Wen Hsi

Commissioning of output factors for uniform scanning proton beams

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

Dosimetric study of uniform scanning proton therapy planning for prostate cancer patients with a metal hip prosthesis, and comparison with volumetric-modulated arc therapy

The main purposes of this study were to 1) investigate the dosimetric quality of uniform scanning proton therapy planning (USPT) for prostate cancer patients with a metal hip prosthesis, and 2) compare the dosimetric results of USPT with that of volumetric‐modulated arc therapy (VMAT). Proton plans for prostate cancer (four cases) were generated in XiO treatment planning system (TPS). The beam arrangement in each proton plan consisted of three fields (two oblique fields and one lateral or slightly angled field), and the proton beams passing through a metal hip prosthesis was avoided. Dose calculations in proton plans were performed using the pencil beam algorithm. From each proton plan, planning target volume (PTV) coverage value (i.e., relative volume of the PTV receiving the prescription dose of 79.2 CGE) was recorded. The VMAT prostate planning was done using two arcs in the Eclipse TPS utilizing 6 MV X‐rays, and beam entrance through metallic hip prosthesis was avoided. Dose computation in the VMAT plans was done using anisotropic analytical algorithm, and calculated VMAT plans were then normalized such that the PTV coverage in the VMAT plan was the same as in the proton plan of the corresponding case. The dose‐volume histograms of calculated treatment plans were used to evaluate the dosimetric quality of USPT and VMAT. In comparison to the proton plans, on average, the maximum and mean doses to the PTV were higher in the VMAT plans by 1.4% and 0.5%, respectively, whereas the minimum PTV dose was lower in the VMAT plans by 3.4%. The proton plans had lower (or better) average homogeneity index (HI) of 0.03 compared to the one for VMAT (HI = 0.04). The relative rectal volume exposed to radiation was lower in the proton plan, with an average absolute difference ranging from 0.1% to 32.6%. In contrast, using proton planning, the relative bladder volume exposed to radiation was higher at high‐dose region with an average absolute difference ranging from 0.4% to 0.8%, and lower at low‐ and medium‐dose regions with an average absolute difference ranging from 2.7% to 10.1%. The average mean dose to the rectum and bladder was lower in the proton plans by 45.1% and 22.0%, respectively, whereas the mean dose to femoral head was lower in VMAT plans by an average difference of 79.6%. In comparison to the VMAT, the proton planning produced lower equivalent uniform dose (EUD) for the rectum (43.7 CGE vs. 51.4 Gy) and higher EUD for the femoral head (16.7 CGE vs. 9.5 Gy), whereas both the VMAT and proton planning produced comparable EUDs for the prostate tumor (76.2 CGE vs. 76.8 Gy) and bladder (50.3 CGE vs. 51.1 Gy). The results presented in this study show that the combination of lateral and oblique fields in USPT planning could potentially provide dosimetric advantage over the VMAT for prostate cancer involving a metallic hip prosthesis. PACS number: 87.55.D‐, 87.55.ne, 87.55.dk

Dosimetric uncertainty in prostate cancer proton radiotherapy

PURPOSE The authors we evaluate the uncertainty in proton therapy dose distribution for prostate cancer due to organ displacement, varying penumbra width of proton beams, and the amount of rectal gas inside the rectum. METHODS AND MATERIALS Proton beam treatment plans were generated for ten prostate patients with a minimum dose of 74.1 cobalt gray equivalent (CGE) to the planning target volume (PTV) while 95% of the PTV received 78 CGE. Two lateral or lateral oblique proton beams were used for each plan. The authors we investigated the uncertainty in dose to the rectal wall (RW) and the bladder wall (BW) due to organ displacement by comparing the dose-volume histograms (DVH) calculated with the original or shifted contours. The variation between DVHs was also evaluated for patients with and without rectal gas in the rectum for five patients who had 16 to 47 cc of visible rectal gas in their planning computed tomography (CT) imaging set. The uncertainty due to the varying penumbra width of the delivered protons for different beam setting options on the proton delivery system was also evaluated. RESULTS For a 5 mm anterior shift, the relative change in the RW volume receiving 70 CGE dose (V70) was 37.9% (5.0% absolute change in 13.2% of a mean V70). The relative change in the BW volume receiving 70 CGE dose (V70) was 20.9% (4.3% absolute change in 20.6% of a mean V70) with a 5 mm inferior shift. A 2 mm penumbra difference in beam setting options on the proton delivery system resulted in the relative variations of 6.1% (0.8% absolute change) and 4.4% (0.9% absolute change) in V70 of RW and BW, respectively. The data show that the organ displacements produce absolute DVH changes that generally shift the entire isodose line while maintaining the same shape. The overall shape of the DVH curve for each organ is determined by the penumbra and the distance of the target in beams eye view (BEV) from the block edge. The beam setting option producing a 2 mm sharper penumbra at the isocenter can reduce the magnitude of maximal doses to the RW by 2% compared to the alternate option utilizing the same block margin of 7 mm. The dose to 0.1 cc of the femoral head on the distal side of the lateral-posterior oblique beam is increased by 25 CGE for a patient with 25 cc of rectal gas. CONCLUSION Variation in the rectal and bladder wall DVHs due to uncertainty in the position of the organs relative to the location of sharp dose falloff gradients should be accounted for when evaluating treatment plans. The proton beam delivery option producing a sharper penumbra reduces maximal doses to the rectal wall. Lateral-posterior oblique beams should be avoided in patients prone to develop a large amount of rectal gas.

Proton‐based chemoradiation for synchronous bilateral non‐small‐cell lung cancers: A case report

In this case report, we present the history and treatment of a 70‐year‐old man with synchronous bilateral non‐small‐cell lung cancers with proton‐beam radiation. Surgical treatment was not feasible and optimized photon intensity‐modulated radiotherapy (IMRT) to the primary tumors would have resulted in unacceptably high normal‐tissue exposures. Proton‐beam radiation enabled radiation dose escalation and concurrent chemotherapy while maintaining normal‐tissue tolerance.

Dosimetric characterization of whole brain radiotherapy of pediatric patients using modulated proton beams

This study was designed to investigate dosimetric variations between proton plans with (PPW) and without (PPWO), a compensator for whole brain radiotherapy (WBRT). The retrospective study on PPW and PPWO in Eclipse and XiO systems and photon plans (XP) using controlled segments in Pinnacle system was performed on nine pediatric patients for craniospinal irradiations. DVHs and derived metrics, such as the homogeneity index (HI), the doses to 2%(D2%) and 5%(D5%) volumes, and mean dose (Dmean) of the whole brain (i.e., PTV), and the organs at risk (OARs) such as lens and skull, were obtained. The PPW plans from both Eclipse and XiO systems uncovered the following advantages: (1) encompassing a cribriform plate area with the 100% isodose line was better than either PPWO or XP, according to calculated two‐dimensional distributions of one patient; (2) the mean value of D5% for lens was reduced to 23.6% of DP from 54.1% for PPWO or 41.6% for XP; and (3) the mean value of Dmean for skull was reduced to 94.8% of DP from either 98.4% for PPWO or 98.3% for XP. However, the PPW plans also exposed several disadvantages including: (1) the HI of PTV increased to 7.7 from 4.7 for PPWO or 3.7 for XP; (2) D2% to PTV increased to 108.8% of DP from 104.8% for PPWO or 105.1% for XP; and (3) D5% to the skull increased to 104.9% of DP from 101.6% for PPWO or 103.4% of for XP. One‐half of the observed variations were caused by different penumbra on lateral profiles and distal fall‐off depth doses of protons in Eclipse and XiO. Because the utilization on the sharp proton distal fall‐off was limited for WBRT, the difference between PPW and PPWO or XP indicated no distinguishable improvement by using a compensator in proton plans. PACS number: 87.55.‐x

Spatial correlation of proton irradiation‐induced activity and dose in polymer gel phantoms for PET/CT delivery verification studies

PURPOSE This work demonstrates a novel application of BANG3-Pro2 polymer gel dosimeter as a dosimetric phantom able to accurately capture both dose and induced activity. METHODS BANG3-Pro2 dosimeters were irradiated with a clinical proton beam using an unmodulated beam and a spread-out Bragg peak (SOBP) modulation, the latter with a Lucite compensator to introduce a range offset in one quadrant of the circular field. The dosimeters were imaged in a nearby positron emission tomography/computed tomography (PET/CT) unit starting within 5 min of beam-off. Induced positron emission (PE) activity along the central axis of the beam was compared to analytical calculations. Dose distributions were read out using an optical CT scanner and were validated against ion chamber measurements and the treatment plan. The offset between the distal fall-off of dose and activity (50% level) was determined over the entire irradiated field. Lateral profiles of PE were correlated to measured dose for the unmodulated beam delivery. RESULTS Measured profiles of PE activity along the central beam axis were found to be within 10% of the predictions of analytical calculations. The depth-dose profiles agreed with the reference values (ion chamber or treatment plan) within 3%. The offset between the depth profiles of dose and activity for the unmodulated beam was 8.4 ± 1.4 mm. For the compensator-based SOBP delivery, the distribution of offsets throughout the field was found to be bimodal, with the mean of 8.9 ± 2.8 mm for the thinner region of the compensator and 4.3 ± 2.5 mm for the thicker region. For the pristine beam delivery, lateral profiles of dose and activity were found to exhibit fair spatial correlation throughout the beam range, with the mean 2D gamma index of 0.42 and 91% of the evaluated pixels passing the test. CONCLUSIONS This work presents the first demonstration of simultaneous and accurate experimental measurement of three-dimensional distributions of dose and induced activity and lays the groundwork for further investigations using BANG3-Pro2 as a dosimetric phantom in PET/CT delivery verification studies.

MO‐A‐213AB‐03: Commissioning of a Clinical Chair for Patients Treated in the Seated Position Using an Inclined Beam Line Treatment Room

PURPOSES A chair, coupled to a robotic patient positioning system (PPS) was manufactured to treat an intracranial tumor in a proton incline beam-line system. Treating patients in the seated position as accurately and efficiently as a treatment table requires the essential functions of isocentric rotation and a weight-sagging-correction algorithm for positioning patients in the seated position. METHODS AND MATERIALS The chair design incorporated a down-slope arm to achieve the desired beam-line height. To overcome this limitation of only 125 degree rotation on PPS, five indexed positions of the seat-base-plate (SBP) were implemented. An in-house developed optical tracking system using a six degree-of-freedom optical camera system was used to align the treatment room coordinate system with the chair coordinate system at all SBP positions. Furthermore, this optical tracking system quantified the sagging effect due to both the height and weight of a variety of patients. RESULTS The optical tracking system can measure accuracy of 0.1 degree and 0.1 mm. The SBP rotating axis was aligned within 0.1 degree to PPS rotating axis. A residual precession of chair rotation was found to be an ellipse with long axis of 2.0 mm and short axis of 1.0 mm. An additional 0.75 mm deviation occurred between rotating of SBP and PPS axes. Sagging tilt of 0.6 degree was found on the SBP for the home position for every additional 162 lbs load. This resulted in a 1.1cm shift (0.65 cm forward and 0.87 cm) for an isocenter 90 cm away from the SBP plate. CONCLUSIONS Using in-house developed optical tracking system, the overall maximum displacement of treatment chair system from isocenter is within 3.0 mm with known sagging characteristics. This characterization is essential to reduce the total treatment time and limited the number of X-rays required for accurate patient alignment in the seated position.

SU‐E‐T‐358: Dosimetric Effects of Beam Angle Arrangements in Lung Proton Therapy

Purpose: The Inclined Beam Line (IBL) is an innovative partial gantry design which provides two beam angles at 30 and 90 degrees with full flexibility of the patient positioning system of the gantry design. Compared to the full gantry design in proton therapy, the IBL is a simplified design allowing for less equipment maintenance, physics quality assurance, and costs. The purpose of this study was to demonstrate that IBL provides sufficient choice of beam angles and efficient beam delivery to treat most protonlung patients.Methods: Eight lung patients who had protontreatment at our center were selected for this study. We designed three treatment plans for each of the eight patients in supine position, using beam arrangements with (1) full gantry, (2) IBL, and (3) hybrid, i.e., a combination of a gantry plan and an IBL plan with the patients being treated with each plan in alternative days. Xio TPS (CMS, St. Louis, MO) was used to design treatment plans. Results: We have compared the dosimetric differences of the three planning strategies. The PTV D95 was within 1% for all three plans for each patient. On average, the lung V20, V10, and V5 were 3.6%/3.7%, 5.1%/3.7%, and 5.1%/1.6% higher for the IBL plans than that for the gantry/hybrid plans, respectively. Cord max dose, esophagus dose, and heart dose showed similar trends. Among all eight patients, IBL plan was not able to meet cord dose limit for only one patient who was treated for right posterior chest wall. Conclusions: Our results showed that seven out of the eight patients (88%) could be treated with full gantry, IBL, or hybrid plans with sufficient target coverage and reasonable critical structure sparing. Therefore, IBL was a sufficient protontreatment delivery method for most of the lung patients in this study.

SU‐E‐J‐76: Clinical Use of a Real‐Time Surface Image‐Guided Positioning and Tracking System in Proton Therapy

Topic of interest: Clinical applications of AlignRT 3-cameras real-time surface image-guided positioning system (IGPS) for positioning patients to reduce the number of X-ray images and tracking intra-fractional movements in proton therapy. PURPOSES To position patients and track the intra-fractional movements, the AlignRT system was implemented in proton incline-beam-line (IBL) at Procure Oklahoma-City center. METHODS The AlignRT3c system was configured near perpendicular to the gantry rotation for accommodating the X-ray IGPS. To evaluate positioning accuracy, more than 10 surfaces of each patient for ten patients with intracranial tumors were acquired after patients positioned by X-ray IGPS. Displacements between acquired surfaces and the reference surface taken at 1st day of treatment were examined. Intra-fractional movements with respiratory was studied with gated surface that allows setting the reference surface for patient at exhale during breathing. Intra-fractional movements due to respiratory were monitored on 10 sections of each patient for three patients with thoracic tumors. RESULTS Accuracy of positioning patient is 2.0 mm at both anterior-posterior and lateral directions, and is 3.5 mm in superior-inferior (SI) direction by aligning the surfaces of masks. Observed larger displacements along SI direction can be due to patients movements within the mask. Periodical displacements within 5 mm compared to its reference were seen for the three patients with thorax tumors. However, 10 mm sharp displacements with a few seconds were observed when patient moved the body. CONCLUSIONS We have implemented the first AlignRT3c IGPS for proton therapy for positioning patients within 2.0 mm, and successfully tracked intra-fractional respiratory motion during treatment after positioning patient.

SU‐E‐T‐301: A Novel Daily QA Device for Proton Therapy

PURPOSE We describe the design and use of a daily QA device for proton therapy. The device is designed for therapists to check the readiness of the IBA Proton Therapy System (IBA, Louvain-la-Neuve, Belgium) during morning QA. The checks include connectivity, positioning, mechanical, imaging and dosimetric parameters of the proton therapy system. METHODS The device consists of a commercial QA device, (rf-DailyQA3 -Sun Nuclear Corporation, Melbourne, FL), in conjunction with a home-made acrylic phantom and mechanical indexing jig. The indexing jig indexes the rf-DailyQA3 to treatment couch. Fiducial markers embedded in the phantom are used for checking the x-ray image and alignment accuracy of the imaging system (VeriSuite, MedCom, Darmstadt- Germany). The rf- DailyQA3 is used to check the proton beam output, range and symmetry, which are acquired during one single beam delivery of 100 monitor units. We developed in-house software to calculate the variation of beam range and symmetry, based on readings from the various ion chambers inside the rf-DailyQA3. RESULTS The device has been employed to perform daily QA since June 2010 at two operational proton treatment centers and will soon be implemented at ProCures New Jersey center. All QA tests are performed by radiation therapists and reviewed by the medical physicist on duty. Due to the simplicity of the device and the associated processing software, the QA time is less than 20 minutes per room. The measurement data collected by the device during daily QA are recorded in the OIS. The integrity of the data is validated by comparing against other independent measurements. CONCLUSIONS The daily QA device has been proven to be robust, reliable and user-friendly. The performance of this system has been proven to be stable and accurate using trend analyses. Key words:proton therapy, daily QA, output, range, symmetry.