Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates: relevance for FLASH radiotherapy.

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From: Canadian Journal of Chemistry(Vol. 100, Issue 4)
Publisher: NRC Research Press
Document Type: Report
Length: 6,838 words
Lexile Measure: 1840L

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Abstract :

Monte Carlo multi-track chemistry simulations were carried out to study the effects of high dose rates on the transient yields of hydronium ions ([H.sub.3][O.sup.+]) formed during low linear energy transfer (LET) radiolysis of both pure, deaerated and aerated liquid water at 25 [degrees]C, in the interval ~1 ps - 10 [micro]s. Our simulation model consisted of randomly irradiating water with N interactive tracks of 300-MeV incident protons (LET ~ 0.3 keV/[micro]m), which simultaneously impact perpendicularly on the water within a circular surface. The effect of the dose rate was studied by varying N. Our calculations showed that the radiolytic formation of [H.sub.3][O.sup.+] causes the entire irradiated volume to temporarily become very acidic. The magnitude and duration of this abrupt "acid-spike" response depend on the value of N. It is most intense at times less than ~10-100 ns, equal to ~3.4 and 2.8 for N = 500 and 2000 (i.e., for dose rates of ~1.9 * [10.sup.9] and 8.7 * [10.sup.9] Gy/s, respectively). At longer times, the pH gradually increases for all N values and eventually returns to the neutral value of seven, which corresponds to the non-radiolytic, pre-irradiation concentration of [H.sub.3][O.sup.+]. It is worth noting that these early acidic pH responses are very little dependent on the presence or absence of oxygen. Finally, given the importance of pH for many cellular functions, this study suggests that these acidic pH spikes may contribute to the normal tissue-sparing effect of FLASH radiotherapy. Key words: liquid water, radiolysis, absorbed dose rate, yields (G values), multiple track model, Monte Carlo multi-track chemistry simulations, hydronium and hydroxide ions, pH, acid spike, FLASH effect. Nous avons realise des simulations Monte Carlo de la trajectoire des particules pour etudier les effets de debits de dose eleves sur les rendements transitoires des ions hydronium ([H.sub.3][O.sup.+]) formes lors de la radiolyse a faible transfert d'energie lineaire (TEL) de l'eau liquide pure, dans des conditions degazees et aerees a 25 [degrees]C, dans l'intervalle d'environ 1 ps a 10 [micro]s. Notre modele de simulation consistait a irradier l'eau de maniere aleatoire avec des protons incidents de 300 MeV decrivant N trajectoires interactives (TEL ~ 0,3 keV/[micro]m) et percutant perpendiculairement l'eau sur une surface circulaire. Nous avons etudie l'effet du debit de dose en faisant varier la valeur N. Nos calculs ont montre que la formation radiolytique d'especes [H.sub.3][O.sup.+] provoque temporairement une acidification tres marquee de l'ensemble du volume irradie. L'ampleur et la duree de cette reponse d'acidification abrupte ( ) dependent de la valeur N. L'intensite culmine a des valeurs de temps inferieures a environ 10 a 100 ns, ou le pH se situe a environ 3,4 et 2,8 pour N = 500 et 2000, respectivement (c.-a-d. pour des debits de dose d'environ 1,9 * [10.sup.9] et 8,7 * [10.sup.9] Gy/s, respectivement). A des temps plus longs, le pH augmente progressivement pour toutes les valeurs de N et revient ultimement a la neutralite (pH = 7), ce qui correspond a la concentration non radiolytique des especes [H.sub.3][O.sup.+] presentes avant l'irradiation. Il importe de souligner que ces reponses rapides d'acidification du pH dependent tres peu de la presence ou de l'absence d'oxygene. Enfin, etant donne l'importance du pH dans de nombreuses fonctions cellulaires, cette etude permet de penser que ces pics d'acidite contribuent peut-etre a l'effet d'epargne des tissus sains qui caracterise la radiotherapie FLASH. [Traduit par la Redaction] Mots-cles : eau liquide, radiolyse, debit de dose absorbee, rendements (valeurs G), modele a trajectoires multiples, simulations Monte Carlo de la trajectoire des particules, ions hydronium et hydroxyde, pH, pic d'acidite, effet FLASH.

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Gale Document Number: GALE|A700235413