam therapy in stereotactic body radiation therapy(SBRT) of early‐stage NSCLCcancer. We investigated in the present study how proton range uncertainties can influence protonSBRTtreatment quality. Methods: Ten medically inoperable patients with stage I NSCLC were included in the current study. The treatment strategy for the SBRTtumors at MGH is to use either i)proton passive scattering or ii)photon 3DCRT for tumors with motion amplitude of less 5mm. The SBRTprotontreatment planning uses either 2 or 3 co‐planar beams. While, SBRTphotontreatment planning uses 2 or 3 non‐coplanar arcs, with a total of 8–10 beams. In the case of the proton beams an additional 3.5%+2mm is added to account for range uncertainties in the proximal and distal ends of the spread‐out‐Bragg‐peak, SOBP. Dosimetric comparisons were performed by defining a high‐ and low‐dose region representing volumes receiving more and less than 50% of the prescription dose, respectively. Results: In high‐dose regions, the volume receiving &#8805;95% of the prescription dose was larger for proton than for photonSBRT, i.e., 46.5 cc versus 33.5 cc (p=0.009), respectively. The conformity indices were 2.46 and 1.56, respectively. For tumors in close proximity to the chest‐wall, the volume of the chest‐wall receiving more‐than‐30Gy (V30) was 7 cc larger for protons than for photons (p=0.06), respectively. In low‐dose regions, the lung V5 and maximum esophagus dose were smaller for protons than for photons (p=0.019 and p<0.001, respectively). Conclusions: Protons generate larger high‐dose regions than photons because of range uncertainties. This can result in nearby healthy organs (such as chest‐wall) receiving close to treatmentdose. Future proton studies should focus on approaches to reduce dose range uncertainties and identify clinical subgroups of patients who will most benefit from the lack of exit dose.

TU‐G‐BRB‐03: Respiratory Motion Effects on Lung Proton SBRT Treatment

Purpose: To investigate the dosimetric impact of the respiratory motion on lung SBRT treatment when proton beams are used. Methods: Ten patients with 4D‐CT scans treated by photon SBRT were recruited for this study. To be consistent with the clinical practice, the PTV density override was performed to account for the tumor movement. Intensity modulated proton therapy (IMPT) plans were generated using our MC based dose calculation and optimization system. To quantify the density variation caused by breathing, the average lung density of inhale, exhale and a middle phase was calculated accordingly for every patient. MC simulation was performed to investigate the effects of density variation and the heterogeneity on the SOBP under simple geometry first. The effects of lung density variation on patient dose calculation were investigated subsequently by recalculating the dose distributions of IMPT plans with a different lung density for every individual phase. To investigate the effect of density override, dose distributions of IMPT plans with and without PTV density override were compared. Results: The density variation (2.3%–120%,average 22%) and heterogeneity cause SOBP distal edge shift, distortion, and a higher exit dose, which consequently leads to underdosage inside the target and a higher dose to the lung/critical organ in the downstream. In reality, the PTV is composed of solid GTV and lung tissue rather than homogeneous tissue with the unity density set in PTV override. Therefore, in patient treatment, part of the proton beam penetrates deeper than expected. This can significantly increase the dose to the adjacent critical organ. Conclusion: The respiratory motion and density variation can significantly affect the critical organ dose and the target coverage, especially for a small tumor with large motion. To encompass the motion, treatment margin and density override should be carefully applied and respiratory gating is necessary for proton SBRT treatment.