Planning study to compare dynamic and rapid arc techniques for postprostatectomy radiotherapy of prostate cancer

PURPOSE

To compare our standard technique for postprostatectomy radiotherapy of prostate cancer, i.e. using two lateral conformal dynamic arcs with volumetric-modulated arc therapy (VMAT) performed with the RapidArc(®) (Varian Medical Systems, Palo Alto, CA, USA). The plans were referred to as DA and RA, respectively.

MATERIALS AND METHODS

The treatment plans of 44 patients receiving adjuvant/salvage radiotherapy in the first months of 2010 were compared. In all cases, the prescribed total dose was 66-68.2 Gy (2.2 Gy per fraction). Both DA and RA plans were optimized in terms of dose coverage and constraints.

RESULTS

Small differences between the techniques were observed for planning target volume (PTV) dose distribution, whereas significant differences in sparing of organs at risk (OARs) were recorded (p < 0.0001). The OAR values (median; 95 % confidence interval, CI) were: rectum: D30 % = 60.7 Gy (59.40-62.04 Gy) and 48.2 Gy (46.40-52.72 Gy), D60 % = 34.1 Gy (28.50-38.92 Gy) and 27.7 Gy (21.80-31.51 Gy); bladder: D30 % = 57.3 Gy (45.83-64.53 Gy) and 46.4 Gy (33.23-61.48 Gy), D50 % = 16.4 Gy (11.89-42.38 Gy) and 17.2 Gy (10.97-27.90 Gy), for DA and RA, respectively. Treatment times were very similar, whereas the monitor units (MU) were 550 ± 29 versus 277 ± 3 for RA and DA, respectively.

CONCLUSION

Dose-volume histograms (DVHs) show improvements in OAR sparing with RA. However, the RA technique is associated with almost double the number of MUs compared to DA. Regarding the PTV, DA is slightly superior in terms of D2 % and dose homogeneity. On the whole, the results suggest that RA be the favorable technique.

The bone anabolic therapy

Teriparatide (TPTD), the amino-terminal parathyroid hormone recombinant peptide [PTH (1–34)], is a drug with a proven anabolic action on the bone, effective in preventing vertebral and non-vertebral fragility fractures. Recent publications have investigated in great detail the TPTD action on the cortical bone, highlighting the increased strength in the critical zone of the hip with high risk of fracture in osteoporotic patients Poole (PLoS ONE 6:e16190, 2011). In November 2002, TPTD was approved by the FDA for use in post-menopausal women and men with osteoporosis at high risk of fracture and in patients with glucocorticoid-induced osteoporosis and, since then, has been used to treat more than 1 million patients worldwide (J Bone Miner Res 27(12):2429-2437, 2012). The unchanged safety profile and the well-known mechanism of action of this drug have led doctors to explore the use of TPTD in other conditions such as delayed fracture healing, non-union, osteonecrosis of the jaw, etc. The positive reports that have resulted from these studies are helping to hypothesize a new perspective on the wider use of this drug, but warrant further clinical investigation to consolidate these results.

Small field segments surrounded by large areas only shielded by a multileaf collimator: comparison of experiments and dose calculation

PURPOSE

Complex radiotherapy fields delivered using a tertiary multileaf collimator (MLC) often feature small open segments surrounded by large areas of the beam only shielded by the MLC. The aim of this study was to test the ability of two modern dose calculation algorithms to accurately calculate the dose in these fields which would be common, for example, in volumetric modulated arc treatment (VMAT) and study the impact of variations in dosimetric leaf gap (DLG), focal spot size, and MLC transmission in the beam models.

METHODS

Nine test fields with small fields (0.6-3 cm side length) surrounded by large MLC shielded areas (secondary collimator 12 × 12 cm(2)) were created using a 6 MV beam from a Varian Clinac iX linear accelerator with 120 leaf MLC. Measurements of output factors and profiles were performed using a diamond detector (PTW) and compared to two dose calculations algorithms anisotropic analytical algorithm [(AAA) and Acuros XB] implemented on a commercial radiotherapy treatment planning system (Varian Eclipse 10).

RESULTS

Both calculation algorithms predicted output factors within 1% for field sizes larger than 1 × 1 cm(2). For smaller fields AAA tended to underestimate the dose. Profiles were predicted well for all fields except for problems of Acuros XB to model the secondary penumbra between MLC shielded fields and the secondary collimator. A focal spot size of 1 mm or less, DLG 1.4 mm and MLC transmission of 1.4% provided a generally good model for our experimental setup.

CONCLUSIONS

AAA and Acuros XB were found to predict the dose under small MLC defined field segments well. While DLG and focal spot affect mostly the penumbra, the choice of correct MLC transmission will be essential to model treatments such as VMAT accurately.