Radioembolization of hepatocarcinoma with (90)Y glass microspheres: development of an individualized treatment planning strategy based on dosimetry and radiobiology.

PURPOSE: The aim of this study was to optimize the dosimetric approach and to review the absorbed doses delivered, taking into account radiobiology, in order to identify the optimal methodology for an individualized treatment planning strategy based on (99m)Tc-macroaggregated albumin (MAA) single photon emission computed tomography (SPECT) images. METHODS: We performed retrospective dosimetry of the standard TheraSphere® treatment on 52 intermediate (n = 17) and advanced (i.e. portal vein thrombosis, n = 35) hepatocarcinoma patients with tumour burden < 50% and without obstruction of the main portal vein trunk. Response was monitored with the densitometric radiological criterion (European Association for the Study of the Liver) and treatment-related liver decompensation was defined ad hoc with a time cut-off of 6 months. Adverse events clearly attributable to disease progression or other causes were not attributed to treatment. Voxel dosimetry was performed with the local deposition method on (99m)Tc-MAA SPECT images. The reconstruction protocol was optimized. Concordance of (99m)Tc-MAA and (90)Y bremsstrahlung microsphere biodistributions was studied in 35 sequential patients. Two segmentation methods were used, based on SPECT alone (home-made code) or on coregistered SPECT/CT images (IMALYTICS™ by Philips). STRATOS™ absorbed dose calculation was validated for (90)Y with a single time point. Radiobiology was used introducing other dosimetric variables besides the mean absorbed dose D: equivalent uniform dose (EUD), biologically effective dose averaged over voxel values (BEDave) and equivalent uniform biologically effective dose (EUBED). Two sets of radiobiological parameters, the first derived from microsphere irradiation and the second from external beam radiotherapy (EBRT), were used. A total of 16 possible methodologies were compared. Tumour control probability (TCP) and normal tissue complication probability (NTCP) were derived. The area under the curve (AUC) of the receiver-operating characteristic (ROC) curve was used as a figure of merit to identify the methodology which gave the best separation in terms of dosimetry between responding and non-responding lesions and liver decompensated vs non-decompensated liver treatment. RESULTS: MAA and (90)Y biodistributions were not different (71% of cases), different in 23% and uncertain in 6%. Response correlated with absorbed dose (Spearman's r from 0.48 to 0.69). Responding vs non-responding lesion absorbed doses were well separated, regardless of the methodology adopted (p = 0.0001, AUC from 0.75 to 0.87). EUBED gave significantly better separation with respect to mean dose (AUC = 0.87 vs 0.80, z = 2.07). Segmentation on SPECT gave better separation than on SPECT/CT. TCP(50%) was at 250 Gy for small lesion volumes (<10 cc) and higher than 1,000 Gy for large lesions (>10 cc). Apparent radiosensitivity values from TCP were around 0.003/Gy, a factor of 3-5 lower than in EBRT, as found by other authors. The dose-rate effect was negligible: a purely linear model can be applied. Toxicity incidence was significantly larger for Child B7 patients (89 vs 14%, p < 0.0001), who were therefore excluded from dose-toxicity analysis. Child A toxic vs non-toxic treatments were significantly separated in terms of dose averaged on whole non-tumoural parenchyma (including non-irradiated regions) with AUC from 0.73 to 0.94. TD50 was ≈ 100 Gy. No methodology was superior to parenchyma mean dose, which therefore can be used for planning, with a limit of TD15 ≈ 75 Gy. CONCLUSION: A dosimetric treatment planning criterion for Child A patients without complete obstruction of the portal vein was developed.

A dosimetric treatment planning strategy in radioembolization of hepatocarcinoma with 90Y glass microspheres.

AIM: Our goal was to limit liver toxicity and to obtain good efficacy by developing a dosimetric treatment planning strategy. While several dosimetric evaluations are reported in literature, the main problem of the safety of the treatment is rarely addressed. Our work is the first proposal of a treatment planning method for glass spheres, including both liver toxicity and efficacy issues. METHODS: Fifty-two patients (series 1) had been treated for intermediated/advanced hepatocellular carcinoma (HCC) with glass spheres, according to the Therasphere® prescription of 120 Gy averaged on the injected lobe. They were retrospectively evaluated with voxel dosimetry, adopting the local deposition hypothesis. Regions of interest on tumor and non tumor parenchyma were drawn to determine the parenchyma absorbed dose, averaged also on non irradiated voxels, excluding tumor voxels. The relationship between the mean non tumoral parenchyma absorbed dose D and observed liver decompensation was analyzed. RESULTS: Basal Child-Pugh strongly affected the toxicity incidence, which was 22% for A5, 57% for A6, 89% for B7 patients. Restricting the analysis to our numerically richest class (basal Child-Pugh A5 patients), D median values were significantly different between toxic (median 90 Gy) and non toxic treatments (median 58 Gy) at a Mann-Withney test, (P=0.033). Using D as a marker for toxicity, the separation of the two populations in terms of area under ROC curve was 0.75, with 95% C.I. of [0.55-0.95]. The experimental Normal Tissue Complication Probability (NTCP) curve as a function of D resulted in the following values: 0%, 14%, 40%, 67% for D interval of [0-35] Gy, [35-70] Gy, [70-105] Gy, [105-140] Gy. DISCUSSION: A limit of about 70 Gy for the mean absorbed dose to parenchyma was assumed for A5 patients, corresponding to a 14% risk of liver decompensation. This result is applicable only to our administration conditions: glass spheres after a decay interval of 3.75 days. Different safety limit (40 Gy) are published for resin spheres, characterized by higher number of particle per GBq (more uniform irradiation, bigger biological effect for the same absorbed dose). CONCLUSION: As result of this study we suggest a constraint of about 70 Gy mean absorbed dose to liver non tumoral parenchyma, corresponding to about 15% probability of radioinduced liver decompensation while still aiming at achieving an absorbed of several hundreds of Gy to lesions.

Need, feasibility and convenience of dosimetric treatment planning in liver selective internal radiation therapy with (90)Y microspheres: the experience of the National Tumor Institute of Milan.

In most centres, the choice of the optimal activity to be administered in selective intra-arterial radioembolization with microspheres is nowadays based on empirical models which do not take into account the evaluation of tumour and non tumour individual absorbed dose, despite plenty of published data which showed that local efficacy is correlated to tumour absorbed dose, and that the mean absorbed dose is a toxicity risk factor. A pitfall of the crudest, empirical tumour involvement method are 20 deaths in a single centre which adopted it to administer the whole liver, or the need of systematic 25% subjective reduction of activity prescribed with body surface area method. In order to develop a possibly safer and more effective strategy based on real individual dosimetry, we examine first external beam liver radiation therapy results. The half century experience has something to be borrowed: the volume effect, according to which the smaller the fraction of the irradiated liver volume, the higher the tolerated dose. Different tolerance for different underlying disease or previous non radiation treatment is to be expected. Radiobiological models experience also has to be inherited, but not their dose reference values. Then we report the published dosimetric experience about (90)Y microsphere radioembolization of primary and metastatic liver tumours. In addition we also present original data from our growing preliminary experience of more refined (99m)Tc MAA SPECT based calculations in hepatocarcinoma patients. This overcame the mean dose approach in favour of the evaluation of dose distribution at voxel level. An insight into dosimetry issues at microscopic level (lobule level) is also provided, from which the different radiobiological behaviour between resin and glass spheres can be understood. For tumour treatment, an attenuation corrected (99m)Tc- SPECT based treatment planning strategy can be proposed, although quantitative efficacy thresholds should be differentiated according to the kind of pathology and previous treatment. For non tumour liver parenchyma, data in favour of a relationship between absorbed dose and dangerous effects are encouraging. Unfortunately in hepato-cellular carcinoma, some confounding factors may hamper the adequate estimation of the risk of toxicity. First there is a lack of consensus about the exact definition of toxicity after (90)Y microsphere radioembolization. Second, for HCC patients, progression of both cancer and cirrhosis can simulate a radioinduced toxicity, making the analysis more complex.