Organ doses from prostate radiotherapy and associated concomitant exposures

Doses delivered by portal imaging quality assurance in routine practice of adjuvant breast radiotherapy worth to by monitored and compensated in some cases

Background

Imaging, in radiotherapy, has become a routine tool for repositioning of the target volume at each session. The repositioning precision, currently infracentimetric, evolves along with the irradiation techniques. This retrospective study aimed to identify practices and doses resulting from the use of high energy planar imaging (portal imaging) in daily practice.

Methods

A retrospective survey of portal images (PIs) was carried out over 10 years for 2,403 patients and for three linacs (1 Elekta SLi, 2 Varian Clinac) for postoperative mammary irradiations. Images were taken using a standardized number of monitor units (MU) for all patients. Due to the variable sensitivities of the detectors and the possibility of adjustment of the detector-patient distance, the number of MU were 3; 2 and 1 respectively, for Elekta SLi^®, Clinac 600^® and Clinac 2100^®. Then, a representative cumulated dose was calculated in simplified reference conditions (5 cm depth, beam of 10 cm × 10 cm, 6 MV), considering the total number of images taken during the whole treatment course. The consistency between the representative doses and the actual absorbed doses received by the patients was verified by simulating a series of typical cases with the treatment plan dose calculation system.

Results

The delivered doses differ significantly between the three linacs. The mean representative dose values by complete treatment were 0.695; 0.241 and 0.216 Gy, respectively, for SLi, Clinac 600 and Clinac 2100. However, 15 patients were exposed to a dose >2 Gy with a maximum dose of 5.05 Gy. The simulated doses were very similar to the representative doses.

Conclusions

A significant dose delivery was highlighted by this study. These representative doses are presently communicated weekly to the radiation oncologist for the radiation protection of their patients. Moreover, they should be taken into account in a possible study of long-term stochastic risks.

Daily On-Line Set-Up Correction in 3D-Conformal Radiotherapy: Is It Feasible?

Aims and background

The aim of this report was to investigate the feasibility in terms of treatment time prolongation of an on-line no-action level correction protocol, based on daily electronic portal image verification.

Methods and study design

The occupation of a linear accelerator (LINAC) delivering 3-D conformal treatments was monitored for two weeks (from Monday to Friday, 10 working days). An electronic portal image device I-View (Elekta, UK) was used for setup verification. Single-exposure portal images were acquired daily using the initial 8 monitor units delivered for each treatment field. Translational deviations of isocenter position larger than 5 mm or 7 mm, for radical or palliative treatments, respectively, were immediately corrected. In order to estimate the extra workload involved with the on-line protocol, the time required for isocenter check and table correction was specifically monitored.

Results

Forty-eight patients were treated. In all, 482 fractions had electronic portal images taken. Two hundred and forty-five setup corrections were made (50.8% of all fractions). The occupation of the LINAC lasted 106 h on the whole. Twelve h and 25 min (11.7% of LINAC occupation time) were spent for portal image verification and setup correction. On the average, 4.3 fractions per hour were carried out.

Conclusions

When used by trained therapists, ideally, portal imaging may be carried out before each fraction, requiring approximately 10% of LINAC occupation time.

Image-guided radiotherapy for prostate cancer with cone beam CT: dosimetric effects of imaging frequency and PTV margin

BACKGROUND AND PURPOSE

This study assesses the effect of frequency of cone beam CT (CBCT) verification imaging on dose-volume parameters during image-guided radiotherapy (IGRT) for prostate cancer. It also investigates the dosimetric impact of reducing the planning target volume (PTV) margin, when daily imaging is used.

MATERIAL AND METHODS

844 CBCT images from 20 patients undergoing radical prostate radiotherapy were included. Patients received a dose of 74Gy in 37 fractions using 7-field intensity-modulated radiotherapy (IMRT). Clinical target volume (CTV) and organs at risk were contoured manually on each slice of every CBCT image. A daily online CBCT verification schedule was compared with a protocol of verification on days 1-3 followed by weekly online imaging. PTV margins of 3mm, 5mm, and 7mm were compared for the daily imaging protocol.

RESULTS

90% of patients had improved target coverage with daily online in comparison to weekly online imaging. A median of 37 fractions per treatment course achieved CTV coverage with daily imaging compared to 34 fractions with a weekly online protocol. 80% of patients had a reduction in rectal dose with the daily protocol. PTV margin reduction to 5mm with adequate target coverage was feasible with daily imaging.

CONCLUSIONS

Daily online CBCT verification improves CTV coverage and reduces rectal dose during IGRT for prostate cancer. Tighter PTV margins could be considered with daily CBCT use.

EURADOS strategic research agenda: vision for dosimetry of ionising radiation

Since autumn 2012, the European Radiation Dosimetry Group (EURADOS) has been developing its Strategic Research Agenda (SRA), which is intended to contribute to the identification of future research needs in radiation dosimetry in Europe. The present article summarises-based on input from EURADOS Working Groups (WGs) and Voting Members-five visions in dosimetry and defines key issues in dosimetry research that are considered important for the next decades. The five visions include scientific developments required towards (a) updated fundamental dose concepts and quantities, (b) improved radiation risk estimates deduced from epidemiological cohorts, (c) efficient dose assessment for radiological emergencies, (d) integrated personalised dosimetry in medical applications and (e) improved radiation protection of workers and the public. The SRA of EURADOS will be used as a guideline for future activities of the EURADOS WGs. A detailed version of the SRA can be downloaded as a EURADOS report from the EURADOS website (www.eurados.org).

Peripheral photon and neutron doses from prostate cancer external beam irradiation

Peripheral photon and neutron doses from external beam radiotherapy (EBRT) are associated with increased risk of carcinogenesis in the out-of-field organs; thus, dose estimations of secondary radiation are imperative. Peripheral photon and neutron doses from EBRT of prostate carcinoma were measured in Rando phantom. (6)LiF:Mg,Cu,P and (7)LiF:Mg,Cu,P glass-rod thermoluminescence dosemeters (TLDs) were inserted in slices of a Rando phantom followed by exposure to 80 Gy with 18-MV photon four-field 3D-CRT technique. The TLDs were calibrated using 6- and 18-MV X-ray beam. Neutron dose equivalents measured with CR-39 etch-track detectors were used to derive readout-to-neutron dose conversion factor for (6)LiF:Mg,Cu,P TLDs. Average neutron dose equivalents per 1 Gy of isocentre dose were 3.8±0.9 mSv Gy(-1) for thyroid and 7.0±5.4 mSv Gy(-1) for colon. For photons, the average dose equivalents per 1 Gy of isocentre dose were 0.2±0.1 mSv Gy(-1) for thyroid and 8.1±9.7 mSv Gy(-1) for colon. Paired (6)LiF:Mg,Cu,P and (7)LiF:Mg,Cu,P TLDs can be used to measure photon and neutron doses simultaneously. Organs in close proximity to target received larger doses from photons than those from neutrons whereas distally located organs received higher neutron versus photon dose.

Measurement-based model of a wide-bore CT scanner for Monte Carlo dosimetric calculations with GMCTdospp software

The aim of this work was to create a model of a wide-bore Siemens Somatom Sensation Open CT scanner for use with GMCTdospp, which is an EGSnrc-based software tool dedicated for Monte Carlo calculations of dose in CT examinations. The method was based on matching spectrum and filtration to half value layer and dose profile, and thus was similar to the method of Turner et al. (Med. Phys. 36, pp. 2154-2164). Input data on unfiltered beam spectra were taken from two sources: the TASMIP model and IPEM Report 78. Two sources of HVL data were also used, namely measurements and documentation. Dose profile along the fan-beam was measured with Gafchromic RTQA-1010 (QA+) film. Two-component model of filtration was assumed: bow-tie filter made of aluminum with 0.5 mm thickness on central axis, and flat filter made of one of four materials: aluminum, graphite, lead, or titanium. Good agreement between calculations and measurements was obtained for models based on the measured values of HVL. Doses calculated with GMCTdospp differed from the doses measured with pencil ion chamber placed in PMMA phantom by less than 5%, and root mean square difference for four tube potentials and three positions in the phantom did not exceed 2.5%. The differences for models based on HVL values from documentation exceeded 10%. Models based on TASMIP spectra and IPEM78 spectra performed equally well.

Investigation into the radiobiological consequences of pre-treatment verification imaging with megavoltage X-rays in radiotherapy

OBJECTIVE

The aim of this study was to investigate the effect of pre-treatment verification imaging with megavoltage X-rays on cancer and normal cell survival in vitro and to compare the findings with theoretically modelled data. Since the dose received from pre-treatment imaging can be significant, the incorporation of this dose at the planning stage of treatment has been suggested.

METHODS

The impact of imaging dose incorporation on cell survival was investigated by clonogenic assay of irradiated DU-145 prostate cancer, H460 non-small-cell lung cancer and AGO-1522b normal tissue fibroblast cells. Clinically relevant imaging-to-treatment times of 7.5 and 15 min were chosen for this study. The theoretical magnitude of the loss of radiobiological efficacy due to sublethal damage repair was investigated using the Lea-Catcheside dose protraction factor model.

RESULTS

For the cell lines investigated, the experimental data showed that imaging dose incorporation had no significant impact on cell survival. These findings were in close agreement with theoretical results.

CONCLUSION

For the conditions investigated, the results suggest that allowance for the imaging dose at the planning stage of treatment should not adversely affect treatment efficacy.

ADVANCES IN KNOWLEDGE

There is a paucity of data in the literature on imaging effects in radiotherapy. This article presents a systematic study of imaging dose effects on cancer and normal cell survival, providing both theoretical and experimental evidence for clinically relevant imaging doses and imaging-to-treatment times. The data provide a firm foundation for further study into this highly relevant area of research.

Doses in sensitive organs during prostate treatment with a 60Co unit

Using thermoluminiscent dosimeters the absorbed dose in the bladder, rectum and thyroid have been evaluated when 200 cGy was applied to the prostate. The treatment was applied with a (60)Co unit. A water phantom was built and thermoluminiscent dosimeters were located in the position where the prostate, bladder, rectum and thyroid are located. The therapeutic beam was applied in 4 irradiations at 0, 90, 180 and 270° with the prostate at the isocenter. The TLDs readouts were used to evaluate the absorbed dose in each organ. The absorbed doses were used to estimate the effective doses and the probability of developing secondary malignacies in thyroid, rectum and bladder.

Peripheral dose measurements with diode and thermoluminescence dosimeters for intensity modulated radiotherapy delivered with conventional and un-conventional linear accelerator

The objective of this paper was to measure the peripheral dose (PD) with diode and thermoluminescence dosimeter (TLD) for intensity modulated radiotherapy (IMRT) with linear accelerator (conventional LINAC), and tomotherapy (novel LINAC). Ten patients each were selected from Trilogy dual-energy and from Hi-Art II tomotherapy. Two diodes were kept at 20 and 25 cm from treatment field edge. TLDs (LiF:MgTi) were also kept at same distance. TLDs were also kept at 5, 10, and 15 cm from field edge. The TLDs were read with REXON reader. The readings at the respective distance were recorded for both diode and TLD. The PD was estimated by taking the ratio of measured dose at the particular distance to the prescription dose. PD was then compared with diode and TLD for LINAC and tomotherapy. Mean PD for LINAC with TLD and diode was 2.52 cGy (SD 0.69), 2.07 cGy (SD 0.88) at 20 cm, respectively, while at 25 cm, it was 1.94 cGy (SD 0.58) and 1.5 cGy (SD 0.75), respectively. Mean PD for tomotherapy with TLD and diode was 1.681 cGy SD 0.53) and 1.58 (SD 0.44) at 20 cm, respectively. The PD was 1.24 cGy (SD 0.42) and 1.088 cGy (SD 0.35) at 25 cm, respectively, for tomotherapy. Overall, PD from tomotherapy was found lower than LINAC by the factor of 1.2-1.5. PD measurement is essential to find out the potential of secondary cancer. PD for both (conventional LINAC) and novel LINACs (tomotherapy) were measured and compared with each other. The comparison of the values for PD presented in this work and those published in the literature is difficult because of the different experimental conditions. The diode and TLD readings were reproducible and both the detector readings were comparable.

Second cancer incidence risk estimates using BEIR VII models for standard and complex external beam radiotherapy for early breast cancer

PURPOSE

To compare organ specific cancer incidence risks for standard and complex external beam radiotherapy (including cone beam CT verification) following breast conservation surgery for early breast cancer.

METHOD

Doses from breast radiotherapy and kilovoltage cone beam CT (CBCT) exposures were obtained from thermoluminescent dosimeter measurements in an anthropomorphic phantom in which the positions of radiosensitive organs were delineated. Five treatment deliveries were investigated: (i) conventional tangential field whole breast radiotherapy (WBRT), (ii) noncoplanar conformal delivery applicable to accelerated partial beast irradiation (APBI), (iii) two-volume simultaneous integrated boost (SIB) treatment, (iv) forward planned three-volume SIB, and (v) inverse-planned three volume SIB. Conformal and intensity modulated radiotherapy methods were used to plan the complex treatments. Techniques spanned the range from simple methods appropriate for patient cohorts with a low cancer recurrence risk to complex plans relevant to cohorts with high recurrence risk. Delineated organs at risk included brain, salivary glands, thyroid, contralateral breast, left and right lung, esophagus, stomach, liver, colon, and bladder. Biological Effects of Ionizing Radiation (BEIR) VII cancer incidence models were applied to the measured mean organ doses to determine lifetime attributable risk (LAR) for ages at exposure from 35 to 80 yr according to radiotherapy techniques, and included dose from the CBCT imaging.

RESULTS

All LAR decreased with age at exposure and were lowest for brain, thyroid, liver, and bladder (<0.1%). There was little dependence of LAR on radiotherapy technique for these organs and for colon and stomach. LAR values for the lungs for the three SIB techniques were two to three times those from WBRT and APBI. Uncertainties in the LAR models outweigh any differences in lung LAR between the SIB methods. Constraints in the planning of the SIB methods ensured that contralateral breast doses and LAR were comparable to WBRT, despite their added complexity. The smaller irradiated volume of the ABPI plan contributed to a halving of LAR for contralateral breast compared with the other plan types. Daily image guided radiotherapy (IGRT) for a left breast protocol using kilovoltage CBCT contributed <10% to LAR for the majority of organs, and did not exceed 22% of total organ dose.

CONCLUSIONS

Phantom measurements and calculations of LAR from the BEIR VII models predict that complex breast radiotherapy techniques do not increase the theoretical risk of second cancer incidence for organs distant from the treated breast, or the contralateral breast where appropriate plan constraints are applied. Complex SIB treatments are predicted to increase the risk of second cancer incidence in the lungs compared to standard whole breast radiotherapy; this is outweighed by the threefold reduction in 5 yr local recurrence risk for patients of high risk of recurrence, and young age, from the use of radiotherapy. APBI may have a favorable impact on risk of second cancer in the contralateral breast and lung for older patients at low risk of recurrence. Intensive use of IGRT increased the estimated values of LAR but these are dominated by the effect of the dose from the radiotherapy, and any increase in LAR from IGRT is much lower than the models' uncertainties.

Neutron dosimetry in organs of an adult human phantom using linacs with multileaf collimator in radiotherapy treatments

PURPOSE

To calculate absorbed doses due to neutrons in 87 organs/tissues for anthropomorphic phantoms, irradiated in position supine (head first into the gantry) with orientations anteroposterior (AP) and right-left (RLAT) with a 18 MV accelerator. Conversion factors from monitor units to μGy per neutron in organs, equivalent doses in organs/tissues, and effective doses, which permit to quantify stochastic risks, are estimated.

METHODS

MAX06 and FAX06 phantoms were modeled with MCNPX and irradiated with a 18 MV Varian Clinac 2100C/D accelerator whose geometry included a multileaf collimator. Two actual fields of a pelvic treatment were simulated using electron-photon-neutron coupled transport. Absorbed doses due to neutrons were estimated from kerma. Equivalent doses were estimated using the radiation weighting factor corresponding to an average incident neutron energy 0.47 MeV. Statistical uncertainties associated to absorbed doses, as calculated by MCNPX, were also obtained.

RESULTS

Largest doses were absorbed in shallowest (with respect to the neutron pathway) organs. In μGyMU(-1), values of 2.66 (for penis) and 2.33 (for testes) were found in MAX06, and 1.68 (for breasts), 1.05 (for lenses of eyes), and 0.94 (for sublingual salivary glands) in FAX06, in AP orientation. In RLAT, the largest doses were found for bone tissues (leg) just at the entrance of the beam in the body (right side in our case). Values, in μGyMU(-1), of 1.09 in upper leg bone right spongiosa, for MAX06, and 0.63 in mandible spongiosa, for FAX06, were found. Except for gonads, liver, and stomach wall, equivalent doses found for FAX06 were, in both orientations, higher than for MAX06. Equivalent doses in AP are higher than in RLAT for all organs/tissues other than brain and liver. Effective doses of 12.6 and 4.1 μSvMU(-1) were found for AP and RLAT, respectively. The organs/tissues with larger relative contributions to the effective dose were testes and breasts, in AP, and breasts and red marrow, in RLAT. Equivalent and effective doses obtained for MAX06/FAX06 were smaller (between 2 and 20 times) than those quoted for the mathematical phantoms ADAM/EVA in ICRP-74.

CONCLUSIONS

The new calculations of conversion coefficients for neutron irradiation in AP and RLAT irradiation geometries show a reduction in the values of effective dose by factors 7 (AP) and 6 (RLAT) with respect to the old data obtained with mathematical phantoms. The existence of tissues or anatomical regions with maximum absorbed doses, such as penis, lens of eyes, fascia (part of connective tissue), etc., organs/tissues that classic mathematical phantoms did not include because they were not considered for the study of stochastic effects, has been revealed. Absorbed doses due to photons, obtained following the same simulation methodology, are larger than those due to neutrons, reaching values 100 times larger as the primary beam is approached. However, for organs far from the treated volume, absorbed photon doses can be up to three times smaller than neutron ones. Calculations using voxel phantoms permitted to know the organ dose conversion coefficients per MU due to secondary neutrons in the complete anatomy of a patient.

Neutron dose equivalent and neutron spectra in tissue for clinical linacs operating at 15, 18 and 20 MV

In this work, the dose equivalent due to photoneutrons and the neutron spectra in tissue was calculated for various linacs (Varian Clinac 2100C, Elekta Inor, Elekta SL25 and Siemens Mevatron KDS) operating at energies between 15 and 20 MV, using the Monte Carlo code MCNPX (v. 2.5). The dose equivalent in an ICRU tissue phantom has been calculated for anteroposterior treatments with a detailed simulation of the geometry of the linac head and the coupled electron-photon-neutron transport. Neutron spectra at the phantom entrance and at 1-cm depth in the phantom, depth distribution of the neutron fluence in the beam axis and dose distributions outside the beam axis at various depths have also been calculated and compared with previously published results. The differences between the neutron production of the various linacs considered has been analysed. Varian linacs show a larger neutron production than the Elekta and Siemens linacs at the same operating energy. The dose equivalent due to neutrons produced by medical linacs operating at energies >15 MeV is relevant and should not be neglected because of the additional doses that patients can receive.

Organ and effective doses from verification techniques in image-guided radiotherapy

The purpose of this work was an evaluation of organ doses and effective doses from three verification techniques in Image-Guided Radiotherapy: from kilovoltage (kV) cone beam computed tomography (CBCT) scans, from two orthogonal kV images and from two orthogonal megavoltage (MV) images for two different treatment sites: pelvis and head and neck (H&N). For comparison reasons, organ doses and effective doses from prostate and H&N radiotherapy were also evaluated. Measurements of organ doses were performed in a male anthropomorphic Rando phantom by means of thermoluminescent dosemeters. In this investigation, measured organ doses from one CBCT scan, from two MV images and from two kV images of pelvis represent typically 1-6, 1-10 and 0.05-1 %, respectively, of organ doses resulting from one fraction of prostate radiotherapy. The maximum effective doses from CBCT scans, kV images and MV images of pelvis are 5.6, 0.8 and 11.9 mSv, respectively.

DART, a platform for the creation and registration of cone beam digital tomosynthesis datasets

Digital tomosynthesis is an imaging modality that allows for tomographic reconstructions using only a fraction of the images needed for CT reconstruction. Since it offers the advantages of tomographic images with a smaller imaging dose delivered to the patient, the technique offers much promise for use in patient positioning prior to radiation delivery. This paper describes a software environment developed to help in the creation of digital tomosynthesis image sets from digital portal images using three different reconstruction algorithms. The software then allows for use of the tomograms for patient positioning or for dose recalculation if shifts are not applied, possibly as part of an adaptive radiotherapy regimen.

Does computed tomography or positron emission tomography/computed tomography contribute to detection of small focal cancers in the prostate?

Prostate cancer is considered to be a multifocal tumor in the majority of patients. Based on histologic data after prostatectomy, there is a growing insight that a considerable number of men who receive a diagnosis in the contemporary setting of prostate-specific antigen screening have unilateral or unifocal disease. With this, the current concept of whole-gland therapy has come into discussion. The need for improvement of intraprostatic tumor characterization is clear. Molecular imaging is one of the areas of research on this aspect. The clinical indications for positron emission tomography (PET)/CT have increased rapidly in the field of oncology and are largely based on fluorodeoxyglucose (FDG) PET. Both conventional CT and FDG PET, however, cannot detect prostate cancer foci <5 mm within the prostate. Dynamic contrast-enhanced CT involves imaging a region of interest rapidly (usually <10 seconds between images) during a bolus intravenous injection of a contrast agent. Through analysis of the contrast enhancement time curves, it is possible to distinguish tissues with different microvascular properties such as cancer. The technologic aspects of both imaging techniques and the clinical results of 11C-choline PET/CT for intraprostatic tumor characterization are discussed. Based on preliminary studies, dynamic contrast-enhanced (DCE)-CT may be a useful tool for localization of prostate tumors and, perhaps more importantly, quantification of therapeutic response in prostate cancer. Validation work is necessary, however, to define its accuracy and role in therapeutic paradigms such as focal therapies, particularly given the current accuracy of MRI. In the future, combining DCE-CT with CT or (11)C-choline PET/CT may be an alternative to MRI, offering a combination of quantitative parameters that may correlate to tumor prognosis as well as cancer localization for focal therapy.

Estimation of organ and effective doses resulting from cone beam CT imaging for radiotherapy treatment planning

In this study, organ doses were measured for various kilovoltage cone beam CT exposures on the Varian Acuity simulator and an alternative method of dose estimation was also assessed. Organ doses were measured by distributing thermoluminescent dosimeters (TLDs) throughout an anthropomorphic phantom, and effective doses were calculated using International Commission on Radiological Protection (ICRP) 60 and ICRP 103 tissue-weighting factors. The ImPACT CT patient dosimetry calculator was also used to estimate doses for comparison with the TLD results. Effective doses of 15.3 mSv (19.4 mSv), 14.3 mSv (9.7 mSv) and 2.8 mSv (3.2 mSv) were calculated from the TLD measurements and ICRP 60 (ICRP 103) weighting factors for breast, pelvis and head acquisitions, respectively. When a 10 cm pencil ionisation chamber was used to measure the CT dose index, the ImPACT calculator was found to provide an adequate estimation of dose when compared with the TLD results. However, the doses for half-fan exposures were found to be overestimated, with the extent of overestimation depending on the radiosensitive organs irradiated. The organ and effective doses reported provide information for justification and optimisation of cone beam CT procedures, and are compared with doses delivered by other imaging devices. The ImPACT calculator may be used to estimate doses from cone beam CT procedures, if the potential for overestimation is acknowledged.

Doses to organs and tissues from concomitant imaging in radiotherapy: a suggested framework for clinical justification

The increasing use of imaging for localization and verification in radiotherapy has raised issues concerning the justifiable doses to critical organs and tissues from concomitant exposures, particularly when extensive image-guided radiotherapy is indicated. Doses at positions remote from the target volume include components from high-energy leakage and scatter, as well as from concomitant imaging. In this paper, simulated prostate, breast and larynx treatments are used to compare doses from both high-energy and concomitant exposures as a function of distance from the target volume. It is suggested that the fraction, R, of the total dose at any point within the patient that is attributable to concomitant exposures may be a useful aid in their justification. R is small within the target volume and at large distances from it. However, there is a critical region immediately adjacent to the planning target volume where the dose from concomitant imaging combines with leakage and scatter to give values of R that approach 0.5 in the examples given here. This is noteworthy because the regions just outside the target volume will receive total doses in the order of 1 Gy, where commensurately high risk factors may not be substantially reduced because of cell kill. Other studies have identified these regions as sites of second cancers. The justification of an imaging regimen might therefore usefully take into account the maximum value of R encountered from the combination of imaging and radiotherapy for particular treatment sites.

Doses to critical organs following radiotherapy and concomitant imaging of the larynx and breast

The development of conformal radiotherapy carries with it the implication of an increased number of imaging procedures at various stages throughout the overall treatment, principally for verification at some, or all, of the treatment fractions. This raises the issue of the balance between the benefit of these additional imaging exposures and the associated risk of radiocarcinogenesis arising from them. As such, it is necessary to appreciate the doses to critical organs for which individual carcinogenic risks have been estimated. In this study, doses to these organs have been measured with lithium fluoride thermoluminescence dosimetry loaded in anthropomorphic phantoms and subjected to realistic radiotherapy treatments of the larynx and breast, including concomitant CT and electronic portal imaging exposures associated with localization and verification of these treatments. Even for large numbers of concomitant images of either modality, arising from imaging at every fraction, the leakage and scatter from the radiotherapy itself is shown to dominate the overall organ dose, with imaging procedures generally contributing 5-20% of the total organ dose.