C. Fiandra

Different IMRT solutions vs. 3D-Conformal Radiotherapy in early stage Hodgkin's lymphoma: dosimetric comparison and clinical considerations

BackgroundRadiotherapy in Hodgkin’s Lymphoma (HL) is currently evolving with new attempts to further reduce radiation volumes to the involved-node concept (Involved Nodes Radiation Therapy, INRT) and with the use of intensity modulated radiotherapy (IMRT). Currently, IMRT can be planned and delivered with several techniques, and its role is not completely clear. We designed a planning study on a typical dataset drawn from clinical routine with the aim of comparing different IMRT solutions in terms of plan quality and treatment delivery efficiency.MethodsA total of 10 young female patients affected with early stage mediastinal HL and treated with 30 Gy INRT after ABVD-based chemotherapy were selected from our database. Five different treatment techniques were compared: 3D-CRT, VMAT (single arc), B-VMAT (“butterfly”, multiple arcs), Helical Tomotherapy (HT) and Tomodirect (TD). Beam energy was 6 MV, and all IMRT planning solutions were optimized by inverse planning with specific dose-volume constraints on OAR (breasts, lungs, thyroid gland, coronary ostia, heart). Dose-Volume Histograms (DVHs) and Conformity Number (CN) were calculated and then compared, both for target and OAR by a statistical analysis (Wilcoxon’s Test).ResultsPTV coverage was reached for all plans (V95% ≥ 95%); highest mean CN were obtained with HT (0.77) and VMAT (0.76). B-VMAT showed intermediate CN mean values (0.67), while the lowest CN were obtained with TD (0.30) and 3D-CRT techniques (0.30). A trend of inverse correlation between higher CN and larger healthy tissues volumes receiving low radiation doses was shown for lungs and breasts. For thyroid gland and heart/coronary ostia, HT, VMAT and B-VMAT techniques allowed a better sparing in terms of both Dmean and volumes receiving intermediate-high doses compared to 3D-CRT and TD.ConclusionsIMRT techniques showed superior target coverage and OAR sparing, with, as an expected consequence, larger volumes of healthy tissues (lungs, breasts) receiving low doses. Among the different IMRT techniques, HT and VMAT showed higher levels of conformation; B-VMAT and HT emerged as the planning solutions able to achieve the most balanced compromise between higher conformation around the target and smaller volumes of OAR exposed to lower doses (typical of 3D-CRT).

Dosimetric predictors of radiation-induced lung injury in stereotactic body radiation therapy

Materials and methods. The aim was to retrospectively investigate correlations between potential predictive parameters and the occurrence of radiation-induced lung injury in patients with primary or secondary lung tumours treated with stereotactic body radiation therapy (SBRT). Sixty patients (63 tumours) underwent SBRT, with a dose of 45 Gy in 3 fractions over 5 days or 26 Gy in single fraction. The following parameters were tested for correlation with Radiation Therapy Oncology Group (RTOG) lung toxicity score: planning target volume (PTV), tumour location, primary vs. metastatic tumour, and Mean Lung Dose (in 2 Gy fractions, MLD2). Normal Tissue Complication Probability (NTCP) values were then estimated. Results. The median follow-up time was 30.9 months (range 6.7–56.7). RTOG grade 0–1 toxicity was observed in 54/63 (85.7%) and grade 2–3 in 9/63 (14.3%) cases. Mean values of MLD2 for RTOG grade 0–1 and 2–3 were respectively 11.2 Gy (95% Confidence Interval (CI) 10.1–12.3 Gy) and 20.3 Gy (95% CI 16.6–23.9 Gy). NTCP mean values for RTOG grade 0–1 and 2–3 were respectively 4% (95% CI 2–5.9%) and 37% (95% CI 11.6–62.3%). Univariate analysis, performed with t-Student test, showed a statistically significant difference between MLD2 values in the two groups (t = 5.93 and p ≤ 0.001). Logistic regression analysis showed a good correlation between MLD2 and toxicity scores 2–3 (p = 0.008, odds ratio 1.5). From logistic regression relationship between the observed rates of grade 2–3 and MLD2, a D50=19.8 Gy and a γ50=2.2 were obtained. From the sigmoid-shaped dose-response relationship between NTCP and MLD2, a D50=22.4 Gy and γ50=2.2 were derived. Discussion. MLD2 is strongly associated to the risk of lung injury. Higher NTCP values are associated with a higher risk, but when comparing the expected to the observed toxicity rate, NTCP seems to underestimate the risk.

Lung stereotactic ablative body radiotherapy: A large scale multi-institutional planning comparison for interpreting results of multi-institutional studies

PURPOSE A large-scale multi-institutional planning comparison on lung cancer SABR is presented with the aim of investigating possible criticism in carrying out retrospective multicentre data analysis from a dosimetric perspective. METHODS Five CT series were sent to the participants. The dose prescription to PTV was 54Gy in 3 fractions of 18Gy. The plans were compared in terms of PTV-gEUD2 (generalized Equivalent Uniform Dose equivalent to 2Gy), mean dose to PTV, Homogeneity Index (PTV-HI), Conformity Index (PTV-CI) and Gradient Index (PTV-GI). We calculated the maximum dose for each OAR (organ at risk) considered as well as the MLD2 (mean lung dose equivalent to 2Gy). The data were stratified according to expertise and technology. RESULTS Twenty-six centers equipped with Linacs, 3DCRT (4% - 1 center), static IMRT (8% - 2 centers), VMAT (76% - 20 centers), CyberKnife (4% - 1 center), and Tomotherapy (8% - 2 centers) collaborated. Significant PTV-gEUD2 differences were observed (range: 105-161Gy); mean-PTV dose, PTV-HI, PTV-CI, and PTV-GI were, respectively, 56.8±3.4Gy, 14.2±10.1%, 0.70±0.15, and 4.9±1.9. Significant correlations for PTV-gEUD2 versus PTV-HI, and MLD2 versus PTV-GI, were observed. CONCLUSIONS The differences in terms of PTV-gEUD2 may suggest the inclusion of PTV-gEUD2 calculation for retrospective data inter-comparison.

Optimized Volumetric Modulated Arc Therapy Versus 3D-CRT for Early Stage Mediastinal Hodgkin Lymphoma Without Axillary Involvement: A Comparison of Second Cancers and Heart Disease Risk

PURPOSE The purpose of this study was to evaluate the risks of second cancers and cardiovascular diseases associated with an optimized volumetric modulated arc therapy (VMAT) planning solution in a selected cohort of stage I/II Hodgkin lymphoma (HL) patients treated with either involved-node or involved-site radiation therapy in comparison with 3-dimensional conformal radiation therapy (3D-CRT). METHODS AND MATERIALS Thirty-eight patients (13 males and 25 females) were included. Disease extent was mediastinum alone (n=8, 21.1%); mediastinum plus unilateral neck (n=19, 50%); mediastinum plus bilateral neck (n=11, 29.9%). Prescription dose was 30 Gy in 2-Gy fractions. Only 5 patients had mediastinal bulky disease at diagnosis (13.1%). Anteroposterior 3D-CRT was compared with a multiarc optimized VMAT solution. Lung, breast, and thyroid cancer risks were estimated by calculating a lifetime attributable risk (LAR), with a LAR ratio (LAR(VMAT)-to-LAR(3D-CRT)) as a comparative measure. Cardiac toxicity risks were estimated by calculating absolute excess risk (AER). RESULTS The LAR ratio favored 3D-CRT for lung cancer induction risk in mediastinal alone (P=.004) and mediastinal plus unilateral neck (P=.02) presentations. LAR ratio for breast cancer was lower for VMAT in mediastinal plus bilateral neck presentations (P=.02), without differences for other sites. For thyroid cancer, no significant differences were observed, regardless of anatomical presentation. A significantly lower AER of cardiac (P=.038) and valvular diseases (P<.0001) was observed for VMAT regardless of disease extent. CONCLUSIONS In a cohort of patients with favorable characteristics in terms of disease extent at diagnosis (large prevalence of nonbulky presentations without axillary involvement), optimized VMAT reduced heart disease risk with comparable risks of thyroid and breast cancer, with an increase in lung cancer induction probability. The results are however strongly influenced by the different anatomical presentations, supporting an individualized approach.

Comparison of Gafchromic EBT2 and EBT3 for patient‐specific quality assurance: Cranial stereotactic radiosurgery using volumetric modulated arc therapy with multiple noncoplanar arcs

PURPOSE Patient-specific quality assurance in volumetric modulated arc therapy (VMAT) brain stereotactic radiosurgery raises specific issues on dosimetric procedures, mainly represented by the small radiation fields associated with the lack of lateral electronic equilibrium, the need of small detectors and the high dose delivered (up to 30 Gy). GafchromicTM EBT2 and EBT3 films may be considered the dosimeter of choice, and the authors here provide some additional data about uniformity correction for this new generation of radiochromic films. METHODS A new analysis method using blue channel for marker dye correction was proposed for uniformity correction both for EBT2 and EBT3 films. Symmetry, flatness, and field-width of a reference field were analyzed to provide an evaluation in a high-spatial resolution of the film uniformity for EBT3. Absolute doses were compared with thermoluminescent dosimeters (TLD) as baseline. VMAT plans with multiple noncoplanar arcs were generated with a treatment planning system on a selected pool of eleven patients with cranial lesions and then recalculated on a water-equivalent plastic phantom by Monte Carlo algorithm for patient-specific QA. 2D quantitative dose comparison parameters were calculated, for the computed and measured dose distributions, and tested for statistically significant differences. RESULTS Sensitometric curves showed a different behavior above dose of 5 Gy for EBT2 and EBT3 films; with the use of inhouse marker-dye correction method, the authors obtained values of 2.5% for flatness, 1.5% of symmetry, and a field width of 4.8 cm for a 5×5 cm2 reference field. Compared with TLD and selecting a 5% dose tolerance, the percentage of points with ICRU index below 1 was 100% for EBT2 and 83% for EBT3. Patients analysis revealed statistically significant differences (p<0.05) between EBT2 and EBT3 in the percentage of points with gamma values<1 (p=0.009 and p=0.016); the percent difference as well as the mean difference between calculated and measured isodoses (20% and 80%) were found not to be significant (p=0.074, p=0.185, and p=0.57). CONCLUSIONS Excellent performances in terms of dose homogeneity were obtained using a new blue channel method for marker-dye correction on both EBT2 and EBT3 GafchromicTM films. In comparison with TLD, the passing rates for the EBT2 film were higher than for EBT3; a good agreement with estimated data by Monte Carlo algorithm was found for both films, with some statistically significant differences again in favor of EBT2. These results suggest that the use of GafchromicTM EBT2 and EBT3 films is appropriate for dose verification measurements in VMAT stereotactic radiosurgery; taking into account the uncertainty associated with Gafchromic film dosimetry, the use of adequate action levels is strongly advised, in particular, for EBT3.

Changes in breast cancer risk associated with different volumes, doses, and techniques in female Hodgkin lymphoma patients treated with supra-diaphragmatic radiation therapy

PURPOSE The contribution of thoracic radiation in increasing secondary breast cancer (BC) risk in female Hodgkin lymphoma patients is well known, and recent changes in radiation therapy volumes, doses and techniques are supposed to minimize it. In this study, we compared different radiation therapy solutions in terms of secondary BC induction risk with the aim of selecting which could be considered the most protective. METHODS AND MATERIALS In 10 female patients under 30 years old we estimated breast cancer risk for different combined treatment solutions (involved field vs involved nodal radiation therapy [IFRT vs INRT], 30 Gy vs 20 Gy, 3-dimensional conformal radiation therapy vs volumetric modulated arc therapy [3DCRT vs VMAT]). The organ equivalent dose (OED) method was used for dose calculation, as OED is directly related to the excess risk. Estimated OED mean values for all options in all patients were then analyzed and compared. RESULTS INRT was significantly associated with a lower OED, regardless of total dose and technique (0.43 vs 1.15, P < .0001). The relative OED reduction from IFRT to INRT was approximately 60%. The dose of 20 Gy resulted in a significant reduction of OED, approximately 25% (0.68 vs 0.9, P < .01). VMAT did not show significantly higher OED when compared with 3DCRT (0.84 vs 0.74, P = .15). The combination of INRT and 20 Gy lead to a decrease in OED of approximately 70% if compared with IFRT 30 Gy. CONCLUSIONS The INRT approach substantially reduces OED, independent of dose and technique; the dose reduction from 30 Gy to 20 Gy also has a significant impact, and as expected INRT-20 Gy resulted to be the solution at lowest risk. No differences were observed when comparing different techniques (3DCRT vs VMAT). The combination of these innovative approaches might lead to a substantial reduction in secondary breast cancer risk in this patient population.

Tracking target position variability using intraprostatic fiducial markers and electronic portal imaging in prostate cancer radiotherapy

PurposeModern radiotherapy has achieved substantial improvement in tumour control and toxicity rates by escalating the total dose to the target volume while sparing surrounding normal tissues. It has therefore become necessary to precisely track tumour position in order to minimise geometrical uncertainties due to setup errors and organ motion. We conducted this prospective evaluation of prostate cancer patients treated with image-guided conformal radiation therapy at our institution. We implanted three fiducial markers (gold seeds) within the prostatic gland in order to quantify daily target displacements and to generate specific margins around the clinical target volume (CTV) to create an appropriate planned target volume (PTV).Materials and methodsBetween April and December 2009, ten patients affected with localised prostate cancer were transrectally implanted with three radio-opaque markers. Each patient underwent a computed tomography (CT) scan for planning purposes following proper bladder and rectum preparation. During treatment two orthogonal images were acquired daily and compared with previously generated digitally reconstructed radiographs. After manual localisation, comparison between the position of the gold seeds on the portal and reference images was carried out, and a set of extrapolated lateral-lateral (LL), anterior-posterior (AP) and cranial-caudal (CC) shift corrections was calculated and recorded. Couch corrections were applied with a threshold of 3 mm displacement.ResultsSystematic and random errors for each direction were calculated either as measured according to displacement of the gold seeds prior to any couch movement and after couch position correction according to the radio-opaque markers. For skin marks, mean systematic and random errors were 0.12+2.94 mm for LL, 1.04+3.37 mm for AP, −1.14+2.71 mm for CC, whereas for seed markers, mean and systematic errors were 0.6+1.5 mm for LL, 0.51+2.45 mm for AP and −0.25+2.51 mm for CC. A scatter plot generated on all measurements after couch repositioning according to gold-seed displacement suggested a confidence range of shift distributions within 5 mm for LL, 8 mm for CC, and 7 mm for AP. The total systematic and random components were then used to calculate proper PTV in patients receiving conventional treatment (7 mm for LL and 9 mm for both AP and CC).ConclusionsProstate positional variability during a course of radiation treatment is strongly influenced by setup and organ motion. Organ tracking through fiducial markers and electronic portal imaging is able to reduce the spread of displacements, significantly contributing to improve the ballistic precision of radiation delivery.RiassuntoObiettivoLa moderna radioterapia ha raggiunto traguardi considerevoli in termini di controllo tumorale e riduzione dei tassi di tossicità associati al trattamento, con la possibilità di erogare dosi importanti al volume bersaglio, risparmiando contemporaneamente i tessuti sani. Risulta, pertanto, focale la caratterizzazione precisa della posizione della lesione neoplastica per minimizzare le incertezze geometriche dovute agli errori di posizionamento ed al movimento d’organo. Abbiamo intrapreso una valutazione prospettica dei pazienti trattati mediante radioterapia conformazionale guidata dalle immagini per carcinoma prostatico, utilizzando tre reperi fiduciali (semi d’oro) impiantati all’interno della prostata per quantificare gli scostamenti giornalieri del volume bersaglio e generare margini specifici attorno al volume bersaglio clinico (CTV) creando un appropriato volume bersaglio di pianificazione (PTV).Materiali e metodiTra aprile e dicembre 2009, 10 pazienti affetti da adenocarcinoma prostatico organo-confinato sono stati sottoposti ad impianto trans-rettale sotto guida ecografica di 3 reperi fiduciali radio-opachi. Ogni paziente è stato sottoposto a tomografia computerizzata (CT) di pianificazione con una adeguata preparazione vescicale e rettale; ad ogni seduta di trattamento, sono state acquisite 2 immagini ortogonali e poi confrontate con immagini radiografiche a ricostruzione digitale. è stata effettuata una localizzazione manuale confrontando la posizione dei reperi fiduciali nelle immagini portali con quella nelle immagini di riferimento. Sono stati estrapolati e registrati gli scostamenti nelle direzioni latero-laterale (LL), anteroposteriore (AP) e cranio-caudale (CC). La correzione della posizione del lettino di trattamento è stata effettuata con una soglia di 3 mm.RisultatiSono stati calcolati l’errore standard e sistematico per ogni direzione, misurando gli scostamenti dei semi d’oro prima del movimento del lettino di trattamento e dopo aver applicato gli spostamenti secondo i reperi fiduciali. Per i reperi cutanei, l’errore sistematico e casuale medio sono stati 0,12±2,94 mm per la direzione LL, 1,04±3,37 mm per la direzione AP e −1,14±2,71 mm per la direzione CC; per i reperi fiduciali, l’errore sistematico e casuale medio sono stati 0,6±1,5 mm per la direzione LL, 0,51±2,45 mm per la direzione AP e −0,25±2,51 mm per la direzione CC. è stato creato un diagramma di distribuzione di tutte le misure ottenute dopo il riposizionamento del lettino in funzione degli scostamenti registrati mediante i reperi fiduciali; l’ambito di confidenza della distribuzione degli spostamenti si trova entro i 5 mm per la direzione LL, entro i 6 mm per la direzione CC ed entro i 7 mm per la direzione AP. L’errore sistematico e l’errore casuale totale sono stati utilizzati per generare margini per ottenere il PTV nei pazienti trattati convenzionalmente (7 mm per la direzione LL; 9 mm sia per la direzione AP che per la direzione CC).ConclusioniLa variabilità della posizione della ghiandola prostatica durante un trattamento radiante è influenzata in modo importante dal posizionamento e dal movimento d’organo. Il monitoraggio della posizione della prostata mediante reperi fiduciali ed immagini portali è in grado di ridurre la dispersione degli scostamenti rispetto all’atteso, contribuendo in maniera significativa al miglioramento della precisione balistica della radioterapia.

Small field output factors evaluation with a microDiamond detector over 30 Italian centers

PURPOSE The aim of the study was a multicenter evaluation of MLC&jaws-defined small field output factors (OF) for different linear accelerator manufacturers and for different beam energies using the latest synthetic single crystal diamond detector commercially available. The feasibility of providing an experimental OF data set, useful for on-site measurements validation, was also evaluated. METHODS This work was performed in the framework of the Italian Association of Medical Physics (AIFM) SBRT working group. The project was subdivided in two phases: in the first phase each center measured OFs using their own routine detector for nominal field sizes ranging from 10×10cm2 to 0.6×0.6cm2. In the second phase, the measurements were repeated in all centers using the PTW 60019 microDiamond detector. RESULTS The project enrolled 30 Italian centers. Micro-ion chambers and silicon diodes were used for OF measurements in 24 and 6 centers respectively. Gafchromic films and TLDs were used for very small field OFs in 3 and 1 centers. Regarding the measurements performed with the users detectors, OF standard deviations (SD) for field sizes down to 2×2cm2 were in all cases <2.7%. In the second phase, a reduction of around 50% of the SD was obtained using the microDiamond detector. CONCLUSIONS The measured values presented in this multicenter study provide a consistent dataset for OFs that could be a useful tool for improving dosimetric procedures in centers. The microDiamond data present a small variation among the centers confirming that this detector can contribute to improve overall accuracy in radiotherapy.

Novel radiotherapy techniques for involved-field and involved-node treatment of mediastinal Hodgkin lymphoma

PurposeHodgkin lymphoma (HL) is a highly curable disease. Reducing late complications and second malignancies has become increasingly important. Radiotherapy target paradigms are currently changing and radiotherapy techniques are evolving rapidly.DesignThis overview reports to what extent target volume reduction in involved-node (IN) and advanced radiotherapy techniques, such as intensity-modulated radiotherapy (IMRT) and proton therapy–compared with involved-field (IF) and 3D radiotherapy (3D-RT)– can reduce high doses to organs at risk (OAR) and examines the issues that still remain open.ResultsAlthough no comparison of all available techniques on identical patient datasets exists, clear patterns emerge. Advanced dose-calculation algorithms (e.g., convolution-superposition/Monte Carlo) should be used in mediastinal HL. INRT consistently reduces treated volumes when compared with IFRT with the exact amount depending on the INRT definition. The number of patients that might significantly benefit from highly conformal techniques such as IMRT over 3D-RT regarding high-dose exposure to organs at risk (OAR) is smaller with INRT. The impact of larger volumes treated with low doses in advanced techniques is unclear. The type of IMRT used (static/rotational) is of minor importance. All advanced photon techniques result in similar potential benefits and disadvantages, therefore only the degree-of-modulation should be chosen based on individual treatment goals. Treatment in deep inspiration breath hold is being evaluated. Protons theoretically provide both excellent high-dose conformality and reduced integral dose.ConclusionFurther reduction of treated volumes most effectively reduces OAR dose, most likely without disadvantages if the excellent control rates achieved currently are maintained. For both IFRT and INRT, the benefits of advanced radiotherapy techniques depend on the individual patient/target geometry. Their use should therefore be decided case by case with comparative treatment planning.ZusammenfassungHintergrund und ZielDas Hodgkin-Lymphom (HL) ist eine Erkrankung mit hohen Heilungsraten. Die Verringerung von Spätkomplikationen und Zweittumoren wird daher immer wichtiger. Zielvolumenkonzepte der Strahlentherapie (RT) verändern sich gegenwärtig und Strahlentherapietechniken entwickeln sich sehr schnell weiter.MethodenDiese Übersichtsarbeit stellt dar, inwiefern die Zielvolumenreduktion hin zum Involved-node(IN)-Konzept und hochentwickelte Strahlentherapietechniken wie die intensitätsmodulierte Strahlentherapie (IMRT) und Protonentherapie, im Vergleich zu Involved-field(IF)-Konzept und 3-D-konformaler Strahlentherapie (3D-RT), die Belastung von Risikoorganen (OAR) mit hohen Dosen reduzieren können und welche Fragen in diesem Kontext noch geklärt werden müssen.ErgebnisseObwohl kein Vergleich aller verfügbaren Techniken auf identischen Patientendatensätzen existiert, entsteht folgendes Bild: Fortgeschrittene Dosisberechnungsalgorithmen (z. B. convolution-superposition/Monte Carlo) sollten im Rahmen der Behandlung des mediastinalen HL zur Anwendung kommen. INRT reduziert unter allen Bedingungen die behandelten Volumina im Vergleich zur IFRT, wobei die Höhe des Vorteils von der jeweiligen INRT-Definition abhängt. Die Anzahl der Patienten, die deutlich von hochkonformalen Techniken wie IMRT gegenüber der 3D-RT hinsichtlich der OAR-Belastung profitiert, ist bei INRT geringer. Die Konsequenz größerer Volumina, die bei modernen Techniken mit eher isotroper Strahlanordnung mit niedrigen Dosen belastet werden, ist unklar. Die Art der verwendeten IMRT-Technik (statisch/Rotation) ist von geringer Relevanz. Alle fortgeschrittenen Photonentechniken resultieren in den gleichen Vorteilen und Nachteilen. Daher muss nur die Modulationstiefe abhängig von den individuellen Behandlungszielen gewählt werden. Die Bestrahlung in tiefer Inspiration wird gegenwärtig evaluiert. Protonentherapie kann theoretisch bei hervorragender Hochdosiskonformalität die applizierte Integraldosis reduzieren.SchlussfolgerungDie weitere Verkleinerung der behandelten Volumina reduziert die Risikoorganbelastung am effektivsten und ohne Nachteile, wenn die gegenwärtig exzellenten Kontrollraten weiterhin erreicht werden können. Sowohl für IFRT als auch INRT hängen Vor- und Nachteile der modernen Strahlentherapietechniken von der individuellen Patientengeometrie ab. Die Entscheidung für die jeweils anzuwendende Technik sollte daher Fall für Fall auf Basis einer vergleichenden Bestrahlungsplanung getroffen werden.

Absolute and relative dose measurements with Gafchromic™ EBT film for high energy electron beams with different doses per pulse

The authors have evaluated the accuracy, in absolute and relative dose measurements, of the Gafchromic™ EBT film in pulsed high-energy electron beams. Typically, the electron beams used in radiotherapy have a dose-per-pulse value of less than 0.1mGy/pulse. However, very high dose-per-pulse electron beams are employed in certain linear accelerators dedicated to intraoperatory radiation therapy (IORT). In this study, the absorbed dose measurements with Gafchromic™ EBT in both low (less than 0.3mGy per pulse) and high (30 and 70mGy per pulse) dose-per-pulse electron beams were compared with ferrous sulfate chemical Fricke dosimetry (operated by the Italian Primary Standard Dosimetry Laboratory), a method independent of the dose per pulse. A summary of Gafchromic™ EBT in relative and absolute beam output determination is reported. This study demonstrates the independence of Gafchromic™ EBT absorption as a function of dose per pulse at different dose levels. A good agreement (within 3%) was found with Fricke dosimeters for plane-base IORT applicators. Comparison with a diode detector is presented for relative dose measurements, showing acceptable agreement both in the steep dose falloff zone and in the homogeneous dose region. This work also provides experimental values for recombination correction factor (Ksat) of a Roos (plane parallel) ionization chamber calculated on the basis of theoretical models for charge recombination.