C Cheng

Range modulation in proton therapy planning: a simple method for mitigating effects of increased relative biological effectiveness at the end-of-range of clinical proton beams

BackgroundThe increase in relative biological effectiveness (RBE) of proton beams at the distal edge of the spread out Bragg peak (SOBP) is a well-known phenomenon that is difficult to quantify accurately in vivo. For purposes of treatment planning, disallowing the distal SOBP to fall within vulnerable tissues hampers sparing to the extent possible with proton beam therapy (PBT). We propose the distal RBE uncertainty may be straightforwardly mitigated with a technique we call “range modulation”. With range modulation, the distal falloff is smeared, reducing both the dose and average RBE over the terminal few millimeters of the SOBP.MethodsOne patient plan was selected to serve as an example for direct comparison of image-guided radiotherapy plans using non-range modulation PBT (NRMPBT), and range-modulation PBT (RMPBT). An additional plan using RMPBT was created to represent a re-treatment scenario (RMPBTrt) using a vertex beam. Planning statistics regarding dose, volume of the planning targets, and color images of the plans are shown.ResultsThe three plans generated for this patient reveal that in all cases dosimetric and device manufacturing advantages are able to be achieved using RMPBT. Organ at risk (OAR) doses to critical structures such as the cochleae, optic apparatus, hypothalamus, and temporal lobes can be selectively spared using this method. Concerns about the location of the RBE that did significantly impact beam selection and treatment planning no longer have the same impact on the process, allowing these structures to be spared dose and subsequent associated issues.ConclusionsThis present study has illustrated that RMPBT can improve OAR sparing while giving equivalent coverage to target volumes relative to traditional PBT methods while avoiding the increased RBE at the end of the beam. It has proven easy to design and implement and robust in our planning process. The method underscores the need to optimize treatment plans in PBT for both traditional energy dose in gray (Gy) and biologic dose (RBE).

Supine proton beam craniospinal radiotherapy using a novel tabletop adapter

To develop a device that allows supine craniospinal proton and photon therapy to the vast majority of proton and photon facilities currently experiencing limitations as a result of couch design issues. Plywood and carbon fiber were used for the development of a prototype unit. Once this was found to be satisfactory after all design issues were addressed, computer-assisted design (CAD) was used and carbon fiber tables were built to our specifications at a local manufacturer of military and racing car carbon fiber parts. Clinic-driven design was done using real-time team discussion for a prototype design. A local machinist was able to construct a prototype unit for us in <2 weeks after the start of our project. Once the prototype had been used successfully for several months and all development issues were addressed, a custom carbon fiber design was developed in coordination with a carbon fiber manufacturer in partnership. CAD methods were used to design the units to allow oblique fields from head to thigh on patients up to 200 cm in height. Two custom-designed carbon fiber craniospinal tabletop designs now exist: one long and one short. Four are in successful use in our facility. Their weight tolerance is greater than that of our robot table joint (164 kg). The long unit allows for working with taller patients and can be converted into a short unit as needed. An affordable, practical means of doing supine craniospinal therapy with protons or photons can be used in most locations via the use of these devices. This is important because proton therapy provides a much lower integral dose than all other therapy methods for these patients and the supine position is easier for patients to tolerate and for anesthesia delivery. These units have been successfully used for adult and pediatric supine craniospinal therapy, proton therapy using oblique beams to the low pelvis, treatment of various spine tumors, and breast-sparing Hodgkins therapy.

SU‐FF‐T‐245: Feasibility Study of Focused and Non‐Focused Photon MLC for Electron IMRT

Introduction: Modulated electron radiation therapy (MERT) could be advantageous for some disease sites. Different modes of MERT have been investigated, such as optimization of energy for each entry angle, electron MLC, etc. Feasibility of using photonMLC for MERT is explored in this study. The depth doses, profiles, penumbra (90%–10%), lateral spread (10%–1%) and radiation leakage for energies 6–21 MeV and at various source‐skin‐distance (80, 90, 100 cm) are investigated. Materials & Methods: Using both a Varian (with non‐focused MLC) and a Siemens (focused MLC) accelerators, beam characteristics at dmax are measured for possible beamlets from 1×1cm2 – 10×10 cm2 for each electron energy at 80, 90 and 100 cm SSD. The profiles are collected using film dosimetry in solid water as well as ion chamber and electron diode measurements in a scanning water tank. Results: For both the focused and non‐focus MLC, profiles obtained with 100 cm ssd exhibit a large penumbra (90–10%) in the range 4.2–7.5 cm for energies 6–21 MeV, the lower the energy, the larger the penumbra. At higher energies and 80–90 SSDs, the penumbra is much reduced. For example, for the focused MLC, it is 23 mm at dmax for 18 MeV, while for the non‐focused MLC, it is 11 mm for the 20 MeV. The percent depth dose(PDD) curves though not as steep as that with an electron cone, are clearly more advantageous compared to a photon PDD with smaller exit dose.Conclusions: The key to MERT with existing photonMLC is to reduce the source‐skin distance, while maintaining sufficient clearance for isocentric treatment. Our measurements indicate that beamlets <10×10, electron energies ⩾ 12 MeV and SSD ⩽ 90cm may provide clinical acceptable combinations for MERT with photonMLC. Focused and non‐focused MLC differ slightly in the beam characteristics.

TU-G-BRD-03: IMRT Dosimetry Differences in An Institution with Community and Academic Model

Purpose: Radiation outcome among institutions can be interpreted meaningfully if the dose delivery and prescription to the target volume is documented accurately and consistently. ICRU-83 recommended specific guidelines in IMRT for target volume definitions and dose reporting. This retrospective study evaluates the pattern of IMRT dose prescription and recording in an academic institution (AI) and a community hospital (CH) models in a single institution with reference to ICRU-83 recommendation. Materials & Methods: Dosimetric information of 625 (500 from academic and 125 from community) patients treated with IMRT was collected retrospectively from the AI and a CH. The dose-volume histogram (DVH) for the target volume of each patient was extracted. Standard dose parameters such as D2, D50, D95, D98, D100, as well as the homogeneity index (HI) defined as (D2-D98)/D50 and monitor units (MUs) were collected. Results: Significant dosimetric variations were observed in disease sites and between AI and CH. The variation in the mean value of D95 for AI is 98.48±4.12 and for CH is 96.41±4.13. A similar pattern was noticed for D50 (104.18±6.04 for AI and 101.05±3.49 for CH). Thus, nearly 95% of patients received dosage higher than 100% to the site viewed by D50 and varied between AI and CH models. The average variation of HI is found to be 0.12±0.08 and 0.11±0.08 for AI and CH model, showing better IMRT treatment plans for academic model compared to community. Conclusion: Even with the implementation of ICRU-83 guidelines, there is a large variation in dose prescription and delivery in IMRT. The variation is institution and site specific. For any meaningful comparison of the IMRT outcome, strict guidelines for dose reporting should be maintained in every institution.

SU‐E‐T‐518: Dose Perturbation at Air‐Tissue Interface in Proton Beam Therapy

PURPOSE The loss of transverse equilibrium along the central axis of the proton beam in the presence of the air/tissue interface creates dose perturbation that has not been fully quantified. This gets magnified in small fields that are used for lung and patch up fields. Air-tissue dose perturbation is studied in a phantom and verified with Monte Carlo simulation. METHODS Air channel of variable thickness that could be found in trachea, larynx and small lesions in lung were studied. To mimic air/tissue interface a simple phantom geometry was used with EBT films. The results confirmed the presence of dose perturbations which were investigating using water phantom in reference condition (10×10 cm2 field, 16 cm range and 10 cm SOBP). A variable air column was created in the front of the phantom. A small volume ion chamber was used to collect high resolution profile data in water. The simulation was performed with 3×10̂7 particles with the Monte Carlo particle transport code FLUKA version 2011.2.10 with cut off energy of 100keV. RESULTS The dose perturbations were visible on film and quantified by ion chamber measurements in water. Dose perturbations at air-tissue interfaces are shown to be significant (-20 to +30%). The measured profiles show significant discontinuities in dose up to +30% in low density medium. The magnitude is dependence on the location and width of the air gap. Under and over dose perturbation pattern is not predicted by treatment planning system (TPS) due to proton transport algorithm and calculation bin. The Monte Carlo simulation confirmed our measured data. CONCLUSIONS Significant dose perturbation exists with high-dose region in low density medium that is not predicated by TPS. The magnitude and shape is position and gap size dependent. This study provides the presence of dosimetric discontinuities that should be evaluated clinically at interfaces.

TU‐D‐ValA‐01: Role of Collimator Angle Optimization in Intensity Modulated Radiation Therapy

Purpose:IMRT optimization involves several treatment parameters producing a complex, unstable and computationally challenging problem during its search of an optimal plan in a reasonable time. Most parameters have been studied in IMRT optimization except the collimator angle, which is investigated in this study. Method and Materials: Five head‐and‐neck and five prostate cases are selected. The head ‐and‐neck and prostate PTVs range between 79.6–441 cm3 and 86.2–250 cm3 respectively, whereas the OAR volumes vary from 11.0–46.3 cm3 and 41.1–312 cm3 respectively. The patients are treated with five or seven fields equally distributed, 0‐ degree collimator angle, using the 1 cm leaf MLC from a Siemens Primus accelerator based on the plan generated using the Nucletron Oncentra treatment planning system. While dose‐volume constraints are kept the same as in the patients initial treatment plan, collimator angles are varied systematically (0–90 degrees) and a new treatment plan is optimized for each collimator angle. The number of beamlets, monitor units (MU) and DVHs for each collimator angle are compared. Results: The variation of the total number of beamlets with collimator angle follows the shape of a parabola and peaks at 45° collimator angle for all patients. However, the MUs appear to be relatively independent of the collimator angle. The PTV dose coverage statistics for each patient are relatively independent of the collimator angle. Similar observations are noted for all the OARs, except for the small structures for which differences could be observed in the DVHs between the different collimator angles. Conclusions:Collimator angle does not play a significant role in IMRT optimization, as long as the PTV overage is adequate. This provides an additional freedom to choose from 0–90 degree of the collimator angle for long fields without compromising the coverage with limited MLC range or treatment time.

SU-E-T-366: Estimation of Whole Body Dose From Cranial Irradiation From C and Perfexion Series Gamma Knife Units

Purpose: The Leksell Gamma Knife (GK) B & C series contains 201 Cobalt-60 sources with a helmet. The new model, Perfexion uses 192 Cobalt-60 sources without a helmet; using IRIS system for collimation and stereotactic guidance to deliver SRS to brain tumors. Relative dose to extracranial organs at risk (OARs) is measured in phantom in this study for Perfexion and C-series GK. Materials & Methods: Measurements were performed in a Rando anthropomorphic phantom on both systems using a large ion chamber (Keithley-175) for each collimator. The Keithley-175 cc ion chamber was sandwiched between phantom slices at various locations in the phantom to correspond to different extracranial OARs (thyroid, heart, kidney, ovary and testis, etc.) The dose measurement was repeated with OSL detectors for each position and collimator. Results: A large variation is observed in the normalized dose between these two systems. The dose beyond the housing falls off exponentially for Perfexion. Dose beyond the C-series GK housing falls off exponentially from 0–20cm then remains relatively constant from 20–40cm and again falls off with distance but less rapidly. The variation of extracranial dose with distance for each collimator is found to be parallel to each other for both systems. Conclusion: Whole body dose is found to vary significantly between these systems. It is important to measure the extracranial dose, especially for young patients. It is estimated that dose falls off exponentially from the GK housing and is about 1% for large collimators at 75 cm. The dose is two-orders of magnitude smaller for the 4mm collimator. However, this small dose for patient may be significant radiologically.

SU‐E‐T‐723: An Effective Atomic Number of the Compounds in Proton Beam Therapy

Purpose: The effective atomic number (Zeff) of a material is defined, conventionally, from the X‐Ray interactions with the material. The interaction of protons in a medium is fundamentally different from photons. Hence applicability of the photon‐based definition may not be justified in proton therapy. The purpose of this study is to define the Zeff of a medium for proton beam therapy. Methods: A robust definition of Zeff is proposed through the proton range shift AR(t) in water due to the presence of a layer of a material of thickness t. This definition assures experimental verification of Zeff. In this study, the proton range shift was calculated with the TRIM (the Transport of Ions in Matter) code for a set of compounds. The ICRU 46 elemental composition for human tissue, dental implant materials and some plastics commonly used in radiotherapy applications were used for Zeff calculation for proton energies from 5 MeV to 250 MeV. The Zeff for protons is compared to those from conventional definitions. Results: The weighted atomic densities Zeff for known material composition assures invariance of the Zeff with initial proton energy in the model. The Zeff for protons differs from those defined for photons substantially. The semi‐empirical model proposed for the calculations of Zeff, is based on the measurable values of the range shift in water in the presence of a material of unknown elemental composition. Zeff may be calculated from the equation Zeff(x) = a exp(−b x), where x = δR(t)/t. Conclusion: A robust definition of effective atomic number of a given medium for proton interaction is proposed based on the measurable value of range shift in water in the presence of the material layer.

SU-E-T-180: Empirical Determination of Output Factors for Proton Therapy Fields Using a Uniform Scanning Proton Beam and 10cm Snout

PURPOSE he output factor of a proton field is affected by energy, width of spread-out Bragg Peak (SOBP), source to measurement point distance, shape and size of the aperture, and the thickness of the compensator. It is generally measured in a water phantom to determine MU needed for each treatment field. This is time consuming and labor intensive. Previous studies employed empirical fits to measured data and then applied correction factors to account for various parameters. In this study, we have developed an empirical model to determine the output factors for proton fields with a 10cm snout using a cubic equation. METHODS Measured output factors with (OFclosed) and without (OFopen) a compensator for 693 fields delivered with the uniform scanning beam were analyzed. The measurements were made for various ranges, SOBP widths, air gaps, aperture shapes and sizes using a 10cm snout. 3D empirical equations for predicting OFclosed and OFopen were determined by fitting the data to several 3D curves and evaluating each fit. RESULTS The proposed model uses a simplified cubic fit. The distribution of closed measured OF vs predicted OF for this model has a mean of 1.000 and 0.013 the standard deviation and agrees within 3% of measured data 88.9% of all fields. The current model being used has a mean of 0.994 and a standard deviation of 0.009 and agrees within 3% of measured data 98.2% of all fields. CONCLUSION We have shown that a simplified cubic fitting of the measured data allows for calculation of output factor with a single equation of the form OF=f(Inverse Square Law Correction, SOBP, Range). This empirical equation may be used to double check the OFclosed obtained with the separate model that is currently used. The model will significantly reduce the QA time required for measurements.

U‐E‐T‐620: Radiobiological Implication of Margin for Target Expansion in Head and Neck IMRT with Daily IGRT

PURPOSE To account for organ motion and set up uncertainties a margin is added to the clinical target volume (CTV) to form the Planning Target Volume (PTV). There exists significant institutional variability of margins employed between CTV to PTV. The margin used has significant implication for extra tissue (PTV-CTV) dose and directly related with normal tissue complication. This study quantifies the setup uncertainties to optimize the PTV margin and its radiobiological implication for target expansion in H&N cancer. METHODS Nine Nine H&N patients treated with IMRT and daily IGRT with on-board-imaging (OBI) were chosen for this study. Using Eclipse treatment planning system (TPS), treatment plans were generated for different margins ranging from 0 to 10 mm subject to the same optimization criterion for PTV and OAR using 6 MV photon beam. The DVH, extra tissue volume and total MU were recorded for all these IMRT plans. NTCP was calculated using the Lyman Kutcher Burman model from DVH data. The daily setup errors from OBI for the entire treatment were also analyzed. RESULTS Analysis of the 9 patients setup with over 800 data points showed that 98% of the points are within ±5mm using daily IGRT. With increasing margin, the PTV volume increases linearly. There is a 4-fold relative increase in extra tissue volume for margin increase from 0-10 mm With increasing PTV margin the NTCP also increases linearly for the parotid glands. Similar patterns were noticed for all other OARs. CONCLUSION With OBI the setup uncertainty could be easily achieved within ±5 mm for 98% of the H&N treatments. Increase in PTV margin increases NTCP. It is concluded that PTV margin should be limited to ±5mm to reduce extra volume irradiation and also reduces NTCP of OARs.