Parallel-opposed Pair Research Articles

A Comparison of Radical Radiotherapy Techniques in the Treatment of the Para-aortic Strip in Stage 2 Testicular Seminoma Patients

Purpose: To compare PTV coverage and organ at risk doses in patients with stage 2 testicular seminoma receiving radical radiotherapy, using a standard open field para-aortic strip parallel opposed pair (POP), a modified POP with MLC and a VMAT technique.

Preclinical dose verification using a 3D printed mouse phantom for radiobiology experiments.

Dose verification in preclinical radiotherapy is often challenged by a lack of standardization in the techniques and technologies commonly employed along with the inherent difficulty of dosimetry associated with small-field kilovoltage sources. As a consequence, the accuracy of dosimetry in radiobiological research has been called into question. Fortunately, the development and characterization of realistic small-animal phantoms has emerged as an effective and accessible means of improving dosimetric accuracy and precision in this context. The application of three-dimensional (3D) printing, in particular, has enabled substantial improvements in the conformity of representative phantoms with respect to the small animals they are modeled after. In this study, our goal was to evaluate a fully 3D printed mouse phantom for use in preclinical treatment verification of sophisticated therapies for various anatomical targets of therapeutic interest. An anatomically realistic mouse phantom was 3D printed based on segmented microCT data of a tumor-bearing mouse. The phantom was modified to accommodate both laser-cut EBT3 radiochromic film within the mouse thorax and a plastic scintillator dosimeter (PSD), which may be placed within the brain, abdomen, or 1-cm flank subcutaneous tumor. Various treatments were delivered on an image-guided small-animal irradiator in order to determine the doses to isocenter using a PSD and validate lateral- and depth-dose distributions using film dosimeters. On-board cone-beam CT imaging was used to localize isocenter to the film plane or PSD active element prior to irradiation. The PSD irradiations comprised a 3×3mm2 brain arc, 5×5mm2 parallel-opposed pair (POP), and 5-beam 10×10mm2 abdominal coplanar arrangement while two-dimensional (2D) film dose distributions were acquired using a 3×3mm2 arc and both 5×5 and 10×10mm2 3-beam coplanar plans. A validated Monte Carlo (MC) model of the source was used as to verify the accuracy of the film and PSD dose measurements. computer-aided design (CAD) geometries for the mouse phantom and dosimeters were imported directly into the MC code to allow for highly accurate reproduction of the physical experiment conditions. Experimental and MC-derived film data were co-registered and film dose profiles were compared for points above 90% of the dose maximum. Point dose measurements obtained with the PSD were similarly compared for each of the candidate (brain, abdomen, and tumor) treatment sites. For each treatment configuration and anatomical target, the MC-calculated and measured doses met the proposed 5% agreement goal for dose accuracy in radiobiology experiments. The 2D film and MC dose distributions were successfully registered and mean doses for lateral profiles were found to agree to within 2.3% in all cases. Isocentric point-dose measurements taken with the PSD were similarly consistent, with a maximum percentage deviation of 3.2%. Our study confirms the utility of 3D printed phantom design in providing accurate dose estimates for a variety of preclinical treatment paradigms. As a tool for pretreatment dose verification, the phantom may be of particular interest to researchers for its ability to facilitate precise dosimetry while fostering a reduction in cost for radiobiology experiments.

Dose Escalation in 5-Fraction Palliative Thoracic Radiotherapy: A Feasibility and Safety Planning Study

Short course palliative radiotherapy is an effective and convenient treatment in patients with incurable advanced lung cancer to manage symptoms. Standard 5-fraction regimens often deliver a lower overall dose, limiting durable local control. We performed a planning study to evaluate the feasibility and safety of 5-fraction dose escalation. In this research ethics board approved study, 10 patients with advanced lung cancer who received palliative thoracic radiotherapy between 2015-2016 were identified from our institutional database. Four 5-fraction plans were generated for each patient: 20 Gy using a parallel opposed pair (POP) beam arrangement, as compared with 25 Gy, 30Gy and 35 Gy using volumetric modulated arc therapy (VMAT). The gross target volume (GTV) was expanded volumetrically by 1 cm to obtain the planning target volume (PTV). The fields for POP plans were defined by extending 8mm from the PTV. VMAT plans were optimized by prioritizing organs at risk (OARs) allowing compromise of PTV coverage (PTV-eval =PTV - overlapping OARs, with an aim for D95≥95%). Conservative dose constraints were selected from literature, with OARs including spinal cord, esophagus, trachea, proximal bronchial tree and brachial plexus. For Lung-eval (lung-GTV), a V20 of ≤10% was targeted, with ≤15% accepted if required. Published radiobiological models were employed to evaluate the likelihood for toxicity to OARs, with a specific focus on esophageal toxicity. The median PTV was 316cc (range 165 -551cc). In the reference 20Gy POP plans, the median lung-eval V20 were 6% (range 1%-22%) and D95≥95% coverage was achieved in 30% cases. For 25Gy VMAT plans, the pre-specified OARs tolerance were met in all cases. For 30-35 Gy VMAT plans, the pre-specified OARs tolerance were met in all except 1 case, which had a lung-eval V20 of 15.5% and 19.5% with the 30 Gy and 35Gy plans respectively. The D95≥95% coverage was achieved in 90 % cases for 25Gy, 20% for 30Gy, and 0% for 35Gy. The coverage evaluated by D90 were as following: 25Gy median 97% (range 97%-100%), 30Gy median 96% (92%-98%), and 35Gy median 88% (range 83-96%). The predicted probability of ≥grade 2 esophagitis for 20Gy, 25Gy, 30Gy and 35 Gy plans were 1.7%, 3.3%, 5.0% and 5.0% respectively, based on the Belderbos normal tissue complication probability model. In this planning study of 5-fraction palliative thoracic radiotherapy, dose escalation from 25 to 35 Gy in 5 fractions appeared feasible using VMAT techniques without significant predicted toxicity. While dose escalation to 25Gy without PTV compromise was feasible in most cases, significant compromise of PTV was required for 30- and 35Gy to minimize predicted toxicity. Based on these data, a dose-escalation clinical trial is planned under the auspices of the Canadian Pulmonary Radiotherapy Investigators (CAPRI) Group.

Accuracy of Dose Calculation for Hemi-Body Treatment at Extended Distance using a Commercial Treatment Planning System

The objective of this study was to assess the accuracy of Monitor Unit (MU) calculation for extended distance hemi-body (HB) treatments in Pinnacle, a commercial treatment planning system. The agreement between planning and delivery of low dose radiation therapy (LD-RT) was assessed with direct comparison to expected doses and tabulated total body irradiation (TBI) calculations. Studies over the past decades indicate that LD-RT has strong potential to be an effective treatment modality for patients, particularly those with Non-Hodgkin’s Lymphoma (NHL), a cancer of the lymphocytes. Past studies also indicate that a LD-RT regime will present minimal toxicity and limited adverse side effects. LD-RT may directly trigger a systemic immune response. This physics-based study aims to provide sufficient conclusions required for prospective clinical studies involving HB irradiation regimes. Specifically, this study may provide reassurance of MU calculation in the Pinnacle system for an upcoming trial regarding non-targeted LD-RT for recurrent prostate cancer. Water Phantom: A plan was created in Pinnacle to deliver 100 cGy to a water phantom with an ion chamber (IC) mount. A percent depth dose (PDD) was obtained. Electrometer readings were recorded with each irradiation of 400 MU at varying IC depths at extended distance. A PDD was created from tabulated data. Anthropomorphic Phantom: A parallel opposed pair (POP) plan was created in Pinnacle to deliver 150 cGy over 10 fractions to the umbilicus of the phantom at 4 m extended source-to-surface distance (SSD). The MU required to deliver 150 cGy, as per Pinnacle were delivered to the phantom using 6 MV photons. Thermoluminescent dosimeters, used to measure exposure using light-emitting crystals, were placed along six reference locations on the phantom (lung, mid-T-spine, abdomen, mid-pelvis, thigh, and mid-abdomen). TLD measurements were then compared to the Pinnacle-derived ROI mean doses. For Experiment 2, TBI calculation factors were used to determine the required MU to deliver 150 cGy to the prescription (Rx) point. The calculated MUs were delivered and TLD readings were recorded to compare the level of agreement of using TBI calculations for HB treatments. Water Phantom: Pinnacle did not accurately estimate dmax at extended distance, however, it did accurately estimate the dose past dmax. Anthropomorphic Phantom: A 10% variation to expected dose was deemed significant. Lung dose did not present significant variation. The abdomen presented borderline significance. Remaining ROIs presented significant variation for both Pinnacle and TBI calculations. Both Pinnacle and TBI calculations were accurate methods of planning HB LD-RT treatment, with insignificant difference. Pinnacle accurately estimated dose to the Rx point but overestimated the dose to remaining ROIs. Overall average variation was borderline significant at 12.1%. At extended SSD, Pinnacle inaccurately estimated the entrance dose and dmax. PDD data supported Pinnacle’s ability to accurately estimate dose at depths greater than dmax. Anthropomorphic phantom studies indicated borderline significant variation, as per the implemented 10% limit. TBI calculations presented similar conclusions. For purposes of HB LD-RT, a borderline 10% variation will have insignificant impact to the patient’s ability to tolerate treatment. Trial eligible prostate cancer patients are currently being recruited for HB LD-RT at the Juravinski Cancer Centre.

161 - A dosimetric comparative analysis of parallel opposed pair (POP) and volumetric modulated arc therapy (VMAT) techniques for high dose palliative radiotherapy (HDPRT) for non-small cell lung carcinoma

161 - A dosimetric comparative analysis of parallel opposed pair (POP) and volumetric modulated arc therapy (VMAT) techniques for high dose palliative radiotherapy (HDPRT) for non-small cell lung carcinoma

Intentional avoidance of the esophagus using intensity modulated radiation therapy to reduce dysphagia after palliative thoracic radiation

BackgroundPalliative thoracic radiotherapy is an effective technique to alleviate symptoms of disease burden in advanced-stage lung cancer patients. Previous randomized controlled studies demonstrated a survival benefit in patients with good performance status at radiation doses of 35Gy10 or greater but with an increased incidence of esophagitis. The objective of this planning study was to assess the potential impact of esophageal-sparing IMRT (ES-IMRT) compared to the current standard of care using parallel-opposed pair beams (POP).MethodsIn this study, 15 patients with lung cancer treated to a dose of 30Gy in 10 fractions between August 2015 and January 2016 were identified. Radiation treatment plans were optimized using ES-IMRT by limiting the max esophagus point dose to 24Gy. Using published Lyman-Kutcher-Burman normal tissue complication probabilities (LKB-NTCP) models, both plans were evaluated for the likelihood of esophagitis (≥ grade 2) and pneumonitis (≥ grade 2).ResultsUsing ES-IMRT, the median esophageal and lung mean doses reduced from 16 and 8Gy to 7 and 7Gy, respectively. Using the LKB models, the theoretical probability of symptomatic esophagitis and pneumonitis reduced from 13 to 2%, and from 5 to 3%, respectively. The median normalize total dose (NTD mean) accounting for fraction size for the GTV and PTV of the clinically approved POP plans compared to the ES-IMRT plans were similar.ConclusionAdvanced radiotherapy techniques such as ES-IMRT may have clinical utility in reducing treatment-related toxicity in advanced lung cancer patients. Our data suggests that the rate of esophagitis can be reduced without compromising local control.

Delivery of Radiation Therapy of Large Target Volumes in Hodgkin's Lymphoma Using IMRT Technique: A Dosimetric Case Study

Radiation therapy techniques for early stage Hodgkin's disease (HD) include simple parallel-opposedpair (POP) and Intensity-Modulated Radiation Therapy (IMRT). This case report compared the dosimetry achieved with POP and IMRT by evaluating target volumes and organs at risk (OAR). It showed that the IMRT technique could be a valuable technique when treating HD. Optimal target coverage can be achieved while at the same time reducing dose given to critical structures such as lung and thyroid.

Dosimetric Comparison of 3D Tangential Radiotherapy of Post-Lumpectomy Breast at Two Different Energies

Introduction: Radiation therapy following breast conserving surgery is one of the most common procedures performed in any radiation oncology department. A tangential parallel-opposed pair is almost always the technique of choice for this purpose. This technique is often performed based on 3D treatment planning. The aim of this study was to compare 3D treatment planning for two different energies (Cobalt 60 versus 6 MV photon beams) in tangential irradiation of breast conserving radiotherapy. In this comparison, homogeneity of isodoses within the breast volume and dose received by lungs were considered. Materials and Methods: In this study, twenty patients with breast cancer treated with conservative surgery were included. A CT scan was performed on selected patients. Three-dimensional treatment planning with 6 MV photon beams was carried out for patients on the Eclipse 3D treatment planning system (TPS). The volumes receiving lower than 95% (Vol 105) (hot areas) of the reference dose, and the volume of lung receiving ≥30Gy (Vol≥30Gy) were derived from dose volume histograms (DVHs). Dose homogeneity index was calculated as: DHI = 100 – (Vol>105 + Vol<95). In the second stage, each patient was replanned with Cobalt 60 and the quantities of interest were obtained from the DVHs. Results: Comparing 3D treatment planning by 6 MV and Cobalt 60 photons, cold areas were reduced (p<0.001), hot areas were decreased (p<0.7), DHI was improved (P<0.001) and Vol≥30Gy was reduced (P<0.017) with 6 MV photons. Discussion and Conclusion: Our results indicate that treatment planning with Cobalt 60 causes more cold areas in the treated volume as compared with 6 MV photons. Cold areas within the target volume of the breast result in reduced tumor control probability. The plan can be optimized to limit dose to lung volume by using 6 MV photons. So lung fibrosis is more likely to occur with Cobalt 60. In addition, DHI was improved with 6 MV photon beams. Improvement of dose homogeneity can lead to higher tumor control probability and cosmetic results, and less skin and lung side effects. Hence, 3D treatment planning with 6 MV photon beams is a more suitable option for patients with breast cancer treated with conserving surgery

A radiotherapy planning study of RapidArc, intensity modulated radiotherapy, three‐dimensional conformal radiotherapy, and parallel opposed beams in the treatment of pediatric retroperitoneal tumors

This planning study compared RapidArc, fixed-field IMRT (cIMRT), 3D conformal radiotherapy (3D-CRT), and a parallel-opposed pair (POP) for children with retroperitoneal tumors. Plans were generated in eight patients to treat the PTV (dose range 19.8-45 Gy) while limiting kidney and liver doses. In selected patients, vertebral body (VB) dose heterogeneity was minimized. Cumulative DVH parameters, monitor units (MU), and treatment times were compared for the four techniques using the Wilcoxon matched pairs test. RapidArc and cIMRT covered target volumes more conformally than 3D-CRT and POP (P = 0.012). There was no difference in the ability to meet kidney dose constraints. A significantly lower volume of the liver received 12 Gy with cIMRT or RapidArc compared with 3D-CRT (P = 0.028). Where VB was included in PTV, VB dose homogeneity was generally within 94-104% of the prescription dose. Time to deliver a single fraction with RapidArc, POP, 3D-CRT, and cIMRT was 1.25 ± 0.01, 1.38 ± 0.10, 2.6 ± 0.45, and 4.02 ± 1.12 min, respectively (P = 0.012). Monitor units for a single fraction with POP, 3D-CRT, RapidArc, and cIMRT were 203 ± 26, 235 ± 32, 325 ± 71, and 665 ± 215, respectively (P < 0.05). POP resulted in favorable MU, treatment time and dosimetry but had poor conformality. 3D-CRT was more conformal but had higher MU and treatment time. RapidArc and cIMRT were generally no better dosimetrically than conformal techniques. RapidArc was dosimetrically very similar to cIMRT, but resulted in a major reduction in time and MU used to deliver the radiation.

THE COMPARISON 2D AND 3D TREATMENT PLANNING IN BREAST CANCER RADIOTHERAPY WITH EMPHASIS ON DOSE HOMOGENEITY AND LUNG DOSE

Introduction: Breast conserving radiotherapy is one of the most common procedures performed in any radiation oncology department. A tangential parallel-opposed pair is usually used for this purpose. This technique is performed using 2D or 3D treatment planning systems. The aim of this study was to compare 2D treatment planning with 3D treatment planning in tangential irradiation in breast conserving radiotherapy. In this comparison, homogeneity of isodoses in the breast volume and lung dose were considered. Material and Methods: Twenty patients with breast cancer treated with conservative surgery were included in this study. The patients were CT scanned. Two-dimensional treatment planning with the Alfard 2D TPS was performed for each patient using a single central CT slice. The data used on the Alfard 2D TPS was imported into the Eclipse 3D TPS, on which 3D treatment planning was performed. Cobalt-60 beams were used in all plans. Results: Comparing 2D and 3D treatment planning, homogeneity of isodoses was improved in 3D treatment planning (p 30Gy was increased in 3D treatment planning (p< 0.01). Discussion and Conclusion: 3D treatment planning is a more suitable option for patients with breast cancer treated with conservative surgery because of improved dose homogeneity in 3D treatment planning. The results of the treatment can be improved with reduced recurrence probability and skin problems.

Novel Application of Helical Tomotherapy in Whole Skull Palliative Radiotherapy

Helical tomotherapy (HT) is a radiation planning/delivery platform that combines inversely planned IMRT with on-board megavoltage imaging. A unique HT radiotherapy whole skull brain sparing technique is described in a patient with metastatic prostate cancer. An inverse HT plan and an accompanying back-up conventional lateral 6-MV parallel opposed pair (POP) plan with corresponding isodose distributions and dose-volume histograms (DVH) were created and assessed prior to initiation of therapy. Plans conforming to the planning treatment volume (PTV) with significant sparing of brain, optic nerve, and eye were created. Dose heterogeneity to the PTV target was slightly higher in the HT plan compared to the back-up POP plan. Conformal sparing of brain, optic nerve, and eye was achieved by the HT plan. Similar lens and brain stem/spinal cord doses were seen with both plans. Prospective clinical evaluation with relevant end points (quality of life, symptom relief) are required to confirm the potential benefits of highly conformal therapies applied to palliative situations such as this case.

In vitro study of cell survival following dynamic MLC intensity‐modulated radiation therapy dose deliverya)

The possibility of reduced cell kill following intensity-modulated radiation therapy (IMRT) compared to conventional radiation therapy has been debated in the literature. This potential reduction in cell kill relates to prolonged treatment times typical of IMRT dose delivery and consequently increased repair of sublethal lesions. While there is some theoretical support to this reduction in cell kill published in the literature, direct experimental evidence specific to IMRT dose delivery patterns is lacking. In this study we present cell survival data for three cell lines: Chinese hamster V79 fibroblasts, human cervical carcinoma, SiHa and colon adenocarcinoma, WiDr. Cell survival was obtained for 2.1 Gy delivered as acute dose with parallel-opposed pair (POP), irradiation time 75 s, which served as a reference; regular seven-field IMRT, irradiation time 5 min; and IMRT with a break for multiple leaf collimator (MLC) re-initialization after three fields were delivered, irradiation time 10 min. An actual seven-field dynamic MLC IMRT plan for a head and neck patient was used. The IMRT plan was generated for a Varian EX or iX linear accelerator with 120 leaf Millenium MLC. Survival data were also collected for doses 1X, 2X, 3X, 4X, and 5x 2.1 Gy to establish parameters of the linear-quadratic equation describing survival following acute dose delivery. Cells were irradiated inside an acrylic cylindrical phantom specifically designed for this study. Doses from both IMRT and POP were validated using ion chamber measurements. A reproducible increase in cell survival was observed following IMRT dose delivery. This increase varied from small for V79, with a surviving fraction of 0.8326 following POP vs 0.8420 following uninterrupted IMRT, to very pronounced for SiHa, with a surviving fraction of 0.3903 following POP vs 0.5330 for uninterrupted IMRT. When compared to IMRT or IMRT with a break for MLC initialization, cell survival following acute dose delivery was significantly different, p < 0.05, in three out of six cases. In contrast, when cell survival following IMRT was compared to that following IMRT with a break for MLC initialization the difference was always statistically insignificant. When projected to a 30 fraction treatment, dose deficit to bring cell survival to the same value as in POP was calculated as 4.1, 24.9, and 31.1 Gy for V79, WiDr, and SiHa cell lines, respectively. The dose deficit did not relate to the alpha/beta ratio obtained in this study for the three cell lines. Clinical data do not show reduction in local control following IMRT. Possible reasons for this are discussed. The obtained data set can serve as a test data set for models designed to explore the effect of dose delivery prolongation/fractionation in IMRT on radiation therapy outcome.

Sci‐Sat AM (1) General‐01: In vitro measurement of cell kill by IMRT fields

Results of the in vitro measurements of cell kill from IMRT fields compared to acute dose delivery are presented. Survival was assayed for three cell lines: Chinese hamster V79 fibroblasts, human cervical carcinoma SiHa and colon adenocarcinoma WiDr. An actual head and neck seven field IMRT plan produced for Varian iX, 120 Millennium MLC linear accelerator was used. The calculated IMRT dose to the point of irradiation in the acrylic phantom was 2.1Gy, verified by measurements with an IC10 ionization chamber. The dose varied within 2% through dimensions of the specimen. Two scenarios were explored: 1. regular IMRT delivery, irradiation time of approximately 5min, and 2. IMRT delivery which required MLC re‐initialization after three fields were delivered, irradiation time of 10min. For comparison the same dose of 2.1Gy was delivered by parallel‐opposed pair (POP) in 75s. Persistent increase in cell survival following irradiation with IMRT fields was observed for all cell lines. This increase varied from small for V79 cells: 0.833±0.018 (95% confidence limits) from POP compared to 0.860±0.040 for IMRT with MLC re‐initialization, to very pronounced for the radiosensitive SiHa cell line: 0.390±0.046 for POP irradiation compared to 0.591±0.080 from IMRT requiring MLC re‐initialization. For WiDr cell line survival was 0.744±0.038 for POP, and 0.809±0.023 for regular IMRT. We observed a persistent, although not always statistically significant increase in cell survival from IMRT fields. While this study does not account for features present in vivo, it provides useful insight into biological consequences of IMRT as assayed by cell survival.

Po‐Thur Eve General‐07: Dosimetry of Small Lung Lesions with EGSnrc Monte Carlo and Treatment Planning Systems

Early stage lung cancer, presenting as a small solitary pulmonary mass, is treated with radiation when concurrent disease precludes a surgical option. These small lesions are usually surrounded by less dense normal lung, which affects the ability to deliver a homogenous dose to the target volume. In low‐density tissue, such as lung, there is increased transmission of photons. In addition, lateral scatter of electrons out of the beam can lead to increased penumbral width. The magnitude of these effects is known to be dependent on beam energy. Some of the commonly used commercial treatment planning systems have had limited success in predicting accurately dose distributions under these highly inhomogeneous conditions.We present a quantitative comparison between Monte Carlo simulation and commercial planning systems for a select range of clinically relevant target geometries and beam parameters. Small water equivalent cylindrical lung tumors of diameter 3 and 5 cm were incorporated within a CT dataset at different locations. A Parallel Opposed Pair (POP) field arrangement with 6MV or 15MV photons and variable field‐edge margins were considered. These plans were calculated using BEAMnrc Monte Carlo code and on two planning systems; ADAC Pinnacle III Version 7.4 and MDS Nordion Theraplan Plus v3.8.The analysis of dose profiles and DVH's show considerable and unique differences between Monte Carlo and the results from each TPS within the tumor and at the junction between tumor and lung. For both planning systems, the severity of these errors, increases with photon energy, and decreases with field size.

SU‐FF‐T‐460: Comparative in Vitro Study of Cell Survival Following IMRT and Acute Dose Delivery

Purpose: To investigate cell kill from IMRT fields compared to acute dose delivery. Method and Materials: Survival was assayed in vitro for three cell lines: Chinese hamster V79 fibroblasts, human cervical carcinoma SiHa and colon adenocarcinoma WiDr. An actual head and neck seven field dynamic IMRT plan produced for Varian iX, 120 Millennium MLC linear accelerator was used. The calculated IMRT dose (Cadplan, Palo Alto) to the point of irradiation in the acrylic phantom was 2.1Gy. This was verified by measurements with an IC10 ionization chamber. Two IMRT dose delivery scenarios were explored: 1. normal delivery, irradiation time of 5min 10s, and 2. IMRT delivery with a 5min break for MLC re‐initialization after three fields were delivered, irradiation time of 10min. For comparison the same dose of 2.1Gy was delivered by parallel‐opposed pair (POP) in 75s, 20s beam‐on time per beam. Survival data were obtained in the dose range up to 10.5Gy to establish the linear‐quadratic survival curves. Results: An increased cell survival following irradiation with IMRT fields was observed for all cell lines. V79 cells showed the smallest increase: 0.833±0.018 (95% confidence limits) from POP compared to 0.860±0.040 for IMRT with MLC re‐initialization. This increase was very pronounced for the radiosensitive SiHa cell line: 0.390±0.046 for POP irradiation compared to 0.591±0.080 from IMRT requiring MLC re‐initialization. Although variable between cell lines, projections made for a 30 fraction treatment showed substantial reduction in cell kill. Conclusions: We observed an increase in cell survival from IMRT fields compared to acutely delivered dose. This increase was persistent, but not always statistically significant. Projections for a fractionated treatment showed that consequences of this increased cell survival are substantial with a very large variation between cell lines. This projection, however, does not account for features present in vivo, for example reoxygenation and reassortment.

More optimal dose distributions for moving lung tumours: A planning study

Background and purposeTarget volumes for moving lung tumours encompass the full range of respiratory motion, increasing the risk of lung complications. Intensity modulated radiotherapy (IMRT) allows for more precise dose distributions. Distributions corresponding to the probability density function (PDF) of tumour motion may better spare lung yet deliver adequate target dose. The planning study purpose is to compare and evaluate different dose distributions on a moving lung tumour: (A) conformal RT (CRT) encompassing the full range of tumour motion, (B) CRT encompassing the modal tumour position only, and (C) an IMRT technique where the dose delivered corresponds to the tumour PDF. Materials and methodsA 5cm diameter spherical target within a rectangular lung equivalent phantom was treated using a parallel-opposed pair technique with a 1.5cm margin around the tumour (in the beam's eye view). Asymmetrical sinusoidal (superior–inferior) target movement (peak-trough=3cm) was simulated for different dose distributions (prescription dose=60Gy). Equivalent uniform dose (EUD) for the tumour and normal tissue complication probabilities (NTCPs) for radiation pneumonitis were evaluated. ResultsThe EUDs were 60.0, 48.5, and 57.9Gy while the NTCPs were 5, 1, and 3% for cases A, B, and C, respectively (assuming survival fraction, SF2Gy=0.5). ConclusionsSince these results rely on unvalidated radiobiologic models, they must be interpreted cautiously. However, more optimized dose distributions for moving lung targets appear feasible and can reduce lung complications with only a negligible impact on the expected EUD and, thus, deserve further study.

Accurate skin dose measurements using radiochromic film in clinical applications

Megavoltage x-ray beams exhibit the well-known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. In addition to films, we also used an Attix parallel-plate chamber and a home-built extrapolation chamber to cover tissue-equivalent depths in the range from 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR-T model GAFCHROMIC films. The correction factors for the exit skin dose due to the build-down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.

Radiotherapy and Perianal Paget's Disease

The aim of this study is to document a case of Extra Mammary Paget's Disease (EMPD), which was treated with radiotherapy at the Suffolk Oncology Centre. We have shown evidence, both in the reviewed literature and in this study, that radiotherapy can be used successfully as a primary form of treatment for this disease.The patient discussed in this paper had EMPD in the perianal region. 40 Gy in 10# using a parallel-opposed pair field arrangement was prescribed. However due to mental and physical frailty only 90% of the dose could be delivered. The patient suffered only minor side effects and three years post-radiotherapy was still disease free. We can therefore say that radiotherapy was successful in this instance.We concluded that there is a definite role for radiotherapy in the management of EMPD and that it can be effective as a primary treatment, where the only other option could be mutilating surgery.

Development of methods for beam angle optimization for IMRT using an accelerated exhaustive search strategy

The purpose of this article is to explore the use of the accelerated exhaustive search strategy for developing and validating methods for optimizing beam orientations for intensity-modulated radiation therapy (IMRT). Combining beam-angle optimization (BAO) with intensity distribution optimization is expected to improve the quality of IMRT treatment plans. However, BAO is one of the most difficult problems to solve adequately because of the huge hyperspace of possible beam configurations (e.g., selecting 7 of 36 uniformly spaced coplanar beams would require the intercomparison of 8,347,680 IMRT plans). An "influence vector" (IV) approximation technique for high-speed estimation of IMRT dose distributions was used in combination with a fast gradient search algorithm (Newton's method) for IMRT optimization. In the IV approximation, it is assumed that the change in intensity of a ray (or bixel) proportionately changes dose along the ray. Evidence is presented that the IV approximation is valid for BAO. The scatter contribution at points away from the ray is accounted for fully in IMRT optimization after the optimum beam orientation has been determined. IVs for all candidate beam angles are generated before the start of optimization. For all subsets of beams selected from a given pool of beams (e.g., 5 of 24 uniformly spaced beams), the distribution of planning scores for the best and the worst plans, optimum angle distributions, dose distributions, and dose-volume histograms (DVH) were analyzed for one prostate and two lung cancer cases. The results of the exhaustive search technique were used to develop a "multiresolution" search strategy. In this approach, a smaller number of beams (e.g., three) is first used to explore the hyperspace of solutions to determine the most preferred and the least preferred directions. The results of such exploration are then used as a starting point for determining an optimum configuration comprising a larger number of beams (e.g., seven). This two-step process is considerably faster than full exhaustive search. The question to be answered was whether the two methods lead to the same or similar solutions. The results of exhaustive search and multiresolution approaches were also compared with a previously published approach that used beam's-eye-view dosimetrics (BEVD). The relative ranks of plans optimized by an accurate dose calculation method were highly correlated with those of the plans optimized by the fast calculation method (i.e., using the IV approximation), which suggests that an approximate dose calculation algorithm can be used effectively for ranking of plans during BAO. We found that dose distributions and DVH of many beam configurations within a specified subset from a given pool of beams (e.g., 5 of 18) may be clinically indistinguishable and acceptable. Their optimized IMRT scores fall in a narrow range, although beam configurations and dose distributions may be different. We used the frequency distributions as a function of beam angles for the best 100 and the worst 100 plans to determine the most and the least preferred beam angles. We found that the most and the least preferred angle distributions for 3 of 18 configurations were very similar to those for 5, 6, 7, or 8 of 18 or 24 configurations, but the size of the search space was much smaller for the 3 of 18 case. Using fewer than three beams was discovered to be inadequate. This information was used to select the most preferred angles and eliminate the least preferred ones before searching for the optimum angles for the remaining beams. For the cases we studies, the multiresolution strategy produced very similar results to the full exhaustive search. Based on the observation that the worst plans had at least one parallel-opposed pair of beams and virtually all of the best plans had none, we were able to further reduce the size of the search space dramatically by using a pool of only nonparallel-opposed equispaced beams (i.e., 7 of 19 instead of 7 of 36). Another observation was that the probability of finding an optimum configuration in a smaller beam pool is substantially lower than in a larger pool (e.g., 5 of 18 vs. 5 of 24). The implication of this BAO is not very important when a large number of beams (nine or more) is used and vice versa. Our results showed that the plans with fewer but optimally placed beams could be as good as or better than plans using a larger number of unoptimized or uniformly placed beams. Exhaustive search with fast IMRT algorithms provides a novel and realistic approach to study the characteristics of IMRT dose distributions as a function of beam angles and to design practical BAO strategies for IMRT planning.

Radiation-Induced Leiomyosarcoma of the Ethmoid Sinus

This high–grade radiation–associated sarcoma involves the paranasal sinuses, anterior skull base, and orbit of the patient's only remaining eye. Leiomyosarcomas are exceedingly rare, high–grade tumors of the head and neck with the potential for distant metastases. Before local and regional management is planned, absence of distant disease must be established. At my institution, a bone scan and computed tomography of the chest, abdomen, and pelvis would be performed to rule out distant disease. For patients with a soft–tissue sarcoma, wide resection and reconstruction followed by radiation offer the best chance of a disease–free survival. In principle, orbital invasion dictates that exenteration be performed in most cases. Unfortunately, this patient has only one eye. The patient must be involved in the decision making, but I would attempt to salvage his vision at the expense of a close surgical margin. However, the patient must be fully prepared for an intraoperative decision to remove the orbit if necessary. Extension of the tumor through the cribriform plate requires a combined intracranial–transfacial surgical approach. In such situations, my preferred method is an anterior craniofacial resection through a bifrontal craniotomy and lateral rhinotomy with a subciliary extension. The resection would include the anterior skull base, ethmoid sinuses, maxillary suprastructure, and the medial wall and floor of the orbit. The involved intraorbital contents would also be resected en bloc. I would make every attempt to preserve the globe and its neurovascular integrity. The patient will not suffer from diplopia even with limited extraocular movement. The palate would be left intact. The likelihood of cervical metastases is low, and I would perform no neck dissection in the absence of clinical disease. The reconstructive goals include reconstitution of dural integrity and restoration of the orbital–facial contour. Postoperative complications should be minimized. A pericranial flap based on the supratrochlear and supraorbital vessels can be used to support the anterior duraplasty. The facial contour can be maintained using calvarial bone grafts to replace the bony orbit. A free vascularized myogenous flap (my preference is the latissimus dorsi muscle) would be positioned to fill the soft–tissue defect, to provide further dural protection, and to vascularize the bone grafts. Postoperative radiation is fundamental to this case, in particular because a close margin is expected if vision is to be preserved. Planning should begin preoperatively in discussion with the radiation oncologist. This patient previously received orthovoltage radiation of 30 Gy, likely delivered using the POP (parallel opposed pair) technique. The dosage to the brain was likely limited, and reradiation is possible. At our center, the patient would be considered for intensity modulated radiation therapy to limit radiation to the retina and optic nerve while delivering more than 60 Gy to the surgical bed. However, postoperative proton–beam radiation has distinct advantages over conventional radiation. In such a case, I would seek an opinion from a center with this capability before surgery. John Yoo M.D. Department of Otolaryngology Head and Neck Oncology–Reconstructive Microsurgery University of Western Ontario London Health Sciences Centre 800 Commissioners Road East London, Ontario, N6A 4G5 Canada John.Yoo@lhsc.on.ca