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Patient-specific quality assurance for prostate cancer patients receiving spot scanning proton therapy using single-field uniform dose

PURPOSE To describe our experiences with patient-specific quality assurance (QA) for patients with prostate cancer receiving spot scanning proton therapy (SSPT) using single-field uniform dose (SFUD). METHODS AND MATERIALS The first group of 249 patients with prostate cancer treated with SSPT using SFUD was included in this work. The scanning-beam planning target volume and number of monitor units were recorded and checked for consistency. Patient-specific dosimetric measurements were performed, including the point dose for each plan, depth doses, and two-dimensional (2D) dose distribution in the planes perpendicular to the incident beam direction for each field at multiple depths. The γ-index with 3% dose or 3-mm distance agreement criteria was used to evaluate the 2D dose distributions. RESULTS We observed a linear relationship between the number of monitor units and scanning-beam planning target volume. The difference between the measured and calculated point doses (mean ± SD) was 0.0% ± 0.7% (range, -2.9% to 1.8%). In general, the depth doses exhibited good agreement except at the distal end of the spread-out Bragg peak. The pass rate of γ-index (mean ± SD) for 2D dose comparison was 96.2% ± 2.6% (range, 90-100%). Discrepancies between the measured and calculated dose distributions primarily resulted from the limitation of the model used by the treatment planning system. CONCLUSIONS We have established a patient-specific QA program for prostate cancer patients receiving SSPT using SFUD.

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

PURPOSE To present our method and experience in commissioning dose models in water for spot scanning proton therapy in a commercial treatment planning system (TPS). METHODS The input data required by the TPS included in-air transverse profiles and integral depth doses (IDDs). All input data were obtained from Monte Carlo (MC) simulations that had been validated by measurements. MC-generated IDDs were converted to units of Gy mm(2)/MU using the measured IDDs at a depth of 2 cm employing the largest commercially available parallel-plate ionization chamber. The sensitive area of the chamber was insufficient to fully encompass the entire lateral dose deposited at depth by a pencil beam (spot). To correct for the detector size, correction factors as a function of proton energy were defined and determined using MC. The fluence of individual spots was initially modeled as a single Gaussian (SG) function and later as a double Gaussian (DG) function. The DG fluence model was introduced to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle, especially from the spot profile monitor. To validate the DG fluence model, we compared calculations and measurements, including doses at the center of spread out Bragg peaks (SOBPs) as a function of nominal field size, range, and SOBP width, lateral dose profiles, and depth doses for different widths of SOBP. Dose models were validated extensively with patient treatment field-specific measurements. RESULTS We demonstrated that the DG fluence model is necessary for predicting the field size dependence of dose distributions. With this model, the calculated doses at the center of SOBPs as a function of nominal field size, range, and SOBP width, lateral dose profiles and depth doses for rectangular target volumes agreed well with respective measured values. With the DG fluence model for our scanning proton beam line, we successfully treated more than 500 patients from March 2010 through June 2012 with acceptable agreement between TPS calculated and measured dose distributions. However, the current dose model still has limitations in predicting field size dependence of doses at some intermediate depths of proton beams with high energies. CONCLUSIONS We have commissioned a DG fluence model for clinical use. It is demonstrated that the DG fluence model is significantly more accurate than the SG fluence model. However, some deficiencies in modeling the low-dose envelope in the current dose algorithm still exist. Further improvements to the current dose algorithm are needed. The method presented here should be useful for commissioning pencil beam dose algorithms in new versions of TPS in the future.

A procedure for calculation of monitor units for passively scattered proton radiotherapy beams

The purpose of this study is to validate a monitor unit (MU) calculation procedure for passively scattered proton therapy beams. The output dose per MU (d/MU) of a therapeutic radiation beam is traditionally calibrated under specific reference conditions. These conditions include beam energy, field size, suitable depth in water or water equivalent phantom in a low dose gradient region with known relative depth dose, and source to point of calibration distance. Treatment field settings usually differ from these reference conditions leading to a different d/MU that needs to be determined for delivering the prescribed dose. For passively scattered proton beams, the proton specific parameters, which need to be defined, are related to the energy, lateral scatterers, range modulating wheel, spread out Bragg peak (SOBP) width, thickness of any range shifter, the depth dose value relative to the normalization point in the SOBP, and scatter both from the range compensator and inhomogeneity in the patient. Following the custom for photons or electrons, a set of proton dosimetry factors, representing the changes in the d/MU relative to a reference condition, can be defined as the relative output factor (ROF), SOBP factor (SOBPF), range shifter factor (RSF), SOBP off-center factor (SOBPOCF), off-center ratio (OCR), inverse square factor (ISF), field size factor (FSF), and compensator and patient scatter factor (CPSF). The ROF, SOBPF, and RSF are the major contributors to the d/MU and were measured using an ion chamber in water tank during the clinical commissioning of each beam to create a dosimetry beam data table to be used for calculating the monitor units. The following simple formula is found to provide an independent method to determine the d/MU at the point of interest (POI) in the patient, namely, (d/MU) = ROF SOBPF. RSF SOBPOCF.OCR.FSF.ISF.CPSF. The monitor units for delivering the intended dose (D) to the POI can be obtained from MU = D / (d/MU). The accuracy and robustness of the above formula were validated by calculating the d/MU in water for many different combinations of beam parameters and comparing it with the corresponding measured d/MU by an ion chamber in a water or water/plastic phantom. This procedure has been in use for MU calculation for patient treatment fields at our facility since May 2006. The differences in the calculated and measured values of the d/MU for 623 distinct fields used for patient treatment during the period of May 2006 to February 2007 are within 2% for 99% of these fields. The authors conclude that an intuitive formula similar to the one used for monitor unit calculation of therapeutic photon beams can be used to compute the monitor units of passively scattered proton therapy beams.

Optimal Treatment Planning for Skull Base Chordoma: Photons, Protons, or a Combination of Both?

PURPOSE We compared dosimetry of proton (PR), intensity modulated radiation therapy (IMRT) photon (PH), and combined PR and IMRT PH (PP) irradiation of skull base chordomas to determine the most optimal technique. METHODS AND MATERIALS Computed tomography simulation scans of 5 patients with skull base chordoma were used to generate four treatment plans: an IMRT PH plan with 1-mm planning target volume (PTV; PH1) for stereotactic treatment, an IMRT PH plan with 3-mm PTV (PH3) for routine treatment, a PR plan with beam-specific expansion margins on the clinical target volume, and a PP plan combining PR and PH treatment. All plans were prescribed 74 Gy/Cobalt Gray equivalents (CGE) to the PTV. To facilitate comparison, the primary objective of all plans was 95% or greater PTV prescribed dose coverage. Plans then were optimized to limit dose to normal tissues. RESULTS PTVs ranged from 4.4 to 36.7 cc in size (mean, 21.6 cc). Mean % PTV receiving 74 Gy was highest in the PP plans (98.4%; range, 96.5-99.2%) and lowest in the PH3 plans (96.1%; range, 95.1-96.7%). PR plans were the least homogeneous and conformal. PH3 plans had the highest mean % volume (V) of brain, brainstem, chiasm, and temporal lobes greater than tolerance doses. The PH1 plans had the lowest brainstem mean % V receiving 67 Gy (V(67Gy); 2.3 Gy; range, 0-7.8 Gy) and temporal lobe mean % V(65Gy) (4.3 Gy; range, 0.1-7.7 Gy). Global evaluation of the plans based on objective parameters revealed that PH1 and PP plans were more optimal than either single-modality PR or PH3 plans. CONCLUSIONS There are dosimetric advantages to using either PH1 or PP plans, with the latter yielding the best target coverage and conformality.

Incorporating partial shining effects in proton pencil-beam dose calculation

A range modulator wheel (RMW) is an essential component in passively scattered proton therapy. We have observed that a proton beam spot may shine on multiple steps of the RMW. Proton dose calculation algorithms normally do not consider the partial shining effect, and thus overestimate the dose at the proximal shoulder of spread-out Bragg peak (SOBP) compared with the measurement. If the SOBP is adjusted to better fit the plateau region, the entrance dose is likely to be underestimated. In this work, we developed an algorithm that can be used to model this effect and to allow for dose calculations that better fit the measured SOBP. First, a set of apparent modulator weights was calculated without considering partial shining. Next, protons spilled from the accelerator reaching the modulator wheel were simplified as a circular spot of uniform intensity. A weight-splitting process was then performed to generate a set of effective modulator weights with the partial shining effect incorporated. The SOBPs of eight options, which are used to label different combinations of proton-beam energy and scattering devices, were calculated with the generated effective weights. Our algorithm fitted the measured SOBP at the proximal and entrance regions much better than the ones without considering partial shining effect for all SOBPs of the eight options. In a prostate patient, we found that dose calculation without considering partial shining effect underestimated the femoral head and skin dose.

Spot-Scanning Proton Therapy Patient- Specific Quality Assurance: Results from 309 Treatment Plans

Abstract Purpose: We report the outcomes of patient-specific quality assurance (PSQA) for spot-scanning proton therapy (SSPT) treatment plans by disease site. Patients and Methods: We analyzed quality assurance outcomes for 309 SSPT plans. The PSQA measurements consisted of 2 parts: (1) an end-to-end test in which the beam was delivered at the prescribed gantry angle and (2) dose plane measurements made from gantry angle 270°. The HPlusQ software was used for gamma analysis of the dose planes using dose-tolerance and distance-to-agreement levels of 2%, 2 mm and 3%, 3 mm, respectively. Passing was defined as a gamma score <1 in at least 90% of the pixels. Results: The overall quality assurance measurement passing rate was 96.2% for the gamma index criteria of 3%, 3 mm but fell to 85.3% when the criteria were tightened to 2%, 2 mm. The passing rate was dependent on the treatment site. With the 3%, 3 mm criteria, the passing rate was 95% for head-and-neck treatment plans and 100% for prostate plans. No signi...

SU‐FF‐T‐94: Clinical Evaluation of Direct Machine Parameter Optimization Algorithm for Head and Neck IMRT Treatment

Purpose: Because many critical structures are in close proximity to target volumes, cancers of the head and neck (H&N) are often suited for treatment with IMRT. However, the time required to generate and deliver a clinically acceptable IMRT plan can be significantly longer than a conventional plan. This study evaluated a new inverse planning algorithm, DMPO (direct machine parameter optimization), with attention to parameter settings, plan quality and treatment efficiency for H&N cancers. Method and Materials: The Pinnacle treatment planning system version 7.4 was used. The DMPO allows users to limit the number of total MLC segments (N) for treatment. After a user-defined number of iterations (n) for pencil beam optimization, the DMPO generates MLC segments for each field for dose calculations using a convolution algorithm. Both the MLC leaf positions and the weight of each segment are then optimized until cost tolerance or iteration number is reached. Treatment plans generated using DMPO were compared with H&N cases that were previously treated using an older version (6.2). The plan quality was compared using cost functions and DVHs of target volumes and critical structures. The total monitor units and MLC segments for treatment were compared for different combinations of n and N. Results: The DMPO provided plans of DVHs similar to clinical cases with significantly less planning time. More importantly, the total MU and MLC segments for treatment delivery were reduced by 40% to 50%. Cost functions changed only slightly on n and N and total MU increased as n increased, but was independent of N. Our preliminary data indicated a combination of n=10–15 with 10 segments per field appeared to be optimal for most H&N cases. Conclusion: The DMPO algorithm generated more efficient plans while providing equal or better quality than the previous plans for IMRT treatment.

SU‐GG‐T‐470: Impact of Daily Patient Setup Variation on Proton Beams Passing through the Couch Edge

Purpose:Proton treatments are sensitive to radiological pathlength in the beam path. The goal is to assess the amount of dose variation caused by the changes of couch edge in the beam during daily treatment. Method and Materials: Daily couch positions were obtained from the R&V system to compute lateral setup variations. Couch positions were simulated by shifting the isocenter and the patients CTimages above the couch‐top for two standard‐deviations (2SD). One and two‐field passive scatter proton plans were designed to cover 95% of ICTV to 74Gy in the reference planning CT. The single beam plan was always chosen to bisect the couch edge. Two‐field plans were designed by adding a second beam not intersecting with the couch. Doses were re‐calculated in the shifted CTimages for 10 patients. Results: The one standard‐deviation of couch lateral variation for 11 patients in 388 daily setups was 0.53cm. We found that lateral couch variations increased for light‐weighted patients (Spearmans correlation=0.76, p=0.007). For single‐beam plans, dose decreased for all organs‐at‐risk when patient setup caused the couch to move into the beam. The reverse effect was true when couch moved out of the beam. The largest change in DVH at any dose level was −1.31% and −0.64% for the ipsilateral and total lungs when the couch moves into the field, and 1.53% and 1.88% when the couch moves out of the field. Compared to the corresponding single‐beam plan, dosimetric uncertainty was reduced to 1.16% and −2.7% with a 2‐beam plan. Conclusion: The overall dosimetric impact is limited if a single beam is allowed to shoot through the couch edge, although attention should be given to light‐weight patients whose position on couch tends to be more uncertain. Selection of a good beam angle for planning is perhaps more important than the couch‐induced dosimetric uncertainties.

SU-E-T-493: Analysis of the Impact of Range and Setup Uncertainties On the Dose to Brain Stem and Whole Brain in the Passively Scattered Proton Therapy Plans

PURPOSE To quantify the impact of range and setup uncertainties on various dosimetric indices that are used to assess normal tissue toxicities of patients receiving passive scattering proton beam therapy (PSPBT). METHODS Robust analysis of sample treatment plans of six brain cancer patients treated with PSPBT at our facility for whom the maximum brain stem dose exceeded 5800 CcGE were performed. The DVH of each plan was calculated in an Eclipse treatment planning system (TPS) version 11 applying ±3.5% range uncertainty and ±3 mm shift of the isocenter in x, y and z directions to account for setup uncertainties. Worst-case dose indices for brain stem and whole brain were compared to their values in the nominal plan to determine the average change in their values. For the brain stem, maximum dose to 1 cc of volume, dose to 10%, 50%, 90% of volume (D10, D50, D90) and volume receiving 6000, 5400, 5000, 4500, 4000 CcGE (V60, V54, V50, V45, V40) were evaluated. For the whole brain, maximum dose to 1 cc of volume, and volume receiving 5400, 5000, 4500, 4000, 3000 CcGE (V54, V50, V45, V40 and V30) were assessed. RESULTS The average change in the values of these indices in the worst scenario cases from the nominal plan were as follows. Brain stem; Maximum dose to 1 cc of volume: 1.1%, D10: 1.4%, D50: 8.0%, D90:73.3%, V60:116.9%, V54:27.7%, V50: 21.2%, V45:16.2%, V40:13.6%,Whole brain; Maximum dose to 1 cc of volume: 0.3%, V54:11.4%, V50: 13.0%, V45:13.6%, V40:14.1%, V30:13.5%. CONCLUSION Large to modest changes in the dosiemtric indices for brain stem and whole brain compared to nominal plan due to range and set up uncertainties were observed. Such potential changes should be taken into account while using any dosimetric parameters for outcome evaluation of patients receiving proton therapy.

SU‐FF‐T‐170: Dosimetric Characterization of a 2D Diode Array Detector in Passive Scattering Proton Beams

Purpose: To evaluate the dosimetric properties of a two‐dimensional (2D) diode array detector in passive‐scattered proton beams (PSPBs). Materials and Methods: The diode array detector, MapCHECK™, was characterized for PSPBs. The relative sensitivity of diodes and absolute dosecalibration were determined using a 250 MeV beam. The measured pristine Bragg curves (PBCs) by MapCHECK™ were compared with the results of an ion chamber (IC) using a range shift method. The water‐equivalent thickness (WET) of MapCHECK™ buildup was also determined. The inverse square dependence, linearity, and other protondosimetric quantities measured by MapCHECK™ were also compared with IC. The change of absolute dose response of MapCHECK™ as a function of accumulated dose was used as an indicator of radiation damage. 2D dose distribution with and without compensator were measured and compared with treatment planning system (TPS) results. To facilitate the comparison, in‐house software called MU Scaler, was developed. Results: The measured PBCs by MapCHECK™ are virtually identical to those measured by IC for 160, 180, and 250 MeV proton beams. The WET of MapCHECK™ buildup is determined to be 1.7 cm. The inverse square result of MapCHECK™ is the same as the IC results, within ±0.4%. The linearity of MapCHECK™ is within 1% compared to IC data for MUs larger than 10. All other dosimetric quantities are within 1.3% of IC results. The absolute dose response of MapCHECK™ has been changed by 7.4% after accumulating total dose of 170 Gy. Good results are observed for 2D dose distribution for patient treatment fields with and without compensator when compared with TPS results. Conclusions: MapCHECK™ is a convenient and useful tool for 2D dose distribution measurements of PSPBs. Variation of dose response of MapCHECK™ with total accumulated dose should be carefully monitored.