Relative biologic effectiveness in terms of tumor response of {sup 125}I implants compared with {sup 60}Co gamma rays

Journal Article · Thu Sep 01 04:00:00 EDT 2005 · International Journal of Radiation Oncology, Biology and Physics

DOI:https://doi.org/10.1016/j.ijrobp.2005.05.009· OSTI ID:20698545

Lehnert, Shirley ^[1]; Reniers, Brigitte ^[2]; Verhaegen, Frank ^[3]

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1. Department of Oncology, Montreal, QB (Canada)
2. Service de Radiotherapie, Hopital Maisonneuve-Rosement, Montreal, QB (Canada)
3. Medical Physics Unit, McGill University, Montreal, QB (Canada)

Purpose: To measure the relative biologic effectiveness (RBE) for {sup 125}I seeds compared with external beam radiotherapy using a clinically relevant in vivo system. Methods and Materials: Photon emission from a detailed source model was simulated using the Monte Carlo code MCNP4C, sampling from a {sup 125}I spectrum. The mouse RIF-1 tumor was treated with either temporary implant of an {sup 125}I seed or with {sup 60}Co gamma rays. The tumors were always the same size at the initiation of treatment, and the endpoint was growth inhibition. Results: The dose-response curve for both modalities was close to linear and was independent of the initial {sup 125}I activity (dose rate) for the range investigated. Calculation of the RBE for tumor response requires assigning a unique value for the tumor dose that is not homogenous but depends on the distance from the {sup 125}I source. Because tumor regrowth will depend on the subpopulation of cells that have the greatest probability of survival (i.e., those at the greatest distance from the {sup 125}I source), one approach is to use the dose to this population. On this basis, the RBE for {sup 125}I compared with {sup 60}Co gamma rays is 1.5. If the {sup 125}I dose is computed as the average dose to the tumor, corrected for the dose that is wasted as overkill in the cell population closest to the center of the {sup 125}I seed, the RBE is 1.4. Conclusion: The result, an RBE of 1.4-1.5 is similar to findings obtained by other methods, supporting the validity of this approach to derive an RBE with validity in a clinical context.

• OSTI ID: 20698545
• Journal Information: International Journal of Radiation Oncology, Biology and Physics, Journal Name: International Journal of Radiation Oncology, Biology and Physics Journal Issue: 1 Vol. 63; ISSN IOBPD3; ISSN 0360-3016
• Country of Publication: United States
• Language: English

Related Subjects

62 RADIOLOGY AND NUCLEAR MEDICINE
BRACHYTHERAPY
DOSE RATES
DOSIMETRY
GAMMA RADIATION
IN VIVO
IODINE 125
MICE
MONTE CARLO METHOD
NEOPLASMS
PHOTON EMISSION
PROBABILITY
RADIATION DOSES
RADIATION SOURCE IMPLANTS
RBE
SAMPLING