Hualin Zhang PhD, Presenter: Nothing to Disclose

To evaluate the impact of fraction dose of hypo-fractionated grid therapy in treating melanoma cancers.

We evaluated melanoma cell and normal tissue cell radiobiologic responses to a Monte Carlo simulated 2D dose distribution of a 6MV photon beam delivered through a commercially available grid collimator in hypo-fractionated grid therapy (fewer fractions and higher dose/fraction, herein 5, 10, 15, 20 and 30 Gy/fraction). The linear-quadratic (LQ) model was used to calculate the cell survival statistics for melanoma and normal tissue cells receiving various doses and numbers of fractions. The melanoma tumors were represented by experimentally determined LQ parameters. Three types of normal tissue, i.e.; radiosensitive, moderate radiosensitive, and radioresistant, were used to estimate normal tissue sparing effects. A therapeutic ratio (TR) was developed to estimate whether grid therapy provided a therapeutic advantage over equivalent open field radiotherapy. A TR greater than 1 would indicate a survival advantage for normal tissue cells.

The melanoma cell and normal tissue cell survival statistics for hypo-fractionated grid therapy with different fraction doses and numbers were calculated. The equivalent open field dose (EOD) to the cancer cells and the normal tissue sparing effect or the therapeutic ratio (TR) were derived. The EOD was found to be only a fraction of the prescribed dose. The TRs were dependent on the size of fraction dose, the number of fractions, and the LQ parameters of neoplastic and normal tissue cells. The TRs increased with the dose per fraction, greater numbers of fractions, and the a/ß ratio. For radiosensitive normal tissue, the TR of a single fraction was greater than 1 when the fraction dose was greater than 10.0 Gy. But in radio-resistant normal tissue, single fraction grid therapy (5 to 30 Gy/fraction) provided no therapeutic advantage in treatment of melanoma.

Our study confirmed that a single fraction of grid therapy has a therapeutic advantage only when the tumor is surrounded by radiosensitive normal tissues. In radioresistant normal tissue, a single fraction of the grid therapy provides no more benefit than open field therapy, but multiple fractions produce TRs greater than 1. At the same dose per fraction, grid therapy provides a greater therapeutic advantage in treating acutely responding melanoma cells because of their larger a/ß values. This result was not seen in late responding melanoma cells.

Zhang, H, Impact of Fraction Dose in Hypo-fractionated Megavoltage Grid Therapy.  Radiological Society of North America 2007 Scientific Assembly and Annual Meeting, November 25 - November 30, 2007 ,Chicago IL. http://archive.rsna.org/2007/6001293.html