Megavoltage Radiation Research Articles

Optimizing the sensitivity and radiological properties of the PRESAGE® dosimeter using metal compounds

The aim of this study is to investigate the radiation-modifying effects of incorporating commercially available bismuth-, tin- and zinc-based compounds in the composition of the PRESAGE® dosimeter, and the feasibility of employing such compounds for radiation dose enhancement. Furthermore, we demonstrate that metal compounds can be included in the formulation to yield water-equivalent PRESAGE® dosimeters with enhanced dose response. Various concentrations of the metal compounds were added to a newly developed PRESAGE® formulation and the resulting dosimeters were irradiated with 100kV and 6MV photon beams. A comparison between sensitivity and radiological properties of the PRESAGE® dosimeters with and without the addition of metal compounds was carried out. Optical density changes of the dosimeters before and after irradiation were measured using a spectrophotometer. In general, when metal compounds were incorporated in the composition of the PRESAGE® dosimeter, the sensitivity of the dosimeters to radiation dose increased depending on the type and concentration of the metal compound, with the bismuth compound showing the highest dose enhancement factor. In addition, these metal compounds were also shown to improve the retention of the post-response absorption value of the PRESAGE® dosimeter over a period of 2 weeks. Thus, incorporating 1–3mM (ca. 0.2wt%) of any of the three investigated metal compounds in the composition of the PRESAGE® dosimeter is found to be an efficient way to enhance the sensitivity of the dosimeter to radiation dose and stabilize its post-response for longer times. Furthermore, the addition of small amounts of the metal compounds also accelerates the polymerization of the PRESAGE® dosimeter precursors, significantly reducing the fabrication time. Finally, a novel water-equivalent PRESAGE® dosimeter formula optimized with metal compounds is proposed for clinical use in both kilovoltage and megavoltage radiotherapy dosimetry.

SU-E-T-312: Optically Stimulated Luminescent Dosimeter Performance in High Dose Rate Brachytherapy.

To investigate the use of optically stimulated luminescent dosimeters (OSLDs) for high dose rate (HDR) Ir-192 brachytherapy. While common in megavoltage radiotherapy applications, the performance of OSLDs using kilovoltage energies, high dose gradient and high dose rate regions ubiquitous to HDR has not been demonstrated. Commercially available nanoDotTM (Landauer, Glenwood, IL) OSLDs - 5mm diameter and 0.2mm thickness - were irradiated using an Ir-192 source in various geometries to determine dose rate dependence. Angular geometries were created using a circular applicator designed for accelerated partial breast irradiation by Accuboost®. Response remained linear for high doses and was independent of dose rate. Specifically, the OSLDs were shown to be linear with dose up to 200 cGy then became slightly supra-linear up to 600 cGy. There was a slight angular dependence for OSLDs which becomes significant in 'edge-on' scenarios. An asymmetry in this angular dependence was discovered, but was attributed to cable curvature, point source approximation, and positioning within its plastic casing. Clinical HDR doses of 300 cGy fall within the linear, dose-rate-independent region. Angular independence can be maintained by avoiding extreme 'edge-on' measurement geometries. Because of re- readability, OSLDs can serve as a permanent record or alternatively be annealed within a few hours using conventional fluorescent light. Lastly, OSLDs are produced for only $5 each. Due to these features, in conjunction with the dosimetric performance, OSLDs should be considered a reliable tool for in vivo HDR brachytherapy measurements.

Long-term outcomes of radiotherapy for stage II testicular seminoma–the Mayo Clinic experience

ObjectivesTo report long-term outcomes of patients with stage II testicular seminoma treated with radiotherapy (RT). Materials and methodsA retrospective review was performed of 52 patients who received megavoltage RT for stage II testicular seminoma at Mayo Clinic between 1974 and 2007. Forty-eight patients (92%) had computed tomography staging. Overall survival (OS), relapse-free survival (RFS), and cause-specific survival (CSS) were determined using the Kaplan-Meier method. Major cardiac event (MCE) was defined as myocardial infarction, coronary artery bypass grafting or stenting, or valve replacement. Second malignancy (SM) was defined as biopsy-confirmed malignancy occurring in the RT field. ResultsThe median patient age at diagnosis was 36 years. Stage was IIA (n = 24), IIB (n = 7), IIC (n = 17), and II not otherwise specified (NOS, n = 4). The median infradiaphragmatic RT dose was 30.7 Gy. Twenty-six patients (50%) received prophylactic mediastinal/supraclavicular (MSCV) RT. The median follow-up was 19 years. Estimates of OS, RFS, and CSS were 94%, 80%, and 96% at 10 years, and 83%, 72%, and 96% at 20 years, respectively. RFS at 10 years for stage IIA, IIB, IIC, and II NOS were 83%, 54%, 81%, and 100%, respectively (log-rank P = 0.21). Ten patients (19%) experienced disease relapse in the MSCV region (n = 7), para-aortic lymph nodes (n = 1), lung (n = 1), or peritoneal cavity (n = 1). Eight patients were successfully salvaged with chemotherapy and/or surgery, while 2 died of seminoma. Risk of MSCV relapse was significantly lower in patients who received MSCV RT vs. those who did not (10-year estimates: 4% vs. 21%, respectively, log-rank P = 0.01). MCE occurred in 10 patients (19%) at a median of 18 years (range 7–30) after RT. SM occurred in 5 patients (10%) at a median of 27 years (range 20–34) after RT. ConclusionsIn patients with stage II testicular seminoma treated with RT, relapse in the irradiated site was uncommon. Infradiaphragmatic RT alone was associated with a significant risk of MSCV failure. Most MCE and SM events occurred more than 20 years after RT, highlighting the importance of vigilant long-term follow-up.

Neoadjuvant chemoradiotherapy and surgery as treatment for oral maxillary squamous cell carcinoma in a dog

A gingival maxillary squamous cell carcinoma was diagnosed in a 12-year-old male Yorkshire Terrier. After a complete diagnostic work-up, including a computed tomography scan, the tumour was staged as T3bN1aM0 and considered non-resectable at presentation. The combination of neoadjuvant megavoltage radiotherapy and neoadjuvant and adjuvant chemotherapy with carboplatin and doxorubicin decreased the size of the tumour, allowing for surgery. The dog was free from local disease for 421 days after which it was euthanased at the owners' request.

The use of megavoltage radiation therapy in the treatment of thymomas in rabbits: 19 cases

An overall median survival time (MST) and prognostic factors in rabbits with thymomas treated with megavoltage radiation therapy (RT) were determined in this multi-institutional retrospective case analysis. Medical records for 19 rabbits with suspected or confirmed thymomas treated with RT were evaluated for data including signalment, haematological and serum biochemistry abnormalities, presence of pleural effusion, radiation plan, body weight, total radiation dose and institution administering RT. Statistical significance of these factors related to overall survival was assessed. An overall MST for all 19 rabbits was 313 days; exclusion of 3 rabbits that died acutely during the first 14 days of RT yielded a MST of 727 days. The only factor associated with a significantly decreased survival time was having a body weight lower than mean body weight of 1.57 kg. Radiation treatment-associated complications were infrequent and included radiation-induced myocardial failure, radiation pneumonitis and alopecia.

A simple quality assurance test tool for the visual verification of light and radiation field congruent using electronic portal images device and computed radiography

BackgroundThe radiation field on most megavoltage radiation therapy units are shown by a light field projected through the collimator by a light source mounted inside the collimator. The light field is traditionally used for patient alignment. Hence it is imperative that the light field is congruent with the radiation field.MethodA simple quality assurance tool has been designed for rapid and simple test of the light field and radiation field using electronic portal images device (EPID) or computed radiography (CR). We tested this QA tool using Varian PortalVision and Elekta iViewGT EPID systems and Kodak CR system.ResultsBoth the single and double exposure techniques were evaluated, with double exposure technique providing a better visualization of the light-radiation field markers. The light and radiation congruency could be detected within 1 mm. This will satisfy the American Association of Physicists in Medicine task group report number 142 recommendation of 2 mm tolerance.ConclusionThe QA tool can be used with either an EPID or CR to provide a simple and rapid method to verify light and radiation field congruence.

Palliative Radiotherapy In Advanced Cancer Of The Cervix

Aim: To assess the role of high dose palliative pelvic radiotherapy in patients with locally advanced/metastatic cancer of the cervix not suitable for radical treatment.Method: Seventy two patients with carcinoma of the cervix receiving palliative radiotherapy to the pelvis from January 2006-December 2010 were included in this study. Patients were treated with parallelopposed pelvic portals with megavoltage radiation monthly up to a maximum of three fractions. The dose per fraction was 8Gy. The patients were assessed for relief of bleeding, vaginal discharge, pain, response and toxicity.Results: The mean age at presentation was 55 years and the mean duration of symptoms was 10 months. A control in vaginal bleeding was observed in 95% patients receiving three fractions. Sixty-six percent patients receiving three fractions had relief from vaginal discharge. Fifty eight percent patients had 100% pain relief after the third fraction. No grade 3-4 acute or late toxicity was observed in evaluable patients on follow up.Conclusion: 8Gy per fraction in two to three monthly fractions is an effective schedule for palliation in patients with advanced cancer of the cervix not suitable for radical radiation.

Position detection accuracy of a novel linac-mounted intrafractional x-ray imaging system

The authors have developed a system that monitors intrafractional target motion perpendicular to the treatment beam with the aid of radioopaque markers by means of separating kV image and megavoltage (MV) treatment field on a single flat-panel detector. They equipped a research Siemens Artiste linear accelerator (linac) with a 41 × 41 cm(2) a-Si flat-panel detector underneath the treatment head. The in-line geometry allows kV (imaging) and MV (treatment) beams to share closely aligned beam axes. The kV source, usually mounted directly across from the flat-panel imager, was retracted toward the gantry by 13 cm to intentionally misalign kV and MV beams, resulting in a geometric separation of MV treatment field and kV image on the detector. Two consecutive images acquired within 140 ms (the first with MV-only and the second with kV and MV signal) were subtracted to generate a kV-only image. The images were then analyzed "online" with an automated threshold-based marker detection algorithm. They employed a 3D and a 4D phantom equipped with either a single radioopaque marker or three Calypso beacons to mimic respiratory motion. Measured room positions were either cross-referenced with a phantom voltage signal (single marker) or the Calypso system. The accuracy of the back-projection (from detected marker positions into room coordinates) was verified by a simulation study. A phantom study has demonstrated that the imaging framework is capable of automatically detecting marker positions and sending this information to the tracking tool at an update rate of 7.14 Hz. The system latency is 86.9 ± 1.0 ms for single marker detection in the absence of MV radiation. In the presence of a circular MV field of 5 cm diameter, the latency is 87.1 ± 0.9 ms. The total RMS position detection accuracy is 0.20 mm (without MV radiation) and 0.23 mm (with MV). Based on the evaluated motion patterns and MV field size, the positional accuracy and system latency indicate that this system is suitable for real-time adaptive applications.

Nasal polyps in dogs: five cases (2005 to 2011)

This series describes five dogs with nasal polyps diagnosed between 2000 and 2011. Clinical signs included reverse sneezing, nasal discharge, epistaxsis, and stertor when breathing. Computerised tomographic findings included soft tissue mass, turbinate destruction, extension through the bony nasal septum and partial lysis of bones surrounding the nasal cavity. Three dogs were treated by dorsal rhinotomy, one dog was treated by ventral rhinotomy, and in one dog the polyp tissue was removed during nasal flushing. Three dogs have no clinical signs of nasal disease. One dog had confirmed recurrence of nasal polyps and was successfully treated with megavoltage radiation. One dog had recurrent nasal disease eight months after dorsal rhinotomy. Nasal polyps are a possible cause of nasal disease in dogs with nasal discharge, epistaxsis and stertor, and a differential diagnosis for dogs with extensive soft tissue lesions of the nasal cavities on computerised tomography. Nasal polyps can be treated successfully by rhinotomy in some cases but may reoccur.

Menhaden oil administration to dogs treated with radiation for nasal tumors demonstrates lower levels of tissue eicosanoids

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) or menhaden oil may reduce inflammatory eicosanoids (prostaglandin E₂, thromboxane B₂, leukotriene B₄, and 11-dehydro thromboxane B₂), matrix metalloproteinases (MMPs), and blood lactate in dogs with nasal carcinomas receiving radiation therapy. We hypothesized that menhaden oil would reduce inflammation from radiation damage and lower blood lactate levels in dogs with nasal carcinoma. In a randomized, double-blind, placebo-controlled clinical study, 12 dogs with malignant carcinomas of the nasal cavity were given dietary menhaden oil (DHA and EPA) or soybean oil (control) and then received radiation therapy. Megavoltage radiation was delivered in 18 fractions to a total dose of 56 Gy. Blood levels of DHA, EPA, insulin, glucose, lactic acid, and MMPs 2 and 9; resting energy expenditure; and inflammatory eicosanoids from nasal biopsies were measured throughout radiation therapy. Samples were obtained from each patient 1 week before the start of radiation therapy, at start of radiation, and 7, 18 (end of radiation therapy), and 42 days after radiation was initiated. Dogs that are fed with menhaden oil had significantly (P < .05) higher plasma concentration of DHA by 500% and EPA by 200% and had significantly lower tissue inflammatory eicosanoids and decreased resting energy expenditure by 20% when compared with controls. Increased plasma DHA was significantly associated (P < .05) with decreased plasma lactic acid and MMPs. These data may suggest that dietary fish oil could reduce some detrimental inflammatory eicosanoids and metabolic consequences of radiation therapy.

Mucosite em pacientes portadores de câncer de cabeça e pescoço submetidos à radioquimioterapia

O objetivo do presente trabalho é classificar o grau de mucosite oral de acordo com os parâmetros internacionais do Common Toxicity Criterion (CTC) em pacientes portadores de tumor de cabeça e pescoço submetidos à radioterapia e quimioterapia concomitantes, e caracterizar um perfil dos pacientes em nosso meio, verificando os hábitos dos indivíduos, as características do tumor, o protocolo de tratamento e a intensidade desta reação aguda. Neste estudo foram avaliados 50 pacientes, submetidos à radioterapia em megavoltagem com doses entre 66 a 70 Gy e quimioterapia com cisplatina ou carboplatina concomitante. Semanalmente foi avaliado o grau de mucosite de acordo com o CTC, uma escala ordinal que apresenta 4 graus. Observou-se interrupção do tratamento por mucosite em 36% do total de pacientes e em 100% dos pacientes diabéticos, o que nos permitiu verificar que esta patologia contribui para a gravidade da mucosite.

Nanodosimetric effects of gold nanoparticles in megavoltage radiation therapy

Background and purposeThe addition of gold nanoparticles (GNPs) to tumours leads to an increase in dose due to their high density and energy absorption coefficient, making it a potential radiosensitiser. However, experiments have observed radiosensitisations significantly larger than the increase in dose alone, including at megavoltage energies where gold’s relative energy absorption is lowest. This work investigates whether GNPs create dose inhomogeneities on a sub-cellular scale which combine with non-linear dose dependence of cell survival to be the source of radiosensitisation at megavoltage energies. Materials and methodsMonte Carlo simulations were carried out to calculate dose in the vicinity of a single GNP on the nanoscale. The effect of this nanoscale dose distribution was then modelled for MDA-MB-231 cells exposed to 2nm GNPs, and compared to experimental results. ResultsDramatic dose inhomogeneities occur around GNPs exposed to megavoltage radiation. When analysed using the Local Effect Model, these inhomogeneities lead to significant radiosensitisation, in agreement with experimental results. ConclusionsThis work suggests that GNP radiosensitisation is driven by inhomogeneities in dose on the nanoscale, rather than changes in dose over the entire cell, which may contribute to the similar radiosensitisation observed in megavoltage and kilovoltage experiments. The short range of these inhomogeneities and the variation in enhancement in different cells suggests sub-cellular localisation is important in determining GNP radiosensitisation.

Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy

The use of solid carbon fiber table materials in radiotherapy has become more common with the implementation of image‐guided radiotherapy (IGRT), since the solid materials give less imaging artifacts than the so‐called tennis racket couchtops. The downside of the solid carbon fiber couch inserts is that they increase the beam attenuation, resulting in increased surface doses and inaccuracies in determine the dose in the patient. The purpose of this study was to evaluate the interaction of 6 and 15 MV photons with eight different couch inserts. The presented results enable direct comparison of the attenuation properties of the studied couchtops. With a direct posterior beam the maximum attenuations reach 3.6% and 2.4% with 6 and 15 M V, respectively. The measured maximum attenuation by a couchtop with an oblique gantry angle was 10.8% and 7.4% at 6 and 15 MV energies, respectively. The skin‐sparing effect was decreased substantially with every couchtop. The highest increases in surface doses were recorded to be four‐ and threefold, as compared to the direct posterior open field surface doses of 6 and 15 MV, respectively. In conclusion, the carbon fiber tabletops decrease the skin‐sparing effect of megavoltage photon energies. The increased beam attenuation and skin doses should be taken into account in the process of treatment planning.PACS number: 07.90.+c

A novel method for megavoltage scatter correction in cone-beam CT acquired concurrent with rotational irradiation

Background and purposeAcquisition of cone-beam CT (CBCT) concurrent with VMAT results in scatter of the megavoltage (MV) beam onto the kilovoltage (kV) detector deteriorating CBCT image quality. The aim of this paper is to develop a method to estimate and correct for MV scatter reaching the kV panel. Material and methodsThe correction method is based on an alternating pulse sequence such that the kV source is enabled in only every other frame. MV scatter, noise and artifacts in the kV-unexposed frames are separated based on their spatial and temporal frequency characteristics and subsequently subtracted from the exposed frames. The method was tested on a phantom and one lung cancer patient scanned on an Elekta Synergy system simultaneous with a 9Gy VMAT delivery. ResultsWithout correction, acquisition during VMAT leads to significant cupping and loss of bone contrast. The novel method removes the signal due to MV scatter. Compared to acquisition without MV beam, the reconstruction quality is identical except for some unavoidable noise due to the scattered MV radiation in the exposed frames. Correction is performed in-line with acquisition without introducing delays. ConclusionsA novel, efficient and effective method was developed to correct for MV scatter in CBCT scans acquired concurrent with rotational radiotherapy.

Therapeutic options to prevent recurrence of an aggressive aneurysmatic bone cyst of the cervical spine of a 16 year old boy - a case report.

The aneurysmatic bone cyst (ABC) is a benign primary bone tumour. If located in the cervical spine, its expansive growth and destructive behaviour may lead to instability and serious neurological impairment. We report a case of a 16-year-old boy with an aggressive ABC in the 7th cervical vertebra. Computertomographic and magnetic resonance imaging revealed the envelopment of the left 7th and 8th spinal nerve along with the anterior displacement of the left vertebral artery. The interdisciplinary surgical strategy consisted of a partially incomplete cyst resection, subtotal spondylectomy with posterior screw-and-rod fixation from C6-Th1, iliac crest bone grafting and anterior plating from C6-Th1. With regard to the high rate of recurrence after incomplete resection published in the recent literature, the patient was postoperatively treated by megavoltage radiotherapy with a total dose of 30Gy (daily dose of 1.8 Gy for 3 weeks). The clinical and radiographic follow-up showed complete recovery of all neurologic impairments and no signs of tumour recurrence at 3, 6 and 12 months after surgery. This case highlights diverse treatment regimens and shall outline the challenge and the problems of the interdisciplinary decision-making in adolescents presenting with ABC in high-demanding anatomical regions.

SU‐E‐T‐305: An Automated Program for Shielding Calculation of a Radiotherapy Vault

Purpose: Shielding calculations for megavoltage radiotherapy facilities with mazes are generally straightforward but highly critical. Excel spreadsheets are often used to expedite the process; but with numerous parameters and variables setting up the spreadsheet can be more time consuming than hand calculations. In busy clinical departments the patient load may leave the physicist unable to dedicate the desirable amount of resources to pay the detailed attention due for a timely and accurate shielding calculation. The purpose of this investigation is to automate this process by streamlining workload calculations, programming lookup tables for easy access, and calculate the necessary dimensions of the primary shielding. The results of a newly written automated shielding calculator program will be compared with hand calculations performed according to NCRP Report 151. Methods: A visual basic program was written, using NCRP Report 151 as a model, to calculate the shielding needed for a linear accelerator in a simple treatment vault. In an attempt to streamline the coding of the program, a set of conditional calculations is used to accurately calculate the thickness of secondary shielding. Results: The visual basic program was able to accurately calculate the thickness of primary and secondary shielding to NCRP Report 151 calculations. Conclusion: The program can be used to calculate shielding thicknesses with accuracy for radiotherapy rooms. To make the program more pragmatic, maze shielding and neutron shielding calculations should be added. The detail of the programming steps to accommodate the NCRP 151 formulas and parameters will be presented.

SU‐E‐T‐240: In‐Vivo Measurement of Shallow Dose for a Spoiled Mega Voltage Radiotherapy Beam

Purpose: Calibration factors for electrons and photons are quite different, especially for MOSFETS with inherent buildups. Megavoltage radiotherapy beam in the presence of a beam spoiler has a large low energy electron component near the surface. A method of calibration of the spoiled beam is proposed. This calibration method will be useful for in‐vivo dose measurement for total body irradiation (TBI). Methods: The ionization charge and deposited dose is measured in water at standard and extended source to surface distance (SSD) for a megavoltage beam in the presence of a beam spoiler at various depths. The contribution of low energy electrons generated by the spoiler is separated from the photon contribution. The calibration factors for the electron and photon parts are weighted according to the proportions of their contributions. The net calibration factor is tested for the composite beam. Results: The calibration factor for the electrons is 1.15 while the calibration factor for the photon component is 1.02. At shallow depths the electron contribution is very high. The resultant calibration factor from a preliminary study is found to be about 1.07 at a depth of 1 cm. Conclusions: A method of determining the calibration factor for a mega voltage spoiled beam is proposed. The method yields a calibration factor for in‐vivo dose measurement for the combined photon and low energy electrons generated by the beam spoiler.

A Case of Localized Acne Following Megavoltage Radiotherapy

A Case of Localized Acne Following Megavoltage Radiotherapy

Cell-Specific Radiosensitization by Gold Nanoparticles at Megavoltage Radiation Energies

Gold nanoparticles (GNPs) have been shown to cause sensitization with kilovoltage (kV) radiation. Differences in the absorption coefficient between gold and soft tissue, as a function of photon energy, predict that maximum enhancement should occur in the kilovoltage (kV) range, with almost no enhancement at megavoltage (MV) energies. Recent studies have shown that GNPs are not biologically inert, causing oxidative stress and even cell death, suggesting a possible biological mechanism for sensitization. The purpose of this study was to assess GNP radiosensitization at clinically relevant MV X-ray energies. Cellular uptake, intracellular localization, and cytotoxicity of GNPs were assessed in normal L132, prostate cancer DU145, and breast cancer MDA-MB-231 cells. Radiosensitization was measured by clonogenic survival at kV and MV photon energies and MV electron energies. Intracellular DNA double-strand break (DSB) induction and DNA repair were determined and GNP chemosensitization was assessed using the radiomimetic agent bleomycin. GNP uptake occurred in all cell lines and was greatest in MDA-MB-231 cells with nanoparticles accumulating in cytoplasmic lysosomes. In MDA-MB-231 cells, radiation sensitizer enhancement ratios (SERs) of 1.41, 1.29, and 1.16 were achieved using 160 kVp, 6 MV, and 15 MV X-ray energies, respectively. No significant effect was observed in L132 or DU145 cells at kV or MV energies (SER 0.97-1.08). GNP exposure did not increase radiation-induced DSB formation or inhibit DNA repair; however, GNP chemosensitization was observed in MDA-MB-231 cells treated with bleomycin (SER 1.38). We have demonstrated radiosensitization in MDA-MB-231 cells at MV X-ray energies. The sensitization was cell-specific with comparable effects at kV and MV energies, no increase in DSB formation, and GNP chemopotentiation with bleomycin, suggesting a possible biological mechanism of radiosensitization.

Patterns of Retreatment by Radiotherapy

Aim To describe patterns of radiotherapy retreatment by cancer type in order to develop methods of modelling retreatment for better service planning and benchmarking. Materials and methods We examined an institutional database of all patients who received their first megavoltage radiotherapy for any type of cancer at the Liverpool and Macarthur Cancer Therapy Centres over the period 1997–2006. The database contains patient demographic data, dates, treatment sites and doses of radiotherapy. The treatment sites were entered as free text and were recoded into a limited number of sites; primary site, bone, brain, soft tissue, other and multiple, so that retreatment sites could be grouped for stratification and analysis. Multiple treatment sites that were part of a single treatment to a primary (for example, primary site and nodes) were recoded as ‘primary’. The total retreatment ratio was defined as the number of episodes of radiotherapy divided by the number of cases in the cohort. Results In the period 1997–2006, 7853 patients had received 9859 episodes of radiotherapy, giving a total retreatment ratio of 26 per 100 patients. In total, 6524 patients (83%) received only one episode of treatment and 1329 received two or more episodes of treatment. The average number of retreatments was 0.26. The tumour type with the highest mean number of retreatments was myeloma (1.05), followed by unknown primary (0.41), lung (0.34) and melanoma (0.32). The lowest mean number of retreatments was for the brain (0.06). The median time between treatment episodes was longest for breast cancer (12.5 months), then colorectal cancer (12.3 months), head and neck cancers (9.0 months), lung cancer (3.8 months) and prostate cancer (5.9 months). Conclusion The retreatment ratio for the cohort was affected by the length of follow-up and by the tumour type. The mean number of retreatments varied from 0.06 for central nervous system malignancy to 1.05 for myeloma, with an average of 0.026.