B Patyal

Water-equivalent path length calibration of a prototype proton CT scanner

PURPOSE The authors present a calibration method for a prototype proton computed tomography (pCT) scanner. The accuracy of these measurements depends upon careful calibration of the energy detector used to measure the residual energy of the protons that passed through the object. METHODS A prototype pCT scanner with a cesium iodide (CsI(Tl)) crystal calorimeter was calibrated by measuring the calorimeter response for protons of 200 and 100 MeV initial energies undergoing degradation in polystyrene plates of known thickness and relative stopping power (RSP) with respect to water. Calibration curves for the two proton energies were obtained by fitting a second-degree polynomial to the water-equivalent path length versus calorimeter response data. Using the 100 MeV calibration curve, the RSP values for a variety of tissue-equivalent materials were measured and compared to values obtained from a standard depth-dose range shift measurement using a water-tank. A cylindrical water phantom was scanned with 200 MeV protons and its RSP distribution was reconstructed using the 200 MeV calibration. RESULTS It is shown that this calibration method produces measured RSP values of various tissue-equivalent materials that agree to within 0.5% of values obtained using an established water-tank method. The mean RSP value of the water phantom reconstruction was found to be 0.995 ± 0.006. CONCLUSIONS The method presented provides a simple and reliable procedure for calibration of a pCT scanner.

Partial Breast Radiation Therapy With Proton Beam: 5-Year Results With Cosmetic Outcomes

PURPOSE We updated our previous report of a phase 2 trial using proton beam radiation therapy to deliver partial breast irradiation (PBI) in patients with early stage breast cancer. METHODS AND MATERIALS Eligible subjects had invasive nonlobular carcinoma with a maximal dimension of 3 cm. Patients underwent partial mastectomy with negative margins; axillary lymph nodes were negative on sampling. Subjects received postoperative proton beam radiation therapy to the surgical bed. The dose delivered was 40 Gy in 10 fractions, once daily over 2 weeks. Multiple fields were treated daily, and skin-sparing techniques were used. Following treatment, patients were evaluated with clinical assessments and annual mammograms to monitor toxicity, tumor recurrence, and cosmesis. RESULTS One hundred subjects were enrolled and treated. All patients completed the assigned treatment and were available for post-treatment analysis. The median follow-up was 60 months. Patients had a mean age of 63 years; 90% had ductal histology; the average tumor size was 1.3 cm. Actuarial data at 5 years included ipsilateral breast tumor recurrence-free survival of 97% (95% confidence interval: 100%-93%); disease-free survival of 94%; and overall survival of 95%. There were no cases of grade 3 or higher acute skin reactions, and late skin reactions included 7 cases of grade 1 telangiectasia. Patient- and physician-reported cosmesis was good to excellent in 90% of responses, was not changed from baseline measurements, and was well maintained throughout the entire 5-year follow-up period. CONCLUSIONS Proton beam radiation therapy for PBI produced excellent ipsilateral breast recurrence-free survival with minimal toxicity. The treatment proved to be adaptable to all breast sizes and lumpectomy cavity configurations. Cosmetic results appear to be excellent and unchanged from baseline out to 5 years following treatment. Cosmetic results may be improved over those reported with photon-based techniques due to reduced breast tissue exposure with proton beam, skin-sparing techniques, and the dose fractionation schedule used in this trial.

Evaluation of the dosimetric properties of a synthetic single crystal diamond detector in high energy clinical proton beams

PURPOSE To investigate the dosimetric properties of a synthetic single crystal diamond Schottky diode for accurate relative dose measurements in large and small field high-energy clinical proton beams. METHODS The dosimetric properties of a synthetic single crystal diamond detector were assessed by comparison with a reference Markus parallel plate ionization chamber, an Exradin A16 microionization chamber, and Exradin T1a ion chamber. The diamond detector was operated at zero bias voltage at all times. Comparative dose distribution measurements were performed by means of Fractional depth dose curves and lateral beam profiles in clinical proton beams of energies 155 and 250 MeV for a 14 cm square cerrobend aperture and 126 MeV for 3, 2, and 1 cm diameter circular brass collimators. ICRU Report No. 78 recommended beam parameters were used to compare fractional depth dose curves and beam profiles obtained using the diamond detector and the reference ionization chamber. Warm-up∕stability of the detector response and linearity with dose were evaluated in a 250 MeV proton beam and dose rate dependence was evaluated in a 126 MeV proton beam. Stem effect and the azimuthal angle dependence of the diode response were also evaluated. RESULTS A maximum deviation in diamond detector signal from the average reading of less than 0.5% was found during the warm-up irradiation procedure. The detector response showed a good linear behavior as a function of dose with observed deviations below 0.5% over a dose range from 50 to 500 cGy. The detector response was dose rate independent, with deviations below 0.5% in the investigated dose rates ranging from 85 to 300 cGy∕min. Stem effect and azimuthal angle dependence of the diode signal were within 0.5%. Fractional depth dose curves and lateral beam profiles obtained with the diamond detector were in good agreement with those measured using reference dosimeters. CONCLUSIONS The observed dosimetric properties of the synthetic single crystal diamond detector indicate that its behavior is proton energy independent and dose rate independent in the investigated energy and dose rate range and it is suitable for accurate relative dosimetric measurements in large as well as in small field high energy clinical proton beams.

Fractionated proton radiotherapy for benign cavernous sinus meningiomas.

PURPOSE To evaluate the efficacy of fractionated proton radiotherapy for a population of patients with benign cavernous sinus meningiomas. METHODS AND MATERIALS Between 1991 and 2002, 72 patients were treated at Loma Linda University Medical Center with proton therapy for cavernous sinus meningiomas. Fifty-one patients had biopsy or subtotal resection; 47 had World Health Organization grade 1 pathology. Twenty-one patients had no histologic verification. Twenty-two patients received primary proton therapy; 30 had 1 previous surgery; 20 had more than 1 surgery. The mean gross tumor volume was 27.6 cm(3); mean clinical target volume was 52.9 cm(3). Median total doses for patients with and without histologic verification were 59 and 57 Gy, respectively. Mean and median follow-up periods were 74 months. RESULTS The overall 5-year actuarial control rate was 96%; the control rate was 99% in patients with grade 1 or absent histologic findings and 50% for those with atypical histology. All 21 patients who did not have histologic verification and 46 of 47 patients with histologic confirmation of grade 1 tumor demonstrated disease control at 5 years. Control rates for patients without previous surgery, 1 surgery, and 2 or more surgeries were 95%, 96%, and 95%, respectively. CONCLUSIONS Fractionated proton radiotherapy for grade 1 cavernous sinus meningiomas achieves excellent control rates with minimal toxicities, regardless of surgical intervention or use of histologic diagnosis. Disease control for large lesions can be achieved by primary fractionated proton therapy.

Dosimetry Aspects of Proton Therapy

High-energy photons and high-energy protons are very different in the ways they interact with matter. These differences lead to distinct advantages of protons over photons for treatment of cancer. Some aspects of proton interactions with tissue that make this modality superior for treating cancer are: (i) Initially, the protons lose energy very slowly as they enter the body; this results in a low entrance dose and low doses to the normal tissues proximal to the tumor. (ii) Near the end of range, protons lose energy very rapidly and deposit all their energy over a very small volume before they come to rest. This is the Bragg peak, a property that results in delivery of the maximum dose to the tumor. (iii) Beyond the Bragg peak, the energy deposited by the protons is zero; no dose is received by normal tissues distal to the tumor. Therefore, protons deliver their maximum dose to the tumor, a low dose to normal structures proximal to the tumor, and no dose to the normal structures beyond the tumor, ideal properties of a radiation modality to treat cancer. One distinct advantage of protons over photons is the ease with which the tumor target can be irradiated conformably to a high dose, and at the same time the normal structures in the vicinity of the tumor can be protected conformably from that high dose. Given the same dose to the tumor via photons and protons, protons inherently deliver less integral dose and, thus, lead to fewer normal-tissue complications. In addition, proton interactions also offer distinct radiobiological advantages over photons. Superior physical and radiobiological proton interactions lead naturally to the concepts of dose escalation and hypofractionation. The superiority of treatment delivery with protons as contrasted with photons is demonstrated by treatment plans.

Analysis of Intensity-Modulated Radiation Therapy (IMRT), Proton and 3D Conformal Radiotherapy (3D-CRT) for Reducing Perioperative Cardiopulmonary Complications in Esophageal Cancer Patients

Background. While neoadjuvant concurrent chemoradiotherapy has improved outcomes for esophageal cancer patients, surgical complication rates remain high. The most frequent perioperative complications after trimodality therapy were cardiopulmonary in nature. The radiation modality utilized can be a strong mitigating factor of perioperative complications given the location of the esophagus and its proximity to the heart and lungs. The purpose of this study is to make a dosimetric comparison of Intensity-Modulated Radiation Therapy (IMRT), proton and 3D conformal radiotherapy (3D-CRT) with regard to reducing perioperative cardiopulmonary complications in esophageal cancer patients. Materials. Ten patients with esophageal cancer treated between 2010 and 2013 were evaluated in this study. All patients were simulated with contrast-enhanced CT imaging. Separate treatment plans using proton radiotherapy, IMRT, and 3D-CRT modalities were created for each patient. Dose-volume histograms were calculated and analyzed to compare plans between the three modalities. The organs at risk (OAR) being evaluated in this study are the heart, lungs, and spinal cord. To determine statistical significance, ANOVA and two-tailed paired t-tests were performed for all data parameters. Results. The proton plans showed decreased dose to various volumes of the heart and lungs in comparison to both the IMRT and 3D-CRT plans. There was no difference between the IMRT and 3D-CRT plans in dose delivered to the lung or heart. This finding was seen consistently across the parameters analyzed in this study. Conclusions. In patients receiving radiation therapy for esophageal cancer, proton plans are technically feasible while achieving adequate coverage with lower doses delivered to the lungs and cardiac structures. This may result in decreased cardiopulmonary toxicity and less morbidity to esophageal cancer patients.

Evaluation of normal tissue exposure in patients receiving radiotherapy for pancreatic cancer based on RTOG 0848

BACKGROUND Pancreatic cancer is a highly aggressive malignancy. Chemoradiotherapy (CRT) is utilized in many cases to improve locoregional control; however, toxicities associated with radiation can be significant given the location of the pancreas. RTOG 0848 seeks to evaluate chemoradiation using either intensity-modulated radiation therapy (IMRT) or 3D conformal photon radiotherapy (3DCRT) modalities as an adjuvant treatment. The purpose of this study is to quantify the dosimetric changes seen when using IMRT or 3D CRT photon modalities, as well as proton radiotherapy, in patients receiving CRT for cancer of the pancreas treated per RTOG 0848 guidelines. MATERIALS Ten patients with pancreatic head adenocarcinoma treated between 2010 and 2013 were evaluated in this study. All patients were simulated with contrast-enhanced CT imaging. Separate treatment plans using IMRT and 3DCRT as well as proton radiotherapy were created for each patient. All planning volumes were created per RTOG 0848 protocol. Dose-volume histograms (DVH) were calculated and analyzed in order to compare plans between the three modalities. The organs at risk (OAR) evaluated in this study are the kidneys, liver, small bowel, and spinal cord. RESULTS There was no difference between the IMRT and 3DCRT plans in dose delivered to the kidneys, liver, or bowel. The proton radiotherapy plans were found to deliver lower mean total kidney doses, mean liver doses, and liver D1/3 compared to the IMRT plans. The proton plans also gave less mean liver dose, liver D1/3, bowel V15, and bowel V50 in comparison to the 3DCRT. CONCLUSIONS For patients receiving radiotherapy per ongoing RTOG 0848 for pancreatic cancer, there was no significant difference in normal tissue sparing between IMRT and 3DCRT treatment planning. Therefore, the choice between the two modalities should not be a confounding factor in this study. The proton plans also demonstrated improved OAR sparing compared to both IMRT and 3DCRT treatment plans.

Evaluation and comparison of New 4DCT based strategies for proton treatment planning for lung tumors

PurposeTo evaluate different strategies for proton lung treatment planning based on four-dimensional CT (4DCT) scans.Methods and MaterialsTwelve cases, involving only gross tumor volumes (GTV), were evaluated. Single image sets of (1) maximum intensity projection (MIP3) of end inhale (EI), middle exhale (ME) and end exhale (EE) images; (2) average intensity projection (AVG) of all phase images; and (3) EE images from 4DCT scans were selected as primary images for proton treatment planning. Internal target volumes (ITVs) outlined by a clinician were imported into MIP3, AVG, and EE images as planning targets. Initially, treatment uncertainties were not included in planning. Each plan was imported into phase images of 4DCT scans. Relative volumes of GTVs covered by 95% of prescribed dose and mean ipsilateral lung dose of a phase image obtained by averaging the dose in inspiration and expiration phases were used to evaluate the quality of a plan for a particular case. For comparing different planning strategies, the mean of the averaged relative volumes of GTVs covered by 95% of prescribed dose and its standard deviation for each planning strategy for all cases were used. Then, treatment uncertainties were included in planning. Each plan was recalculated in phase images of 4DCT scans. Same strategies were used for plan evaluation except dose-volume histograms of the planning target volumes (PTVs) instead of GTVs were used and the mean and standard deviation of the relative volumes of PTVs covered by 95% of prescribed dose and the ipsilateral lung dose were used to compare different planning strategies.ResultsMIP3 plans without treatment uncertainties yielded 96.7% of the mean relative GTV covered by 95% of prescribed dose (standard deviations of 5.7% for all cases). With treatment uncertainties, MIP3 plans yielded 99.5% of mean relative PTV covered by 95% of prescribed dose (standard deviations of 0.7%). Inclusion of treatment uncertainties improved PTV dose coverage but also increased the ipsilateral mean lung dose in general, and reduced the variations of the PTV dose coverage among different cases. Plans based on conventional axial CT scan (CVCT) gave the poorest PTV dose coverage (about 96% of mean relative PTV covered by 95% isodose) compared to MIP3 and EE plans, which resulted in 100% of PTV covered by 95% isodose for tumors with relatively large motion. AVG plans demonstrated PTV dose coverage of 89.8% and 94.4% for cases with small tumors. MIP3 plans demonstrated superior tumor coverage and were least sensitive to tumor size and tumor location.ConclusionMIP3 plans based on 4DCT scans were the best planning strategy for proton lung treatment planning.

Conditions for reliable time-resolved dosimetry of electronic portal imaging devices for fixed-gantry IMRT and VMAT.

PURPOSE The continuous scanning mode of electronic portal imaging devices (EPID) that offers time-resolved information has been newly explored for verifying dynamic radiation deliveries. This study seeks to determine operating conditions (dose rate stability and time resolution) under which that mode can be used accurately for the time-resolved dosimetry of intensity-modulated radiation therapy (IMRT) beams. METHODS The authors have designed the following test beams with variable beam holdoffs and dose rate regulations: a 10 × 10 cm open beam to serve as a reference beam; a sliding window (SW) beam utilizing the motion of a pair of multileaf collimator (MLC) leaves outside the 10 × 10 cm jaw; a step and shoot (SS) beam to move the pair in step; a volumetric modulated arc therapy (VMAT) beam. The beams were designed in such a way that they all produce the same open beam output of 10 × 10 cm. Time-resolved ion chamber measurements at isocenter and time-resolved and integrating EPID measurements were performed for all beams. The time-resolved EPID measurements were evaluated through comparison with the ion chamber and integrating EPID measurements, as the latter are accepted procedures. For two-dimensional, time-resolved evaluation, a VMAT beam with an infield MLC travel was designed. Time-resolved EPID measurements and Monte Carlo calculations of such EPID dose images for this beam were performed and intercompared. RESULTS For IMRT beams (SW and SS), the authors found disagreement greater than 2%, caused by frame missing of the time-resolved mode. However, frame missing disappeared, yielding agreement better than 2%, when the dose rate of irradiation (and thus the frame acquisition rates) reached a stable and planned rate as the dose of irradiation was raised past certain thresholds (a minimum 12 s of irradiation per shoot used for SS IMRT). For VMAT, the authors found that dose rate does not affect the frame acquisition rate, thereby causing no frame missing. However, serious inplanar nonuniformities were found. This could be overcome by sacrificing temporal resolution (10 frames or 0.95 s/image): the continuous images agreed with ion chamber responses at the center of EPID and the calculation two-dimensionally in a time-resolved manner. CONCLUSIONS The authors have determined conditions under which the continuous mode can be used for time-resolved dosimetry of fixed-gantry IMRT and VMAT and demonstrated it for VMAT.

SU‐E‐T‐100: How to Improve the Dose Accuracy for Gantry Angle Dependent Patient Specific IMRT QA Using 2D Ion Chamber Array with Octavius Phantom

PURPOSE To determine the cross calibration factors which can predict more accurate dose distribution for fixed beam IMRT QA using Octavius phantom. METHODS The ion chamber based Octavius 2D-array detector (PTW, Freiburg, Germany) is a step in the right direction to measure the absolute dose and dose distribution for patient specific IMRT QA. However, the directional dependency of this detector made it less than desirable for angle dependent IMRT QA. We evaluated the new Octavius system (PTW, Freiburg, Germany) for angle dependent IMRT QA which compensates the response due to directional dependency. The system is designed for full arc VMAT QA, but does not always work for the discrete angle IMRT QA due to non-averaging of errors caused by directional dependence of detectors. The proposed method uses correction factors for each gantry angle. The dose for a 10cm × 10cm open field for each gantry angle was calculated by treatment planning system and measured using the Octavius phantom. The correction factors were determined at each gantry angle and the dose distribution was renormalized at each angle using correction factors. RESULTS The discrepancy between measured and planned dose per monitor unit depended on the gantry angle and were in the range of +-4% using the PTW method. Using our method, uncertainty due to the detector angle dependency was eliminated. CONCLUSIONS The new method removes the angle dependency of ion chamber based 2D array detector for the fixed beam IMRT QA. It provides fast, accurate and more realistic results for angle dependent IMRT QA.