Table 2.
Instructions for the use of hippocampal blocking in the radiation treatment planning of hippocampus-avoidance whole brain radiation therapy with simultaneous integrated boost (HA-WBRT+SIB)
| 1 | Define structures for optimization | |||||
| HAR: the union of the two hippocampi with an isotropic margin of 7 mm | ||||||
| Ring_1 cm: a 1 cm isotropic expansion of HAR from which HAR is subtracted | ||||||
| BMs: the union of all PTVBM amenable to SIB | ||||||
| BMs +5 mm: a 5 mm isotropic expansion of BMs | ||||||
| PTVWB-HAR: PTVWB with HAR subtracted | ||||||
| PTVWB-HAR-BMs: PTVWB with HAR and BMs +5 mm subtracted | ||||||
| 2 | Set parameters for treatment fields | |||||
| The isocenter is placed between the two hippocampi | ||||||
| Two complete arcs with a collimator angle near 0° (e.g., 5°) plus two complete arcs with a collimator angle near 90° (e.g., 85°) | ||||||
| In the first step the field openings are adapted to the PTVWB. In the second step the x‑jaws are closed such that the two arcs with identical collimator angle only have a small overlap between 2 cm and 3 cm (Fig. 1). This allows the MLC to block the hippocampi at all angles without also blocking larger parts of the PTV | ||||||
| 3 |
Set objectives—e.g., according to the following template: (prescription dose for PTVWB/PTVBM_1,2,3 …/PTVBM_4,5,6 … of 30 Gy/51 Gy/42 Gy) |
|||||
| Structure | Type | Dose [Gy] | Priority | gEUDa | Blocking | |
| PTVWB-HAR-BMs | Upper | 40.00 | 1000 | – | No | |
| Upper | 30.00 | 1000 | – | |||
| Lower | 27.00 | 1000 | – | |||
| Lower | 29.20 | 1000 | – | |||
| Lower | 29.50 | 1000 | – | |||
| Lower | 28.50 | 1000 | – | |||
| Upper gEUD | 33.00 | 1000 | 40.0 | |||
| Upper gEUD | 32.00 | 1000 | 20.0 | |||
| Upper gEUD | 31.00 | 1000 | 10.0 | |||
| GTVBM_1,2,3 … | Lower | 51.00 | 1000 | – | No | |
| PTVBM_1,2,3 … | Upper | 53.00 | 1000 | – | No | |
| Lower | 51.00 | 1000 | – | |||
| Lower | 50.00 | 1000 | – | |||
| Upper gEUD | 52.00 | 1000 | 40.0 | |||
| Lower gEUD | 51.50 | 1000 | −40.0 | |||
| GTVBM_4,5,6 … | Lower | 43.00 | 1000 | – | No | |
| PTVBM_4,5,6 … | Upper | 43.00 | 1000 | – | No | |
| Lower | 41.80 | 1000 | – | |||
| Lower | 41.30 | 1000 | – | |||
| Upper gEUD | 43.00 | 1000 | 40.0 | |||
| Lower gEUD | 42.50 | 1000 | −40.0 | |||
| Hippocampus left, right | Upper | 8.00 | 800 | – | Yes | |
| Upper | 6.00 | 800 | – | |||
| Mean | 8.00 | 700 | – | |||
| Brainstem | Upper | 33.00 | 1000 | – | No | |
| Lower | 28.50 | 1000 | – | |||
| Upper gEUD | 30.30 | 1000 | 40.0 | |||
| Upper gEUD | 30.00 | 600 | 40.0 | |||
| Chiasm | Upper | 31.00 | 1000 | – | No | |
| Lower | 28.01 | 100 | – | |||
| Upper gEUD | 30.50 | 1000 | 40.0 | |||
| Optic nerves left, right | Upper | 31.00 | 1000 | – | No | |
| Eye left, right | Mean | 7.00 | 600 | – | No | |
| Upper gEUD | 11.00 | 600 | 10.0 | |||
| Lens left, right | Upper | 5.00 | 800 | – | No | |
| Upper gEUD | 5.00 | 1000 | 40.0 | |||
| Upper gEUD | 4.00 | 800 | 5.0 | |||
| Ring_1 cm | Upper | 33.00 | 700 | – | No | |
| Lower | 27.50 | 700 | – | |||
| Lower | 28.50 | 700 | – | |||
| Lower | 29.00 | 900 | – | |||
| Lower | 28.50 | 900 | – | |||
| Upper gEUD | 33.00 | 600 | 40.0 | |||
| z_low (see below) | Lower | 29.00 | 1000 | – | No | |
| Lower | 28.50 | 1000 | – | |||
| Lower | 28.00 | 1000 | – | |||
| z_high (see below) | Upper | 33.00 | 1000 | – | No | |
| Upper gEUD | 32.00 | 1000 | 40.0 | |||
| z_out (see below) | Upper gEUD | 26.00 | 800 | 10.0 | No | |
| Upper gEUD | 29.00 | 1000 | 40.0 | |||
| 4 | Fine tuning | |||||
| Convert 28.5 Gy isodose to structure and subtract the result from PTVWB-HAR to define the areas in which the PTV is underdosed. The corresponding volume is found as z_low in the objective template | ||||||
| Convert 28.5 Gy isodose to structure and crop the result from PTVWB to define the areas outside the PTV which receive a high dose. The corresponding volume is found as z‑out in the objective template | ||||||
| Convert 32 Gy isodose to structure and crop the result from BMs +5 mm with an additional margin of 5 mm to define the areas in which the PTV is overdosed. The corresponding volume is found as z‑high in the objective template | ||||||
| Optimize and repeat these steps until the result is satisfactory. Similar volumes can of course be used to optimize the dose inside the SIB volumes. However, according to our experience this is hardly necessary | ||||||
WB whole brain, SIB simultaneous integrated boost, HAR hippocampus avoidance region, BMs brain metastases, RES resection cavity, PTV planning target volume, GTV gross tumor volume, MLC multileaf collimator, gEUD generalized equivalent uniform dose, gEUDa weighting parameter a of the generalized equivalent uniform dose