1. INTRODUCTION Radiation-induced radiation-induced sensori-neural hearing loss (SNHL) had long been recognized as a complication of radiotherapy (RT) for head and neck tumours, if the auditory pathways had been included in the radiation fields. It is believed that radiation therapy was a much larger etiologic factor of hearing loss than suspected and should clearly be recognized as a major factor in the etiology of adult hearing disorders (Mencher et al, 1995). The incidence of SNHL after radiotherapy for nasopharyngeal carcinoma (NPC) has been reported to be as high as 24% (Kwong et al, 1996). NPC is common among the Chinese and the main modality of treatment for NPC is RT. NPC is therefore common in Singapore, with a prevalence rate of 10.8 and 3.7 (agestandardized) per 100,000 per year for males and females respectively (Seow et al, 2004). In conventional RT of tumours located in the head and neck region, the auditory pathways are often included in the radiation fields. In NPC, many patients with post-RT hearing loss are encountered, both conductive and sensori-neural in nature (Low & Fong, 1998). While conductive hearing losses can normally be effectively overcome by wearing appropriate hearing aids, the use of hearing aids in SNHL may possibly result in sounds that are amplified but distorted (Low, 2005). Therefore, radiation-induced SNHL is of particular concern in Singapore, where the post-treatment quality of life is increasingly being emphasized. Today, we are faced with a number of clinical issues related to the effects of radiation on the sensori-neural audiory system. Do we know enough about these effects, so as to offer feasible solutions to the important clinical problems? Specifically: 1. Can the high incidence of radiation-induced SNHL be reduced after RTof head and neck tumours, if the auditory pathways had to be included in the radiation fields? This may be possible if the cellular and molecular basis of radiation-induced ototoxicity is known. 2. In patients with significant SNHL and whose auditory pathways had been irradiated before, cochlear implantation may be clinically indicated to restore hearing. In cochlear implantation, a pre-requisite for a successful outcome is intactness of the retro-cochlear pathways. In conventional modern-day RT, even with effective shielding of the brainstem, the cochlear nerve and ganglion are normally still at risk. Therefore, will radiation damage the retro-cochlear pathways, such that cochlear implants cannot work? 3. Combined chemo-radiotherapy using cisplatin (CDDP) has increasingly been used to treat advanced head and neck cancers like NPC (Wee J et al, 2005; Plowman, 2002). As CDDP is also known to be ototoxic, patients receiving combined radiation and CDDP therapy may be at high risk of losing significant hearing. Therefore, will the usefulness of combined therapy in head and neck cancers, be limited by unacceptable synergistic ototoxic effects? The relevant literature on the effects of radiation on the sensori-neural auditory system and related topics will be reviewed, focusing on the issues that have important clinical relevance. This thesis aims to address some of the gaps in knowledge that have the potential to improve clinical outcomes in the immediate and near future. 2. LITERATURE REVIEW 2.1 Characteristics of radiation-induced SNHL 2.1.1. Animal studies The effects of radiation on the inner ear was documented as early as 1905, when Ewald placed radium beads in the middle ear of pigeons and noted labyrinthine symptoms. Girden & Culler (1933) were first to evaluate the effects of radiation on hearing. Experimenting on dogs subjected to various X ray doses, a hearing impairment averaging 5.5 dB was recorded. Novotny (1951) studied the ionizing effects of radiation in guinea pigs and found a hearing impairment of about 8.4dB at 4000 Hz. Kozlov (1959) also noted a hearing imparment of 3.9-9.1dB over 500-8000 Hz in guinea pigs. Gamble et al (1968) subjected the ears of 50 guinea pigs to 500-6000 rads of radiation and found decreased sensitivity of cochlear microphonics of 20-40dB, particularly in the high frequencies. This was consistent with microscopic findings by Keleman (1963), who demonstrated damage to Organ of Corti and cochlear duct in rats, after doses of 100-3000 rads. Tokimoto & Kanagawa (1985) in their study on guinea pigs concluded that even in the absence of gross anatomical cochlear changes, post-irradiation functional hair cell defects could exist Histologically, Keleman (1963) studied temporal bones in rats which had received 1003000 rads of radiation. Haemorrhage was noted to be the most prominent finding with destruction of the cochlear duct, organ of corti and its surrounding elements. Gamble et al (1968) reported early changes of the stria vascularis, accompanied by significant inflammatory responses in the inner ears of guinea pigs which had received 6000 rads of radiation. 2.1.2 Human studies Several clinical studies have recorded SNHL in patients who have had RT for head and neck malignancies, where inner ear structures were included in the radiation fields. Leach (1965) observed SNHL in some of the 56 patients who had received 3000-12000 rads of RT for different head and neck cancers. Morretti (1976) retrospectively studied 137 postirradiated NPC patients, and found to have SNHL of at least 10dB. However, there had also been studies which suggested that radiation did not result in significant SNHL (Evans et al, 1988). In a systematic review of the literature, Raajmakers & Engelen (2002) explained that the conflicting results were attributed to variations in patient groups, size, study design, follow-up period, radiotherapy techniques and presentation of audiometric results. From the pooled data generated from their systematic review, they concluded that about one-third of patients who had received 70 Gy in Gy per fraction near the inner ear, developed hearing loss of 10dB or more at kHz. Schnecht & Karmody (1966) reported the histological features of a deafened man who had received 5,220 rads of radiation to the region of the ears several years ago. Degeneration of the Organ of Corti was noted, with atrophy of the basilar membrane, spiral ligament and stria vascularis. Progressive hearing loss across the various frequencies had been attributed to obliterating endarteritis and eventual fibrosis, leading to vascular compromise (Morreti, 1976). 2.1.3 Prevalence & epidemiology Mencher et al (1995) lamented that it was difficult to establish the exact prevalence of radiation-induced hearing loss because statistics relating to ototoxicity was often not well kept or easily interpreted. Not only was hearing not often considered important in the face of life-threatening diseases, hearing losses were frequently not recorded in those who did not survive. Clinical studies had reported varying prevalence, depending on factors such as dose, period of follow-up and definition/criteria used for hearing loss (Raajmakers & Engelen, 2002). In NPC where the radiation dose received by the ear is relatively high, the incidence of post-RT SNHL had been reported to be as high as 24% (Kwong et al, 1996). In SNHL after RT in NPC patients, sex and age were found to be independent prognostic factors (Kwong et al, 1996). Males were noted to be more susceptible than females in developing SNHL after radiation. Older patients were at greater risk, as pre-existing degenerative changes could have made them more vulnerable to radiation injury (Wang et al, 2003). Post-RT middle ear effusion was identified to be another predicting factor, as toxic materials from chronic inflammation could affect the inner ear (Oh et al, 2004). However, it is argued that the development of post-RT middle ear effusion could have been just another manifestation of radiation damage related to individual variation in susceptibility to radiation (Kwong et al, 1996). 2.1.4 Effect of radiation dose Gamble et al (1968) found in guinea pigs that the extent of inner ear injury correlated with the radiation dose applied. Bohne et al (1985) confirmed in chinchillas that the higher the radiation dose received, the greater the damage to the inner ear. A systematic review on human studies reported increasing loss with increasing dose, starting at about 40 Gy applied in Gy per fraction (Raajmakers & Engelen, 2002). In fact, guidelines for tolerance doses in normal tissues are being used in clinical practice (Emami et al, 1991). It is noteworthy that early human studies had reported the effects of very high doses of radiation to the ears, doses that are no longer used in clinical practice today (Thibadoux et al, 1980; Talmi et al, 1988). Although these could have potentially provided rare opportunities to study the effect of excessively high doses of radiation on the inner ear in humans, documentation of the audiometric data was so poor that it was impossible to draw conclusions on the relationship between high radiation dose and SNHL. 2.1.5 High frequency hearing loss Hearing in the high frequencies were consistently found to be more affected than hearing in the lower frequencies after irradiation (Raajmakers & Engelman, 2002; Talmi et al, 1989). This corresponded to histological observations made in animals that the basal part of the cochlea (which respond to higher frequency sounds) was usually more damaged by radiation than the apical part. Keleman (1963) demonstrated in rats that the apical turn of the cochlea was least affected by radiation doses of 100-3000 rads. Winter (1970) similarly reported in guinea pigs that post-irradiated apical hair cells remained intact while the outer hair cells of the basal turns were affected. Early threshold shifts at high stimulus frequencies are indicators of probable subsequent shifts to low frequencies (Schell et al, 1989). 2.1.6 Early vs late onset Traditionally, radiation-induced SNHL is regarded to have either early or late onsets (Talmi et al, 1989). The existence of early-onset radiation-induced cochlear damage had been convincingly demonstrated in both animals and humans. Winter (1970) reported in guinea pigs, hair cell damage as early as hrs following radiation of 4,000-7,000 rads. Tokimoto & Kanagawa (1985) demonstrated in guinea pigs that sensori-neural loss appeared 3-10 hrs depending on dose administered, and outer hair cells in the basal turn of the cochlea were destroyed about hrs after radiation. In humans, SNHL often occurred near the end or shortly after the completion of fractionated RT (Linskey & Johnstone, 2003). Early-onset SNHL due to inflammatory causes or transient functional disturbances in the stria vascularis, may recover with time (Kwong et al, 1996; Linskey & Johnstone, 2003). In late-onset hearing loss, Schuknecht & Karmordy (1966) observed marked atrophy of the cochlear stria vascularis in a patient who had developed hearing loss years after radiotherapy (5200 rads) for carcinoma of the ear. Grau et al (1991) studied 22 NPC patients with well-documented pre- and post-RT hearing levels over 7-84 months, and found SNHL (especially the higher frequencies) developing 12 months post-RT. Merchant et al (2004) were of the opinion that radiation-induced SNHL could only occur years after irradiation. Delayed-onset hearing loss was found to correlate with age, preexisting SNHL and radiation dosage (Honore et al, 2002). Late-onset radiation-induced SNHL had generally been attributed to progressive vascular compromise from radiationinduced vasculitis obliterans (Morretti, 1976). However, after a review of the relevant literature, Sataloff et al (1994) were not convinced that late-onset hearing loss existed at all. They argued that although significant hearing losses over the whole spectrum of the speech range were often seen several years after radiation, there was little or no convincing evidence to support the notion that hearing loss developing several years following radiation was causally related to radiation therapy itself. In summary, although early-onset radiation-induced SNHL and its progressive nature have generally been well accepted, late-onset radiation-induced SNHL attributed to progressive vascular compromise has not been convincingly shown. Therefore, late-onset radiation-induced hearing loss may well be a later manifestation of progressive earlyonset hearing loss. 2.2 Effect of radiation on retro-cochlear pathways 2.2.1 The sensori-neural auditory pathways According to Hackney (1987), the hair cells of the Organ of Corti transduce vibrations within the cochlea into neural signals. Outer hair cells are contractile and may contribute to mechanical feedback processes, whilst the inner hair cells are apparently the primary sensory cells being innervated by the majority of the afferent fibres. These run in the cochlear nerve to the brain stem where they bifurcate, projecting cochleotopically to the dorsal and ventral cochlear nuclei. A divergence continued in the main routes taken by the ascending pathways; one runs bilaterally from the ventral cochlear nucleus to the superior olivary complex and then to the inferior colliculi, the other runs from the dorsal cochlear nucleus to the contralateral inferior colliculus. 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The MAPK/JNK signalling pathway offers potential therapeutic targets for the prevention of acquired deafness.Curr Drug Targets CNS Neurol Disord;3:325-32. 190 [...]... demonstrated in rabbits, loss of ganglion cell bodies and cochlear ganglia after more than 40 Gy of gamma radiation On the other hand, Keleman (1963) did not observe any effect on the cochlear nerve fibres after single doses of photon radiation less than 20 Gy As the effects of radiation on nervous tissues could be related to dose, Bohne et al (1985) studied the effects of ionising radiation on the. .. seen that the existing literature gave conflicting views on the effect of radiotherapy on the retro-cochlear pathways It is pointed out that many of the studies were retrospective and were based on small numbers of patients Prospective studies on the long-term effects beyond 1 year after irradiation, have not been done 2. 3 Combined effects of radiation and CDDP on SNHL 2. 3.1 Combined chemo-radiotherapy... Although the brainstem is well shielded during RT, the retro-cochlear auditory pathways at the level of the spiral ganglia (located in the modiolus of the cochlea) and cochlear nerve remain at risk, as these structures are not effectively shielded during treatment There have been only limited studies on the effects of radiation on the spiral ganglia and auditory nerve, and the results have not been conclusive... 5,000 rads In these patients, 54% had pronounced deterioration of thresholds and/ or speech discrimination; BERA was abnormal in all but one ear However, because the post-irradiation hearing results are often confounded by 16 the tumour effect on the cochlear nerve (Kaplan et al, 20 03), acoustic neuroma is not a suitable clinical model to study the effects of radiation on the retro-cochlear auditory pathways... (Adrian, 20 02) The Bcl -2 family consists of a group of proteins that function as a checkpoint for cell death and survival signals at the level of the mitochondria Bcl -2 family members can be characterized as either anti-apoptotic (eg Bcl -2) or pro-apoptotic (eg Bax) Upon receiving the stress signal, the proapoptotic proteins in the cytoplasm, BAX and BID, bind to the outer membrane of the mitochondria... based on their prospective study of 22 patients evaluated prior to and 7-84 months after radiotherapy for NPC They found that auditory brainstem evoked responses in 4 patients were severely abnormal and 2 had clinical signs of brainstem dysfunction Grau et al (19 92) had shown by dose response analysis, a correlation between total radiation doses received by the brainstem and the incidence of pathologic... 20 05) 32 2.5 Radiation- induced apoptosis 2. 5.1 Radiobiology The biological effects of radiation can be mediated via a direct or an indirect mechanism (Withers, 19 92) When radiation is absorbed in a biological material, the atoms in the cellular target itself may be ionized or excited, thus initiating the chain of events that leads to a biological change This is the so-called direct action of radiation; ... chincillas to 40 to 90 Gy of radiation, fractionated at 2 Gy per day The animals were sacrificed two years after completion of treatment and the temporal bones were studied microscopically Dose dependent degeneration of sensory and supporting cells as well as loss of 8th nerve fibres in the Organ of Corti, were observed In ears exposed to 40-50Gy of radiation, the incidence of nerve fibre damage was... Vougiouka, 20 03) The hydrolysis product is believed to be the active species reacting mainly with glutathione in the cytoplasm and the DNA in the nucleus, thus inhibiting replication, transcription and other nuclear functions A number of additional properties of CDDP are now emerging, including activation of signal transduction pathways leading to apoptosis Firing of such pathways 18 may originate at the. .. chemo-radiotherapy is increasingly being used clinically to treat advanced head and neck cancers In RT of tumours in the head and neck region, the auditory pathways are often included in the radiation fields and radiation- induced SNHL may result CDDP, widely used as an effective anti-neoplastic drug for these cancers, is also well known to cause ototoxicity Therefore, in combined therapy, the synergistic . Gy of gamma radiation. On the other hand, Keleman (1963) did not observe any effect on the cochlear nerve fibres after single doses of photon radiation less than 20 Gy. As the effects of radiation. and were based on small numbers of patients. Prospective studies on the long-term effects beyond 1 year after irradiation, have not been done. 2. 3 Combined effects of radiation and CDDP on. number of clinical issues related to the effects of radiation on the sensori-neural audiory system. Do we know enough about these effects, so as to offer feasible solutions to the important clinical