Commissioning and Quality Assurance in IOeRT

Transcription

1 Advanced Course on IORT (IntraOperative Radiation Therapy) Trieste, May 3, 2012 The Abdus Salam International Centre for Theoretical Physcs Commissioning and Quality Assurance in IOeRT Eugenia Moretti AOUUD SMM Udine -Italy

2 Key words Acceptance Commissioning Quality Controls

3 To begin : References Guidelines for quality assurance in intra-operative radiation therapy ISS (2003) Intraoperative Intraoperative electron radiation beam therapy radiation using therapy: technique, mobile dosimetry electron linear and dose accelerators: specification Report Report of AAPM of AAPM Radiation Radiation therapy Therapy committee Committee Task force Task Group No 48 No. 72 (1995) (2006)

4 IOeRT equipment Non dedicated (stationary) linacs Dedicated (mobile) linacs Italian choice

5 Dedicated linacs Novac (NRT, Italy) Liac (Sordina, Italy) Mobetron (IntraOp, USA)

6 MOBETRON DOSIMETRIC DATA Energy (MeV) 4, 6, 9, 12 Applicator size (cm) Base angle Material Field Flatness ± 5% Dose Rate 3 10 (5 mm) 0, 15, 30, 45 Steel 10 Gy/min 9 GHz X-band Technology in tandem (λ=3 cm) UNIT DATA Isocentric modality/c-arm gantry 5 degree of motion of gantry head Power Supply Weight SOFT-DOCKING SYSTEM 230 VAC 50 Hz 2.5 kva 1250 kg

7 NOVAC DOSIMETRIC DATA Energy (MeV) 3, 5, 7, 9 Applicator size (cm) Base angle Material 3,4,5,6,8,10 0, 22.5,30, 45 PMMA Long (Short) Term Output Stability 2% ( 1%) Field Uniformity 5% Field Symmetry 2% Dose Rate X-Ray Contamination 1% (6 39) Gy/min INSTALLATION DATA Min dimension 180 x 95 x 233 cm 3 Max dimension 240 x 95 x 233 cm 3 Weight Control Console dimension Control Console weight Power Consumption Power Supply HARD-DOCKING SYSTEM 600 kg 80 x60 x 110 cm kg < 1kW 230 VAC 50 Hz 2.5 kva

8 LIAC DOSIMETRIC DATA Energy (MeV) 4, 6, 8, 10, (12) Applicator sizes (cm) Base angle Material 3,4,5,6,8,10 (12) 0, 15, 30, 45 PMMA Long (Short) Term Output Stability 3% ( 0.3%) INSTALLATION DATA Unit Dimension 210 x 80 x 180 cm 3 Unit Weight 60 x 80 x 120 cm 3 Control Console dimension Control Console weight 400 kg 180 kg Short Term Output Stability 0.3% Air dissipate power Power Supply < 0.8 kw 230 VAC 50 Hz 2.5 kva Field Uniformity ± 3% (± 9%) Field Symmetry 3% Dose Rate (10 25) Gy/min X-Ray Contamination 0.3% HARD-DOCKING SYSTEM

9 1. ACCEPTANCE Advanced Course on IORT, Trieste May 3, 2012

10 Dedicated linacs -Acceptance Radiation survey at the testing site as soon as the accelerator is stable and after a preliminary output calibration Radiation safety for estimating the attenuation through the beam stopper Testing of interlocks and safety features according to the manufacturer s acceptance procedures Mechanical testingfor verifying the full range of gantry motion (all the movements: rotational, translational) and beam stopper

11 Dedicated linacs -Acceptance Docking system test (soft-docking): Mobetron is equipped with an optical docking system (laser mounted on the beam collimation system to guide the operator in the docking of the gantry with the electron applicator) Beam characteristics tuning: adjustments of the beam energy, output rate, flatness, symmetry of reference applicator performed by installation engineer Options and accessories evaluation

12 Acceptance summary AAPM TG72 Table V.1 Typical procedures required for acceptance testing of a mobile IORT unit

13 Dedicated linacs Materials The lack of a gantry isocenter and a surgical bed with precise movement control may imply additionaltimetosetupawaterphantom. The beam stopper on a mobile IORT unit may also prevent the use of the WP support system (table support). It may be necessary to build a special low table that straddles the beam stopper. Another solution may be 3D mini-water phantom.

14 Dedicated linacs: where do we make measurements? The shielding of a mobile accelerator is designed for infrequent use for IORT, not for continuous use in an unshielded room. It is strongly preferred to establish a dedicated vault or location for acceptance, commissioning and annual quality assurance activities.

15 2. Commissioning Advanced Course on IORT, Trieste May 3, 2012

16 Advances Course on IORT, Trieste May 3, 2012 The physics in IOeRT (con t) The IORT requires special dosimetrical determinations, which are sometimes differentin comparison to those, associated with conventional external-beam radiotherapy. The main reason stems from the fact that a single high-dose of radiation is delivered to a selectively defined volume of tissue, whose extension and depth are directly determined in the operating theatre. ISTISAN 03/1 EN, Guidelines for quality assurance in iort (2003)

17 The physics in IOeRT (con t) ItisalsointheoperatingtheatrethattheIORTteamselectsthe shape and the diameter of the applicator, the energy and the isodose of reference more suitable for assuring the therapeutic prescription. Since there is no possibility of using a TPS and there is little time to make the dosimetric calculations, it is necessary that all the physical data for every type of applicator and energy employed, are available in a format of fast consultation and easyuse( ) ISTISAN 03/1 EN, Guidelines for quality assurance in iort (2003)

18 The physics in IOeRT (con t) A further difference between IORT and external RT is related to the use of specific applicators that contributes to the determination of the physicalgeometrical characteristics of the electron beams ( ) Such applicators ( ) can be of circular section, with diameters between 3 12 cm, or of rectangular section, with dimensions up to cm. The circular applicators can have an oblique distal part that is tilted with respect to the geometric axis of the beam, with angles ranging from 15 to 45 (base bevelled applicators) ( ) ISTISAN 03/1 EN, Guidelines for quality assurance in iort (2003)

19 The physics in IOeRT 10 MeV Another difference with external RT electron beams derives from the high-dose per pulse delivered by some types of dedicated accelerators 2 13 cgy/p vs 0.1 cgy/p

20 IOeRT Dosimetry in reference conditions International dosimetric protocols can be used, with some precautionsfor the dosimetry of non-dedicated accelerators, operating with specific applicator for IORT. Nevertheless, it is difficult to achieve the same accuracy in the determination of the absorbed dose to water, typical of the conventional non IORT treatments, as the use of specific applicatorsdoes not allow a total conformity with the reference conditions specified in the protocols. In the case of dedicated accelerators, characterised by a high-dose/pulse, it is impossible to follow the recommendations of the protocols for dose determination with an ionisation chamber, due to the problems of ion recombination inside the gas of IC.

21 Absolute dosimetry (ref&not-ref) taking into account the dose/p Standard linacs & MOBETRON Mobile linacs with high-dose/p NOVAC & LIAC IC (p-p) Fricke, alanine dosimetry, radiochromic films IAEA TRS398 IC withalternative compensation methods for ionrecombination

22 Dedicated linacs workload We have to know the operational conditions of the unit to perform a correct characterization (the energy, the dose per pulse and hence the dose rate be kept fairly stable during data collection) The point is that: these linacs are not designed for long beam-on times

23 Dosimetry in reference conditions A square section applicator (10x10 cm 2 ) or a circular applicator (dia-10 cm) with a plane base is recommended. The more complex treatment set-ups of IORT give electron fields where the lateral-scatter equilibrium is degraded compared with conventional beams. The position, direction, charge and energy of the ionizing particles of such beams will be different from those collimated with the conventional systems. They present an energetic spectrum downgraded towards low energies and a wider angular distribution in comparison to the standard beams. Bjork P et al. PMB 47,2002; PMB 48, 2004

24 So, can we continue referring it like dosimetry in reference conditions? The passage of the electrons through the IORT collimation system results in a large number of scattered electrons in the clinical beam, and these electrons may have a large influence on the absorbed dose distribution in the patient Bjork et al. PMB 47; 2002

25 Reference conditions in IOeRT More deviations are observed with mobile linacs equipped with hard-docking collimation system where could be not present filter or bending magnet Mobile linacs PDD show higher surface dose (especially for the lower energies) and less steep dose gradients (especially for the higher energies) 9 MeV NOVAC CLINAC 2100C/D E O (MeV) R max (mm) R 90 (mm) R 50 (mm) R p (mm) 39 45

26 IORT electron beams Monte Carlo simulations Björk et al. Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations in a setup for intraoperative radiation therapy, PMB 47, 2002 Conventional linac (Elekta SL25) EGS4 code (1999) 6 MeV 12 MeV

27 IORT electron beams Monte Carlo simulations Pimpinella et al. Dosimetric characteristics of electron beams produced by a mobile accelerator for IORT, PMB 52, 2007 Dedicated linac (NOVAC7) BEAMnrc code (2005) 9 MeV

28 IORT electron beams Monte Carlo simulations Iaccarino et al. Monte Carlo simulation of electron beams generated by a 12 MeV dedicated mobile IORT accelerator, PMB 56, 2011 Dedicated linac (LIAC2) EGS4nrc/OMEGA code (2010) 12 MeV

29 Relative dosimetry Standard linacs If the dosimetry protocols are applied directly to degraded e-beams, an additional uncertainty can be introduced in the dose determination (due to the strong energy dependence of the water-to-air SP-ratio with IC) 1% 2% It is preferable to use detectors having a small energy and angular dependence (p Si-diodes or diamond detectors)

30 Diodes &diamonds bbb Björk et al, PMB, 27, 2000

31 Mobile linacs with high-dose per pulse Absolute dosimetry in reference conditions A square section applicator (10x10 cm 2 ) or a circular applicator (dia-10 cm) with a plane base is recommended Due to the high density of electric charge produced in the chamber s volume per radiation pulse, the correction factor for ion recombination can be largely overestimated (up to 20%, Piermattei, PMB, 45, 2000) if the correction methods recommended by the international protocols are used (TVA)

32 Mobile linacs with high-dose per pulse Fricke absolute dosimetry: gold standard ISS recommends the absolute dosimetric system of Fricke (chemical dosimeter based on a solution of iron sulphate) ISS also recommends that such system be managed by a Primary Metrological Institute or by a recognised Calibration Centre due to high criticality typical of chemical dosimetry, until to guarantee the required accuracy in the dose measurement. Reference depth = R max (due to its large size and to the perturbation in the radiation beam by its walls, it is better to measure at a depth where the dose gradient is low).

33 Sealed glass vials (FeSO 4 ): V = 1.1 cm 3 Ø ext =8.8 mm; Ø int =7.8 mm; l=24 mm Fricke Dosimetry Optical absorption measurements for dose assessment (spectro-photometer λ= 304 nm) Composite uncertainty: 1.6 % (1 σ) Dose range: Gy Usually 3 vials for app+energy Not easy use, long times

34 Mobile linacs with high-dose per pulse other dosimeters (con t) Fricke have some disadvantages that limit their use low spatial resolution, low sensitivity, high-cost post-irradiation reading process Dosimetric systems can be employed whose sensitivity is independent from - dose-rate - beam energy - angle of incidence of the electron beam

35 Mobile linacs with high-dose per pulse other dosimeters Alaninedosimetry (De Angelis et al. On measuring the output of an IORT mobile dedicated accelerator Radiat Prot Dos, 2006). Radiochromic films (C. Fiandra et al. Absolute and relative dose measurements with Gafchromic EBT film for high energy electron beams with different doses per pulse Med Phys, 35, 2008) Optimum should be.. the ionization chamber(good sensitivity, practical use, real-time dose indication) mainly italian dosimetric problem.

36 The Renaissance of IC Advanced Course on IORT, Trieste May 3, 2012

37 Electron beam dosimetry The plane-parallel IC is the dosimetry device recommended for the pulsed high-energy (>3 MeV) electron beam calibration (IAEA TRS-398) D w,q (z ref ) = M corr N D, w, Qref k Q, Qref k s Q user beam quality, Q ref reference beam quality, z ref = 0.6R g cm -2 (R 50 in g cm -2 ) M corr = M k T,p k h k pol (k pol ) N D, w, Qref is the IC-reading (M) corrected for T/p (k Tp ), humidity (k h ), polarity is the calibration factor in terms of the absorbed dose to ref k Q, Qref k s corrects for the difference between Q ref and the actual Q corrects for the lack of complete charge collection due to ion recombination

38 Electron beam dosimetry The correction for ion recombination Boag theory [Boag and Currant, B.J.R.1980] f k -1 s charge collection efficiency of the ionization chamber 2 µd q u = V cavity gas This is not valid for mobile linacs with θ high-dose/pulse k s 1 values u (considerable = f free-electron ln 1 + epu -1 component) p p -free-electron fraction [IC properties (V), not dose pulse] µ(mv C -1 ) costant depending on the IC d (m) distance between IC plates r (C m -3 ) density ion charge per pulse liberated in the cavity gas V (V) voltage supply IC For typical dose per pulse values: q θ 0 (u 0) (usual p-p IC) k s u ln 1 + u ( ) 1 + u 2

39 Di Martino et al. method (Med Phys, 32, 2005) In this work the dependence of k s on the doseper-pulse value is derived, based on the general equation that describes the ion recombination in the Boag theory. A new equation for k s, depending on known or measurable quantities, is presented. The new equation is experimentally tested by comparing D w measured with p-p IC to that measured using dose-per-pulse independent dosimeters, such as radiochromic films and Fricke dosimeters.

40 Di Martino et al. method (Med Phys, 32, 2005) D w (z ref ) comes from Fricke Dosimetry

41 Di Martino et al. method (Med Phys, 32, 2005) k s = βd w, θ, eff ln 1+ epβd w, θ, eff p 1 The free-electron fractions p(fz IC) represents the incognita The p value was estimated for the p-p ICs Markus PTW (type 23343) and Roos PTW (type 34001) by fitting(with this eq.) the k s values evaluated at a different dose per pulse. Agreement with chemical dosimetry <3% Roos p-p IC

42 Di Martino et al.method It is not possible by this method to determine the collection efficiency of an ion chamber unless this chamber has been previously calibrated in high-dose-per-pulse beams by an absorbed dose standard independent of the dose per pulse

43 Laitano et al. method (PMB, 51, 2006) The aim of the work was to determine chamber collection efficiencies in high-dose-per-pulse beams starting from the Boag et al expressions and not requiring any chamber calibration. The implementation of the Boag et al. procedure requires however to take a decision on the three models they propose and on the numerical values for the physical parameters entering the relevant equations that strictly refer to the experimental conditions of interest. The 1st objective of the study was then to choose these appropriate values for Boag-equations. The 2nd objective was to assess, by means of experimental independent methods, the validity of each of the three models on which the collection efficiency expressions are based.

44 Ion recombinations models Starting point: More accurate models of the charge collection process, incorporating the freeelectron component, developed by Boag et al.(pmb, 1996) [based on the results by Hochhäuser et al. (JPD,1994) and Balk et al. (PMB,1996)]. Huxley et al. (1974), Hochhäuser et al. (JPD, 1994) Balk et al. (PMB, 1998) ( ) + c( 1- e -de ) τ = a 1- e -be Hochhäuser et al. (1994) ' 1 u ln 1+ epu 1 f f '' p ''' = λ + 1 (1 p) MODEL u I u ln p 1+ eλ(1-λ)u 1 λ λ = 1 - (1- p) - p = ωτ d 1 - e- u = µd2 r V [ ] MODEL II MODEL III d ωτ p free-electron fraction w = a + b -ne -ce -ce d ce ne ( 1- e ) 1+ n n c

45 Ion recombinations models Laitano et al. performed TVA (Two Voltage Analysis) methodon the basis of these more accurate Boag recombination models The equations for f', f'', f'''are rearranged to be able to be solved (numerically by iterative methods) for the parameter u 1, once the pair of charge values Q 1,Q 2 corresponding to the voltage V 1, V 2, was measured (MODEL I, II, III) Q 1 = V ln + ep 1 u 1 1 λ Q 1 1 p u 1 u ln ln 1 + [ 1 p (1- e λ 1 (1- λ 1 )u 1 Q p 1 )u = λ - 1 ( ) ] = 1 Q V1 Q 2 Q 2 ln 1 + p 2 u 1 p 2 u 1 + V 2 ln + V 2 1 λ 2 (1 V 1 p 2 )u 1 2 u 1 + V 1 ln e λ 2(1- V λ2 ) 1 u 1 V 2 V 1 V λ 2 p 2 MODEL I III

46 Laitano et al. method uncertainty: 3% (1 SD)

47 6 MeV dia (cm) cgy/p k TVA s k' s k'' s k''' s k''' s - MODEL III 8 MeV dia (cm) cgy/p k TVA s k' s k'' s k''' s MeV dia (cm) cgy/p k TVA s k' s k'' s k''' s Munich 2009

48 6 MeV dia (cm) cgy/p k TVA s k''' s % Advanced Course on IORT, Trieste May 3, MeV dia (cm) cgy/p k TVA s k''' s % MeV dia (cm) cgy/p k TVA s k''' s % dramatic discrepancy! Munich 2009

49 Dosimetry in non reference conditions For every applicator, energy and SSD of clinical interest PDD along the clinical axis of the beam (R max, R p, R 90, R 50, D s, D x ) OAP (in two orthogonal max, R 90, R 80, R 50 Pdd% A10_0 4 MeV 6 MeV 8 MeV 10 MeV depth (mm) Isodose curves in the two principal orthogonal planes containing the clinical axis of the beam Output (cgy/mu) the reference depth on the clinical axis of the beam Correction factors (such as air-gap factor)

50 Dosimetry in non reference conditions CLINICAL AXIS J.R. Palta et al.,, IJROBP, 1995

51 Dose distribution Standard linacs & MOBETRON: p-si diodes, diamond detectors, radiochromich films Mobile linacs with-high dose per pulse: p-si diodes, radiochromich films Particular attention must be given both to the synchronisation of the detector s signal with the pulsed beam and to the characteristics of the used electronics

52 Dose distribution & ICs The recommendation of not using ICs is due to the facts that the dependence of the ion recombination effects on the depth is not known with sufficient accuracy, and that it is uncertain how such dependence may influence the measurement of the PDD. Pimpinella et al. (PMB, 52, 2007) calculated the Spencer-Attix water-to-air SP (s w,air ) ratios of the NOVAC7 beams on the basis of the energy distributions obtained by MC simulations. Compared to the corresponding s w,air values from TRS-381&398 IAEA codes, they resulted to deviate up to 1% (PTW IC).

53 Output (cgy/mu) The calibration of the electron beams (dose per MU) must be performed for each applicator, energy and SSD of clinical use, beacause it depends on the energy of the beam and on the dimensions of the applicator It is recommended to employ a WP In particular conditions and with the adoption of suitable correction factors, also solid phantoms can be used

54 Standard linacs & Mobetron Output (cgy/mu) Measurement depth = R max (on the clinical axis of the beam) IC should present low angular dependence For tilted applicators, due to the asymmetry of the beam, small size detectors are recommended Dosimeters whose response is independent from the beam s incidence angle are recommended (alanine dosimeters, radiochromic films, TLD or even small size ionisation chambers with the exception of plane-parallel IC).

55 Mobile linacs with high-dose per pulse Output (cgy/mu) Measurement depth = R max (on the clinical axis of the beam) The high dose-rate by some dedicated linacs cannot permit to adopt the standard dosimetry protocols for IC dosimetry see De Martino method or Laitano method Gold standard: absolute Fricke dosimetric system With Fricke dosimeters, due to its large size, it may be necessary (low energy electron beams, bevelled applicators) to apply a correction factor for non uniform dose distribution inside the dosimeter Relative dosimetric systems with a response independent from the dose-rate (alanine dosimeters or radiochromic films) have the advantage of a small thickness and size, but also a low sensitivity, allowing to be irradiated with a dose next to the clinically employed one ( 10 Gy).

56 Absolute dosimetry-bevelled applicators Karaj et al.method (Med Phys, 34, 2007) The authors adapted the method described in the previous work for p-p IC for the cylindrical IC, obtaining a general formula based-on the Boag theory for the k s and the value of the free-electron fraction p for the small cylindrical-ic CC01 (IBA, V= cm 3 ) CC01-dosimetry were compared to p-p IC dosimetry (Markus, PTW) and radichromic dosimetry (EBT, ISP) Overall uncertainty estimated on D w 3.1%

57 Output factors OFs are referred to reference applicator Iaccarino et al. (PMB, 56, 2011) compared the estimation of OFs by using three different dosimeters [Advanced Markus, PinPoint (PTW), PPC05 (IBA)] with the corresponding MC-calculated values for the set of applicators of LIAC2. They assessed the absolute dose according to IAEA TRS398, with the exception of the ion-recombination (Laitano et al. method)

58 OFs B e v e ll e d A p p F l a t A p p

59 Radiation leakage It is recommended to estimate the % of the radiation scattered through the applicator s walls, as a function of the beam energy and of the distance from the walls and from the base of the applicator Solid phantom plus radiochromic films or TLD % Radiochromic cut-out 9 MeV 7 MeV 5 MeV 3 MeV distance from the wall

60 Correction factors Determination of correction factors to the output taking into account the possible presence of an air gap between the entrance surface of the patient and the base of the applicator. The air gap factor is the ratio of dose with an air gap to the dose without one at d max. Air gap factors are measured at the appropriate d max for each combination applicator+beam energy. In order to increase the accuracy in the calculation of the number of MU in presence of partial shielding inside the irradiation field, it is also recommended to introduce the corresponding appropriate correction for the output factors (dose/mu).

61 Accessories for special applications (e.g. beam blocker discs for IORT breast) Al or PTFE: to reduce electrons Pb or steel: to reduce X-rays Determination of attenuation (<3%) 90 IC R 9 0

62 3. Quality controls Advanced Course on IORT, Trieste May 3, 2012

63 Periodic QA for standard linacs In addition to routine QA applied to the equipment used in conventional external- RT, specific periodic tests have to be performed for linacs used for IORT (applicators and docking system), in order to verify the stability of all the parameters that can become critical in an intra-operative context.

64 Quality Assurance of a dedicated IOeRT-linacs When adapting the recommendations of QA protocols of traditional linacs to mobile linacs, the physicist needs to deal with some conflicting aspects. These units are transported each day of use. They have not collimator jaws and bending magnets (to reduce weight and radiation leakage): these design elements simplify the system, but they make the electron beam energy more dependent on variations in RT-power generation and coupling to the accelerator. Reasons to perform more frequent beam measurements than with conventional installations equipment is used in ORs with no added shielding

65 Challenges in the developing a QA process 1. Radiation safety considerations limit the beam time for QA as much as possible 2. We have to learn to work in a sterilized ambient Fast and accurate

66 QA output&energy constancy Typically the machine is housed in a storage area where it is powered only to maintain the vacuum. The morning of treatment, it is transported to the OR. There is an expectation that it can be simply powered-up and after a minimal warm-up it can irradiate. We are assuming that the act of transportation does not affect the calibration. Besides, the QA tests are performed either the evening before or the early morning of treatment and so many hours can pass between QA and treatment. The question is: how can the daily QA-results represent the performance of the treatment delivery? In order to test the stability of linac performance, it should collect data concerning reproducibility and energy constancy in these different phases (after transportation, after some hours of inactivity during the day or overnight etc).

67 Daily-QA : what does daily mean? The practical question is: whento set up the machine and to do the QA checks? Because of multidisciplinary nature, single-dose procedure, the tolerance for machine downtime is very low, but the need to move and set up the machine adds complexity and the possibility of malfunctions. It is better to test the basic operation of the machine on the day before its intended use. AAPMrecommends that dosimetric QA follows on the day of treatment, preferably early enough to permit some troubleshooting if needed ISS recommend to perform dosimetric QA within 24 hrs before the treatment.

68 Daily-QA with IORT-dedicated linacs:..hard work Not having a place adequately shielded in the operating block implies that QC are performed outside normal working hours, verifying that nobody is present in the adjacent areas of the operating theatre. This is a labor intensive process that can be simplified if experience with the machine demonstrates its reliability. Physicist who wakes-up early for QA before-iort..but when the going gets tough.

69 QA -Output&energy constancy We can check by using a solid phantom in which a practical dosimeter with high reproducibility (IC) can be placed at two depths: near d max (output) and in a point on PDD in the 50 80% range (ratio of readings -> energy) Reference applicator

70 Daily-QA : how many MUs? The total number of MUs used for daily QA may exceed the number of MUs used for treatment. Given that the machine is prepared for use more often than it is actually used, more beam time (and ambient radiation) may be allocated to QA than to treatment. Hence, there is good reason to carefully evaluate which readings and how many MUs per readingare necessary for QA. Use of a dual channel dosimeter to simultaneously acquire readings at two depths would be advantageous.

71 Periodic QA: ISS guidelines summary

72 Periodic QA: AAPM guidelines summary every before year the treatment every month

73 QA during IORT treatment In the OR, the RO may verbally indicate the key details of the prescription to the person who will perform the MU calculation and program the machine. Two likely sources of error: 1)the treatment planner may misunderstand the physician s instructions; 2)the planner may make a mistake in the calculation or in programming the treatment console. (..) Having more than one person check the critical elements in a single-shot (and high-dose) is crucial. The team tasked with implementing IORT must recognize these potential errors and design procedures to prevent them. The planner can use both PC AAPM or manual TG72 calculation methods, thus double-checking the mechanics of the calculation. The calculation can be written out in such a way that the physician can check that the prescription has been properly understood. A second person (the physician or another physicist) can check that the energy&mus have been properly programmed.

74 IVD: central role in the IOeRT-QA process In vivo dosimetry (IVD) represents an important QA tool to estimate the actual dose delivered to the target and to OAR. In particular for IOeRT, because of the high dose deliveredin a single fraction and the impossibility of elaborating a patient-specific RT-plan as in conventional RT. An on-line activesystem (e.g. MOSFETS) offers the opportunity to detect and correct, before the end of treatment, the delivered dose, if the predicted and measured dose differs more than a fixed value defined as the action level. TLD The deviation can be attributed to: 1) the dose distribution may change owing to irregular treatment surfaces, biologic fluid accumulation (buildup effect), backscattering discs 2) fluctuations of output machine 3) treatment error.

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